Multiplexing

S-108.199 OPTICAL COMMUNICATIONS AND INSTRUMENTS SDH and WDM, 10.3.2004 Antti Pietiläinen

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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

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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

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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

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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.

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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.

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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

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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

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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.

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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

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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)

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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

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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.

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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

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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

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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.

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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

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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

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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

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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

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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.

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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

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270 bytes

Regenerator section overhead

5 6

7 8 9 10

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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