Download Optical network management with OSC

Optical network management with OSC*
Fengqing Liu, Qingji Zeng, Shilin Xiao, Xu Zhu, Jie Xu
Electronic Engineering Department, Shanghai Jiaotong University,
Shanghai, 200030, P. R. China
ABSTRACT
How to transport the control and management information of optical networking is a major concern these days. We
compare several means and arrive at a conclusion that OSC (optical supervisory channel) is a better choice. The content
of OSC is given and OSC channel wavelength, bit rate and coding method are discussed. An OSC example of WDM
metro network, which demonstrates the processing operation of overheads in OSC, FDI behavior for three types of
failure, and Self-management of OSC subsystem, are illuminated. To fulfill the demand of intelligent and distributed
management of optical transport network, an OSC of OC-3 (155Mbps) channel bit rate may be needed in the near
future.
Keywords: OSC, OTN (Optical Transport Network), FDI, WDM metro ring.
1. INTRODUCTION
With the unprecedented growth in data traffic, demand for bandwidth is seemly limitless. Optical transport network
(OTN) is being adopted for this purpose. And optical networking technology has been developed to circumvent the data
rate bottleneck caused by the electronic device in OEO transition. To implement optical networking, some characteristic
information of OTN must be transported between network elements and sent to network management system. There are
various means to provide for these and are summarized in the table below.
Table 1 several methods
In-fiber
Separate network
In-band
Optical Channel Overhead
Optical Channel
Out of band
Optical Supervisory Channel (OSC)
Data Communications Network (DCN)
The OSC is a separate channel, which carries overhead information for network management purposes. Management
messages for the OMS and OTS layers, together with management messages for the Och layer that are transported via a
channel non-associated implementation, may share an OSC1.
Comparing these methods, we can arrive at the conclusion that OSC is a better choice. The advantage of OSC can be
summarized as follows:
1. In the equipment without electronic interface, such as line optical amplifiers, the control and management
information of them cannot be sent with in-band method. That is why the OSC is presented.
2. OSC (Optical supervisory channel) is physically diverse from the associated data bearing channel. OSC ’s health is
independent from that of data bearing channel (that is optical channel). So when the data bearing channel fails, the
This paper is supported by National Natural Science Foundation of China. The number is 69990540.
E_mail: [email protected] Tel: 86-21-62932166 Fax: 86-21-62820892.
Fiber Optic Components, Subsystems, and Systems for Telecommunications, Suning Tang,
Xiaomin Ren, Editors, Proceedings of SPIE Vol. 4604 (2001) © 2001 SPIE · 0277-786X/01/$15.00
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3.
4.
control channel may still work and vice versa. For example, communications between nodes is possible if the
failure is specific to the OCH and not the entire fiber or a node.
Unlike DCN (Data Communication Network), OSC doesn’t need a separate network. It can utilize the existing fiber
network. As OTN may transverse some place where there is no existing data networks, it is convenient to use OSC.
OSC can transport OTS and OMS overhead, so it has the inherent ability to carry common information of multiple
wavelength, even multiple links. This can reduce the information that needs to be communicated between nodes.
As more and more fibers and more available wavelengths in a fiber between adjacent nodes will be provisioned,
control and management issues will become complex and error-prone. OSC can make this simple and makes the
OTN more scalable. This is very useful for GMPLS, which extends MPLS to IP/WDM integrated network and
need implementing LSP hierarchy and link bundling for scalability2.
The paper is organized as follows. In section 2 we will discuss the content of Optical Supervisory Channel and both
frame-based and message-based methods will be used. In section 3 an example of using OSC to manage WDM metro
network is given, which specifies how to process the OSC overhead and describes the scenario of FDI sending over the
OSC. In section 4, we mainly deal with the self-management of itself. Finally, we discuss the future trend of OSC,
which will eat up about 100Mbps bandwidth.
2. OSC contents
It has been agreed that OMS (Optical Multiplex Section) and OTS (Optical Transport Section) overhead are carried on
the Optical Supervisory Channel. Some Optical Channel overhead, such as optical channel trace, need to be associated
with Och. However, there are some locations where Och associated overhead are not available, but there are needs for
OMS terminations to perform an OCh maintenance function. Otherwise, there must be a standby transmitter to be
capable of generating an above-line-rate signal in order to create the byte that houses the OCh maintain information3. In
such an instance, 2.5 Gb/s capable transmitter would be necessary to send a 2 Mb/s signal. It is very expensive and low
cost-effective ratio.
In another view, OSC will need to carry several separate information streams, including control and management
information. Control information has low latency requirement, which includes the signaling for both data bearing
channel and OSC channel itself. Fault management information including FDI and APS is an example of it.
Management information is relative time insensitive information, which includes OMS, Och FTFL messages. OTS,
OMS DCC (Data communication channel) may be time-critical or time insensitive according to what are transferred in
it. Besides these information, there are OTS, OMS order wire bandwidth requirements and some other information.
According to information that belongs to different layers (OTS, OMS, Och) and used for data bearing channel or optical
supervisory channel, the contents of OSC can be depicted as figure 1. FDI/BDI is used to suppress the downstream or
upstream alarming messages in the corresponding layer, respectively. FTFL (Fault Type and Fault Location) messages
are used to determine the fault type and fault location of corresponding layer. Data communication channel has multiple
usages, which can be used to transport wavelength provision information, control information of the OXC and OADM,
even they can be used to download software between nodes. The overhead for OSC mainly includes
protection-switching signaling for OSC subsystem, the states and commands that OSC subsystem and network manager
want to exchange. There are also some bits that are reserved for national and carrier use.
There are two different means to transport these separate streams, frame-based and message-based. We use both, for
they have different advantages for different information. Fixed bit assignments of a frame are used to carry the low
latency information, which can be easily examined and has the advantage of low latency. An OSC of 2Mbps, using
PCM30/32 frame structure is used in our implementationof Metro ring. Message-based protocols, a simplified TCP/IP
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protocol stacks, are used for those time insensitive information. A fixed amount of transport capacity in a frame is
reserved for the message-based protocol and variable contents can be defined in it. That is, some information will be
packaged and transported though the fixed amount of bandwidth, such as DCC.
OTS OH
OMS OH_P
OMS OH_S
FAS OTS TTI OTS OW OTS DCC OTS Reserved
OMS TTI OMS OW OMS FDI/BDI
OMS APS OMS DCC OMS FTFL OMS Reserved
OSC APS OMS OSC FDI/BDI OSC APS
OCH unassociated OH_P
OCH1 FDI/BDI OCH1 FTFL
• • •
OCHn FDI/BDI OCHn FTFL
OCH Reserved
OCH unassociated OH_S
OCH1 OSC FDI/BDI
Figure 1
•••
OCHn OSC FDI/BDI
Content of the OSC
OMS OH_P means OMS overhead for OMS payload. OMS OH_S means OMS overhead for OSC channel. So is for
other similar expression. OSC states and commands from network manager are defined in OMS DCC as a type of
message.
3. An OSC example of WDM metro network
We will illuminate an OSC implementation of WDM metro network. It is a three-nodes 4-fiber BLSR Ring with 16
wavelengths per fiber. Two of three nodes are OADMs. Another is OXC. Using of OXC is just for scalable purpose to
interconnect with other future WDM rings. It functions like an OADM at present. All these three nodes fulfill mainly
OTS and OMS layer function as defined by ITU-T. They can Add/Drop and switch wavelength channels, but they don’t
have the ability to process the associated overhead of OCH layer. In this architecture, there is no optical line amplifier.
So this network includes a very simplified OTS layer, including just FAS series to delimiter the frame. The overhead of
OMS and OCH layer is just like figure 1. Someone may argue that reserving a fixed bandwidth for the FTFL message is
too bandwidth consuming for three-node network. We keep it in our frame structure for the compatible and scalable
consideration. Figure 2 (Two fibers are pictured, another two is similar and omitted) shows the node structure, the
structure of OSC controller and segregation of OSC subsystem (control plane) and WDM network (data plane).
OSC is a physically independent subsystem. OSC and WDM data network just share fibers and 1510/1550 multiplexer
and demultiplexer, which are passive and reliable. But they operate in different wavelength and have different internal
structure. To make the failure of data channel not infect the control channel, OSC channel wavelength should be chosen
to be different from data channel wavelength band, which is near 1550nm. OSC channel wavelength, data bit rate and
channel coding is not yet standardized. For channel wavelength, there are several choices. It can be selected in 1510nm,
1310nm or 1480nm. Corning corporation, which OA operates in 1400nm, select 1625nm wavelength as OSC channel
wavelength. We use 1510±10 nm, for it is normally used and has a compatible advantage in interoperability with other
carriers. To a three-node WDM metro ring, 2Mbps is enough. CMI (Coded Mark Inversion) is adopted for OSC in our
metro ring. And a PCM30/32 frame structure is used. Some bytes of the frame are assigned to transport the time crucial
information. Another bytes are used to transport message-based variable contents, such as DCC.
We demonstrate the processing operation of overheads in OSC when a fault happens.
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O/E
NE
Manager
E/O
NE
OSC controller
Add/Drop
module
Manager
OSC
Device
interface
controller
OW
1550/1510
Demultiplexer
1550/1510
Multiplexer
E/O
(a) OADM Node structure
O/E
(b) OSC controller
OADM
Data plane
OSC control plane
(1550nm band)
(1510 nm band)
(C) Physically separation of OSC
subsystem and WDM optical network
OADM
OXC
Figure 2
OSC node structure and segregation of OSC network and WDM optical network
Prior to the fault the OSC OH will be processed as follows:
1). OTS OH and OMS OH will terminate at the OMS layer. A new OTS OH and OMS OH will be generated.
DROP
ADD
OTS OH OMS OH
DATA BEARING CHANNEL
OTS OH OMS OH
OTS/OMS
OTS/OMS
OH Rev.
OH Trans.
OCH un. OH-P
OCH un. OH-S
NO FAULT
OCH Fault detection (OCH LOS-P)
OCH un. OH-P
OCH OH-P Trans.
OCH un. OH-S
OCH OH-S Trans.
OSC channel Fault Detection( OCH LOS_S)
Figure 3 the overhead processing operation in a node
2). OCH unassociated OH-p and OCH unassociated OH_S will be passed through unchanged. Their contents will
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be changed only if a data channel failure or OSC channel failure occurs.
A special device (such as OSA, Optical spectrum Analyzer) will detect the data bearing channel failure, and notify the
OSC controller through internal mechanism. OSC controller will detect failure of the OSC channel. OCH FDI-P or
OCH FDI-S and associated FTFL messages will be inserted into OSC OCH overhead. PD (Photonical Detector) will
find fiber cut. It will make OMS FDI, OMS FTFL transmitted. Figure 3 shows the overhead processing operation in a
node.
Figure 4 demonstrates the way to deal with three possible failure of the WDM metro ring. As there is no node with Och
associated overhead accessing capability, OCH FDI and OCH FTFL are transported in OSC channel and processed by
OSC controller. Optical transmitter/receiver communicate with OSC through internal mechanism. Although there are
only three nodes, we use the backward indication of failure for future consideration.
OMS FDI_P, OMS FDI_S ,
OMS FTFL via OSC
OADM
OTS_TT
OMS_TT
OMS BDI, OMS
FTFL via OSC
OTS_TT
OCHi FDI, OCHi FTFL
via OSC
OADM
a) Fiber cut
OMS FDI_S, OCHi
FTFL via OSC
OADM
Figure 4
via OSC
OTS_TT OTS_TT
OADM
OMS_TT OMS_TT
OADM
OTS_TT
OMS_TT
b) payload channel failure
OTS_TT
OMS_TT
OADM
OMS FTFL
via OSC
OTS_TT
OMS_TT
OCHi channel
faiure
OCHi FTFL
Fiber cut
OTS_TT
OADM
OMS_TT
OADM OMS_TT
OSC channel
failure
OADM
c) OSC channel failure
OTS_TT
OMS_TT
FDI Behavior for three types of failure in WDM metro ring
4. Self-management of OSC subsystem
OSC is a subsystem of OTN network. It has its own management information to transport. We defined dedicated bytes
for them in OSC frame. Alarm, self-protection signaling, command/state are the information that need to be
communicated in the OSC channel.
OSC is a lower bit rate channel, so it can be monitored in electrical domain to get more details to decide protection or
not. Separation of data channel and control channel protection has some other advantages.
OSC channel failure may be caused by OSC channel failure, OSC optical transmitter/receiver or OSC controller failure.
We consider only the former two failures. There are two signals to indicate it. One is LOL (Loss Of Light), detected by
optical transmitter/receiver. The other is LOF (Loss Of frame), detected by OSC controller.
Topology of OSC ring in normal conditions is as figure 5(a). When any OSC work channel between two nodes fails,
protection switching will be activated. To reduce the information that needs to be transported we use the sate-command
model. States of OSC channel (just refer to the channel between upstream node and it) are kept in the node. Only
Protection request (or ACK) are sent in APS bytes. According to its states and the signaling in OSC channel, each node
decides whether it initiate protection.
When protection actions are finished, the topology of OSC subsystem is like figure 5 (b). Besides the work channel ring,
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another ring will be formed as soon as the failure is repaired. So these two nodes can detect signal of frame sync and
one of them will activate restoration back to the normal states.
Node 1
Node 1
E/O
O/E
O/E
E/O
E/O
O/E
work ring
work ring
E/O
O/E
protection ring
protection ring
Failure
Node 2
Node 3
Node 2
Node 3
O/E
E/O
O/E
E/O
O/E
E/O
O/E
E/O
E/O
O/E
E/O
O/E
E/O
O/E
E/O
O/E
a) Topology of OSC ring in
b)
Topology
of
OSC
ring
after
protection has been finished
normal conditions
Figure 5
protection of OSC subsystem.
5. Conclusion and future trend
In this paper, we compare the advantage and disadvantage of in-band and out of band methods. OSC contents are
discussed. Both frame-based and message-based methods are used to transport them. Finally, an example of metro
WDM ring is given, including self-protection of OSC subsystem, OSC overhead processing process and FDI behavior
for three types of failure.
To manage a big intelligent and distributed optical network, there is a lot more information to be transported. These
information include OTS Layer Alarm messaging, OMS Layer Data Control, Network Topology Probing, Trouble
Shooting, Maintenance, OSC Protection and others, which will eat up about 100Mbps bandwidth3. So a 2Mbps channel
is not enough and an OC-3 link may be used to transport them. To interface with internal devices, Ethernet or fast
Ethernet technology may be used.
REFERENCES
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2.
3.
200
G.9591 Optical transport network physical layer interfaces
A. Banerjee et al., “Generalized Multiprotocol Lable Switching: An Overview of Routing and Management
Enhancements” IEEE Comm. Mag., Vol. 39, no1., Jan. 2001.”
Laszio I. Szerenyi, Optical networking – OSC requirements, T1X1.5/2000-138
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