Sdh Basics Overview

SDH BASICS OVERVIEW

Overview Introduction to PDH Transport Network Path and Section Virtual Containers Synchronous Transport Module Protection Rings and Capacity SDH Multiplexing Structure PDH Mapping SDH Bytes

Introduction to PDH The Transport Plane  Offers a Transport Service between the service and access layers as well as between all of the physical locations on each of these planes  Each of the telephone or data switches on the service plane relies on the transport plane to carry traffic between them as well as to and from end users. Hence the terminology TRANSPORT PLANE

A Transport Network is used simply to Transport signals to their required destination and to deliver them in the same format as they were received 

TRANSPORT PLANE

PSTN

ATM

Service

TRANSPORT PLANE

SDH

Access PBX

Large business Telephone

The Transport Network The Transport Network provides an ability to carry Traffic between the points  Just providing a set of links between Telephone Sites is not Sufficient. The following are the Key issues to be considered : 

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Resilience : The ability of the network to cope with a loss or failure of a link or node, alternative routes to be provided Set up Cost (Investment) : Minimization is usually required Services Supported : The network must be able to carry any services

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Future Proof (Upgrade, growth) : The network must be able to evolve to satisfy new requirements, without disrupting existing services Operation, Administration and Maintenance (OA & M): The system proposed must integrate into existing processes and contribute to minimizing operational costs

Connection Point Logical Communication Path

Multiplexing 

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In the early days of Telephony, each telephone circuit required it’s own set of Copper wires to connect two Telephones together As demand increased, this technique became very expensive and impractical Multiplexing is a method to combine more than one Telephone channel onto the same pair of wires

Telephone

Before

Telephone

Telephone

Telephone

Telephone

Telephone

Telephone

Telephone One line needed per connection

Several Channels transmitted on one line After Use one line (MULTIPLEXING)

Several Channels transmitted on one line 1

Multiplexed Signal

1 30

30 European Standard 30 Telephone lines multiplexed into a 2 Mbps signal

Several Channels transmitted on one line 

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Transmission systems that are designed to European standards combine 30 Telephone Channels to create a multiplexed signal This allows vendors to make multiplexers that can interwork This signal is multiplexed using a technique called Time Division Multiplexing TDM combines these signals which are in Digital form This conversion is done using PCM (Sampling, Quantizing, Encoding) The transmission speed of this signal is 2.048 Mbps As far as Transport Networks are concerned the 2 Mbps signal is the lowest signal that is carried and therefore, forms the building blocks for higher orders of Multiplexing

Multiplexing Principles   

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The input signals to the Multiplexer are called Tributaries(Tribs) The output signal (multiplexed signal) is called the Aggregate For the tributaries to be multiplexed all tribs must be operating at exactly the same speed The speed of the aggregate must be at least the sum of all the trib speeds, this is because the multiplexer must transmit one byte from all tribs in the same length of time that it takes to receive one byte from one tributary

Multiplexing Principles

Input signals are called Tributaries(Tribs)

Mux

The output signal is called the aggregate

30 Channel PCM Frame Time Slots (TS)

0

16 Channels for speech or data 1-15

TS0 for frame alignment

TS16 for signaling

31 Channels for speech or data 16 - 30

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Although the frame contains 30 Telephone channels, a total of 32 Time slots are multiplexed. TS 0 is used to transmit a set pattern of bits continuously so that the receiver can locate and lock on to the start of the frame, in this way all the channels can be demultiplexed and connected to the correct tributaries. TS 16 is used to carry signaling information from the telephones to the Telephone Exchange and vice versa, such as off hook, on hook, dial tone and ring tone Frame Size : 32 Bytes (256 Bits) Frame Duration : 125 us (1/8,000th of a second) Frame Rate : 32 x 64 Kbps = 2.048 Mbps

PDH Limitations 

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The problem in a plesiochronous network is illustrated by considering what a network operator has to be able to provide a business customer with a 2 Mbps leased line If the high speed channel passes near to the customer, the operation of providing him a single 2 Mbps from within that channel is not simple The use of justification bits at each levels in the PDH, means that identifying the exact location of the frame from a single 2Mbps line within a 140 Mbps channel is impossible To access a single 2 Mbps line the 140 Mbps channel must be demultiplexed to it’s 64 constituent 2 Mbps lines via 34 and 8 Mbps Once the required 2 Mbps has been identified and extracted , the channels must be remultiplexed back up to 140 Mbps for onward Transmission

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As we know there are separate bit rate hierarchies in North America and Europe This caused a lot of difficulty providing cross Atlantic communication due to incompatibility of the transmission speeds Only the digital telephone channel itself operated at the same rate of 64 Kbps

Tributary signals can only be accessed at the level from which they were multiplexed 34 140

140 34

34

140M LTE

34

8

8 Inflexible : Slow provision of services

Real Estate: A lot of space required for equipment Spares ; More equipment means more spares to be held

2

Unreliable

8

8

Expensive : Lots of equipment required

2

Customer

140M LTE

Difficulty interworking between Europe and North America

North American Bit Rates X4 1.5 Mbps

X7 6 Mbps

X6 275 Mbps

45 Mbps

X 24 DS1

DS3

DS2

64 Kbps

Max Transmission rate limited to 565 Mbps

European Bit Rates X 32

X4

X4 2 Mbps

First Order (E1)

8 Mbps

Second Order (E2)

X4 34 Mbps

Third Order (E3)

X4 140 Mbps Fourth Order (E4)

565 Mbps

Not Specified by CCITT

Transport Network 

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The equipment used to implement a point to point connection is called a Terminal Multiplexer or line system A terminal mux offers fixed connections between end user termination points An SDH terminal takes in a number of electrical and optical signals (tributaries )and transmits them on the line as a single signal (aggregate) The tributary signals may be 2,34,45 or 140 Mbps SDH terminals can also handle synchronous tributaries from other SDH multiplexers All the traffic received from the aggregate is terminated by the terminal mux (hence the name terminal)

1 Mux

63

1

Fixed Network (Line System) Mux

63 1 to 1 63 to 63

Terminal Multiplexers

Cross Connect 

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A simple point to point network does not allow changes to be made to the way the traffic is delivered between the points, and is therefore inflexible A flexible network allows changes to be made to the way the points are connected and can respond to new connection requests faster than a fixed network A cross connect, is a piece of equipment which provides flexible connections between it’s termination points

1

1

63

63 1

Point of flexibility (Cross Connect) X

1

1 to 63 2 to 1 63

62 to 63 63 to 2

63

Add Drop Multiplexer  

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At each location the signal can get on or get off the Mux The key difference between a Terminal Mux and an ADM is that the ADM is equipped with two aggregate units and sufficient cross connect functionality to cross connect the traffic from one aggregate port to the other or from an aggregate port to a tributary port This also enables the signals to be transmitted from the aggregate in two directions (East & west) A terminal mux can also be provided with an additional aggregate unit in order to provide an alternative route for resilience

1

Terminal Mux

Terminal Mux

Flexible Network Add Drop Mux

63

63

X

1

X

62

2 Add Drop Multiplexers

X

62

Network Survivability 

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In the case of a point to point line system, resilience can be introduced by duplicating the link This would require the Terminal Mux’s to be equipped with an additional aggregate unit Maximum protection will be achieved if these two links are separately routed In the case of a flexible network implemented with a cross connect, the cross connect is the major point of vulnerability With a bus structure resilience can be achieved by adding an extra link

An ADM with protection ring

Alternate route for protection 1 ADM

ADM

ADM Ring X

X

63

X

1

X

62

X

2 62 Add Drop Multiplexers

63

Ring Topology ADM

ADM

ADM

ADM Ring ADM

Ring Topology • •

Provides resilience to traffic Ideal for maximizing capacity

Line System Terminal Mux

Terminal Mux

Optical Alternate Route Line System (Point to Point ) •

Resilience can be provided by adding an additional fiber

Transport Plane Transport Plane can be broken down into different distinct components  Collector Function : At the edge of the network, there is a need to collect traffic. Collector rings provide the network interface for all access applications including local exchanges, PABX, Access Multiplexers etc  Bandwidth Management Function : This tier provides routing, grooming and consolidation of the traffic between the collector rings and the high speed backbone.  High Speed Backbone: This tier provides a backbone function providing reliable high speed transport between geographical regions and locations

Path and Section Path and Path overhead  In transmission a path is defined as a circuit joining two nodes that may pass through a number of intermediate nodes.  In SDH, extra capacity is reserved to carry monitoring and management information associated with the path  Extra information associated with the path (generated at the originating termination point and processed at the terminating termination point) is called the path overhead This helps to know  The quality of the overall end to end transmission  The existence of the path between two terminating points  Allows one end to communicate to the other end that there is trouble in the signal received

Path and Path overhead Path A path is an end to end circuit

helps to

SDH allows information to be associated with the path (Path overhead)

Path overhead -Path trace -Error monitoring -Far end error

Section and Section Overhead        

A section can be considered as one stage of an end to end path Defined as node to node transmission A path may be made up of a number of sections One section may also be the path The SDH also reserves some extra capacity within the defined bit rates to carry information relating to the section This extra information associated with a section is called the section overhead The section overhead allows control of node to node transmission such as transmission quality The section overhead also provides extra communication channels, one of them being the ECC (Embedded communication channel) used for network management Data Communication Network.

Section and Section overhead Sections Section is node to node transmission

SDH allows information to be associated with the Section (Section overhead)

Path overhead -Protection Switching -Error monitoring Channels for -Network Management -Maintenance Phone link

Virtual Containers  

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SDH Multiplexers package the signals into small containers There are different sized containers for tributary signals with different bit rates. Each container remains a defined size irrespective of any variations in the bit rate of the PDH signal that it contains The POH is then added so that the signal and the quality of it’s transmission are all traceable POH now stays with the VC through all intermediate multiplexers until it reaches it’s destination SDH multiplexer where it is removed and processed Any PDH signal which is packaged by an SDH multiplexer, is transported and delivered at the same bit rate as it entered the SDH network

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If you put a 34 Mbps signal in, then you will get a 34 Mbps out and not just some of it’s constituent signals Signal to send eg: 2 Mbps

Virtual Container = Signal + POH

Container is a defined. size and designed to carry a specific signal Path Overhead VC

Path Overhead -Path Trace -Error Monitoring -Far End Error -VC Composition

Information stays with the path (ie end to end circuit control)

Nesting of VCs   

A VC can contain other VCs but only nested two deep Largest container is designed to carry all smaller containers Smaller containers cannot be put inside intermediate sized containers prior to being put into the largest container Virtual Containers

VC Nesting

Path Overhead VCs can be nested, but only 2 deep

VC Numbering Scheme 

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There is no container for the 8 Mbps signal, therefore the VC-2 does not need a second digit ie VC – 21 The VC – 3 is designed to carry both signals at the third order, so once again does not require a second digit The north american hierarchy does not have a fourth order and so the VC-4 also does not need a second digit

VC Numbering Scheme

1st Order

2nd Order

3rd order

4th Order

1

North America

1.5 Mbps

VC - 11

2

Europe

2 Mbps

VC - 12

1

North America

6 Mbps

VC -2

2

Europe

8 Mbps

No Container

1

Europe

34 Mbps

2

North America

45 Mbps

1

Europe

140 Mbps

2

North America

N/A

VC – 3

VC – 4

VC packaging  

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One 2 Mbps signal is packaged into a VC 12 63 x 2 Mbps signals can go into a VC 4 (they are grouped in 3 sets of 21) Alternatively a VC 4 can contain some VC 12 or VC 3 A 140 Mbps signal takes up a whole VC4

2 Mbps

VC 12

34 Mbps

VC 3

3 x VC -3

140 Mbps

VC 4

Content Combinations

63 x VC 12 (3 x 21)

VC - 4

VC 3 VC 3 Largest Container

VC Packaging

21 VC 12 21 VC 12 21 VC 12

VC -4

VC 3 VC 3 VC 3

21 VC 12

140 Mbps

VCn and PDH Rates 

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The smallest container is for a 2 Mbps signal There is no container for the standard 8 Mbps signal The VC number reflects the PDH hierarchy level

Bit Rate

PDH Europe

SDH

name

container

140 M

E4

VC 4

34 M

E3

VC 3

8M

E2

2M

E1

64 K

E0

VC 12

Synchronous Transport Module (STM) 

Several VCs are placed in and carried by a STM VC – 4s are carried in a Transport Module

VC – 4

POH

STM - n

Smallest Transport Module : STM -1 = 155 Mbps (Including Payload)

Transport Module 

The SOH is related to the STM in the same way as the POH is related to the VC Transport Module Contains

(VC – 4 Or n x VC -4 )

Section Overhead

Only one transport module sent per section

Higher Bit Rates 

SDH offers higher bit rates at STM-4, STM-16 and STM-64 Payload VC – 4 VC – 4

VC – 4

VC – 4

Still only one Section Overhead

STM -4 = 4 x STM-1 When higher bit rates are required, the STM-1 is multiplexed up to an STM-4. This is equivalent to 4 x STM-1 and operates at 622 Mbps. The payload area Is increased to 4 x VC-4, but still there is only one STM this time it is STM-4

European Bit rates

SDH line Transmission rates are 155 Mbps 622 Mbps 2.5 Gbps 10 Gbps You can see that the fastest standardized PDH rate is only 140 Mbps SDH therefore offers much higher capacity systems than standard PDH Line signals can be electrical or optical Electrical signals are used for short distances( eg between shelves in a rack) for interconnection of equipment on the same site Electrical signals are sent on copper wires, while optical signals used for longer distances (from 40 Km) require optical fibers

European Bit Rates Bit Rate

PDH Europe

SDH

Name

Container

Transport

10 G

STM - 64

2.5 G

STM - 16

622 M

STM - 4

155 M

STM - 1

140 M

E4

VC4

34 M

E3

VC3

8M

E2

2M

E1

64 K

E0

VC 12

The SDH and SONET Comparison Bit Rate

PDH North America Name

rate

PDH Europe Name

rate

SDH Container

Transport

SONET Container

Transport

10G

STM-64

STS/OC192

2.5G

STM-16

STS 48/OC 48

622M

STM-4

STS 12/OC 12

467M

STS 9/OC 9

155M

STM-1

140M

E4

140M

VC4

51M 45M

STS 1/OC 1 DS3/T3

45M

34M

E3

34M

8M

E2

8M

E1

2M

E0

64K

6M

STS 3/OC 3

DS2/T2

VC3

6M

2M 1.5M

DS1/T1

1.5M

64K

DS0/T0

64K

VC12

Protection Rings and Capacity 

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A rings capacity is the maximum number of paths that the ring can support A ring’s maximum capacity occurs only when all of the traffic enters the ring at one node and exits the ring at an adjacent node

STM-1 ring

VC 12 63

Ring Capacity = Max no of paths which can be set up What is the maximum capacity of this STM-1 ring ?

Two main types of protection Dedicated Protection Ring A ring with Dedicated path Protection called a DP Ring is shown in figure

STM-1 ring

VC 12

VC 12 63

63

Traffic sent both ways

DP Ring How many fully protected VC 12 paths can be set up? 63 Paths Traffic is sent simultaneously on both sides of the ring (also called East and West), hence one path uses a complete lane on this 63 lane motorway 

STM-16 Ring How many fully protected VC4 paths can be set up in the STM-16 ring? 16 Paths

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The traffic is sent both ways around the ring, and so in a sense it is duplicated The receiver will now be receiving the same signal on two ports The decision has to be made as to which signal should be selected to be passed to the tributary port Initially the decision to select is normally random, and carried out by a automatic change over switch When to switch from one port to another, is based on fault reporting and error detection functions of the multiplexer, and is switched automatically

Shared Protection Ring 

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Shared protection is achieved by reserving half the line capacity on each section for protection ie 8 STM-1s are allocated for traffic and 8 STM-1s for protection

STM-16 ring Half of each section capacity is for protection

9 -16 for protection VC 4/STM-1 VC 4/STM-1 1-8 for traffic

Traffic sent one way only How many fully protected VC-4 paths can be set up? 48 Paths

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SP ring protects a path section around the ring Each section is responsible for ensuring that the traffic transported gets to the other node Under normal operating conditions, the traffic is sent in one direction (East or West) only using the 8 STM-1s for normal traffic Shared protection protects all traffic in the ring ( it is not possible to protect only some paths, all paths are protected)

SP Ring protection Re routed Traffic

STM-16 ring

9 -16 for protection VC 4/STM-1 VC 4/STM-1 1-8 for traffic

Traffic sent one way only In case of failure, the signal is rerouted to the good side

DP Ring Versus SP Ring 8

STM-16 Ring

8 8

8 8

6 Nodes Max paths = 16

8 6 Nodes

Max paths = 48

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2. 3.

An operator can choose a SP for the capacity advantages that it provides. This means the operator can obtain : Better Network utilization Capital investment saving

DP Ring – For hubbing traffic (all to the same point) and also the circuitry is simple, used in collector ring SP Ring – For uniform Traffic and also the circuitry is complex, used in backbone ring

SDH Multiplexing Structure Xn

STM-n

AUG

AU4

C4

VC4 X3

X1

TUG 3

X3

AU3

140M

TU3

VC3

VC3

C3

34M 45M

X7 X7

TUG 2

X1

TU2

VC2

C2

6M

TU 12

VC12

C12

2M

TU 11

VC11

C11

1.5M

X3 X4

SDH Multiplexing Structure The first level of the SDH is at 155.52 Mbps and is known as a Synchronous Transport Module level 1(STM-1) Signal  Higher rates are integer multiples of the first level bit rate and are denoted by the corresponding multiplication factor of the first level rate.  At present , the following rates constitute the synchronous hierarchy STM-1 = 155 Mbps STM-4 = 622 Mbps STM-16 = 2.5 Gbps STM -64 = 10 Gbps  The SDH allows for any of the current transmission rates except 8 Mbps to be mapped into containers, called VCs  The containers can be combined into std formats in order to form the payload of the STM-1 signal  Different containers can be mixed, allowing for different rates to be carried simultaneously within the same structure 

SDH Multiplexing elements Container(C-n), n = 1 to 4 This is the basic element of the STM signal consisting of a group of bytes allocated to carry the Transmission rates defined in ITU-T recommendation G.702 (i.e. 1.5 Mbps and 2 Mbps) 

Virtual Container (VC-n), n = 1 to 4 The lower order VC-ns (n = 1 or 2) are built up of the basic container (C-n, n =1 or 2) plus additional capacity to carry POH information The higher order VC-ns (n = 3 or 4) are built up of either a single basic container (C-n, n = 3 or 4) or an assembly of Tributary unit groups , together with the appropriate POH information The POH information includes VC path performance monitoring, signals for maintenance purposes, and alarm status indicators 

SDH Multiplexing elements Tributary Unit (TU-n), n = 1 to 3 This element consists of a VC plus a Tributary unit pointer and provides adaption between the lower order path layer and the higher order path layer The pointer value indicates the phase alignment of the VC wrt to the higher order VC it is added to. The location of the pointer is fixed wrt to this higher order VC The TU can be considered as the space within the higher order VC which is reserved for the lower order VC payload. 

SDH Multiplexing elements Tributary Unit Group (TUG-n), n = 2 or 3 This element is formed by a group of identical TUs or TUGs allowing mixed capacity payloads to be constructed  Administrative Unit (AU-n), n =3 or 4 This elements consists of a VC-n (n = 3 or 4) plus an AU pointer and provides adaption between the higher order paths and the multiplexer section layer . The pointer value indicates the phase alignment of the VC-n with respect to the STM-1 frame. The location of the pointer is fixed within the STM-1 frame structure The AU can be considered as ‘the space’ within the STM frame which is reserved for the high order VC payload 

SDH Multiplexing elements Administrative Unit Group (AUG) This element is formed by a group of byte interleaved AU s. The AUG has a fixed position in the STM payload  Synchronous Transport Module Level 1(STM-1) This Is the basic element of the SDH and comprises a single AUG and the SOH information. The STM-1 frame structure comprises an array of 270 columns by 9 rows of 8 bytes (2,430 Bytes) 

PDH Mapping 2,430 Bytes STM-n

2,358 Bytes 2,349 Bytes

Xn AUG

+ 72 Bytes SOH

AU 4 + 9 Bytes Pointer

3 x 774 + 2 Columns of stuffing = 9 Bytes POH

VC 4

X3

774 Bytes TUG - 3 7 x 108 + 2 Columns of stuffing

X7 108 Bytes TUG - 2

32 Bytes

2 Mbps Mapping Multiplexing Multiplexing(SDH) Aligning

X3

Pointer

POH

TU12

VC12

36 Bytes

35 Bytes

Stuff C12 34 Bytes

STM-1 frame structure 9 Bytes

261 Bytes

RSOH 9 Bytes

AU Pointers

Payload 2,349 Bytes

MSOH

Frame Size = 2,430 Bytes Frame Duration = 125 Us

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SOH – Carries control and management information Administrative pointers to cater for problems in timing variations The payload area for carrying subscriber data

SDH Bytes 

VC 12 POH (V5 Byte)

RDI

RFI 1

2

3

4

5

6

7

8

REI

Signal Label BIP 2 BIP 2 – Bit interleaved parity Used to provide an error monitoring function for the VC 12 path REI – Remote error Indication Used to communicate detected BIP-2 errors back to the VC12 path originator RFI – Remote failure indication Signal Label – Used to indicate payload mapping and equipped status RDI - Remote Defect Indication Used to indicate certain detected TU path alarms to the VC 12 path originator

VC-4 POH (HO POH) J1 = Path Trace Used to provide a fixed length string which is transmitted repetitively So that the receiving terminal can verify connection to the intended transmitter B3 = Path BIP Provides an error monitoring function for the VC-4 path C2 = Signal Label Used to indicate the composition of the VC4 payloads G1 = Path Status Used to convey path terminating status and performance Information back to the VC-4 path originator F2 = Path User Channel User communication purposes between path elements H4= Multiframe indicator Provides a generalized multiframe indicator for payloads K3 = Spare N1 = Network operator Byte May be used for tandem connection maintenance

VC4 Payload 2,349 Bytes

VC4 Path overhead

RSOH 

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A1,A2 = Framing ( These bytes are used for frame alignment purposes) J0 RS Path Trace = Regenerator section path trace B1 BIP = Used to provide an error monitoring function for a regenerator section E1 Order Wire = Used to provide an order wire channel which may be accessed at regenerators and multiplexers F1 user channel = Reserved for user purposes D1 to D3 DCCr = The Data communication channel bytes provides a 192 Kbps regenerator data channel.

MSOH B2

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B2 BIP = Used to provide an error monitoring function for the multiplex section K1, K2 APS Channel = These bytes are used for APS protocol E2 Order Wire = Used to provide an order wire channel which may be accessed only at multiplexer section terminations. M1 MS REI Errors = Multiplex section REI for detected B2 errors D4 to D12 DCCm = The Data communication channel bytes provides a 576 Kbps multiplex telemetry channel. S Sync = Synchronous status messaging Z1,Z2 Spare = Function not allowed

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