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S-108.199 OPTICAL COMMUNICATIONS AND
INSTRUMENTS
SDH and WDM, 10.3.2004
Antti Pietiläinen
1
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
How the future of layering looked 4 years ago
Transport/Application Layer
IP
Trend towards simplification
of protocol stacks
From
this
Towards
this
FR
ATM
PDH
SDH Transport
IP
ATM
IP
Digital Signal
(FR, FOTS, PDH)
WDM/Optical Networking Layer
Time
Current
Systems
2
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
Current
R&D
Future
Deployment
(3-6 yrs)
How the future looks now
From
this
Transport/Application Layer
IP
Trend towards simplification
of protocol stacks
Towards
this
FR
ATM
PDH
IP
GFP
SDH Transport
Ethernet
IP
GFP
OTN
Current
Systems
3
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
OTN
Current
R&D
and future deplyment
Acronyms of previous page
• IP = Internet protocol
• FR = frame relay
• PDH = plesiochronous digital hierarchy
• ATM = asynchronous transfer mode
• SDH = synchronous digital hierarchy
• GFP = generic framing procedure
• WDM = wavelength division multiplexing
• OTN = optical transport network
4
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
Digital signal
• Analog telephone connection carries the frequencies between 300 Hz and
3400 Hz.
• The signal can be converted into a digital signal by sampling 8-bit
samples at 8000 samples/s.
• Channels are multiplexed together using time domain multiplexing. Each
channel gets one-byte timeslot in every frame. Frame rate is 8000 frames
per second. 30 or 31 channels fit into 32 time slots where one or two time
slots are used for frame alignment and signaling. The total bit rate of E1
frame is 2.048 Mbit/s.
• The ANSI frame T1 includes 24 64-kbit/s channels. A single frame has 24 x
8 =192 payload bits and one framing bit.
• The 64-kbit/s channels collected into one E1 or T1 are digitized using the
same clock. Thus, the channels are synchronized with each other.
Ramaswami has assumed the framing bit as a stuffing bit, which could
accommodate bit rate differences, so p. 365 has an error.
• However, individual E1 and T1 signals are not necessarily synchronized
with each other.
5
© NOKIA
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Short history multiplexing hierarchy, PDH
• PDH – plesiochronous digital hierarchy. Multiplexing signals, which are
running at almost the same speed.
• First standards in the second half of 1960s.
• Tributaries and higher order bit streams are allowed to deviate from a
pre-defined bit rate by a specified amount, for example at 2 Mbit/s the
value is 50 ppm.
• Justification (bit stuffing) process is therefore required, which brings all
the tributaries up to the same bit rate before multiplexing takes place.
6
© NOKIA
Level
North America
Europe
Japan
0
DS or T
0.064 Mbit/s
E
0.064 Mbit/s
0.064 Mbit/s
1
1544 Mbit/s
2048 Mbit/s
1544 Mbit/s
2
3
4
6312 Mbit/s
44736 Mbit/s
139264 Mbit/s
8448 Mbit/s
34368 Mbit/s
139264 Mbit/s
6312 Mbit/s
32064 Mbit/s
97728 Mbit/s
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
PDH multiplexer mountain
• When a signal is demultiplexed, the stuffed bits are removed. Each level
knows how to demultiplex the next level.
• To extract a 2-Mbit/s signal from a high- bit rate stream, a “multiplexer
mountain” is required.
34 Mbit/s
140 Mbit/s 34 140
MUX
832
MUX
8 Mbit/s
28
MUX
2 Mbit/s Drop/Add
7
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
2 Mbit/s
28
MUX
34 - 140 Mbit/s
140
MUX
832
MUX
Foundation of SDH/SONET
• Developed during late 1980s
• SDH - Synchronous digital hierarchy (Europe, Japan, Intercontinental
cables, ITU recommendation)
• SONET – Synchronous optical network (North America, ANSI standard)
• All clocks in a network are synchronized to a single master clock.
• All bit rates are integer multiples of the basic bit rates and no bit stuffing
is required.
• A lower bit rate stream can be extracted from a multiplexed SONET/SDH
stream in a single step.
8
© NOKIA
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Other advantages of SONET/SDH
• Management
• Incorporates extensive management information for managing the
network
• Performance monitoring
• Identification of connectivity and traffic type
• Identification and reporting of failures
• Data communication channel
• Interoperability
• PDH standard did not specify standard format on the transmission link.
Therefore different vendors used different line coding, optical
interfaces etc. In SDH standardization is more complete. However,
there are still some problems in connecting equipment from different
vendors
• Network availability
• Incorporate specific network topologies and specific protection
techniques and associated protocols to provide high availability.
Restoration time after failure can be less than 60 ms.
9
© NOKIA
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Multiplexing
• The basic SONET bit rate is 51840 Mbit/s and the basic SDH bit rate is
155.420 Mbit/s. The highest bit rate standardized so far is 39813.120
Mbit/s.
Optical Level
Electrical
Level
Line Rate
(Mbps)
SDH
Equivalent
OC-1
(ANSI)
STS-1
Payload
Overhead
Rate (Mbps) Rate
(Mbps)
51.840
50.112
1.728
-
OC-3
STS-3
155.520
150.336
5.184
STM-1
OC-12
STS-12
622.080
601.344
20.736
STM-4
OC-48
STS-48
2488.320
2405.376
82.944
STM-16
OC-192
STS-192
9953.280
9621.504
331.776
STM-64
OC-768
STS-768
39813.120
38486.016
1327.104
STM-256
(ANSI)
10
© NOKIA
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SDH Multiplexing structure (ITU-T G.707)
x1
x1
AUG-4-256c
STM-256 AUG-256
STM-64
STM-16
STM-4
STM-1
STM-0
x1
x1
x1
x4
x1
AUG-64
x4
AUG-16
x4
AUG-4
x1
AUG-1
AU-4
SONET
Pointer processing
Multiplexing
Aligning
Mapping
AU-3
C-4-64c
2396.16M
C-4-16c
599.04M
VC-4-4c
C-4-4c
150.336M
VC-4
149.8M
x3
x 1 ETSI ETSI
ETSI TUG-3
SONET x 3
x1
9584.64M
VC-4-16c
AUG-4-4c
x1
C-4-256c
VC-4-64c
AUG-4-16c
x4
x1
VC-4-256c
AUG-4-64c
x1
38338.56M
VC-3
SONET
TU-3
ETSI x 7
x1
x7
TUG-2
SONET
x4
© NOKIA
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
DS4 139M
E4 139M
C-3
DS3 45M
E3 34M
C-2
DS2 6M
SONET
TU-2
x3
VC-2
TU-12 VC-12
C-12 E1 2M
TU-11 VC-11
C-11 DS1 1.5M
ETSI
SONET
11
ETSI
VC-3
C-4
Procedures in SDH framing
• SDH mapping: A procedure by which tributaries are adapted into Virtual
Containers at the boundary of an SDH network.
• SDH multiplexing: A procedure by which multiple lower order path layer
signals are adapted into a higher order path or the multiple higher order
path layer signals are adapted into a multiplex section.
• SDH aligning: A procedure by which the frame offset information is
incorporated into the Tributary Unit or the Administrative Unit when
adapting to the frame reference of the supporting layer.
12
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Cross-connecting VCs
• VCs arrive inside STMs into a cross connect. The VCs are recovered and
cross-connected to another port and aligned into another STMs.
Connection
points
In 1 STM-1 1
In 2
STM-4
1
flexible
matrix
Connection
points
1
2
2
3
4
3
4
1
STM-4 Out 1
STM-1
Out 2
VC-4 pipes :
continuous stream
of virtual containers
13
© NOKIA
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Aligning virtual containers into TU (transport unit)
or AU (administrative unit)
• Virtual container is a frame (frame
rate 8000 frames/s).
• The virtual containers are
synchronous because the clock is
derived from the master clock of
the network.
• However, virtual containers may
travel a long way and go through
many links.
• Each link transmits frames
between two nodes at a frame
rate of 8000 frames/s. The bit rate
of all consecutive links may be the
same but the frame boundaries
occur at different moments. Thus,
VCs float inside the TUs or AUs and
a pointer indicates where the
starting point of a VC is within a TU
or AU
14
© NOKIA
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Pointer
TU-12
frame
VC-12
frame
Pointer
TU-12
frame
Justification bytes and pointer slip
• Sometimes the whole network is not synchronized and VCs may be
running at a slightly faster or slower rate than the TU or AU.
• Therefore the pointer must slip one byte every now and then. For
allowing slightly faster rate there has to be an extra byte in the TU or AU
that may take the last byte of a frame when there is a negative slip where
the VC shifts to a location one byte earlier than before and would
otherwise overwrite a byte. Correspondingly, in the case of positive split,
one byte in the TU or AU is jumped over and left empty.
15
© NOKIA
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Building an STM-1 from E2s
STM-1
Multiplex x1 + MSOH, 2.88 Mbit/s, and RSOH, 1.728 Mbit/s
AUG-1
Multiplex x1
AU-4
Alignment of VC-4 into AU-4
and add AU pointer + 3 justification bytes
VC-4
Multiplex x3 + add VC-4 POH, 576 kbit/s
(higher order VC POH)
TUG-3
TUG-2
TU-12
Multiplex x7
Multiplex x3
Alignment of VC-12 into TU-12 and add 2-byte TU pointer+justification byte
+ one empty byte for every 4 frames, 64 kbit/s
VC-12 POH, 64 kbit/s
VC-12 Add
(lower order VC POH)
C-12
Add stuffing bits, 128 kbit/s
2.048 Mbit/s tributary signal in
16
© NOKIA
STM = Synchronous transfer module
MSOH = Multiplexer section overhead
RSOH = Regenerator section overhead
AUG = Administrative unit group
AU = Administrative unit
TUG = Tributary unit group
TU = Tributary unit
POH = Path overhead
VC = Virtual container
C = Container
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
Mapping
Moving pointer in a cross connect
Pointer
Pointer
TU-12
frame
TU-12
frame
VC-12
frame
TU-12
frame
Multiplex section n
17
© NOKIA
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Pointer
TU-12
frame
VC-12
frame
Multiplex section n + 1
Recovering an E2 from an STM-1
STM = Synchronous transfer module
MSOH = Multiplexer section overhead
RSOH = Regenerator section overhead
AUG = Administrative unit group
AU = Administrative unit
TUG = Tributary unit group
TU = Tributary unit
POH = Path overhead
VC = Virtual container
C = Container
STM-1
Demultiplex ÷ 1, read MSOH and RSOH
AUG-1
Demultiplex ÷ 1
Recovery of VC-4 from AU-4
AU-4 (read AU pointer + strip justification bytes)
VC-4
TUG-3
TUG-2
Demultiplex ÷3 read VC-4 POH
Demultiplex ÷7
Demultiplex ÷ 3
of VC-12 from TU-12
TU-12 Recovery
(read TU pointer + strip justification byte
VC-12 Read VC-12 POH
C-12
Remove stuffing bits
2.048 Mbit/s tributary signal out
18
© NOKIA
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SDH sublayers
(VC) path
Path
Multiplexer
section
Line
Regenerator
section
Terminal
multiplexer
SONET:
Section
Regenerator
Tributaries
Add/drop
multiplexer
Terminal
multiplexer
Tributaries
•Path, Multiplexer section and Regenerator section each carry
management information, which is terminated at the endpoints of the
path or section.
19
© NOKIA
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STM-1 frame
• The resulting STM-1 frame
1 2 3
1
2
3
4
4 5 6
7
AU pointers
125 µs
Multiplex
section
overhead
8
9
20
© NOKIA
270 bytes
Regenerator
section
overhead
5
6
7 8 9 10
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
SDH physical interface example
• V-16.2
STM-16, 2.5 G
1550 nm
• Fiber type G.652 standard single-mode fiber
• Loss 22-33 dB
• Allowed dispersion 2400 ps/nm
• Fiber has attenuation of <0.275 dB/km and dispersion 17 ps/nm/km
21
© NOKIA
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Optical communications cumulative market size
Cumulative
Market
size
• The hype reached its peak in 2000
• Current estimates indicate solid but lower growth
Access wave
Estimate 2000
Metro wave
Core wave
Estimate 2004
1990
22
© NOKIA
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2000
2005
2010
Laboratory experiments conquering dispersion limits
• Chromatic dispersion limit using standard single-mode fiber: 2.5 Gbit/s 1000 km, 10 Gbit/s - 60 km. Most fiber laid in the ground is standard
single mode fiber.
• 1 Terabit/s (100 × 10 Gbit/s) transmission over 7300 km non-zero
dispersion shifted fiber, Electronics Lett. 35 (1999) p.2212.
• 1.28 Tbit/s (32 x 40 Gbit/s) over 1000 km non-dispersion-shifted fiber
using dispersion compensation fiber, Electronics Lett. 37 (2001) p.43.
• DWDM 40 x 40G transmission over trans-Pacific distance (10,000 km)
using CSRZ-DPSK, enhanced FEC and all-Raman amplified 100 km
UltraWave™ fiber spans,” Technical Digest of OFC 2003, post-deadline
paper, PD18, (2003).
23
© NOKIA
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WDM layer below SDH layer
• WDM has introduced functionality, such as optical add/drop multiplexers
and optical regeneration below SDH in layered model.
• Optical transport network (OTN) layer serves SDH client layer.
Path LAYER
TDM
Electrical
Layers
Multiplex Section LAYER
Regenerator Section LAYER
Optical Channel LAYER
WDM
Optical
Layers
Optical Multiplex Section LAYER
Optical Transmission Section LAYER
Fibre LAYER
24
© NOKIA
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Optical sections within a single SDH regeneration section
OPTICAL
MULTIPLEX
SECTION
OPTICAL
MULTIPLEX
SECTION
OPTICAL
OPTICAL
OPTICAL
TRANSMISSION TRANSMISSION TRANSMISSION
SECTION
SECTION
SECTION
TRIBUTARY
SIGNALS
TRIBUTARY
SIGNALS
WDM
MULTIPLEXER
OPTICAL
AMPLIFIER
OPTICAL
AMPLIFIER
OPTICAL CROSS-CONNECT
Optical Channel
SDH REGENERATOR SECTION
25
© NOKIA
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WDM
DEMULTIPLEXER
Optical channel frame
• STM-16 + FEC = OTU1, STM-64 + FEC = OTU2, STM-256 + FEC = OTU3
• Other payload mappings available, for example Generic framing procedure.
OTU type
OTU nominal bit rate
OTU1
255/238 × 2 488 320
kbit/s
OTU2
255/237 × 9 953 280
kbit/s
OTU3
255/236 × 39 813 120
kbit/s
OTU bit rate tolerance
±20 ppm
NOTE – The nominal OTUk rates are approximately: 2 666 057.143 kbit/s
(OTU1), 10 709 225.316 kbit/s (OTU2) and 43 018 413.559 kbit/s (OTU3).
OTU = Optical channel transport unit, FEC = Forward error correction
Source ITU-T G.709 (03/2003)
26
© NOKIA
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Optical transport network functionality
• Service provisioning
• Protection
• Performance monitoring
• Wavelength conversion
• Multiplexing and grooming
• Bit rate transparency (with provisions)
• High bit rate more sensitive to dispersion and attenuation
• Very high bit rate consumes more optical bandwidth
27
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WDM network elements
• Optical add/drop multiplexers (OADM)
Ring network
28
© NOKIA
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Linear network
WDM network elements
• Optical crossconnects (OXC)
OXC
OADM
Mesh
OADM
OXC
29
© NOKIA
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OADM
Protection at different optical layers
OXC
• Optical channel protection
• A backup route is planned for a path
• Optical multiplex section protection
• A link between two OXCs has a backup
route.
OADM
OADM
OXC
OADM
Trail Protection
1+1
1:1
m:n
Optical Transmission
Section Layer
30
© NOKIA
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1+1
1:1
(end-to-end
trail, ffs)
Optical Channel Layer
Optical Multiplex
Section Layer
Subnetwork Protection
(meshed
networks)
(ring
protection)
(ring
protection)
(meshed
networks)
m:n
(section)
(ring
protection)
(ring
protection)
OADM architectures
• Parallel where all the wavelengths are separated and multiplexed back
λw
λ1, λ2, …, λw
λ2
λ1
Drop
31
© NOKIA
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λ1, λ2, …, λw
λ2
λ1
Add
OADM architectures
• Modular version of parallel architecture
Band 4
λ1, λ2, …, λw
λ1, λ2, …, λw
Band 3
Band 2
Band 1
λ1 λ2
Drop
32
© NOKIA
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Add
OADM architectures
• Serial
λ1, λ2, …, λw
Drop
λ1, λ2, …, λw
λ1 Add
λ2
λ3
• Band drop
λ1, λ2, …, λw
λ1, λ2, …, λw
Drop
Add
λ1, λ2, λ3
33
© NOKIA
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OADM architectures
• Reconfigurable parallel architecture
Optical switch
λ1, λ2, …, λN
λ1, λ2, …, λN
λN
34
© NOKIA
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λ2
λ1
Optical crossconnect (OXC)
OLT (optical line terminal)
OXC
35
© NOKIA
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Different scenarios for OXC deployment
OLT (optical line
terminal)
• Electrical switch core
O E O
O E O
O E
O E
E O
E O
O E O
O E O
O E O
O E O
O E
O E
E O
E O
O E O
O E O
Electrical core
• Optical switch core surrounded by O/E/O converters
36
© NOKIA
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
O E O
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
Optical
core
Different scenarios for OXC deployment
• Optical switch core connected directly to transponders in OLT
O E O
O E O
O E O
O E O
Optical core
O E O
O E O
OLT
O E O
O E O
• Optical switch core directly connected to multiplexer in OLT
OLT
Optical core
37
© NOKIA
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Optical core wavelength plane OXC
λ1, λ2, λ3
λ1
λ1, λ2, λ3
λ2
λ1, λ2, λ3
λ1, λ2, λ3
Local add
Local drop
λ1
38
© NOKIA
λ2
S_108_199_10_3_2004.ppt / 10.3.2004 / APi
λ3
λ3
λ2
λ1
Reading instructions
• SDH, pp. 364-381. SDH multiplexing hierarchy instead of SONET
multiplexing hierarchy.
• WDM, pp. 403-430 and optical network layering and the principle of
optical protection.
39
© NOKIA
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