SDH Principle

1

Unitrans D&T GroupUnitrans D&T Group

SDH PrincipleSDH Principle

2 Frame Structure & Multiplexing

3 Overhead

1 SDH over view

2

B Advantages of SDH over PDH

A Limitation of PDH

Limitations of PDHLimitations of PDH

1. Interface• Electrical Interfaces

• There are only regional standards, instead of universal standards

• Optical Interfaces

• No unified standards for optical line equipments, manufacturers develop equipment according to their own standards.

3

European Series565Mb/s565Mb/s

139Mb/s139Mb/s

34Mb/s34Mb/s

8Mb/s8Mb/s

2Mb/s2Mb/s

×4

×4

×4

×4

Japanese Series North American Series1.6Gb/s1.6Gb/s

400Mb/s400Mb/s

100Mb/s100Mb/s

32Mb/s32Mb/s

6.3Mb/s6.3Mb/s

1.5Mb/s1.5Mb/s

274Mb/s274Mb/s

45Mb/s45Mb/s

6.3Mb/s6.3Mb/s

×4×4

×4

×4

×6

×7

×3

×5

64Kb/s64Kb/s

×24 ×30

×3

×3



2. Multiplexing Method• Asynchronous Multiplexing

• Code rate justification is required for matching and accepting clock difference.

• The locations of the low-rate signals in high-rate signals are not regular nor fixed.

4

140/34 Mb/s 34/140Mb/s

34/8 Mb/s 8/34 Mb/s

8/2 Mb/s 2/8 Mb/s

2 Mb/s

MultiplexingDemultipexing

Adding and Dropping in PDH



3. Operation and Maintenance• PDH signal frame structure has very few overhead bytes for

Operation, Administration, and Maintenance (OAM).

4. Network Management Interface • No universal network management interface for PDH network.

5

A Limitations of PDH

B Advantages of SDH over PDH

Definition of SDHDefinition of SDH

• A hierarchical set of digital transport structures, standardized for the transport of suitably adapted payloads over physical transmission networks.

• SDH defines the frame structure, multiplexing method, transmission rate, and interface code pattern.

6

Advantages of SDH over PDHAdvantages of SDH over PDH

1. Interface• Electrical Interfaces

• Can be connected with existing PDH networks

• Optical Interfaces

• SDH provides a set of standard rate levels – STM-N (N = 4n = 1, 4, 16, 64……).

• The basic signal transmission structure level is STM-1 with a rate of 155Mb/s.

• Optical interfaces adopt universal standards. Line coding of SDH signals involves scrambling, instead of inserting redundancy codes.


2. Multiplexing Method• Low-rate SDH signals → High-rate SDH

• Signals via byte interleaved multiplexing method

• PDH signals → SDH

• Synchronous multiplexing method and flexible mapping structure

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3. Operation Maintenance• Abundant overhead bits are used for OAM.

• Unnecessary to add redundancy bits to monitor the line performance during line coding.

4. Compatibility• SDH network and existing PDH network can work together.

• SDH network can accommodate the signals of other hierarchies such as ATM, FDDI, and Ethernet.

1 SDH Overview

3 Overhead


8

Level Rate Mbit/s

STM-1

STM-4

STM-16

STM-64

155.520

622.080

2488.320

9953.280

STM-256 39813.12

SDH Signal RatesSDH Signal Rates

• Byte-oriented block structure

• Frame transmission rate: 125µs (8000 frames/sec)

Transmission Direction

1

3

45

9

RSOH

STM-N Payload(including POH)

9×N 261×N270×N

MSOH

AU PTR

9 x 270 x N Bytes

SDH Frame StructureSDH Frame Structure

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• Payload – area for services transmission in STM-N

2M, 34M, and 140M signals are packed and carried in the payload of STM-N frame over SDH network. If STM-N frame is a truck, the payload area is the carriage of the truck.

• Path Overhead (POH) – after packing low rate signals, POH is added to monitor the transmission of every packet. This process is like attaching a label on the packet.



• Section Overhead (SOH) – monitors the whole STM-N frame, i.e. monitor performance of all packages in the carriage of the truck.

• Regenerator Section Overhead (RSOH) – monitors the whole STM-N frame.

• Multiplex Section Overhead (MSOH) – monitors each STM-1 of the STM-N frame.

• RSOH, MSOH, and POH compose the integrated monitoring system of SDH.

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SDH Frame StructureSDH Frame Structure• AU Pointer (AU-PTR) – used for alignment of lower rate signals in

the payload of STM-N frame to accurately locate the payload.

• AU-PTR is added in transmitting end, when the signal is packed into the payload of STM-N frame. The process is like setting a coordinate value to identify where the package is in the carriage.

• At receiving end, the low rate signal is dropped from STM-N frame according to the AU-PTR value. The process could compare to getting the package from the carriage according to above coordinate value.

• Since packages are byte interleaved, the entire payload could be dropped once the first package is identified through alignment.


• When the rate of the signal to multiplex is lower, like 2M & 34M, 2-level pointer alignment is necessary.

• First, packing the low rate signals, like 2M or 34M into apacket;

• Secondly, aligning the signal in the packet by TU Pointer (TU-PTR);

• Thirdly, multiplexing the above lower rate packet into another higher rate packet by AU-PTR.

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2-Level Pointer Alignment

AU PTR

TU PTR

2M

34M

Synchronous Multiplexing StructureSynchronous Multiplexing Structure

• Multiplexing Structure• Low-order SDH frame → High-order SDH frame: 4 in 1 byte

interleave

• PDH → STM-N: synchronous multiplexing and flexible mapping

• ITU-T G.709 defines a complete set of multiplexing structures, in which multiplexing of PDH signal into STM-N frame is not unique and every country or area adopts one particular structure.

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Virtual Container

STM-N140Mb/s

45 Mb/s34 Mb/s6.3Mb/s

2Mb/s

1.5Mb/s

x3

x7

x7

x1x3

C-11

C-12

C-2

C-3

C-4

VC-11

VC-2

VC-3

VC-3

VC-4

TU-11

TU-12

TU-2

TU-3

TUG-2

TUG-3

AUG

AU-3

AU-4

VC-12

x3

x4

x1

Container

Tributary Unit

Administrative Unit

Tributary Unit Group

Administrative Unit Group

Synchronous Transmission Module

Alignment

Multiplexing

Mapping

xNITU-T G.709 Multiplexing StructureITU-T G.709 Multiplexing Structure

Multiplexing Structure in ChinaMultiplexing Structure in China

Mapping

Alignment

Multiplexing

STM-N AUG AU-4 VC-4

TU-3 VC-3 C-3

C-4

TUG-2 TU-12 VC-12 C-12

TUG-3

xN

139264 kb/s

34268 kb/s44736 kb/s

2048 kb/s

x3

x7

x3

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Multiplexing of 140M into STM-1Multiplexing of 140M into STM-1

• C-4: Container-4 is the standard information structure for 140M signal, implementing rate adjustment.

• VC-4: Virtual container-4 is the standard information structure corresponding to C-4, monitoring the real-time performance of the carried 140M signal.

1

140MRate adjustment/ packing C-4

1 2609

125 µs

Add POH for monitoring/ packing VC-4

POH

1 1

9

125 µs1 261

Next page


• AU-4: Administrative Unit 4, the information structure corresponding to VC-4.

• Multiplexing process: 140M → VC-4 → AU-4 → STM-1

• Consequently, only one 140M signal can be multiplexed to STM-1

Pointer alignment

1 270

AU PTR AU-41 9

10 270

Add SOH

RSOH

MSOHPayload

AU PTR

1

9

1 270×N1

9

STM-N

125 µs 125 µs

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• C-3: Container 3 is the standard information structure for 34M signal, implementing rate adjustment.

• VC-3: Virtual Container-3 is the standard information structure corresponding to C-3, monitoring the real-time performance of the carried 34M signal.

1 1

34M C-3

1 849

125 µs

Add POH for monitoring/packing VC-3

POH

9

125 µs1 85

Next page

Rate adjustment/ packing


• TU-3: Tributary Unit-3, the standard information structure corresponding to VC-3, implementing the first level pointer.

• TUG-3: Tributary Unit Group-3, the standard information structure corresponding to TU-3.

• Multiplexing process: 34M → VC-3 → TU-3 → TUG-3, 3×TUG-3 → VC-4 → STM-1

• Consequently, three 34M signals can be multiplexed to one STM-1.

First level pointer alignment

86

Fill in gapTU-3

1H1H2H3

1 861

9

H1H2H3

R

TUG-3 Byte interleave

1

9

×3

9

11 261

POH

R R VC-4

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• C-12: Container-12 is the standard information structure for 2M signal, implementing rate adjustment. Four basic frames compose a multi-frame.

• VC-12: Virtual Container-12 is the standard information structure corresponding to C-12, monitoring the real-time performance of the carried 2M signal.

• TU-12: Tributary Unit-12, the standard information structure corresponding to VC-12, implementing the first level pointer.

125 µs

2M

Basic frame POH

速率适配

1 4

C-12

1

9

Add POH for monitoring

First level pointer alignment

VC-12 TU-12

1 1 441 1

9 9

Next Page

Rate adjustment


• TUG-2: Tributary Unit Group-2. TUG-3: Tributary Unit Group-3.

• Multiplexing process: 2M → C-12 → VC-12 → TU-12; 3×TU12→TUG-2; 7×TUG-2 → TUG-3; 3×TUG-3 → VC-4 → STM-1.

• Consequently, 63 ( = 3x7x3) 2M signals can be multiplexed into STM-1. The multiplexing structure of 2M is 3-7-3 structure.

1 12

TUG-2

1

9

R R TUG-3

1 86

Byte interleave

Byte interleave

x 7x 3 x 3

Byte interleave

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1 SDH Overview


3 Overhead Bytes

Overhead BytesOverhead Bytes

• Overhead bytes implements the monitoring functions to ensure proper transport of the payload.

Overhead

SOH

POH

RSOH

MSOH

High Order POH

Low Order POH

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Section Overhead of STM-1Section Overhead of STM-1A1 A1 A1 A2 A2 A2 J0 ›› ››

B1 ‹‹ ‹‹ E1 ‹‹ F1 ›› ››

D1 ‹‹ ‹‹ D2 ‹‹ D3

Administrative Unit Pointer (AU-PTR)

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

S1 M1 E2 ›› ››

Transmission DirectionRSOH

MSOH

9 Columns

9Rows

›› Reserved for National Use ‹‹ Media Dependent Bytes

A1, A2 BytesA1, A2 Bytes

• Framing Alignment Bytes: A1, A2• To identify the initial location of a frame

• A1=F6 H, A2=28 H

OOF is reported

OOF lasts 3 m seconds

LOF is reported

A1, A2 cannot be detected for five consecutive frames.

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J0 ByteJ0 Byte

• Regenerator Section Trace Byte: J0

• As the Regenerator Section Access Point Identifier, it ensures that a section receiver can verify its continued connection to the intended transmitter.

• A 16-byte frame is defined for the transmission of the Section Access Point Identifiers.

F1 ByteF1 Byte

• User Channel Byte: F1 • Provides a 64 kb/s data/voice channel for special maintenance

purposes.

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D1-D12 BytesD1-D12 Bytes

• Data Communication Channel (DCC) Bytes: D1 - D12• DCC is the channel for transmission of OAM information

among NEs and NMS.

• 192kb/s (3 x 64 = 192) channel is defined using bytes D1, D2, and D3 as a Regenerator Section DCC.

• 576kb/s (9 x 64 = 576) channel is defined using bytes D4 to D12 as Multiplex Section DCC.

E1, E2 BytesE1, E2 Bytes

• Orderwire Bytes: E1, E2 • E1 and E2 are used to provide 64 kb/s channels for voice

communication.

• E1 is accessed at Regenerators as well as at all multiplex points

• E2 is accessed only at Multiplexers

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B1 ByteB1 Byte

• Bit Interleaved Parity (BIP-8) Byte: B1• B1 is for regenerator section error monitoring.

• BIP-8 is computed over all bits of the regenerator section of STM-N frame.

• BIP-8 Principle:

• B1 is computed in unit of 8 bits.

• Monitoring partition: bit column.

• Even parity is generated by setting the BIP-8 bits so that there is an even number of 1s in each partition of the signal.

B1 ByteB1 Byte• B1 Byte Principle

• At transmitting side, the BIP-8 is computed over all bits of the STM-N regenerator before scrambling, and the result is placed in byte B1 of the preceding frame.

• At receiving end, the BIP-8 is computed over all bits of the regenerator after de-scrambling. This result is then Exclusive OR with the B1 byte result received in later frame.

• If the value of Exclusive OR operation is zero, there is no bit block error. But if the result is not zero then there may be errors in transmission. A1 00110011

A2 11001100 A3 10101010 A4 00001111

B 01011010

BIP-8For example

BIP-8 is computed over a frame of signal composed of 4 bytes.

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B1 ByteB1 Byte• At transmitting end A, BIP-8 is computed over all bits of the first

frame, and result is placed in byte B1 of the second frame. At receiving end B, the BIP-8 is computed over all bits of the first frame, and then exclusive OR with the B1 byte of the second frame. The number of 1s of exclusive OR operation indicate transmission errors

1stframe

A B

1st frame1st frame

Transmitting end

2ndframeNth

frame

Nth frame

2ndframe2nd

frame

Receiving end

B2 BytesB2 Bytes

• Bit Interleaved Parity 24 (BIP-Nx24) Byte: B2• B2 is for multiplex section error monitoring.

• The BIP24 is computed over all bits of the STM-N frame except for the first three rows of SOH.

• BIP24 Principle:

• B2 is computed in unit of N x 24.

• Monitoring block: bit column.

• Even parity is generated by setting the BIP24 bits so that there is an even number of 1s in each block of the signal.

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B2 BytesB2 Bytes• B2 Byte Principle

• At transmitting end, the BIP-Nx24 is computed over all bits of the STM-N frame except for the first three rows of SOH, and the result is placed in 3 bytes B2 of the preceding frame before scrambling.

• At receiving end, the BIP- Nx24 is computed over all bits of the frame except for the first three rows of SOH, and then Exclusive OR with the B2 bytes of the later arrived frame.

• If the value of Exclusive OR operation is zero, there is no bit block error. Any mismatch in result indicates transmission errors. 11001100 11001100 11001100

01011101 01011101 0101110111110000 11110000 11110000BIP24

01100001 01100001 01100001

For example

BIP-N×24 is computed over a frame of signal composed of 9 bytes.

M1 ByteM1 Byte

• Remote Error Indication (MS-REI) Byte: M1• A return information from receiving end detecting MS-BBE to

transmitting end.

• Convey the count of interleaved bit blocks that have been detected in error by BIP-24 in receiving end.

• The transmitting end will report a corresponding performance event, MS-REI.

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K1, K2 BytesK1, K2 Bytes

• Automatic Protection Switching (APS) Bytes: K1 & K2

• Last three bits of K2 byte indicates alarm type

• 111 indicates MS-AIS alarm (Multiplex Section Alarm Indication Signal) at receiving end.

• 110 stands for MS-RDI alarm (Multiplex Section Remote Defect Indication) at transmitting end.

S1 ByteS1 Byte

• Synchronization Status Message Byte: S1 (b5-b8)• S1 is used to implement clock source protection and switching

function.

• The value corresponding to b5-b8 indicates the quality of synchronization. The smaller values indicates better quality of the clock sources.

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Section OverheadSection Overhead

• Byte Interleaving of Section Overhead

When STM-1 frames are multiplexed into STM-N, the byte interleave multiplexing of AU Pointer and Payload is different from Section Overhead. In the former case, all bytes are interleaved.For the latter, only the first STM-1 frame’s section overhead is reserved, while remaining STM-1 frame’s Section Overheads are omitted except bytes like A1, A2, B2.

Path OverheadPath Overhead

• Classifications

• High Order Path Overhead (HPOH)

• Low Order Path Overhead (LPOH)

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High-order Path OverheadHigh-order Path Overhead

VC4

1

1

261

9

J1B3C2G1F2H4F3K3N1

Path Trace BytePath BIP-8 ByteSignal Label BytePath Status BytePath User Channel BytePosition Indicator BytePath User Channel Byte

APS Channel ByteNetwork Operator Byte

J1 ByteJ1 Byte

• Path Trace Byte: J1

• The first byte of VC4

• Pointed by AU-PTR

• Required to be matched at transmitting and receiving ends

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B3 ByteB3 Byte

• Path BIP-8 Code: B3

• Implements higher order VC’s error monitoring

• Monitoring principle: BIP-8 even parity

• The value of B3 byte needs to be compared at both transmitting and receiving ends. Any inconsistency between two results means transmission errors in VC-4.

C2 ByteC2 Byte

• Signal Label Byte: C2

• Indicates the composition and type of multiplexing structure

• Examples:

• 00H means unused

• 02H means multiplexing structure is 3xTUG-3

• Indicate the information about the payload type

• Required match at both ends.

27

G1 ByteG1 Byte

• Path Status Byte: G1

• Indicating high order VC transmission status

• Return message from receiving end to transmitting end

• HP-REI: Higher Order Path Remote Error Indication (sum of receiving error block of VC4)

• HP-RDI: High Order Path Remote Defect Indication

H4 ByteH4 Byte

• Multi-frame Indicator Byte: H4

• Indicate the multi-frame types and location of the payload.

• For 2M PDH to SDH multiplexing structure, H4 indicates the current frame is which frame of the multi-frame, allowing Rx to find TU-PTR and drop 2M signals.

• H4 value: 00H-03H

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Low-order Path OverheadLow-order Path Overhead

• V5: Path status, Path BIP-2, and Signal Label Byte

• J2: Low order path trace byte

• N2: byte for network operator usage

• K4: APS byte for low order path

11

9

500µs VC12 Multi-frame

V5 J2 N2

VC12 VC12VC12

4K4

VC12

V5 ByteV5 Byte

• Path Status, Path BIP-2, and Signal Label Byte: V5

• The first byte of VC-12 multi-frame

• Pointed by TU-PTR

• Monitor error block, signal label, path status

• Error block monitoring: b1-b2

• Return path status message: b3, b8

• Signal label: b5-b7

• Similar to B3, C2, and G1

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• Understanding SOH and POH?• Both SOH and POH are bytes for Operation, Administration,

and Maintenance (OAM), which ensure reliable and flexible transmission.

• SOH and POH monitor and administrate transmission at different layers (or levels). RSOH and MSOH are for regenerator section and multiplex section respectively. Whereas,HPOH and LPOH are for VC-4 / VC-3 and VC-12 respectively.


Comparison • LPOH – to monitor small package (VC-12)

• HPOH – to monitor large package (VC-3/VC-4)

• MSOH – to monitor the carriage of the truck (STM-1)

• RSOH – to monitor the motorcade which consists of trucks(STM-4 / STM-16 / STM-64)

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RegeneratorSTM-N STM-N Terminal Multiplexer

STM-NSTM-N

PDH

Add / Drop Multiplexer

STM-NSTM-N

PDH SDH

PDH

SDH

PDH

SDH

Digital Cross-connect

SDH Network ElementsSDH Network Elements

Overhead ManagementOverhead Management

2M34M

140M

LO - PTE HO - PTE

STM-1 STM-N

REG

••••••

STM-N

DXC

••••••

STM-N

ADMREG

2M

140M

LO - PTEHO - PTE

STM-1STM-N STM-N 34M

LOW-ORDER PATH

HIGH-ORDER PATH

REGENERATOR

SECTION

REGENERATOR

SECTION

STMPDH

REGENERATOR

SECTION

REGENERATOR

SECTION

MULTIPLEX

SECTION

MULTIPLEX

SECTION

MULTIPLEX

SECTION

LPOHHPOH

RSOH RSOH/MSOH RSOHRSOH/MSOH RSOH/MSOH

LPOH

HPOHRSOH/MSOH

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Overhead ManagementOverhead Management

LPOH

VC-12

HPOH

VC - 4MSOH RSOH RSOH RSOH MSOH

HPOH

VC - 4

LPOH

VC-12

Low Order Path

High Order Path

Multiplex Section

RegeneratorSection

RegeneratorSection

V5J2K4N2

J1, N1B3, C2G1, K3F2 - F3

H4

B1 B1

B2

B3

V5

B2, E2K1 - K2

D4 - D12S1, M1

A1 - A2J0, B1E1, F1D1 - D3

V5J2K4N2

J1, N1B3, C2G1, K3F2 - F3

H4

B2, E2K1 - K2

D4 - D12S1, M1

A1 - A2J0, B1E1, F1D1 - D3

A1 - A2J0, B1E1, F1D1 - D3

A1 - A2J0, B1E1, F1D1 - D3

Transmission Line

LOFLOSRS-TIM

SignalA1/A2

J0RS-BBEB1

RegSection

MS-AISM1 MS-REI

K2 6-8

Multiplex Section

AIS

AIS

MS-RDIK2 6-8

AU-AISAU-LOP

AU PtrAU Ptr

MS-BBEB2M1 MS-REI

HP-TIMHP-UNEQHP-SLM

J1C2C2

HP-RDIG1 5 - 7

HP-BBEB3HP-REIG1 1 - 4

AIS

AIS

HP-REIG1 1 - 4TU-LOMTU-AISTU-LOP

H4 7 - 8 AISTU PtrTU Ptr

LP-TIMLP-UNEQLP-SLM

J2V5 5 - 7

AIS

V5 5 - 7LP-RDILP-RFI

V5 8V5 4

HO Path LO Path

LP-BBE

LP-REILP-REI

V5 1 - 2

V5 3V5 3

Event Detection

Event Registration

OR (Logical Function)

Signal FlowAlarm Indication Sent

Error Indication ReceivedError Indication Sent

Transmission Section Line STM Path VC Path

Events Management

32

R-LOS R-LOF

MS-EXC MS-AIS

AU-LOP AU-AIS HP-UNEQ HP-TIM HP-SLM

TU-AIS

Hierarchy of Common AlarmsHierarchy of Common Alarms

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SDH Principle

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