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Project P709
Planning of Full Optical Network
Deliverable 2
Basic factors influencing optical networks
Volume 3 of 5: Annex B – Protection in the Optical Layer
Suggested readers:
 PNOs studying potential upgrade possibilities for their SDH networks
 System engineers and network planners
 Experts on standard bodies of ITU-T SG13 (Q 19), SG15 (Q 16, 17, 20) and ETSI
TM-1WG2/WG3
 Researchers engaged in the field of optical transmission networks and technologies
For full publication
May 1999
EURESCOM PARTICIPANTS in Project P709 are:
 Finnet Group
 Swisscom AG
 Deutsche Telekom AG
 France Télécom
 MATÁV Hungarian Telecommunications Company
 TELECOM ITALIA S.p.a.
 Portugal Telecom S.A.
 Telefonica S.A.
 Sonera Ltd.
This document contains material which is the copyright of certain EURESCOM
PARTICIPANTS, and may not be reproduced or copied without permission
All PARTICIPANTS have agreed to full publication of this document
The commercial use of any information contained in this document may require a
license from the proprietor of that information.
Neither the PARTICIPANTS nor EURESCOM warrant that the information
contained in the report is capable of use, or that use of the information is free from
risk, and accept no liability for loss or damage suffered by any person using this
information.
This document has been approved by EURESCOM Board of Governors for
distribution to all EURESCOM Shareholders.
© 1999 EURESCOM Participants in Project P709
Deliverable 2
Volume 3: Annex B – Protection in the optical layer
Preface
(Prepared by the EURESCOM Permanent Staff)
The advances in optical fibre transmission technology over the past years have kept
pace with the demand for increased bandwidth. In particular the introduction of the
WDM technology enables Telecom Operators to upgrade the capacity of their
networks by an order of magnitude. The evolution of photonics makes the
development of optical switching and routing structures in the core and metropolitan
part of the transport network possible.
As a consequence, the development of an optical network infrastructure will enable
the flexible, reliable and transparent provision of transport services for any type of
traditional and innovative services and applications. Taking into consideration the
current trends, the objective of network planning is to find the best possible balance
between network implementation cost, network flexibility, network availability and
survivability, subject to service requirements and topological constraints.
The aim of the P709 EURESCOM Project is to investigate a number of alternative
strategies for the planning of the optical transport network - with massive deployment
of WDM, OADM, and small size OXC- that will be used in a middle term future.
This is the second Deliverable (D2) of P709. D2 summarises the most important
factors that have to be taken into account when preparing the planning of optical
networks. Restoration and protection techniques implemented in optical networks are
assessed in terms of requirements, constraints on network planning and upgrading, as
well as their interaction with client layer functionalities. A study of resource
allocation and impact on network planning and upgrading is also presented.
We should remind the reader that the first P709 Deliverable (D1) provided an
overview over network architectures, which potentially may be used in the future and
D3 will give an analysis of the existing network planning methods, plus guidelines for
planning future optical networks.
The present Deliverable (D2) is a very useful study for Optical Network planners &
system engineers, and experts on Standard Bodies of ITU-T SG15 and ETSI TM1
(WG2 & WG3).
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
page ii (vi)
Deliverable 2
© 1999 EURESCOM Participants in Project P709
Deliverable 2
Volume 3: Annex B – Protection in the optical layer
Table of contents
Preface ............................................................................................................................ i
Table of contents .......................................................................................................... iii
Abbreviations ................................................................................................................. v
1
Introduction ............................................................................................................ 1
2
Optical protection techniques ................................................................................ 2
2.1 Identification of protection techniques ........................................................ 2
2.1.1 Optical channel protection .............................................................. 2
2.1.2 Optical multiplex section protection ............................................... 2
2.1.3 Dedicated protection ....................................................................... 3
2.1.4 Shared protection ............................................................................ 3
2.2 Examples of optically protected rings ......................................................... 3
2.2.1 Optical Channel Dedicated Protection Ring (OC-DPRing) ........... 3
2.2.2 Optical Channel Shared Protection Ring (OC-SPRing) ................. 4
2.2.3 Optical Multiplex Section Dedicated Protection Ring (OMSDPRing) .......................................................................................... 5
2.2.4 Optical Multiplex Section Shared Protection Ring (OMSSPRing) ........................................................................................... 5
3
Protection of interconnected optical network domains.......................................... 7
3.1 Selected reference network configurations.................................................. 7
3.2 Interconnection of sub-networks and protection ......................................... 7
3.3 Protection with optical drop and continue functionality ........................... 10
3.3.1 Two-level OMS-SPRing architecture ........................................... 10
3.3.2 Two-level OMS-SPRing-Optical Mesh architecture .................... 13
3.3.3 Two-level Optical Mesh-OMS-SPRing architecture .................... 13
3.4 Conclusions................................................................................................ 15
4
Wavelength conflicts under protected state in OMS-SPRings ............................ 16
4.1 Protection in OMS-SPRings ...................................................................... 16
4.1.1 Architecture .................................................................................. 16
4.1.2 Single Span Failure in OMS-SPRing............................................ 17
4.1.3 Multiple Span Failure in OMS-SPRing ........................................ 18
4.1.4 Node Failure in OMS-SPRing ...................................................... 19
4.2 Misconnections in OMS-SPRings ............................................................. 20
4.2.1 Introduction ................................................................................... 20
4.2.2 Misconnections without low-priority traffic ................................. 21
4.2.3 Misconnections with low-priority traffic ...................................... 22
4.2.4 Wavelength squelching ................................................................. 24
4.3 Conclusion ................................................................................................. 24
5
Identification of constraints on network planning ............................................... 26
5.1 Protection of individual channels .............................................................. 26
5.1.1 Coloured Section Ring (CSR)....................................................... 26
5.1.2 CSR-CSR hierarchical network .................................................... 28
5.1.3 Optical Multiplex Section Shared Protection Ring (OMSPR) ..... 28
5.2 Protection Priority Classes......................................................................... 29
5.2.1 Priority classes .............................................................................. 30
5.2.2 Other performance priority classes definition .............................. 30
© 1999 EURESCOM Participants in Project P709
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5.3
5.4
5.5
Deliverable 2
Protection against simultaneous failures .................................................... 31
5.3.1 General characteristics of single domain optical protection ......... 31
5.3.2 Protection against simultaneous failures in interconnected
optical network domains ............................................................... 32
Existing infrastructures in optical network protection ............................... 34
5.4.1 Existing point-to-point WDM systems.......................................... 34
5.4.2 Existing SDH systems ................................................................... 36
Conclusions ................................................................................................ 36
5.5.1 Protection of individual channels .................................................. 36
5.5.2 Protection priority classes ............................................................. 37
5.5.3 Protection against simultaneous failures ....................................... 37
5.5.4 Existing infrastructures in optical network protection .................. 38
6
Identification of constraints on network upgrading ............................................. 39
6.1 Upgrading Coloured Section Rings ........................................................... 39
6.1.1 Capacity extension ........................................................................ 39
6.1.2 Change in structure ....................................................................... 40
6.1.3 Changing the protection ................................................................ 41
6.2 Upgrading CSR-CSR Hierarchical Networks ............................................ 42
6.2.1 Inserting new nodes within a station ............................................. 42
6.2.2 Inserting new nodes ....................................................................... 44
6.2.3 Inserting new rings on the lower level .......................................... 44
6.2.4 Changing protection ...................................................................... 45
6.3 Conclusions ................................................................................................ 45
7
Conclusions .......................................................................................................... 46
References .................................................................................................................... 48
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© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
Abbreviations
ADM
Add Drop Multiplexer
AIS
Alarm Indication Signal
AU-N
Administrative Unit (level) N
CSR
Coloured Section Ring
DXC
Digital Cross Connect
EDFA
Erbium Doped Fibre Amplifier
ETSI
European Telecommunications Standards Institute
HO
Higher Order
HOP
Higher Order Path
ITU-T
Telecommunication standardization sector
Telecommunication Union
LO
Lower Order
LOP
Lower Order Path
MS
Multiplex Section
MSP
Multiplex Section Protection
MS-SPR
Multiplex Section Shared Protection Ring
OA
Optical Amplifier
OADM
Optical Add Drop Multiplexer
OC
Optical Channel
OCH
Optical Channel
OC-DPR
Optical Channel Dedicated Protection Ring
OC-SPR
Optical Channel Shared Protection Ring
OMS
Optical Multiplex Section
OMS-DPR
Optical Multiplex Section Dedicated Protection Ring
OMS-SPR
Optical Multiplex Section Shared Protection Ring
OTM
Optical Terminal Multiplexer
OTS
Optical Transmission Section
OXC
Optical Cross Connect
P
Protection
RS
Regenerator Section
SDH
Synchronous Digital Hierarchy
SDXC
Synchronous Digital Cross Connect
SNCP
Sub Network Connection Protection
STM-N
Synchronous Transport Module (level) N
© 1999 EURESCOM Participants in Project P709
of International
page v (vi)
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TM
Terminal Multiplexer
W
Working
WDM
Wavelength Division Multiplexing
page vi (vi)
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© 1999 EURESCOM Participants in Project P709
Deliverable 2
1
Volume 3: Annex B – Protection in the optical layer
Introduction
Survivability will be of utmost importance in future high capacity all-optical
networks. Different strategies can be used to improve the survivability of the transport
network. Two main approaches are protection and restoration. Protection uses a preassigned capacity between nodes to replace the failed or degraded transport entities.
Restoration can use any capacity available between nodes to find a transport entity
that can replace the failed one. Compared to restoration techniques automatic
protection is much faster but utilises less efficiently the available network resources.
Automatic protection can be applied to different network architectures such as point
to point, ring and meshed networks. Automatic protection is widely used in SDH
networks. The availability of configurable optical add drop multiplexers will allow
network operators to introduce survivable optical ring networks. Several types of
optical ring protection can be identified depending on architectural layer where
protection is implemented. In meshed networks optical sub network connection
protection can be applied.
The selected protection strategy has a strong impact on network planning and network
upgrading possibilities. Therefore, it is extremely important to understand the
characteristics of different optical protection methods when high capacity optical
networks are planned.
© 1999 EURESCOM Participants in Project P709
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2
Optical protection techniques
2.1
Identification of protection techniques
Deliverable 2
Optical protection techniques can be classified according to the architectural layer
were protection is implemented, and also in terms of how the capacity is allocated to
service and protection channels. The combination of these characteristic results is
four possible types of protection.
The following table presents examples of optical architectures for these different
types of protection:
Layer\Capacity
Dedicated
Shared
Optical Channel
OC-DPRing
OC-SPRing
1+1 link (OC)
1:1 link (OC)
OMS-DPRing
OMS-SPRing
1+1 link (OMS)
1:1 link (OMS)
Optical Multiplex Section
Table I – Protection in optical architectures
Detailed descriptions of the above optically protected architectures can be found in
Deliverable 1 of EURESCOM Project P615. The comparison of performance and
requirements of some of these architectures can be found in Deliverable 2 of the same
Project.
We will now characterise the different types of protection that can be applied in
optical networks, from the layer and capacity points of view.
2.1.1
Optical channel protection
Optical channel protection is implemented in the Optical Channel (OC) layer. Each
optical channel can be protected individually. This allows the selection of which
channels to protect in the multiplex. The protection wavelength is transmitted in the
protection fibre, in the direction opposite to the service channel (ring case), or over a
physically independent path (mesh case).
If the optical layer supports SDH client signals, then the OC-protection is functionally
equivalent to SDH MS-protection. As such, in this case a proper consideration of
advantages and disadvantages should be taken before deciding to replace the SDH
protection by the OC-protection. The possibility of keeping protection both in the
SDH and optical layers could also be considered, in order to improve the survivability
of the network. However, in this case, a proper interworking of the protection
mechanisms should be ensured.
2.1.2
Optical multiplex section protection
Optical multiplex section protection is implemented in the Optical Multiplex Section
(OMS) layer. In this case, all channels in the multiplex are simultaneously protected.
This may lead to a great simplification of the client layer electronic equipment, since
now there is no need for electronically implemented protection systems for the
individual client signals (e.g.: SDH protection), which imply the use of duplicated line
interfaces, control electronics and software. Also, the reliability of the network may
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© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
increase due to the simplification of the electronic equipment and to the more
efficient protection. OMS-protection is especially interesting in networks supporting
large numbers of optical channels.
2.1.3
Dedicated protection
Dedicated protection means that the entity to be protected is protected using dedicated
resources. For example, in a ring the OCs are duplicated when inserted, one going in
the clockwise direction, while the other goes in the anti-clockwise direction. The
receiver selects the best signal by means of a switch. If the duplication and selection
of channels is done in the OC layer, we have an OC-DPRing. If the duplication and
selection is done in the OMS layer, we have an OMS-DPRing. A 2-fibre ring supports
bi-directional communication in an OC-DPRing, since each channel can be
transmitted clockwise or anti-clockwise. However, a bi-directional OMS-DPRing
needs four fibres, since the channels in the multiplex are processed as a whole.
In meshes, made up of point-to-point links, two physically disjoint pairs of fibres must
be used to implement bi-directional communications with dedicated protection, one
pair supporting the working channels and the other supporting the protection
channels. The demands are transferred to the channels in the protection fibre if the
working fibre fails.
2.1.4
Shared protection
Shared protection means that the working channels share the protection capacity.
Under normal conditions all demands are routed over the working channels, and the
protection capacity can support low-priority traffic, though this results in a more
complex node implementation.
Optical rings with shared protection have the capacity of the fibres divided between
service and protection channels. As such, a 2-fibre Shared Protection Ring (SPRing)
is a bi-directional ring, since one fibre supports clockwise traffic and the other
supports anticlockwise traffic.
Shared protection for optical meshes (in point-to-point links) would use a similar
sharing of the fibre capacity: half the channels in one of the fibre pairs would support
half of the demands, and half of the channels in the other fibre pair would support the
remaining demands. The remaining half of the fibre capacities would serve as
protection capacity. If one fibre pair failed, the working channels would be transferred
to the protection channels of the other fibre pair, keeping the link in service.
2.2
Examples of optically protected rings
In this section we will describe particular optical ring architectures that implement all
possible combinations of optical protection characteristics.
2.2.1
Optical Channel Dedicated Protection Ring (OC-DPRing)
The OC-DPRing is an example of ring architecture with optical routing and optical
protection. Optical routing allows a virtual mesh topology by allocation of different
wavelengths to each possible link connecting pairs of nodes. The OC-DPRing with 4fibres allows bi-directional traffic with 1+1 protection. It offers greater flexibility than
its 2-fibre counterpart, and requires fewer wavelengths. The total number of
© 1999 EURESCOM Participants in Project P709
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wavelengths needed can be reduced if wavelengths are re-used. The following figure
represents an OC-DPRing with wavelength re-use (e.g.: adjacent spans use the same
wavelength).
Protection in this architecture is implemented in the OC layer. Optical switches are
used to connect traffic to the working or protection fibres, according to the state of the
ring (normal or failure conditions, respectively). Care should be taken when
wavelengths are re-used: to avoid wavelength conflicts in the protection fibres under
failure conditions, dual-ended protection switching should be implemented.
Figure 1. OC-DPRing with wavelength re-use
2.2.2
Optical Channel Shared Protection Ring (OC-SPRing)
This ring uses two fibres for bi-directional communication between the nodes. Under
normal working conditions each fibre carries a different wavelength: 1 clockwise, 2
anti-clockwise. If a section or node fails, the adjacent nodes will re-route the
wavelengths to the complementary arc of the ring, sharing the capacity of the fibres
between the two optical channels. Therefore, we have shared protection at the OC
level (Figure 2).
SDH ADM
SDH ADM
A
B
C
A
B
C
F
E
D
F
E
D
Figure 2. The OC-SPRing under normal and section failure conditions
Regarding traffic routing, this is done electrically by the SDH equipment, as the
optical channels only exist between adjacent nodes, corresponding to physical nodeto-node sections.
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© 1999 EURESCOM Participants in Project P709
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Deliverable 2
2.2.3
Optical Multiplex Section Dedicated Protection Ring (OMS-DPRing)
An OMS-DPRing can be a 2-fibre or 4-fibre ring. The traffic routing is implemented
at the OC level, by means of allocating different wavelengths to the different node-tonode links. The protection is implemented at the OMS layer hence all channels are
simultaneously protected in case of failure. Since we have dedicated protection, in the
2-fibre ring one of the fibres is dedicated to protection of the traffic circulating in the
opposite direction in the service fibre. When a section or node failure occurs, the
nodes adjacent to the failure will re-route the traffic from the service fibre to the
protection fibre, along the complimentary arc of the ring.
2.2.4
Optical Multiplex Section Shared Protection Ring (OMS-SPRing)
This architecture is a 2- or 4-fibre ring, where routing is done at the OC level, as
direct node-to-node logical links can be established using different wavelengths. This
allows the implementation of full logical meshes, connecting every node to every
other node in the ring (Figure 3).
A
A
B
D
C
D
B
A
B
D
C
C
Figure 3. The OMS-SPRing under normal and section failure conditions. Central
scheme represents node connectivity based on wavelength routing
Protection is implemented at the OMS layer, the capacity of each fibre being shared
between service and protection wavelengths. For example, in the clockwise fibre the
first half of the wavelength spectrum (e.g.: channels 1 to 8 in a 16-channel ring) is
used for service traffic, while the second half is used for protection of the service
channels in the anti-clockwise fibre. In the anti-clockwise fibre things are inverted:
the first half of the spectrum is used for protection of the service channels in the
clockwise fibre, while the second half is used for service traffic (Figure 4).
Failure
conditions
Normal
conditions
service
channels
protection
channels
service
channels
protection
channels
protection
channels
service
channels
protection
channels
service
channels
Figure 4. Shared protection of optical channels in the OMS-SPRing
© 1999 EURESCOM Participants in Project P709
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If a section or node fails, the adjacent nodes re-route the traffic to the complementary
arc of the ring. The service wavelengths of the clockwise fibre are re-routed to the
protection wavelengths of the anti-clockwise fibre, and the service wavelengths of the
anti-clockwise fibre are re-routed to the protection wavelengths of the clockwise fibre.
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© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
3
Protection of interconnected optical network domains
This chapter describes the general principles for protection of interconnected optical
network domains. Only the selected optical network architectures defined in
Deliverable 1 of P709 will be studied. The main characteristics of the protection of
these interconnected optical domains will be described.
3.1
Selected reference network configurations
Deliverable 1 of P709 selected the following as reference network configurations:

Two-level CS-Ring with network SDH connectivity

Two-level OMS-SPRing network with limited optical connectivity

Two-level OMS-SPRing-Optical Mesh architecture

Two-level Optical Mesh-OMS-SPRing architecture with full optical connectivity
All the selected reference network configurations use dual node interconnection
because in large capacity core networks disjoint alternative routing is an essential
requirement.
3.2
Interconnection of sub-networks and protection
This section studies the characteristics of the dual node interconnection of optical
domains, with special focus in the case of interconnected rings. Only two fibre
architectures will be analysed.
As a starting point, it is important to clarify the concepts of physical interconnection
and logical interconnection between two rings. Two rings that “touch” each other in
two (one) nodes are physically interconnected in a dual-node (single-node) way: we
have a dual-node (single-node) physical interconnection. A wavelength that is routed
through two single-node physically interconnected rings (Figure 5 for both OCHDPRings and OMS-SPRings) is protected within each ring, dropped from the first ring
by the OADM I1 and added into the second ring by the OADM I3. In this case, the
scheme applied at the interconnection is a single-homing interconnection. On the
contrary, a wavelength that is routed through two dual-node physically interconnected
rings, and uses both interconnection nodes to cross the border between rings,
performs a “dual-homing interconnection”.
Node A
Node A
I1
I1
I3
I3
Node B
a)
Node B
b)
Figure 5. Single-homing interconnection a) OC-DPRings, b) OMS-SPRings
© 1999 EURESCOM Participants in Project P709
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Trying to apply to optical rings the concepts proposed by the ETSI document
DTR/TM-03041 (that deal with SDH network protection interworking), two kinds of
dual-homing architectures can be applied at the interface between two dual-node
physically interconnected rings:

Virtual ring architecture

Drop and continue architecture (see section 0)
Node A
Node A
I1
I2
I1
I2
I3
I4
I3
I4
Node B
a)
Node B
b)
Figure 6. Dual-homing interconnection based on virtual ring architecture, a)
OC-DPRings, b) tentative application to OMS-SPRings
The Virtual Ring Architecture (VRA) can only be applied to OC-DPRings. The
routing and protection scheme is shown in Figure 6a). As it can be seen the same
wavelength connection is protected as if the two rings were a single one and both
interconnection nodes are used to cross the border between rings (i.e. dual-homing
interconnection).
A possible application of the virtual ring architecture concept to OMS-SPRings is
shown in Figure 6b). This proposal duplicates the wavelength from A to B, routing
the wavelengths in a route disjoint way. In practice, the protection at the OMS layer
(within the OMS-SPRings) is combined with route diversity at the OCH layer. The
protection mechanism recovers single link or node failures within each ring, while the
route diversity strategy improves the connection survivability in case of failure of an
interconnection link between the two rings. However, if both wavelengths carry the
same client layer connections (i.e. they are a copy of the other) this solution is worse
than the VRA over OCH-DPRings, because it requires twice the bandwith without
improving availability (in a first approximation). In fact both VRA over OCHDPRings and the solution of Figure 6b) can survive against a single link/node failure
between A and B. However, in general, when route diversity is applied, each route
between a pair of nodes carries only half of the client layer connections between these
two nodes. That has two consequences:

the number of wavelengths to be carried in the optical layer, in principle, halves
or, otherwise stated, no protection redundancy is applied on client layer
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connections (although this can rise the problem of a low utilisation of the OCHs
when the client layer demands are not high enough);

the client connection resilience against failures should be evaluated in a twolayer perspective.
In practice, under these conditions, the architecture of Figure 6b) carries half the
client layer connections between nodes A and B over the 1, and the other half over
2. So it recovers:

all the client connections in case of single node/link failure within each ring

only half the client connections in case of failure of an interconnection link
This improves the performance in terms of resource requirements, which is now as
good as the VRA over OCH-DPRings, but we decrease the interconnection resilience.
It should be noticed that the scheme in Figure 6b) cannot be considered a dualhoming interconnection, as each wavelength crosses the border between rings through
a single node.
There are other architectures that can be used to improve the performances of VRA
over OCH-DPRings without introducing drop-and-continue: they use a single-homing
solution over dual-node physically interconnected rings. Figure 7 shows these
architectures in the case of OCH-DPRings (uni-directional, but it works for bidirectional OCH-DPRings as well) and OMS-SPRings. The logical interconnection is
a single-homing one in both cases because each wavelength is dropped from the ring
by one and only one OADM.
Node A
Node A
I1
I2
I1
I2
I3
I4
I3
I4
Node B
a)
Node B
b)
Figure 7. Improved virtual ring architectures a) OC-DPRings, b) OMS-SPRings
The proposal here is to carry in each wavelength half the client layer connections, so
that the resource requirements can be, in principle, the same of the VRA over OCHDPRings (or better, in the case of OMS-SPRings). But here each Survivable SubNetwork (SSN, i.e. ring) can recover a single node/link failure, allowing multiple
failures (one per ring) between node A and B. But both these architectures are weaker
than the VRA over OCH-DPRings at the interconnection nodes: only half the client
layer connections survive against an interconnection failure. If protecting locally the
© 1999 EURESCOM Participants in Project P709
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interconnection OCHs between I1 and I3, and between I2 and I4, with a 1:N protection,
in order to overcome this weakness, is feasible and cost-effective, these solutions are
generally the best ones without d&c.
3.3
Protection with optical drop and continue functionality
This section studies the characteristics of the dual node interconnection of optical
domains when optical drop and continue functionality is used. Only four-fibre
architectures with optical connectivity will be analysed.
Four-fibre OMS-SPRings require four fibres for each span of the ring. Working and
protection channels are carried over different fibres: two multiplex sections
transmitting in opposite directions carry the working channels while two multiplex
sections, also transmitting in opposite directions, carry the protection channels. This
permits the bidirectional transport of working traffic. The difference between OMSSPRings and OMS-DPRings is that in OMS-SPRings no dedicated resources (e.g.
duplication of OCs) are used.
3.3.1
Two-level OMS-SPRing architecture
The two-level OMS-SPRing architecture with dual node interconnection is shown in
Figure 8. It consists of one upper level OMS-SPRing with termination node A (ring 1)
and two lower level OMS-SPRings with termination nodes B and C (rings 2 and 3).
Interconnection and routing are done at the optical layer.
For the interconnection of termination nodes B and C the signal is sent using
wavelength 1 from node B to the primary interconnection node L1. Drop and continue
functionality is used at node L1 to drop the signal towards the primary interconnection
node U1 in ring 1 and to continue the same signal to the secondary interconnection
node L2. In node L2 the signal is dropped to the secondary interconnection node U2 in
ring 1. From node U2 the signal is fed to node U1 where a service selector is used to
select between the signals coming from nodes L1and U2. The selection is then fed
through termination node A to the primary interconnection node U3. Drop and
continue functionality is used at node U3 to drop the signal towards the primary
interconnection node L3 in ring 3 and to continue the same signal to the secondary
interconnection node U4. In node U4 the signal is dropped to the secondary
interconnection node L4 in ring 3. From node L4 the signal is fed to node L3 where a
service selector is used to select between the signals coming from nodes L 4 and U3.
The selection is then fed to termination node C.
In the other direction of transmission the signal is sent using the same wavelength 1
from node C to the primary interconnection node L3 where drop and continue
functionality is used to send the signal to both node U3 and node L4. In node L4 the
signal is dropped to the secondary interconnection node U4 from which it is fed to the
primary interconnection node U3 where a service selector is used to make a selection
between the signals coming from nodes L3 and U4. The selected signal is then fed
through node A to the primary interconnection node U1. Drop and continue
functionality is used at node U1 to drop the signal towards the primary interconnection
node L1 in ring 2 and to continue the same signal to the secondary interconnection
node U2. In node U2 the signal is dropped to the secondary interconnection node L2 in
ring 2. From node L2 the signal is fed to node L1 where a service selector is used to
select between the signals coming from nodes U1 and L2. The selection is then fed to
termination node B.
page 10 (48)
© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
The drop-and-continue function can be simply implemented with a an optical splitter.
The simplest way to implement the service-selector function is to use an optical
switch activated by power monitoring.
This interconnection architecture provides protection against the failure of one or both
interconnecting nodes (each on different ring, but on the same interconnect), or the
connection between the two interconnecting nodes. The architecture can also protect
against a single failure in each of the rings provided that these failures are not
terminating node failures, or these failures do not combine to affect both
interconnections between the rings. Within the OMS-SPRings, any failure outside the
termination, primary or secondary nodes is handled in a standard fashion. The nature
of the failure (cable cut or node failure) has no impact on the configuration of the
termination, primary or secondary nodes. A cable cut failure is shown in Figure 9. A
failure of the primary node in the OMS-SPRing results in a secondary connection of
the signal. This is also shown in Figure 9. During a failure of the secondary node the
signal at the termination node is unaffected, since it receives its signal from the
primary node.
A
Ring 1
U1
U3
U2
U4
Upper level network
Lower level network
L1
L2
Ring 2
1
1
B
L4
L3
Ring 3
1
1
C
Working
Protection
Optical add-drop multiplexer
Service selector
Drop and continue
Figure 8. Two-level OMS-SPRing architecture with dual node interconnection
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
For the interconnection of termination nodes A and B or A and C other wavelengths
(2, 3) have to be used. One wavelength is needed for each traffic demand in this
architecture. The same wavelength can be used for both directions of transmission. In
some cases the same wavelength might also be used for other connections which do
not overlap with the ones where the wavelength is already used. In failure situations
this may, however, cause wavelength conflicts in OMS-SPRings when the normal ring
protection procedure is used.
A
Ring 1
U1
U3
U2
U4
Upper level network
Lower level network
L1
L2
Ring 2
1
1
B
L4
L3
Ring 3
1
1
C
Working
Protection
Optical add-drop multiplexer
Service selector
Drop and continue
Figure 9. Two-level OMS-SPRing architecture: response to complete primary
node failure in Ring 1 and cable cut in Ring 3
page 12 (48)
© 1999 EURESCOM Participants in Project P709
Deliverable 2
3.3.2
Volume 3: Annex B – Protection in the optical layer
Two-level OMS-SPRing-Optical Mesh architecture
The two-level OMS-SPRing-Optical Mesh architecture is shown in Figure 10. It
consists of two Optical Meshes with termination nodes B and C and one upper level
OMS-SPRing with termination node A. Interconnection and routing are done at the
optical layer.
For the interconnection of termination nodes A and B the signal is sent using
wavelength 1 from node A to the primary interconnection node U1. Drop and
continue functionality is used at node U1 to drop the signal towards the primary
interconnection node L1 and to continue the same signal to the secondary
interconnection node U2. In node U2 the signal is dropped to the secondary
interconnection node L2. From nodes L1 and L2 the signal is fed using separate routes
to termination node B where a selection is made between the two signal paths. In the
other direction of transmission the signal is sent using the same wavelength 1 from
node B on separate routes to the primary interconnection node L1 and to the secondary
interconnection node L2. From nodes L1 and L2 the signal is sent to nodes U1 and U2
respectively. From node U2 the signal is fed to the primary interconnection node U1
where a service selector is used to make a selection between the signals. The selected
signal is then fed to termination node A. For the interconnection of termination nodes
A and C wavelength 2 is used in a similar way as shown in Figure 10.
The architecture can protect against any single failure between the termination nodes.
Within the OMS-SPRing, any failure outside the termination, primary or secondary
nodes is handled in a standard fashion. In this architecture one wavelength is needed
for each traffic demand. The same wavelength may also be used in other parts of the
network when there is no possibility of wavelength conflicts in the normal or
protected state. In order to improve the survivability of the meshed networks against
simultaneous failures more wavelengths should be used.
3.3.3
Two-level Optical Mesh-OMS-SPRing architecture
The two-level Optical Mesh-OMS-SPRing architecture with dual node
interconnection is presented in Figure 11. It consists of two lower level OMS-SPRings
(rings 1 and 2) with dual node interconnection to an upper level Optical Mesh.
Interconnection and routing are done at the optical layer.
For the interconnection of termination nodes B and C the signal is sent using
wavelength 1 from node B to the primary interconnection node L1. Drop and
continue functionality is used at node L1 to drop the signal to the primary
interconnection node U1 and to continue the same signal to the secondary
interconnection node L2. From node U1 the signal is sent to node U3 and further to
node L3 in ring 2. In node L2 the signal is dropped to the secondary interconnection
node U2. From node U2 the signal is fed to node U4 and further to node L4 in ring 2.
From node L4 the signal is sent to node L3 where a service selector is used to select
between the signals coming from nodes L4 and U3. The selection is then fed to
termination node C. In the other transmission direction the signal is sent using the
same wavelength 1 from node C to node B in a similar way.
The architecture can protect against any single failure between nodes B and C. This
architecture can also protect against a single failure in each of the rings provided that
these failures are not terminating node failures, or these failures do not combine to
affect both interconnections between the rings. For this level of protection only one
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
Deliverable 2
wavelength is needed for each traffic demand. Survivability in the meshed network
can be improved by using more wavelengths.
1 2
A
U1
U3
U2
U4
Upper level network
Lower level network
L1
L2
L4
B
1 1
1
2
L3
C
2
2
Working
Protection
Optical add-drop multiplexer
Optical cross connect
Service selector
Drop and continue
Figure 10. Two-level OMS-SPRing-Optical Mesh architecture with dual node
interconnection
page 14 (48)
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
A
U1
U3
U2
U4
Upper level network
Lower level network
L1
L2
Ring 1
1
1
B
L4
L3
Ring 2
1
1
C
Working
Protection
Optical add-drop multiplexer
Optical cross connect
Service selector
Drop and continue
Figure 11. Two-level Optical Mesh-OMS-SPRing architecture with dual node
interconnection
3.4
Conclusions
To improve the survivability of interconnected optical network domains the number
of used wavelengths has to be increased. The increase of the number of wavelengths
could be avoided by introduction of optical drop and continue functionality and
optical switching functionality in the interconnection nodes. In two fibre OMSSPRings two wavelengths, one for each direction of transmission, are needed for each
traffic demand between domains in order to provide protection against any single
failure between connected nodes. In four fibre OMS-SPRings one wavelength is
needed for each traffic demand between domains in order to provide protection
against any single failure between connected nodes. The same wavelength can be used
in both directions of transmission. To further improve the protection of the
interconnections against simultaneous failures the number of wavelengths has to be
increased.
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
4
Deliverable 2
Wavelength conflicts under protected state in OMSSPRings
This chapter identifies and analyses wavelength conflicts under protected state in
OMS-SPRings. This study is based on the analysis of the conditions leading to
"wrong connections" in SDH MS-SPRings under protection, translating these
conditions to the frequency domain (wavelengths).
4.1
Protection in OMS-SPRings
This section overviews the two-fibre OMS-SPRing architecture. The main
characteristics of the architecture are briefly described. Then, the behaviour of the
architecture under the different types of failures that can be protected against is
studied. The types of failures considered are: single span, multiple span and node
failures.
4.1.1
Architecture
In the two-fibre OMS-SPRing, the capacity on each of the two fibres is shared by
working and protection traffic. For example, a sixteen-wavelength WDM system
would have eight channels allocated for working traffic and eight for protection traffic
on each of the two fibres. The working channels in one fibre are protected by the
protection channels, travelling in the opposite direction around the ring, in the other
fibre. In the event of a failure, working traffic on one fibre would be switched over to
the protection capacity on the other fibre. Therefore the eight working channels on
one fibre have the same wavelengths as the eight protection channels on the other
fibre and vice versa.
In summary, the main characteristics of the OMS-SPRing architecture are:
fibres:
2
physical topology:
ring
logical topology:
mesh
routing:
optical channel and SDH path layer
protection:
optical multiplex section shared protection (dual-ended
switching)
span failure:
protected
multiple span failure:
protected
node failure:
protected
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© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
4.1.2
Single Span Failure in OMS-SPRing
a)
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
B
"
C
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
b)
"
service
"
F
E
D
A
B
C
F
E
D
Service
Fibre failure
Protection
NOTE: A two-fibre ring only uses ring switches to restore traffic.
Figure 12. Two-fibre OMS-shared protection ring architecture. a) working state,
b) protection state under single span failure
In the example considered here the working traffic is bidirectionally transported
between nodes A and D under normal operating conditions (Figure 12a). If a failure
occurs in the span between nodes B and C, both nodes adjacent to the failed span loop
(bridge and switch) the working traffic to the protection channels in the other fibre.
The working traffic is, then, transported on the long path, to bypass the failed span
(Figure 12b).
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
4.1.3
Deliverable 2
Multiple Span Failure in OMS-SPRing
a)
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
B
"
C
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
"
service
"
F
E
D
A
B
C
F
E
b)
D
S
e
r
v
i
c
e
F
i
b
r
e
f
a
i
l
u
r
e
P
r
o
t
e
c
t
i
o
n
Figure 13. Two-fibre OMS-shared protection ring architecture. a) working state,
b) protection state under multiple span failures
In the previous figure, a multiple span failure (in spans A-B and B-C) is illustrated. In
this situation, the traffic to node B is lost. All the remaining traffic is recovered.
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© 1999 EURESCOM Participants in Project P709
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Deliverable 2
4.1.4
Node Failure in OMS-SPRing
a)
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
B
"
C
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
"
service
"
F
E
D
A
B
C
F
E
b)
D
S
e
r
v
i
c
e
N
o
d
e
f
a
i
l
u
r
e
P
r
o
t
e
c
t
i
o
n
Figure 14. Two-fibre OMS-shared protection ring architecture. a) working state,
b) protection state under node failure
The consequences of a node failure, are quite similar to the situation presented in the
previous point. As before, the traffic to node B is lost. All the remaining traffic is
recovered.
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
4.2
Misconnections in OMS-SPRings
4.2.1
Introduction
Deliverable 2
As it was said before, this study is based on the analysis of the conditions leading to
“wrong connections” in SDH MS-SPRings under protection, translating these
conditions to the frequency domain (wavelengths). So, it is now appropriate to present
a summary about misconnections in SDH MS-SPRings.
In an SDH MS-SPRing, in order to perform a ring switch, the protection channels are
essentially shared among each span of the ring. Also, extra traffic may reside in the
protection channels when the protection channels are not currently being used to
restore working traffic transported on the working channels. Thus, each protection
channel time slot is subject to use by multiple services (services on the same time slot
but on different spans, and service from extra traffic).
With no extra traffic in the ring, under certain multiple span failures, such as those
that cause node(s) isolation, services (on the same time slot but on different spans)
may contend for access to the same protection channel time slot. This yields a
potential for misconnected traffic.
With extra traffic in the ring, even under single point failures, a service on the
working channels may contend for access to the same protection channel time slot that
carries the extra traffic. This also yields a potential for misconnected traffic.
So, how can a potential misconnection be identified?
A potential misconnection is determined by identifying the nodes that will act as the
switching nodes for a bridge request, and by examining the traffic that will be affected
by the switch. The switching nodes can be determined from the node addresses in the
K1 and K2 bytes. The switching nodes determine the traffic affected by the protection
switch from the information contained in their ring maps and from the identification
of the switching nodes.
Each of the ring maps, then, shall contain at minimum:
1.
information regarding the order in which the nodes appear on the ring,
2.
the AU-4 time slot assignments for traffic that is both terminated at that node and
passed-through that node,
3.
for each of these AU-4 time slots, the node addresses at which the traffic enters
and exits the ring,
4.
and an optional indication of whether the AU is being accessed at the lower order
VC level somewhere on the ring.
Inserting the appropriate AU-AIS (an “all-ones” loading) in those time slots where
misconnected traffic could occur shall squelch potential misconnections. For rings
operating at an AU-4 level, this squelching occurs at the switching nodes. For rings
using lower order VC access, squelching locations are under study.
So, a misconnection exists when a node extracts a channel, which must be delivered
to another node.
In the two-fibre OMS-SPRing case, an analogy between wavelengths and time slots
(AU-4 of the SDH case) will be used. As such, a misconnection can occur in two
distinct situations. The first situation is characterised by a multiple span failure
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© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
without low-priority traffic in the protection channels. The second situation presents a
single span failure with low-priority traffic in the protection channels. The use of
extra low-priority traffic in the protection channels of an OMS-SPRing is still under
discussion, since it brings additional complexity to the node equipment. However, the
discussion in this document will not take these implementation matters into account,
as it is kept at a functional level.
4.2.2
Misconnections without low-priority traffic
a)
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
B
"
C
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
F
"
service
"
E
D
b)
A
F
B
C
E
D
S
e
r
v
i
c
e
N
o
d
e
f
a
i
l
u
r
e
P
r
o
t
e
c
t
i
o
n
Figure 15. Two-fibre OMS-shared protection ring architecture. a) working state,
with wavelength reuse b) misconnection under protected state
This example shows a bidirectional communication between nodes C and D and
nodes D and F, with wavelength reuse (Figure 15a). In the case of failure of node D,
for example, nodes E and C loop (bridge and switch) the traffic. So, node F extracts
the traffic inserted by node C, and node C extracts the traffic inserted by node F,
resulting in a misconnection (Figure 15b).
A possible solution, is similar to the one adopted in SDH MS-SPRing. In SDH MSSPRings inserting the appropriate AU-AIS (an “all-ones” loading) in those time slots
where misconnected traffic could occur shall squelch potential misconnections. For
example, for rings operating at an AU-4 level, this squelching occurs at the switching
nodes.
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
Deliverable 2
So, in this particular case, nodes E and C must perform the squelching of the traffic
that will be misconnected, (“a wavelength squelching”) before the ring switch set-up
(Figure 16). A discussion of how to perform this “wavelength squelching” is
presented in section 4.2.4.
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
B
"
C
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
"
service
"
Squelching
F
E
Squelching
D
S
e
r
v
i
c
e
N
o
d
e
f
a
i
l
u
r
e
P
r
o
t
e
c
t
i
o
n
Figure 16.Two-fibre OMS-shared protection ring architecture: example of
misconnection
4.2.3
Misconnections with low-priority traffic
To study this subject, two bidirectional connections, between nodes D and F, and
nodes A and C, are considered. Between nodes D and F, the communication is
supported by bidirectional working traffic in service wavelengths 1 and 2. Between
nodes A and C, the communication is supported by bidirectional extra traffic (lowpriority) in protection wavelengths 1 and 2 (Figure 17a).
When a span failure occurs between nodes E and D, how does the protection scheme
work?
Nodes D and E loop (bridge+switch) the working traffic (Figure 17b). So, when node
E bridges the working traffic to the protection channels, the consequence is that node
A extracts the traffic inserted by node F. When, node E performs the switch, node F
extracts the low-priority traffic inserted by node A.
A similar situation occurs in node D. When node D bridges the working traffic to the
protection channel, the node C extracts the traffic inserted by node D. When node D
performs the switch, it extracts the traffic inserted by node C.
To avoid this problem, nodes A and C must perform the squelching of the extra traffic
(“a wavelength squelching”), before the ring switch set-up (Figure 18).
page 22 (48)
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
a)
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
"
B
C
1

(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
F
b)
F
"
service
"
E
 1
A
B
 
E
D
C
D
S
e
r
v
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e
F
i
b
r
e
f
a
i
l
u
r
e
P
r
o
t
e
c
t
i
o
n
Figure 17. Two-fibre OMS-shared protection ring architecture. a) working state,
with extra traffic in the protection channels b) misconnection under protected
state
(1,3, 5,...) clockwise service wavelengths
A
(,, ,...)
"
protection
"
B
C
Squelching
(1,3, 5,...) anti-clockwise protection wavelengths
(,, ,...)
F
 
"
service
"
Squelching
E
D
Service
Fibre failure
Protection
Figure 18. Two-fibre OMS-shared protection ring architecture: example of
misconnection
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
4.2.4
Deliverable 2
Wavelength squelching
In the previous two sub-sections, it was shown that the optical channels subject to
misconnections under protected state in an OMS-SPRing should be “squelched”. This
operation is the frequency domain equivalent of the AU-4 (time domain) squelching
performed in SDH MS-SPRings. In the SDH case the squelching is done by loading
the misconnected AU-4 with an AIS signal (an “all-ones” loading). In the optical
architecture case this operation cannot be done directly on the optical channel because
there is no access, in the optical channel layer, to the client layer signal.
A possible way of performing the wavelength squelching is by associating an
overhead channel to each optical channel, and reserving some capacity in this
overhead channel for an AIS-like indication. Initial studies regarding the types of
alarm signals needed by the optical layers have already been performed in
EURESCOM Project P615, for example, where such a signal has been considered
[see P615 Deliverable 1, Annex B]. The overhead channels associated to the optical
channels could be transported by a specific wavelength, perhaps outside the gain band
of the optical amplifiers.
When a failure occurs that could lead to misconnected wavelengths, the nodes
initiating the switching procedure should check which wavelengths can be affected.
As in the SDH, this is done by looking at the ring maps which now contain the
following information:
1. logical ordering of nodes in the ring
2. identification of the optical channels being terminated at each node
3. identification of the optical channels passing through each node
After identifying the optical channels that can be misconnected, the switching nodes
should insert the AIS-like indication in the proper place in the associated overhead
channels, so that the affected receiving nodes can avoid using the misconnected
optical channels. Of course, all overhead channels should be accessible to all nodes in
the ring. This would be possible if the wavelength supporting the overhead
information is electrically terminated at all nodes. It should be noted that the optical
channels are still kept in the optical domain, being only terminated in the nodes they
interconnect.
4.3
Conclusion
In this chapter the problem of wavelength conflicts, or misconnections, under
protected state in OMS-SPRings has been addressed. The approach followed in this
study consisted of establishing an analogy with the SDH MS-SPRing case.
In the SDH case, misconnections occur at the AU-4 level under certain failure
conditions: node or multiple span failure without low-priority traffic in the protection
AUs, and single span failure with low-priority traffic in the protection AUs. To avoid
the misconnections, the nodes squelch the AUs that will be misconnected by inserting
an AIS signal in these AUs. The decision on which AUs to squelch is done taking into
account which nodes are performing the protection switching and also the node
connectivity defined in the ring maps.
To study the OMS-SPRing case, wavelengths become the analogous of the time slots
in SDH. As such, misconnections will now occur at the optical channel level under
page 24 (48)
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
conditions similar to the SDH case: node or multiple span failures without lowpriority traffic in the protection wavelengths, and single span failures with lowpriority traffic in the protection wavelengths. The proposed solution for the
misconnections is based on wavelength squelching, using the overhead channel
associated to the optical channels to support an Optical Channel AIS signal. This
solution would maintain the independence of the optical layers from the client layers,
as the client signals would not be directly manipulated.
An issue left for further study is the proper way of linking the AIS alarm at the
Optical Channel Layer to the management system of the client signal (typically SDH),
in order that the client would be able to reject the misconnected signals.
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
Deliverable 2
5
Identification of items with impact on network planning
5.1
Protection of individual channels
Certain channels of an hierarchical network may have higher importance requiring
very high availability that otherwise cannot be ensured. On the other hand, the
structure itself may have weak points making the network in certain failure
constellations vulnerable. The purpose of this section is to investigate how individual
channels can be protected on optical networks.
5.1.1
Coloured Section Ring (CSR)
CSRs have self-healing features, i.e. in case of a single failure, automatic protection is
performed. The routing and protection is performed entirely on SDH MS layer. The
structure provides protection for span failure.
In CSRs one wavelength is assigned to each span. If the traffic goes via nonneighbouring nodes, o/e/o conversion is made at the intermediate nodes, and the
traffic is passed through by the SDH ADMs on electrical layer. This makes the CSRs
vulnerable against the failure of these intermediate nodes since the ring cannot
perform protection.
Figure 19 depicts the protection architecture according to ETSI TS 101 010. It can be
seen that neighbouring protection “blocks” are chained in case of a connection going
through several MSs (which means several wavelengths). The gaps between
protection “blocks” indicate the weak points of the system.
network
layer
service
LOP
HOP
MS
RS
OCH
OMS
OTS
k-1
k
k+1
k+2
geographical
location
Figure 19. Protection layer interconnections of CSR: Chained intra-layer
network protection
This is the reason why the CSR in preferably designed by choosing the right logical
node order. It means that the wavelengths are assigned to the main traffic flows
between nodes to minimise the need for transiting at nodes electrically. This results
that the logical order may differ from the physical order.
If a high priority connection is established via such transiting nodes, the ring does not
protect against the failure of this node. Therefore additional protection shall be
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provided. This additional protection can be made in SDH layer on the HO or LO path
level (1+1 path protection) (Figure 20).
ADM
ADM
APS on VC
layer
passing through
on VC layer
OADM
OADM
CS-Ring
OADM
OADM
working
protection
ADM
ADM
Optical add-drop multiplexer
Figure 20. CSR - Path protection on VC level
This situation is shown in Figure 21 by protection architecture. The whole MS chain
is covered by an upper protection “block” in the HOP (or LOP) layer. This protection
structure is called hybrid interlayer protection.
network
layer
service
LOP
HOP
MS
RS
OCH
OMS
OTS
k-1
k
k+1
k+2
geographical
location
Figure 21. Protection layer interconnections of CSR using 1+1 HO path
protection: Hybrid inter-layer network protection
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5.1.2
Deliverable 2
CSR-CSR hierarchical network
The situation is slightly different when CSRs are interconnected forming a CSR-CSR
hierarchical network. In this structure, two interconnection nodes serve as gateways to
the upper/lower network level.
On the lower level, the main traffic flows are pointing towards these hub nodes,
therefore a unique wavelength can be assigned toward each hub node avoiding the
intermediate o/e/o conversions. If this cannot be made, SDH 1+1 path protection can
be established.
On the higher level ring it is very difficult to choose a node order that eliminates the
o/e/o conversions, hence the application of extra protection seems to be more
frequently necessary.
5.1.3
Optical Multiplex Section Shared Protection Ring (OMSPR)
In OMSPRings the protection is entirely performed on optical layer. The structure
gives protection not only against span and node failures, but against double span
failures as well. In other words, OMSPR exhibits higher availability, having no weak
point as CSR has.
network
layer
service
LOP
HOP
MS
RS
OCH
OMS
OTS
k-1
k
k+1
k+2
geographical
location
Figure 22. Protection layer interconnections of OMSPR: Single intra-layer
network protection
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Deliverable 2
network
layer
service
LOP
HOP
MS
RS
OCH
OMS
OTS
k-1
k
k+1
k+2
geographical
location
Figure 23. Protection layer interconnections of OMSPR using 1+1 MSP: Hybrid
inter-layer network protection
As the protection architecture shows it in Figure 22, the entire connection is covered
by a continuous block indicating that there is no such weak point in the system as in
CSRs. Therefore, additional protection does not seem to be necessary. Yet, the
possibility is given: extra protection can be made on electrical layer (1+1 MSP or
HO/LO path protection). (See Figure 23)
5.2
Protection Priority Classes
Implementing a network survivability policy should consider the possibility of
providing different grades of service in what concerns network recovery performance,
and create the concept of survivability priority classes. These would be defined by
different efficiencies (100%, 50%, 25%, 0%) and recovery times (50ms, 200ms, 1min,
no guaranteed time), and be charged accordingly. More variants can be offered
combining these two characteristics. The user could also be given the possibility of
defining their own subclasses. The guaranty of a certain survivability performance
means that in the worst case that is the performance available to the user, and that
better performance maybe available.
Users contracting the optical transport service without any guaranteed network
recovering performance, could simple rely on their own electrical recovery strategies,
e.g. SDH protection techniques.
Recovery time is directly related to the chosen technique for implementing
survivability, and depends on the failure detection speed, on the efficiency of used
protocol and algorithms. When protection is used the limit is 50ms (SDH imposition),
but for lower efficiency classes, longer time could be tolerated; if restoration is used
recovery time for 100% efficiency is around 200ms. Efficiency is dependent of
available resources for network recovery and efficiency of used protocols (protection)
and algorithms (restoration).
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5.2.1
Deliverable 2
Priority classes
The obvious classes to provide in terms of recovery efficiency are the following:
Highest priority class I: (100%, 50ms) protection for path or link failures in ring
architectures.
The user would have protection of all its traffic, within the SDH time limit, for path
failures: this would cover multiple link and node failures within the same path, but not
terminal node failure. In order to cover this type of failure node equipment duplication
could be necessary. This could be implemented by 1+1 architectures (the most
expensive option), but also by 1:1 and 1:n and n:m architectures.
Highest priority class II: (100%, 50ms) protection for path or single link failure in
meshed architectures.
100% protection efficiency, within SDH time limit, assured for path or single link
failures. This could be implemented by 1+1 architectures (the most expensive option),
but also by 1:1 and 1:n and n:m architectures. In meshed architectures multiple link
failure coverage with 100% single link protection depends on the type of resource
allocation and on the actual failure scenario. One way to assure this feature is to
consider and dimension rings overlayed with the meshed structure for protection
purposes, subject to the restriction of path hop count limit and path length limit.
Less than 100% protection classes make use of protection capacity reserved,
dedicated or shared (i.e., excluded the 1+1 case).
Medium priority class: (50%, 50ms) One simple way to provide this is, use 1:1
protection, and split the user traffic between working and protection channels. In this
case, 50% of traffic is fully protected and the other 50% makes use of protection
capacity, therefore being unprotected. In a 1:1 scheme a user can therefore contract
two channels with 50% protection. When a failure occurs, the unprotected traffic is
discarded in order to assure protection to the protected traffic. The option of defining
what traffic to protect and what traffic to discard can be given to the user (user
subclass definition).
The use of shared protection schemes n:m also allows offering such a service class.
However, the optical channels cannot be fractioned. Therefore, this percentage
performance for optical protection in such schemes has to take that particularity into
account. For instance with a 1:2, scheme one can have one channel in class 100% and
two channels unprotected; two channels in class 50% and 1 channel unprotected;
three channels in class 30% of protection.
Unprotected traffic: This traffic will be the one carried in the protection channels,
being discarded in the case of failure, in order to allow the protection of working
channels.
5.2.2
Other performance priority classes definition
Architectures with n:m protection schemes provide a good basis for establishing
protection priority classes. Fixed a network architecture, the population of each
protection class has a limit. Completely filling the 100% performance class will
prohibit the offering of lower performance class services, if the network was first
dimensioned to assure 100% protection. The number of priority classes to offer
simultaneously, their grading and their limit population should be parameter for
network planning.
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Complex architectures mesh-ring, ring-mesh or ring-ring, are potential promising for
the provision of diversified protection grades, as was already referred for the case of
meshed with overlayed rings. In these architectures an additional parameter
influencing the protection scheme will be the interconnection of the two levels of the
architecture, and in particular, what traffic, how much, in what failure scenarios and
how will be protected by the other level of the architecture.
Mixed survivability techniques can also be possible for these networks, including
protection and restoration; this will hopefully allow the optimisation of spare
resources use at the cost, additional management complexity, more intelligence in the
network and lower recovery speed. However, this last factor cannot be very penalising
as for many users an interruption of about 200ms can be tolerable, and therefore this
can result on a variant of offered survivability priority classes.
5.3
Protection against simultaneous failures
Protection mechanisms are usually designed to protect against single failures. The
network should be planned in such a way that the impact of simultaneous failures is
minimised. However, protection against simultaneous failures may be necessary in
certain parts of the network and should also be taken into account in network
planning. This section studies the impact of protection against simultaneous failures
on network planning. The focus is in the protection of interconnected optical domains
described in Deliverable 1 of P709.
5.3.1
General characteristics of single domain optical protection
5.3.1.1
Optical ring protection
Unidirectional link failure and bi-directional link failure are single point failures.
Section or link failure and an optical component failure are examples of this kind of
failures. As is well known single domain CS-Ring and OMS-SPRing architectures
protect against any single point failure.
Multiple failures are single point failures occurring at more than one physical location
in a ring. These failures are either link failures, nodal failure or combination of them.
A node failure can be considered as a bi-directional link failure occurring on both
sides of the node. In some cases, depending on the physical location of the failures,
the traffic can be protected against multiple failures in both CS-Ring and OMSSPRing. However, the ring architecture cannot protect against terminating node
failures and usually part of the total traffic will be lost also under other multiple
failure conditions. Multiple failures may also result in ring segmentation. Therefore,
protection at client layer is sometimes required if there is a need to fully protect the
client layer traffic against multiple failures in optical ring architectures.
5.3.1.2
Optical sub-network connection protection
Optical SNCP architecture protects against any single point failure. The traffic can
also be fully protected in some multiple failure cases, depending on the physical
location of the failures. However, the optical SNCP cannot protect against terminating
node failures and also in many other multiple failure cases the traffic is lost.
Therefore, protection at client layer is sometimes required if there is a need to fully
protect the client layer traffic against multiple failures in optical SNCP architecture.
© 1999 EURESCOM Participants in Project P709
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5.3.2
Deliverable 2
Protection against simultaneous failures in interconnected optical network
domains
This section examines if additional resources are needed for protection against
multiple failures in interconnected optical network domains. Only the selected dual
node interconnection architectures defined in Deliverable 1 of P709 will be studied.
Each individual optical sub-network of the interconnected optical network domains
protects against multiple failures according to the description given in the previous
section. Therefore, only the protection of interconnected traffic against simultaneous
failures will be discussed in this section.
5.3.2.1
Two-level CS-Ring architecture
The interconnection of two nodes located in different CS-Rings can be provided with
one wavelength by using optical drop and continue functionality and optical selectors
in the interconnection nodes. This architecture provides protection against several
multiple failure cases. However, the architecture cannot protect against terminating
node failures, or failures affecting simultaneously on both interconnections between
the rings.
In the case where drop and continue functionality is not available two wavelengths are
needed to provide the interconnection of two nodes located in different CS-Rings.
This architecture can protect against any single point failure and also against some
failures occurring simultaneously in the network. The architecture cannot protect
against terminating node failures, or failures affecting simultaneously on both
interconnections between any two rings, or failures affecting simultaneously on
certain combinations of interconnections between the two lower level rings and the
upper level ring. The survivability of the network can be improved by using four
wavelengths to provide the interconnection of two nodes located in different CSRings. This architecture provides protection against several multiple failure cases.
However, the architecture cannot protect against terminating node failures, or failures
affecting simultaneously on both interconnections between the rings.
The described dual node interconnected two-level CS-Ring architectures cannot
provide protection against all multiple failure cases. Therefore, protection at client
layer is sometimes required if there is a need to fully protect the client layer traffic
against multiple failures in the optical network.
5.3.2.2
Two-level OMS-SPRing architecture
The interconnection of two nodes located in different OMS-SPRings can be provided
with two wavelengths by using optical drop and continue functionality and optical
selectors in the interconnection nodes. This architecture protects against several
multiple failure cases. However, the architecture cannot provide protection against
terminating node failures, or failures affecting simultaneously on both
interconnections between the rings.
In the case where drop and continue functionality is not available two wavelengths are
needed to provide the interconnection of the two nodes located in different OMSSPRings. This architecture can protect against any single point failure and also against
some failures occurring simultaneously in the network. The architecture cannot
provide protection against terminating node failures, or failures affecting
simultaneously on both interconnections between any two rings, or failures affecting
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Volume 3: Annex B – Protection in the optical layer
simultaneously on certain combinations of interconnections between the two lower
level rings and the upper level ring. The survivability of the network can be improved
by using four wavelengths to provide the interconnection of the two nodes located in
different OMS-SPRings. This architecture protects against several multiple failure
cases. However, the architecture cannot provide protection against terminating node
failures, or failures affecting simultaneously on both interconnections between the
rings.
The described dual node interconnected two-level OMS-SPRing architectures cannot
provide protection against all multiple failure cases. Therefore, protection at client
layer is sometimes required if there is a need to fully protect the client layer traffic
against multiple failures in the optical network.
5.3.2.3
Two-level OMS-SPRing-Optical Mesh architecture
The interconnection of two nodes located in different optical SNCP meshes can be
provided with two wavelengths by using optical drop and continue functionality and
optical selectors in the interconnection nodes. This architecture protects against
several multiple failure cases. However, the architecture cannot provide protection
against terminating node failures, or failures affecting simultaneously on both
interconnections between the ring and mesh, or certain combinations of failures
occurring simultaneously in one interconnection node of the OMS-SPRing and in the
links or nodes of the optical SNCP mesh.
In the case where drop and continue functionality is not available two wavelengths are
needed to provide the interconnection of the two nodes located in different optical
SNCP meshes. This architecture can protect against any single point failure and also
against some failures occurring simultaneously in the network. The architecture
cannot provide protection against terminating node failures, or failures affecting
simultaneously on both interconnections between the ring and the mesh. In addition,
the architecture cannot protect against failures affecting simultaneously on certain
combinations of interconnections between the two lower level meshes and the upper
level ring, or certain simultaneous failures occurring in the optical SNCP meshes. The
survivability of the network can be improved by using four wavelengths to provide the
interconnection of the two nodes located in different optical SNCP meshes. This
architecture protects against several multiple failure cases. However, the architecture
cannot provide protection against terminating node failures, or failures affecting
simultaneously on both interconnections between the ring and mesh. In addition, the
architecture cannot protect against certain combinations of failures occurring
simultaneously in one interconnection node of the OMS-SPRing and in the links or
nodes of the optical SNCP mesh.
The described dual node interconnected two-level OMS-SPRing-Optical Mesh
architectures cannot provide protection against all multiple failure cases. Therefore,
protection at client layer is sometimes required if there is a need to fully protect the
client layer traffic against multiple failures in the optical network.
5.3.2.4
Two-level Optical Mesh-OMS-SPRing architecture
The interconnection of two nodes located in different OMS-SPRings can be provided
with two wavelengths by using optical drop and continue functionality and optical
selectors in the interconnection nodes. This architecture provides protection against
several multiple failure cases. However, the architecture cannot protect against
© 1999 EURESCOM Participants in Project P709
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terminating node failures, or failures affecting simultaneously on both
interconnections between the ring and mesh, or certain combinations of failures
occurring simultaneously in one interconnection node of the OMS-SPRing and in the
links or nodes of the optical SNCP mesh.
In the case where drop and continue functionality is not available two wavelengths are
needed to provide the interconnection of the two nodes located in different OMSSPRings. This architecture can provide protection against any single point failure and
also against some failures occurring simultaneously in the network. The architecture
cannot protect against terminating node failures, or failures affecting simultaneously
on both interconnections between the ring and the mesh. In addition, the architecture
cannot provide protection against failures affecting simultaneously on certain
combinations of interconnections between the two lower level rings and the upper
level mesh, or certain simultaneous failures occurring in the optical SNCP mesh. The
survivability of the network can be improved by using four wavelengths to provide the
interconnection of the two nodes located in different OMS-SPRings. This architecture
provides protection against several multiple failure cases. However, the architecture
cannot protect against terminating node failures, or failures affecting simultaneously
on both interconnections between the ring and mesh. In addition, the architecture
cannot provide protection against certain combinations of failures occurring
simultaneously in one interconnection node of the OMS-SPRing and in the links or
nodes of the optical SNCP mesh.
The described dual node interconnected two-level Optical Mesh-OMS-SPRing
architectures cannot provide protection against all multiple failure cases. Therefore,
protection at client layer is sometimes required if there is a need to fully protect the
client layer traffic against multiple failures in the optical network.
5.4
Existing infrastructures in optical network protection
Existing infrastructures should be in some way utilised when choosing the protection
mechanisms for the optical network. One possibility is to use existing point-to-point
WDM systems for the interconnection of network domains. In some cases also SDH
connections and SDH protection may be used. This section presents some possible
solutions for using existing systems for protection in the optical network.
5.4.1
Existing point-to-point WDM systems
Point-to-point WDM systems have been installed by several operators to fulfil
growing capacity needs in the existing optical fibre network. These systems have
usually been installed in optical cables where the traffic demand is very high and the
number of available fibres is small. Because the investments in these systems are
reasonably large it should be possible to utilise them when planning new optical
network structures. These existing systems may be used in a meshed network, for
construction of ring structures and for interconnection of network domains.
5.4.1.1
Meshed network
In a meshed
restoration or
used for 1+1
nodes and an
page 34 (48)
optical network protection can be implemented by using optical
dedicated protection methods. Existing point-to-point systems can be
protection when fast protection switching is needed between certain
alternative optical cable route is available between these nodes. The
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
limiting factor might be the capacity of the systems, because the WDM systems
installed in the first stage usually carry only 4 or 8 bi-directional optical channels. In
this case the existing point-to-point systems could be upgraded to the required
capacity if it is technically possible and cost effective. Another possibility is to
provide protection only for a certain part of the wavelengths according to the capacity
available in the point-to-point system.
5.4.1.2
Ring structures
Existing point-to-point WDM systems may be utilised in optical network architectures
also when creating new ring structures. This is illustrated in Figure 12. Point-to-point
WDM systems from node A to nodes B and C can be used when constructing a ring
A-B-C. For this purpose the optical terminal multiplexers in nodes A, B and C have to
be replaced by add-drop multiplexers. This means that from the original systems only
the optical line amplifiers are used. If the capacity of the systems is also upgraded, the
same line amplifiers cannot necessarily be used. When OMS-SPRing protection is
going to be used an extra pair of fibres is needed between the nodes of the ring.
B
B
b)
a)
A
C
C
A
OTM
OADM
OA
Figure 24. Existing point-to-point WDM systems (a), used for constructing a ring
structure (b)
As a conclusion, it seems that constructing ring structures by using existing point-topoint systems is not a very cost effective solution. It can also include some technical
problems depending on the technology used in the existing systems.
5.4.1.3
Interconnection of domains
Existing point-to-point systems may also be used for the interconnection of domains
in the architectures defined in Deliverable 1 of P709 (two-level OMS-SPRing, OMSSPRing - Optical Mesh, Optical Mesh - OMS-SPRing). This is useful only when the
interconnecting link is so long that the normal interfaces of the OADMs or OXCs are
not sufficient. Normally this is not the case because the domains to be interconnected
are usually situated geographically close to each other. One reason for using point-topoint systems for the interconnection of domains could be the requirement for dual
node interconnection. In this case even a longer route may be used for the secondary
interconnection - at least as a temporary solution - if it is the only possible alternative.
A limiting factor is then the maximum number of consecutive optical amplifier
sections between regeneration points.
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5.4.2
Deliverable 2
Existing SDH systems
If protection at the optical layer cannot be implemented it is possible to use existing
SDH systems and equipment for protection in some parts of the network. This can be
realised using different ring structures and dedicated protection or SDH restoration in
a meshed network. SDH systems can also be used for the interconnection optical
domains. One alternative is to use SDH restoration as a secondary protection when
dedicated optical protection is used. This gives an opportunity to provide protection
against two simultaneous failures in certain parts of the network.
Optical network protection based merely on the use of SDH connections can only be
an interim solution. If the target of planning is a fully optical network the protection
should not be based on the use of the client layer. However, protection in the SDH
layer can be used in addition to optical protection to improve the availability of the
network. In this case the interworking of the two protection methods must be
controlled in such a way that the possibility of conflicts is minimised.
5.5
Conclusions
5.5.1
Protection of individual channels
The basic optical architectures implementing different protection methods may
exhibit weak points, leading to vulnerability. On the other hand, there may be very
important interconnections between sub-domains of a network, the availability of
which has to be improved.
Coming from these two reasons, the possibilities of protecting individual channels are
investigated. In this discussion an individual channel is an electrical client signal. The
target of this examination is to find out whether it makes sense to have individual
channel protection in addition to the existing network protection or not.
The results are shown in Table II, where various basic optical architectures and
hierarchical networks are considered.
In CS-Rings, the protection is performed entirely on the SDH MS layer. The weak
point of a CS-Ring can be the SDH node where some signals are transited on the
electrical path layer. However, this weak point can be eliminated by either:

using 1+1 path protection on the HOP or LOP layer, or

using the extra flexibility features of CS-Rings, which makes it possible to rearrange the logical order of nodes in a way that transiting through the electrical
layer is no longer necessary.
Anyway, individual channel protection is always possible in the client layer (SDH
path protection or MSP) regardless of the optical architecture used. But it does not
make sense in every case. When the network has weak points it can, obviously, be
considered. But when the architecture is robust enough it does not seem to be
necessary.
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Weak point
eliminate weak point
protect against
double failure
(note 1)
individual prot.
possibility
Worth individual
channel protection?
CSR
yes
1+1 SDH path prot. and/or
changing the logical node
order
no
1+1 SDH path prot.
proposed
OMSPR
no
n.a.
no
1+1 SDH path prot.
or MSP
not proposed
CSR-CSR
normal routing
yes
1+1 SDH path prot. and/or
changing the logical node
order
only on the same
route
1+1 SDH path prot.
proposed
OMSPR-OMSPR
normal routing
no
n.a.
only on the same
route
1+1 SDH path prot.
or MSP
not proposed
note 1:
Naturally, no ring can fully survive two span or node failures, independently of the ring type. However, in hierachical networks, when
the failures happen on different layers, imunity agains such double failure can be achieved.
Table II - Conclusions on protection of individual channels
5.5.2
Protection priority classes
Network survivability performance is of utmost importance as aggregate traffic rates
increase both for the network operator and for the user. Such an increase of traffic
will occupy network resources initially planned to assure network protection.
The definition of protection priority classes is possible, and suits both the operator
and the user. The first one will be able to release some previously allocate resources
for protection, will not degrade its reliability image for the user, and will get a
revenue which is proportional to the degree of offered service. The users will be able
to select the type of protection suited to its own needs and only be charged
accordingly to that. A migration to a higher level of protection its always possible if
the its protection requirements change.
A possible definition based on percentage of user traffic protected and recovery delay
is presented in section 5.2.1, and a proposition of simple implementation with existing
protection schemes done. More elaborated schemes based on n:m protection, two
level complex architectures and mixed protection restoration techniques seem very
promising for offering diversified grades of survivability, however their planing
maybe very complex and there are no available dedicated tools for this purpose.
5.5.3
Protection against simultaneous failures
With the use of optical drop and continue functionality additional wavelengths are not
needed for the interconnection of optical network domains. By this way protection
against several, but not all, multiple failure cases can be achieved. Additional
wavelengths may still be needed to further improve the survivability of the network.
Nevertheless, the network cannot be protected against all simultaneous failure cases.
In the case where drop and continue functionality is not available additional
wavelengths are not necessarily needed for the interconnection of optical network
domains. By this way protection against all single point failures and only some
multiple failures can be achieved. The survivability of the network against multiple
failures can be improved, to the level that the use of drop and continue functionality
provides without additional wavelengths, by doubling the number of wavelengths
used to interconnect the two nodes located in different optical network domains. This
can provide protection against several, but not all, multiple failure cases. Additional
wavelengths may still be needed to further improve the survivability of the network.
Nevertheless, the network cannot be protected against all simultaneous failure cases.
© 1999 EURESCOM Participants in Project P709
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Full protection against all multiple failure cases is hard to achieve with reasonable use
of resources. Therefore, protection at client layer is sometimes required if there is a
need to fully protect the client layer traffic against multiple failures in the optical
network.
5.5.4
Existing infrastructures in optical network protection
The best way to utilize existing point-to-point WDM systems is to implement 1+1
optical protection between certain nodes when fast protection switching is needed. It
can also be used in addition to other optical protection methods in some parts of the
network. The small capacity of the existing point-to-point systems may limit the level
of protection.
The technology used in the existing WDM systems may crucially limit their usage in
ring structures. If optical terminal multiplexers cannot be easily upgraded to OADMs
the construction of ring structures is not cost-effective. Interconnection of network
domains using point-to-point systems is useful only when the distances between the
domains are very long.
Existing SDH systems and equipment can be used for protection in the optical
network as an interim solution when fully optical protection methods are not
available. It is also possible to use SDH layer for secondary protection in order to be
able to protect certain connections against two simultaneous failures.
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6
Identification of constraints on network upgrading
This chapter investigates the upgrading possibilities of Coloured Section Rings and
hierarchical network consisting of CSRs. First the upgrading possibilities of CSRs are
discussed. Based on these results, a more complex hierarchical network is examined.
6.1
Upgrading Coloured Section Rings
If the capacity of the CSR is fully utilised, and new links are needed between certain
ring nodes, we can insert new nodes within the existing stations. This possibility is
investigated in chapter 0.
In principle, the structure of the ring can be modified either by inserting totally new
nodes at different stations, or turning the ring into a mesh, or changing the CSR into a
real optical ring (OMS-SP Ring). These possibilities are considered in chapter 0.
Another upgrading issue can be the modification of the existing protection mechanism
discussed in chapter 0.
The upgrading possibilities mentioned above are summarised in Table III.
In this document the results of EURESCOM Project P615 are implicitly used
concerning the CSR features. Wherever these results are used a clear reference is
given to the relevant P615 document.
Capacity extension
inserting new node
within station
Structure upgrade
inserting new nodes
Change in protection
1+1 to 1:1
(secondary traffic)
change to mesh
change to OMS-SP-Ring
Table III - CSR upgrading possibilities
6.1.1
Capacity extension
The capacity of an existing CSR can be increased by inserting a new node within the
station. The solution is depicted in Figure 25 and Figure 26.
In the presented solution, only one new connection is shown (two additional nodes),
therefore instead of ADM, terminal multiplexers (TM) are used. However, the TMs
can be changed to ADMs later on, by inserting another wavelength.

If optical amplifiers are used, the maximum number of nodes is limited to 9 (see
P615 Deliverable 1, Volume 2, Chapter 3.4.2: physical limitations of CSR).
Therefore, by the upgrade we cannot exceed this number.

There is no interruption in the traffic during the insertion of new OADMs thanks
to the self-healing feature of CSR.

Since the maximum number of nodes in CSRs is 9 (see P615 Deliverable 1,
Volume 2, Chapter 3.4.2), the ring upgrade must be made accordingly.

In optically amplified CSRs, the maximum number of wavelengths in CSR is not
a limitation factor, therefore it is not necessary to be considered in the upgrade.

Two additional OADMs per wavelengths are needed. The power budget has to
have enough safety margin to work properly with these additional insertion
losses.
© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
Station A
TM
Deliverable 2
Station B
ADM
ADM
CS-Ring
new 
TM
ADM
ADM
new equipment
Station D
Station C
Figure 25. Inserting new nodes into CSR as capacity extension
W
W
W
TM
ADM
P
OADM
OADM
OA
OADM
OADM
new equipment
P
P
OA
OADM
OADM
new 
Figure 26. Inserting a node into CSR
6.1.2
Change in structure
6.1.2.1
Inserting new nodes
An example can be seen in Figure 27. In the figure, 3 new nodes are put into an
existing CSR.

The number of necessary additional wavelength is equal to the number of new
nodes.

There is no interruption in the traffic, provided the nodes are inserted one after
the other.

As the maximum number of nodes in optically amplified system is 9 (see P615
Deliverable 1, Volume 2, Chapter 3.4.2), we should consider that in the upgrade.
page 40 (48)
© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2

If any of the new nodes has to have an optical connection to any of the existing
nodes, it requires to change the ADM to a DXC at the existing node as well. This
should be avoided as it means traffic interruption.
Station E
Station A
Station B
ADM
ADM
ADM
Station F
A
D
M
CS-Ring
ADM
ADM
Station G
E, F, G
Station D
new stations
ADM
Station C
new s
new equipment
Figure 27. Three new nodes of an existing CSR
6.1.2.2
Changing CSR to mesh or OMS-SP Ring
Changing CSR to mesh or OMS-SP Ring is not possible since the optical elements of
the CSR are entirely different, and their working principles are absolutely different.
6.1.3
Changing the protection
The CSR utilises 1+1 protection handled by the add-drop multiplexers on the SDH
client layer. In this case, the entirely protection capacity is reserved, and unused in
normal operation.
Changing the 1+1 protection to 1:1 scheme can make sense, as we can use the
reserved capacity for low priority traffic. This traffic having secondary importance
will immediately be lost when a network failure occurs, and the working traffic is
switched to the protection routes. (See Figure 28.)
With this solution, additional traffic can be routed by the same ring, and therefore in
terms of capacity extension, it is relating to the capacity extension of the ring
discussed in chapter 6.1.1.
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
Changing 1+1 to 1:1 protection requires that the ADM equipment shall be able to
handle the 1:1 MSP scheme that is quite common in SDH. This modification in the
ADMs can be made without influencing the existing traffic.
normal secondary normal
traffic
traffic
traffic
W
W
ADM
P
P
OADM
OA
OADM
OA
OADM
OADM
Figure 28. Using 1:1 MSP in the ADM of a CSR node
6.2
Upgrading CSR-CSR Hierarchical Networks
In this section only upgrading is considered relating to transit demands, i.e. demands
crossing the upper level CSR are taken into account. The other upgradings are
identical to the separate CSR upgrading issues discussed in section 0.
If the capacity of the network is fully utilised, and new links are needed between
nodes located on different rings, we can either insert new wavelengths or insert new
nodes within the existing stations, as discussed in section 0. These two possibilities
applied especially to the CSR-CSR network are investigated in the following chapters.
The structure can be modified by inserting totally new nodes at different stations.
Such modifications such as changing the CSR-CSR hierarchical network to OMSSPR - OMS-SPR or other structure is not considered, as it is not possible (see section
0).
The modification of the existing protection mechanism discussed in section 0 is also
relevant to CSR-CSR networks.
These upgrading possibilities are summarised in Table IV.
Capacity extension
inserting new node
within station
Structure upgrade
inserting new nodes
Change in protection
1+1 to 1:1
(secondary traffic)
Table IV - CSR-CSR upgrading possibilities
6.2.1
Inserting new nodes within a station
The capacity of an existing CSR-CSR network can be increased also by inserting new
nodes within the stations. The principle of the solution is shown in Figure 29. The
changes within the stations are identical with of Figure 26.

The wavelength allocation can be made separately for each ring.
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Volume 3: Annex B – Protection in the optical layer

The limitations in number of wavelengths and number of nodes are the same as
in separate CSRs, discussed in chapter 6.1.1.2, as the rings are optically
separated.

There is no interruption in the traffic during the insertion of new OADMs thanks
to the self-healing feature of CSR.

Two additional OADMs per wavelengths are needed. The power budget has to
have enough safety margin to work properly with these additional insertion
losses.

If inserting new nodes step by step, there is no traffic interruption.

Because of inserting two nodes on the lower level as the originating and end
point of the traffic, four new nodes insertion is required on the higher ring.
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
Station A
ADM
ADM
Lower level CS-Ring
new s
DXC
DXC
new equipment
I1
I2
Upper level CS-Ring
I3
I4
DXC
DXC
Lower level CS-Ring
ADM
ADM
Station B
Figure 29. Inserting new nodes within stations into CSR-CSR network
6.2.2
Inserting new nodes
Inserting entirely new nodes is the same as within the station, therefore the same are
valid as described in chapter 6.2.2.
6.2.3
Inserting new rings on the lower level
A new lower level ring can be integrated into the network, using the existing upper
level ring for transiting. On the upper level, new hub nodes can be inserted or just
simply new nodes at the existing hub stations.
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© 1999 EURESCOM Participants in Project P709
Volume 3: Annex B – Protection in the optical layer
Deliverable 2
These all can be made on the same principles described in previous chapters.
6.2.4
Changing protection
Each CSR of the network utilises 1+1 protection handled by the add-drop
multiplexers or DXCs. In this case, the entirely protection capacity is reserved, and
unused in normal operation.
Changing the 1+1 protection to 1:1 scheme can make sense, as we can use the
reserved capacity for low priority traffic. This traffic having secondary importance
will immediately be lost when a network failure occurs, and the working traffic is
switched to the protection routes. (See Figure 28.)
With this solution, additional traffic can be routed via the network.
Changing 1+1 to 1:1 protection requires that the ADM and DXC equipment shall be
able to handle the 1:1 MSP scheme. This modification can be made without
influencing the existing traffic.
6.3
Conclusions
This section addresses the upgrading possibilities of Coloured Section Rings and
hierarchical networks consisting of CS-Rings. In the CS-Ring hierarchical network
the rings are optically separated. Therefore, the same rules for network upgrading are
valid in both cases:

Inserting new nodes into a CS-Ring hierarchical network, either in a new station
or within a station, can be made without any problem, as far as some rules (such
as the maximum number of nodes or power budget) are kept in mind.

It makes sense to use 1:1 protection in the SDH ADMs and DXCs, to transport
additional secondary traffic, if the required additional tributary ports are
available in the SDH equipment.
The upgrading issues of CS-Ring - CS-Ring networks are summarised in the table
below.
degree of "intervention"
traffic interruption
limitation
benefit
Inserting new
node
low
no
max. 9 nodes in total
(note 1)
new capacity
Changing protection:
1+1 to 1:1
low
no
new capacity for
secondary traffic
note 1: according to Deliverable 1 of P615, the maximum number of
nodes in a CS-Ring using OAs is 9, due to power budget limitations
Table V - Upgrading CSR-CSR network
© 1999 EURESCOM Participants in Project P709
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7
Deliverable 2
Conclusions
This document has studied protection techniques for optical networks. Focus has been
on two-level network architectures. The architectures that have been studied use dual
node interconnection because in large capacity core networks disjoint alternative
routing is necessary.
It has been found that the introduction of optical drop and continue functionality and
optical switching functionality in the interconnection nodes is an effective means of
improving the survivability of interconnected optical network domains. In the case of
interconnected OMS-SPRings this functionality provides protection against the failure
of one or both interconnecting nodes (each on different rings, but on the same
interconnection), or the connection between the two interconnecting nodes. The
architecture can also protect against a single failure in each of the rings provided that
these failures are not terminating node failures, or these failures do not combine to
affect both interconnections between the rings.
The problem of wavelength conflicts, or misconnections, under protected state in
OMS-SPRings has been addressed. In this case, wavelengths are analogous to the time
slots in SDH. As such, misconnections will occur at the optical channel level under
conditions similar to the SDH case: node or multiple span failures without lowpriority traffic in the protection wavelengths, and single span failures with lowpriority traffic in the protection wavelengths. The proposed solution for the
misconnections is based on wavelength squelching, using the overhead channel
associated to the optical channels to support an Optical Channel AIS signal. This
solution would maintain the independence of the optical layers from the client layers,
as the client signals would not be directly manipulated.
An issue left for further study is the proper way of linking the AIS alarm at the
Optical Channel Layer to the management system of the client signal (typically SDH),
in order that the client would be able to reject the misconnected signals.
The CS-Ring architecture presents a weak point when traffic electrically transits
through a node, as this traffic is unprotected against failures in that node. This
weakness can be avoided either by re-ordering the nodes logically, in order to avoid
transits through the electrical layer, or by the implementing 1+1 path protection for
this traffic.
Network survivability is of utmost importance as aggregate traffic rates increase. A
possible protection priority class definition based on percentage of user traffic
protected and recovery delay is presented in this document. Also a proposition of
simple implementation with existing protection schemes is done. More elaborated
schemes based on n:m protection, two level complex architectures and mixed
protection restoration techniques seem very promising for offering diversified grades
of survivability, however their planing maybe very complex and there are no available
dedicated tools for this purpose.
Full protection against all multiple failures is hard to achieve in two level optical
network architectures without wasting wavelengths. Most efficient protection against
multiple failures can be achieved by using optical drop and continue functionality in
the interconnecting nodes. However, protection at client layer is sometimes required if
there is a need to fully protect the client layer traffic against multiple failures in the
optical network.
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© 1999 EURESCOM Participants in Project P709
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Volume 3: Annex B – Protection in the optical layer
The best way to utilise existing point-to-point WDM systems is to implement 1+1
optical protection between certain nodes when fast protection switching is needed.
The small capacity of the existing point-to-point systems may limit the level of
protection. The technology used in the existing WDM systems may crucially limit
their usage in ring structures. If optical terminal multiplexers cannot be easily
upgraded to OADMs the construction of ring structures is not cost-effective.
Interconnection of network domains using point-to-point systems is useful only when
the distances between the domains are very long.
© 1999 EURESCOM Participants in Project P709
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Deliverable 2
References
[1]
ITU- T Recommendation G.842 – Interworking of SDH network protection
architectures (04/97)
[2]
ETSI TS 101 010 V1.1.1 (1997-11)
Transmission and Multiplexing (TM);
Synchronous Digital Hierarchy (SDH);
Network protection schemes;
Interworking: rings and other schemes
[3]
EURESCOM P615 PIR 2.2, document BT22-03a.doc – Choice of network
study cases and specifications of comparison scenarios
[4]
EURESCOM P615 PIR 2.2, document BT22-05a.doc – Traffic uniformity,
planning complexity and traffic interruption aspects of the OMS-SPRING
architecture with wavelength reuse
[5]
EURESCOM P615 PIR 2.3, document TE23-04c.doc – Dimensioning results
of ring architectures
[6]
ITU-T Recommendation G.841 – Types and Characteristics of SDH Network
Protection Architectures (07/95)
[7]
EURESCOM P615 Deliverable 1 – Understanding Optical Network
Architectures, Vol. 2
[8]
ETSI TS 101 009 V1.1.1 (1997-11)
Transmission and Multiplexing (TM);
Synchronous Digital Hierarchy (SDH);
Network protection schemes;
Types and characteristics
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© 1999 EURESCOM Participants in Project P709