sdh fundamentals

TelecommunicationsTechniquesCorporation

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SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94 1

SDH FUNDAMENTALS

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2 SDH FUNDAMENTALS (0039.p65) • v1.0 • 11/94

NOTICE

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TELECOMMUNICATIONS TECHNIQUES CORPORATION PROVIDES THIS MATERIAL“AS IS” AND MAKES NO WARRANTY OF ANY KIND, EXPRESSED OR IMPLIED,INCLUDING BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OFMERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.TELECOMMUNICATIONS TECHNIQUES CORPORATION SHALL NOT BE LIABLE FORERRORS CONTAINED HEREIN OR CONSEQUENTIAL DAMAGES (INCLUDING LOSTPROFITS) IN CONNECTION WITH THE FURNISHING, PERFORMANCE OR USE OF THISMATERIAL WHETHER BASED ON WARRANTY, CONTRACT OR OTHER LEGAL THEORY.

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Table of Contents

Page

Objectives ............................................................................................................................................................... 4

Advantages and Benefits of SDH ........................................................................................................................... 5

Network Overview ................................................................................................................................................. 6

Framing Structure Overview ................................................................................................................................ 14

Differences Between SDH and SONET Technologies ........................................................................................ 49

SDH Abbreviations .............................................................................................................................................. 51

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Objectives

The objectives of the SDH FUNDAMENTALS Training Manual are to enable a trainee to:

1. Describe the structure of SDH signals, including SDH overhead.

2. Explain the advantages of SDH systems over PDH systems.

3. Describe the three major SDH networks elements.

4. Mention three important differences between SDH and SONET technologies.

NOTE TO THE READER: To achieve these objectives, it is assumed the reader is familiar with PDH

technology.


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Advantages and Benefits of SDH

Advantages and benefits of SDH over PDH are:

• Reduced network installation, operation, and maintenance costs.

• A flexible transmission structure able to handle existing as well as new signals both simultaneously or non-

simultaneously without equipment modification or replacement. Examples of new signals include ATM

and IEEE 802.6 for MAN.

• High- and low-level compatibility among transmission line equipment manufacturers.

• Increased integrated maintenance and network management capabilities.

• Direct access to lower speed tributaries within a signal without need to multiplex/demultiplex the entire

high-speed signal.

• A unified worldwide standard, which eliminates signal conversion at international borders, dramatically

reducing the cost of international connections.

• SDH specification does not impose limits on the transmission capacity, so SDH will be able to satisfy both

current and future needs.

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Network Overview

Line/RadioSystems

DXC/EA Line/RadioSystems

SM

SM

SM

TR

TR

TR

SM

SM

SM

TR

TR

TR

NNINNINNINNI

TR: TributariesSM: Synchronous Multiplexer

NNI: Network-Node InterfaceDXC: Digital Cross-Connect System

EA: External access equipment

Location of the Network Node Interface

SDH, which stands for Synchronous Digital Hierarchy, is a group of ITU-T specifications describing the

Network Node Interface (NNI). The NNI is the interface between the transmission facility and the network

node which performs signal termination, switching, cross-connection or multiplexing/demultiplexing.

The transport networks built to comply with these specifications conform a system for the transmission of high-

speed signals based upon PDH, ATM, and other signals.

Previously to SDH, the specifications used are what is understood as Plesiochronous Digital Hierarchy (PDH).


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Line/RadioSystems

DXC/EA Line/RadioSystems

SM

SM

SM

TR

TR

TR

SM

SM

SM

TR

TR

TR

NNINNINNINNI

TR: TributariesSM: Synchronous Multiplexer

NNI: Network-Node InterfaceDXC: Digital Cross-Connect System

EA: External access equipment

Network Overview - Operations, Administration, Maintenance, and Provisioning (OAM&P)

Location of the Network Node Interface

One of the important benefits of SDH in respect to PDH is the enhanced operations, administration, maintenance,

and provisioning (OAM&P) capabilities made available. These capabilities allow network operators to provide

centralized control of the network, to better measure the quality of the information transferred, to implement

emergency connections and to allocate extra capacity for future, yet unknown services.

This is achieved in SDH by increasing the amount of overhead within the SDH frame structure (respect to PDH),

by specifying interfaces to access the Telecommunications Management Network (TMN), and by specifying basic

functionality of the elements in a network.

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Network Overview - Network Elements

Synchronous Digital Cross-Connect System

The principal component in an SDH network is the Digital Cross-Connect System (DCS or DXC), called

Synchronous Digital Cross-Connect System. The reasons for this are essentially the same as for PDH networks:

Often, not all the traffic carried to a network node by a high-capacity transmission link may be switched there;

some of the capacity may be required for Public Switched Telephone Network (PSTN) traffic routed through to

other nodes and for private circuits.

Also, the introduction of broadband services will necessitate the provision of channels carrying traffic at much

higher rates than 64 kbit/s.

SDHsignals

SDHsignals

M/N


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Synchronous Multiplexers

As in PDH, SDH uses multiplexers to combine lower-speed signals into higher-speed signals. These

multiplexers are typically referred to as synchronous multiplexers. There are two types of synchronous

multiplexers: Access and SDH multiplexers.

Access multiplexers have tributary (input) interfaces compatible with PDH and other signals (such as ATM,

FDDI, and IEEE 802.6 for MAN), and aggregate (output) parts compatible with SDH signals.

SDH multiplexers have both tributary interfaces and aggregate ports compatible with SDH signals.

Tributaries(PDH, ATM,IEEE 802.6)

SDHlines

Terminal Multiplexer

Higher orderSDH

signals

Terminal Multiplexer

Lower orderSDH

signals

symbol means "fiber optic line"

SDHlines

SDHlines

Tributaries (PDH, ATM, IEEE 802.6)

Higher orderSDH

signals

Lower order SDH signals

Higher orderSDH

signals

ACCESS MULTIPLEXERS

SDH MULTIPLEXERS

Add/Drop Multiplexer

Add/Drop Multiplexer

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Standard synchronousRegenerator

Drop & Insert synchronousRegenerator

Synchronous Regenerators

As in PDH systems, an SDH network may include signal regenerators, called synchronous regenerators.

However, fewer regenerators are needed in an SDH system than in a comparable PDH system. The reason for

this is that the use of optical transport medium makes the distances between regenerators (in excess of 70 km,

depending on the type of fiber used) much larger than in comparable PDH systems.

Unlike PDH regenerators, SDH regenerators must synchronize to the incoming frame structure. When a fault

occurs, for example, “Loss of Signal” or “Loss of Frame Synchronization,” synchronous regenerators produce a

proper SDH frame to keep the system “alive.” Also, SDH regenerators may provide access to the incoming

traffic; these are called Drop and Insert regenerators.


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Network Overview - Transmission Media

SDH Ring

To achieve cost reductions and increased reliability, SDH specifies the use of single-mode optical fiber across

the network. However, initial deployment of SDH systems have had to fit into a digital-distribution frame

regime dominated by coaxial plesiochronous interconnections at 34 Mbit/s and 140 Mbit/s. For this reason,

coaxial cable is currently being used for SDH connections within stations.

Many PTTs have invested heavily in coaxial cable for the transmission network. For this reason, copper cable

may be expected to continue in use for at least five more years. In addition, ITU-T has not specified optical

monitoring points, so optical/electrical signal conversion will still be needed.

In addition to coaxial cable and optic fiber, SDH radio systems have also been developed using new frequency

bands and using new modulation schemes in order to contain the bandwidth for compatibility with existing

frequency plans.

TR

TR

TR

TR

: Optic Fiber lines

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Network Overview - Timing Issues

timing signal

2 Mbit/stributaries

8 Mbit/sbuffer

timing signal

8 Mbit/s≈ 8 Mbit/s

PDH Network Node PDH Multiplexer

PDH Timing Issues

In PDH systems, differences in timing at a given hierarchical level (8 Mbit/s, for example), are handled by

using buffers. Multiplexing lower-level signals into higher-order signals uses the technique of bit justification

which allows frequency equalization of plesiochronous tributaries.

Unlike PDH, SDH systems make provision for differences in timing only at the first SDH level. There are no

provisions for handling differences in frequency at higher-rate SDH signals. The reason is that higher-order

SDH signals are synchronized by a highly stable, accurate network clock.

At the first SDH level, incoming PDH and other signals are synchronized with the framing structure by using

bit justification. Once this has occurred, subsequent frequency variations are handled by using a new technique

that does not use bit-justification.


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Network Overview - A Typical SDH Network

PDHDXS

FlexibleMux

FlexibleAccessSystem

64kDXS

(2-34)M links

(PSTN and Private Circuits)

PDHand

B-ISDN

2Mlinks

(voiceand data)

DigitalSwitch

(LE)

2M

STM-1 STM-1

STM-1 STM-1

STM-1 STM-43 140 Mbit/s links3 34 Mbit/s links

140 Mbit/s

34 Mbit/s

64klinks

(2-34)Mlinks

34 M

140 M4 STM-1s

"Regenerator Section"

STM-1

"RegeneratorSection"

"Multiplex Section"

"Path"

SkipMux

STM-1140 Mbit/s

Example of a Regional Implementation of SDH

The figure above shows a typical SDH network configuration. Three terms are especially important when

describing an SDH network: “Path”, “Multiplex”, and “Regenerator”.

“Path” is used to name any section of the SDH network between the points were an SDH signal is originated

and terminated.

“Multiplex Section” is used to name any section of the SDH network between two points where multiplexing or

demultiplexing of an SDH signal occurs, but no origination and termination occurs. If STM-4/STM-16 and

above multiplexers are used, then the network sections they delimit are also Multiplex sections.

“Regenerator Section” is used to name any section of the SDH network between two points with regeneration

capabilities (synchronous multiplexers and synchronous regenerators).

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Framing Structure Overview - Comparison of PDH and SDH

PDHDXS

FlexibleMux


64kDXS

(2-34)M links

(PSTN and Private Circuits)

PDHand

B-ISDN

2Mlinks

(voiceand data)

3 140 Mbit/s links3 34 Mbit/s links

140 Mbit/s

64klinks

(2-34)Mlinks

34 M

140 M

SkipMux

PDH

PDH

PDH

PDH

140 Mbit/s

How PDH and SDH Coexist in a Typical Network

As previously discussed, SDH technology is used in the interface between the transmission facility and the

network node which performs signal termination, switching, cross-connection or multiplexing/demultiplexing.

Before SDH, the technology used was PDH.

Although from the technology point of view PDH and SDH are different, the functionality and

applications of both PDH and SDH are essentially similar. For example, as in the case of PDH, SDH

framing is based upon a Frame Alignment signal, which is a binary word ranging from 7 to 12 bits in

PDH and a multiple of 6 octets in SDH.


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Framing Structure Overview - Introduction to STM-1 Frame

STM-1 Frame Structure

The basic SDH frame, called synchronous transport module at level 1 (STM-1) frame, is represented above.

The STM-1 frame consists of nine equal segments of serial bits. Each of these segments is composed of 270

octets. For each segment, the first 9 octets are used for “overhead” transport, and the remaining 261 octets are

used for “payload” transport. The length of an STM-1 frame is 270 octets/segment x 9 segments = 2430 octets.

The aggregate bit rate is therefore:

Aggregate bit rate = 2430 octets x 8 bits/octet ——> = 155.52 Mbit/s125 µ seconds

PDHDXS

STM-1 STM-1

STM-1 STM-1

STM-1

4 STM-1s

STM-1

STM-1

STM-1

STM-1 Signals Within a Typical Regional Network

1 2 43 5 6 7 8 9

Frame Period = 125 µ seconds

OVERHEAD PAYLOAD

9 bytes 261 bytes

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Framing Structure Overview - Multiplexing a 140 Mbit/s Signal

C-4

VC-4

AU-4

AUG

STM-1 Frame

VC-4 Path Overhead

VC-4 Pointer

Section Overhead

Container for139264 kbit/s signal

Steps in Multiplexing a 140 Mbit/s signal into the STM-1 Frame

The diagram above shows the steps used to multiplex a 140 Mbit/s PDH signal into the STM-1 frame. Each of

those steps will be discussed separately next.


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PDHDXS

STM-1 STM-44 STM-1s

C-4

VC-4

AU-4

AUG

STM-1 Frame

140M

VC-4 Path Overhead

VC-4 Pointer

Section Overhead

Multiplexing a 140Mbit/s Signal

In a PDH-SDH link containing a 140 Mbit/s signal, the STM-1 overhead and pointers are processed in the link

portion indicated in the figure above. Path overhead is processed at the nodes where virtual containers are

originated and terminated.* The VC-4 pointer is processed at the synchronous multiplexers and DCSs, and the

section overhead is processed at both synchronous multiplexers and line repeaters.

* (originating and terminating synchronous multiplexers)

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C-4

VC-4VC-4 POH


C-4 and VC-4 Origination

In order to multiplex a 140 Mbit/s PDH signal into an STM-1 SDH signal, the first step is to synchronize the

PDH signal with the STM-1 structure. This is done by using the technique of “bit justification”. The created

signal is referred to as a “container”. For a 140 Mbit/s signal, this container is called a “C-4”.

The next step in the multiplexing process is to add overhead to the container just created. This overhead, which

is a group of 9 octets, is called “higher-order Path Overhead (POH)”. The C-4, along with its POH is referred to

as Virtual Container, order 4 (VC-4).


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VC-4 Origination

Each of the nine VC-4 POH octets receives a unique name: J1, B3, C2, G1, F2, H4, Z3, Z4, and Z5. Any two

of these 9 octets are separated by 269 octets in the STM-1 frame. A brief description of each follows:

J1 (Path Validation and Trace): The octet is used to repetitively transmit a known pattern so the receiver can

verify its continued connection with the intended transmitter.

B3 (Path Error Monitoring): A bit interleaved parity of depth eight (BIP-8) is used to monitor for errors. This

works as follows: The number of ones in the position n in each octet of the previous VC-4 before scrambling is

counted. If the result is odd, then the corresponding bit in B3 is set to one. If the result is even, then the

corresponding bit is set to 0. At the receiver, even parity is verified.

C2 (Signal Label): This octet is reserved to indicate the composition of the VC-4, which could contain a 140

Mbit/s signal, ATM cells, MAN signals (DQDB protocol) or even FDDI signals. It also indicates if no valid

signal is being carried.

VC-4VC-4 POH

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VC-4 Origination

G1 (Path Status): This octet conveys back to a VC-4 path originator the path terminating status and

performance. The first four bits return the count of BIP-8 errors Far End Block Errors (FEBE), and therefore

can be thought of as advanced 2 Mbit/s PDH REBEs. Bit 5 generates a Far End Receiver Failure (FERF) alarm

similar to PDH remote alarms. A FERF is sent in the opposite direction if signal failure occurs, or if Path Trace

Mismatch (checked with J1 octet) occurs. Bits 6, 7 and 8 are not currently used.

VC-4VC-4 POH

SDH PathOriginator/TerminatorVC-4 Path FEBE

BIP-8 errors SDH PathOriginator/Terminator

VC-4 Path LOPor

VC-4 Path AISor

Path Trace Mismatch

VC-4 Path FERF

FEBE1 2 3 4

FERF UNUSED5 6 7 8

F2, Z3 (Path User Channels): These two octets are reserved for user communication purposes between path

elements and are payload dependant.

H4 (Position Indicator): This octet provides a generalized position indicator for payloads and can be payload

specific.

Z4 (Spare octet): This octet is reserved for future uses.

Z5 (Network Operator octet): This octet is reserved for specific management purposes, such as tandem

connection maintenance.


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AU-4 Origination

The next step in the multiplexing process of a 140 Mbit/s PDH signal is to make provision for differences in

frequency and phase between VC-4s. This is achieved by adding a “VC-4 pointer” to the existing VC-4. The

VC-4, along with its pointer, is called “Administrative Unit, order 4,” or AU-4.

The VC-4 pointer is a 10-bit string that indicates the position of the first octet of the VC-4 (J1 octet). Given that

the transmission frequency of an STM-1 is fixed, the VC-4 pointer allows VC-4 transmission rate and phase to

be controlled within certain limits. (Allows the VC-4 to “float” inside the STM-1).

There are three ways to change the value of a VC-4 pointer; those three will be reviewed in the next page.

AU-4VC-4 Pointer

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VC-4 Pointer Value Definition

The first method to change a pointer value consists in incrementing or decrementing it by one unit at a time with

indication; the second, by forcing an arbitrary, non-unit change; and the third, by detecting a new pointer value

for three consecutive frames.

The first method of pointer value change is indicated by the bits in the pointer itself. When the value of

the pointer needs to be increased by one, 5 bits invert their value. To increase the value of the pointer by

one unit, bits 1,3,5,7, and 9 of the pointer are inverted. To decrease it, bits 2,4,6,8, and 10 of the pointer

invert their value.

The second method of pointer value change is achieved by the use of the New Data Flag (NDF) word. This is a

4-bit word indicated in the figure above by NNNN. When using this method, the normal NDF value of 0110 is

replaced by the value 1001. The new present pointer value takes place immediately.

The third method of pointer value change calls for accepting a new pointer value if it is received by three

consecutive frames.

N N N N S S 1 0 1 0 1 0 1 0 1 0

unspecifiedbits

VC-4 pointerNew DataFlag


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How a VC-4 Pointer indicates the location of a VC-4

Contrary to PDH, which uses positive justification only to modify information speed, pointers allow

information speed and phase modification by using both positive and negative justification. Positive

justification allows transmission of “less” information within the frequency-constant STM-1 signal, whereas

negative justification allows transmission of “more” information within the frequency-constant STM-1 signal.

Typically, phase adjustments are implemented by using the New Data Flag (NDF) word. Also, in order to

compensate for frequency variations within the synchronous network, frequent pointer value adjustments are

used. Because of this, timing problems in a synchronous network can be pinpointed by observing the values of

pointers over time. A frequent variation would indicate potential timing problems.

N N N N S S 0 0

2 bytes

0 0 0 0 0 0 0 0

N N N N S S 0 0 0 0 0 0 0 0 0 1

2 bytes

NDF

NDF

VC-4 pointer

VC-4 pointer

3 bytes

3 bytes

VC-4 starts here

VC-4 starts hereinfo. from previous VC-4

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AUG Origination

There is an SDH alarm dependent upon pointer activity. This alarm is called AU-4 Path AIS. Whenever an

SDH Path originator/terminator receives an AU-4 Path AIS, or an AU-4 Path LOP, it will transmit AU-4 Path

AIS, which is “all ones” in the entire AU-4, including the VC-4 pointer.

The next step in the multiplexing process of a 140 Mbit/s PDH signal is to generate an Administrative Unit

Group (AUG). No additional information is added to an AU-4 to create an AUG. This structure is part of SDH

primarily to remain compatible with the North American SONET structure.

AU-4

AUG

VC-4 Pointer

SDH PathOriginator/Terminator

AU-4 Path AISor

AU-4 Path LOPAU-4 Path AIS


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STM-1 FrameSection Overhead

STM-1 Frame Origination

An AUG is multiplexed into an STM-1 signal by adding additional overhead, called Section overhead. Section

overhead includes framing information and information for maintenance, performance monitoring, and other

operational functions. The SOH includes both Regenerator Section Overhead (RSOH) , which is terminated at

regenerator functions, and Multiplex Section Overhead (MSOH), which passes transparently through

regenerators and is terminated where the AUGs are assembled and disassembled (synchronous multiplexers).

The section overhead is organized in octets. An explanation of the Section overhead follows.

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STM-1 Frame

A1, A2 octets (Framing octets): Three A1 octets (11110110) and three A2 octets (00101000) are used to create

a Frame Alignment Signal (FAS) for frame alignment purposes. The used sequence is A1A1A1A2A2A2.

C1 octet (STM-N identifier): Indicates the level of the frame (1, 4, 16, etc.). Particularly useful when higher-

level signals are transported.

B1 octet (Regenerator Section Error Monitoring): Implements a bit interleaved parity eight (BIP-8) code using

even parity. The BIP-8 is computed over all bits of the previous STM-1 frame after scrambling and is placed in

B1 before scrambling.

E1, E2 octets (Orderwire channels): Use these two 64 kbit/s channels for voice communications. E1 is part of

the RSOH, and E2 is part of the MSOH.

F1 octet (User Channel): This 64 kbit/s channel is reserved for user purposes, for example, to provide

temporary data/voice channel connections for special maintenance purposes.

D1-D12 octets (Data Communication Channels): These channels can be used for alarms, maintenance, control,

monitoring, administration, and other purposes. These channels are divided into one 192 kbit/s channel at the

Regenerator section, and one 576 kbit/s channel at the Multiplex section.

B2 octets (Multiplex Section Error Monitoring): Three octets are used to implement a BIP-24 code using even

parity. The whole previous STM-1 frame, except the RSOH are included in the calculation.

FAS

A1 A1 A1 A2 A2 A2

2430 bytes


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STM-1 Frame

K1, K2 octets (Automatic Protection Switching, APS): Those two octets are allocated to the function of

coordinating protection switching across a set of multiplex sections organized as a protection group. Bits 6, 7,

and 8 of K2 octet are used to signal MS-FERF (110) and MS-AIS (111).

Z1 octet (Synchronization status octet): Bits 1-4 of these three octets are unused. Bits 5-8 of those three octets

are used for synchronization status messages.

Z2 octet (Additional octet): This byte is reserved for functions not yet defined.

There are 39 additional octets as part of the section overhead whose use is still not defined.

FAS

A1 A1 A1 A2 A2 A2

2430 bytes

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SDH MSElement

LOS, LOF,MS-AIS or

Excessive Errors

MS-FERF

MS-AIS

At the Multiplex Section level of an STM-1 SDH signal, there are two important alarms:

a) Multiplex Section AIS: Referred to as MS-AIS, this alarm is signaled by setting bits 6, 7 and 8 of the K2

octet equal to “1”. MS-AIS should be generated when signal is lost, when frame alignment is lost, when MS-

AIS is received, or when an excessive BER, as calculated using B2 octets, is detected.

b) Multiplex Section FERF: Referred to as MS-FERF, this alarm is signaled towards the opposite direction of

the data flow by setting bits 6, 7 and 8 of the K2 octet to 110. MS-FERF should be generated when signal is

lost, when frame alignment is lost, when MS-AIS is received, or when an excessive BER, as calculated using

B2 octets, is detected.


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C-3

VC-3

TU-3

TUG-3

VC-4

VC-3 Path Overhead

VC-3 Pointer


AU-4

AUG

STM-1 Frame

VC-4 Path Overhead

VC-4 Pointer

SectionOverhead

3 x

Steps in multiplexing a 34 Mbit/s signal into the STM-1 frame

The diagram above shows the steps used to multiplex a 34 Mbit/s PDH signal into the STM-1 frame. Following

is a separate discussion of these steps.

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C-3

VC-3

TU-3

TUG-3

VC-4

VC-3 Path Overhead

VC-3 Pointer


AU-4

AUG

STM-1 Frame

VC-4 Path Overhead

VC-4 Pointer

SectionOverhead

3 x

PDHDXS

(2-34)M links

(PSTN andPrivate Circuits)

STM-1

STM-1

STM-43 140 Mbit/s links3 34 Mbit/s links

140 Mbit/s

34 Mbit/s

(2-34)Mlinks

STM-1

1

2

1 , 2

STM-1

1 , 2

Multiplexing a 34 Mbit/s signal

The figure above shows an example of where in a PDH-SDH link containing a 34 Mbit/s signal the STM-1

overhead and pointers are processed. Path overhead is processed at the nodes where the respective virtual

containers are originated and terminated (originating and terminating synchronous multiplexers). The pointers

are processed at the synchronous multiplexers and DCCs, and the section overhead is processed at both

synchronous multiplexers and line repeaters.


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C-3

VC-3

TU-3

VC-3 POH

VC-3 Pointer


C-3, VC-3 and TU-3 Origination

The first three steps in multiplexing a 34 Mbit/s PDH signal into an STM-1 are similar to the first three steps in

the multiplexing of a 140 Mbit/s signal. The 34 Mbit/s signal is first synchronized with the STM-1 signal, then

Path Overhead identical to that used in a 140 Mbit/s signal is added, and then a VC-3 pointer is added to the

created Virtual Container, to create a Tributary Unit, order 3 (TU-3).

The alarms for a VC-3 Path are similar to those of a VC-4 Path.

The only major difference between a VC-4 pointer and a VC-3 pointer is that when adjusting the value of a VC-

4 pointer by one, the position of the payload is moved by three octets, but when adjusting the value of a VC-3

pointer by one, the position of the payload is moved by one octet. The alarms found in an AU-4 are similar to

those in a TU-3.

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TU-3

TUG-3

VC-4VC-4 Path Overhead

3 x

TUG-3 and VC-4 Origination

The next step in the multiplexing process is to generate a Tributary Unit Group of order 3 (TUG-3). To

generate a TUG-3, no pointer processing occurs or extra overhead is added to the existing TU-3s. The reason

for having this extra step in the multiplexing process is to allow an STM-1 signal to carry mixed capacity

payloads made up of different size TUs. This makes an STM-1 signal more flexible as a transport structure.

The next step in the multiplexing process is to create a VC-4. This is done by octet-interleaving three TUG-3s,

and adding a higher-order Path Overhead. In general, the contents of the involved 3 TUG-3s may or may not be

the same type of signals (2 Mbit/s or 34 Mbit/s signals).

From this point on, the remaining multiplexing steps are similar to the ones discussed for a 140 Mbit/s signal.


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Steps in multiplexing a 2 Mbit/s signal into the STM-1 frame

The diagram above shows the steps used to multiplex a 2 Mbit/s PDH signal into the STM-1 frame. Each of

those steps will be discussed separately next.

C-12

VC-12

TU-12

TU-12

VC-4

VC-12 Path Overhead

VC-12 Pointer


AU-4

AUG

STM-1 Frame

VC-4Path Overhead

VC-4 Pointer

SectionOverhead

3 x

TUG-2

TUG-3

7 x

3 x

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Multiplexing a 2Mbit/s signal

The figure above shows an example of where in a PDH-SDH link containing a 2 Mbit/s signal the STM-1

overhead and pointers are processed. Path overhead is processed at the nodes where the respective virtual

containers are originated and terminated (originating and terminating synchronous multiplexers). The pointers

are processed at the synchronous multiplexers and DCCs (not shown), and the section overhead is processed at

both synchronous multiplexers and line repeaters.

C-12

VC-12

TU-12

TU-12

VC-4

VC-12 Path Overhead

VC-12 Pointer


AU-4

AUG

STM-1 Frame

VC-4Path Overhead

VC-4 Pointer

SectionOverhead

3 x

TUG-2

TUG-3

7 x

3 x

PDHDXS

64 kDXS

STM-1

STM-1

34 Mbit/s

2

1 , 2


FlexibleMux

DigitalSwitch(LE)

2M

1 , 2

2M link

1


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C-12 Origination

Unlike 140 Mbit/s and 34 Mbit/s PDH signals, which are asynchronous in nature, 2 Mbit/s signals in a PDH

network may be either synchronous or asynchronous. The reason for this is that PDH network synchronization

is carried with 2048 kHz reference signals.

The advantages of 2 Mbit/s asynchronous multiplexing are an unrestricted payload, timing transparency, and

minimum mapping delay, while its main disadvantage is the inability to access 64 kbit/s timeslots within the 2

Mbit/s signal.

There are two ways to multiplex a synchronous 2 Mbit/s signal: bit-synchronous and octet-synchronous

multiplexing. Bit-synchronous multiplexing is typically used with unframed signals that are synchronized with

the SDH transport signal. Framed signals, however, can also be multiplexed with bit-synchronous mapping.

Byte-synchronous multiplexing provides timeslot visibility and is therefore appropriate where 64 kbit/s signals

are to be added, dropped, or cross-connected directly from an STM-1 signal.

C-12Container for2048 kbit/s signals

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C-12

VC-12VC-12 Path Overhead


VC-12 POH: • • • • • •V5 J2 Z6 Z7

VC-12 Origination

The next step in the multiplexing process is to add overhead to the C-12 container just created. This overhead

consists of 4 octets, called V5, J2, Z6 and Z7. It is referred to as “lower-order Path Overhead (POH)”. These

octets are added at the beginning of the C-12 container. The C-12 along with the added octets is referred to as

Virtual Container, order 12, VC-12.

The first VC-12 Path Overhead octet called V5 will be described next.


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Description of V5 byte

BIP-2 (Path error monitoring): Bits 1 and 2 are used for error monitoring using Bit Interleaved Parity (BIP).

Bit 1 is set such that parity of all odd-numbered bits (1, 3, 5 and 7) in all octets in the previous VC-12 is even.

Bit 2 is set similarly for the even-numbered bits (2, 4, 6 and 8). At the receiver, even parity is verified.

FEBE (far end block error): Bit 3 is a VC-12 Path Far End Block Error (FEBE) indication. This bit is set to

one and sent back to the VC-12 originator if one or more errors are detected by the BIP-2.

Remote Failure Indication: Bit 4 is a VC-12 path Remote Failure Indication (RFI). This bit is set to one if a

failure occurs. A failure is a Defect* that persists beyond the maximum time allocated to the protection

mechanism.

Signal Label: This three-bit code is used to indicate to the receiver whether a valid 2 Mbit/s signal is being

transported in the VC-12, and what type of multiplexing is being employed (asynchronous, bit or octet

synchronous).

FERF (Far End Receive Failure): This bit is set to one if the associated receiver in the SDH Path element

receives a TU-12 Path AIS or TU-12 Path LOP.

*In this context, Defect is understood as defined in ITU-T Recommendation G.826.

Bit Number

BIP-2 FEBE RFI SIGNAL LABEL

1 2 3 4

FERF

85 6 7

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• • • • • •V5 J2 Z6 Z7

VC-12 Path Overhead

J2 octet is provisionally used to repetitively transmit an identifier signal so that a path-receiving terminal can

verify its continued connection to the intended transmitter. The E.164 numbering format is used. Similar to J1

octet in higher-order POH.

Z6 octet is used to provide a tandem-connection monitoring function. Similar to Z5 octet in the higher-order

POH.

Z7 octet is reserved for future use.


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TU-12 Origination

The following alarms are used in association with 2 Mbit/s Virtual Containers:

The next step in the multiplexing process of a 2 Mbit/s PDH signal is to make provision for differences in

frequency and phase between VC-12s. This is achieved by adding a “VC-12 pointer” to the existing VC-12.

The VC-12, along with its pointer, is called “Tributary Unit, order 12,” or TU-12.

The VC-12 pointer is very similar to the VC-3 and VC-4 pointers in structure and function. As in the case of

VC-3 pointers, single-octet increments for VC-12 pointers are possible.

VC-12

TU-12VC-12 Pointer

SDH PathElement

VC-12 Path FEBE

BIP-2 errorsSDH Network

Element

TU-12 Path LOPor

TU-12 Path AIS

VC-12 Path FERF

TU-12 Path AIS

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TU-12 Origination

There are two possible multiplexing modes of the TU-12 structure: Floating and Locked multiplexing modes.

Floating mode implies the use of the VC-12 pointer previously discussed to allow frequency and phase

alignment of VC-12s. The V5 octet is generated and used for the purposes previously described. Note that if

this terminology is used, then the VC-4 for a 140 Mbit/s and the VC-3 for a 34 Mbit/s PDH signals employ

Floating mode.

In contrast, Locked mode of a TU-12 implies that the VC-12 pointer is not used (in fact, the pointer is set to

zero), the VC-12 occupies a fixed location within the higher-order VC (VC-4), and no VC-12 POH is

generated. This mode of multiplexing is essentially equivalent to mapping N x 64 kbit/s signals directly into the

VC-4. This multiplexing mode was introduced as a potentially less expensive structure for implementing

subnetworks with 64 kbit/s flexibility as the need for pointer processing could be avoided.

VC-12

TU-12VC-12 Pointer


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TUG-2 and TUG-3 Origination

The next step in the multiplexing process is to generate a Tributary Unit Group of order 2 (TUG-2) by octet

interleaving three TU-12s. For this, no pointer processing occurs or extra overhead is added. One reason for

having this step in the multiplexing process is to allow an STM-1 signal to carry any of the first-level PDH

signals (European 2 Mbit/s or American 1.544 Mbit/s), and also to be compatible with second-level PDH

signals.

The following step in the multiplexing process is to generate a Tributary Unit Group of order 3 (TUG-3) by

octet interleaving seven TUG-2s. Again, here no pointer processing occurs or extra overhead is added. From

this point on, subsequent multiplexing of a 2 Mbit/s signal into an STM-1 signal is similar to the multiplexing of

a 34 Mbit/s PDH signal.

TU-123 x

TUG-2

TUG-3

7 x

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Framing Structure Overview - Multiplexing Other Signals

B-ISDN in a typical Regional SDH Implementation

Payloads that require more than one C-4 can also be transported at the STM-1 level. This is done by

concatenating AU-4s. Sets of X contiguous AU-4s may have their payloads locked together by setting the

pointer value in all but the leading AU-4 to a specific state known as the Concatenator Indicator (CI). Pointer

adjustments indicated for the leading AU-4 are then replicated in all the concatenated AU-4s in the set,

maintaining bit sequence integrity over the whole broadband payload. Such a set of concatenated AU-4s is

designated AU-40Xc.

An STM-1 signal can transport not only synchronous signals (such as 2 Mbit/s synchronous) and

plesiochronous signals (most PDH signals), but can also transport asynchronous signals. An example of such

asynchronous signals that is likely to become common in the future is B-ISDN based upon Asynchronous

Transfer Mode (ATM).

PDHDXS

PDHand

B-ISDN


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Framing Structure Overview - Higher Order SDH Signals

PDHDXS

STM-4

Use of an STM-4 link in a typical Regional SDH Implementation

Network growth and the demand for broadband services are leading to very high-rate optical transmission

systems at 622 Mbit/s, 2488 Mbit/s, and possibly beyond. These should ultimately replace 140 Mbit/s and 565

Mbit/s systems. Consequently, there is a requirement for synchronous transport modules operating at rates

higher than 155 Mbit/s.

These higher-order synchronous transport modules can be assembled by further multiplexing. At each stage,

four tributaries are combined by extracting the payload from each, recalculating their pointer values, then phase

aligning and octet interleaving them and finally adding a new section overhead.

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Framing Structure Overview - Higher-Order SDH Signals

Byte interleaving STM-1s to form STM-4 and STM-16

The resulting digit rates are 4n x 155.52 Mbit/s. STM-N is the generic term for these higher-rate modules. For

example, STM-4 is at 622.08 Mbit/s. STM-16 is at 2488.32 Mbit/s and can carry 16 times the payload of an

STM-1. The resulting hierarchy is extendible to arbitrarily-high bit rates.

The multiplexing explained before needs no addition of information, because the 155.52 Mbit/s signals are bit

and frame synchronized. This allows the use of the Frame Alignment Signal (FAS) from the lower-order level

to align at the higher level.

1234

4 3 2 1 . . .

5678

8 7 6 5 . . .

9101112

13141516

12 11 10 9 . . .

16 15 14 13 . . .

STM-4s

STM-1s . . . 11

7

3 14 10 6 2 13 9 5 1 . . .

STM-16


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Framing Structure Overview - Framing Synchronization Strategy

In-FrameState (IF)

OOF LOF

A B

. . .

A: 5 consecutive frames with FAS errors occurB: 3 mseconds without detection of two consecutive non-FAS errored frames

For STM-N; N=1,4,16 signals, the framing synchronization strategy for a network element is as follows:

a) Frame synchronization is declared when two consecutive frames without FAS errors are received. This

condition is called In-Frame (IF) state.

b) If In-Frame state has been achieved, when five consecutive frames with FAS errors are detected, the receiver

equipment declares Out-of-Frame (OOF) state.

c) When in Out-of-Frame state, if two consecutive frames without FAS errors are not received within 3

milliseconds, the network equipment declares Loss-of-Frame (LOF) state.

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Framing Structure Overview - Common STM-1 Representation

1 2 43 5 6 7 8 9

OVERHEAD

PAYLOAD

123456789

Common STM-N Frame Representation

The diagram above shows how an STM-1 frame is typically rearranged to display the information in a block-

like form. This representation is normally used because it is more compact than a serial bit stream

representation.

In this representation, the STM-N frame is represented as a block of 9 rows by 270 x N columns. The first 9 x

N octet columns of the STM-N contain overhead information, and the remaining columns contain path

overhead, virtual container pointers, and users information.


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J1

B3

C2

G1

F2

H4

Z3

Z4

Z5

Section Overhead

Section Overhead

PDH 140M orStructured SDH Signal

1

3

4

5

9

Regenerator

Multiplex

VC-4POH

9 x N 261 x N

270 x N columns (bytes)

9 rows

A1 A1 A1 A2 A2 A2 C1

B1 ∆ E1 F1

D1 D2 D3

B2 B2 B2 K1 K2

D4 D5 D6

D7 D8 D9

D10 D11 D12

Z1 Z1 Z1 Z2 Z2 Z2 E2

Bytes reserved for national use

Unscrambled bytes. Therefore care should be takenwith their content

Media dependent bytes

9 r

ow

s

RSOH

MSOH

9 bytes

AU-n pointers

∆

∆ ∆

∆

∆

∆


Common STM-N Frame Representation

The figure above shows the block representation as used by the ITU-T. Note the location of the Section

Overhead in this representation. If the STM-1 signal carries a 140 Mbit/s signal, then the corresponding VC-4

POH is also part of the STM-1 signal as shown above.

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If three 34 Mbit/s signals are carried within an STM-1 signal, then the ITU-T representation is as shown in the

following figure:

J1

B3

C2

G1

F2

H4

Z3

Z4

Z5

Container-3

85 columns

86 columns

TUG-3 = TU-3

VC-3

VC-3 POH

H1

H2

H3

Fixe

d st

uff

86 columns

VC-12

TUG-2 TUG-2

TUG-3(7 x TUG-2)

3 VC-12s

Fixe

d st

uff

NPI

POH

POH

TU-12PTRs

If 63 2 Mbit/s signals are carried within an STM-1 signal, then the corresponding representation is as shown in

the following figure.


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Differences Between SDH and SONET Technologies

Most of the important differences between SONET and SDH were influenced by the need of a transport system

able to support the main European PDH rates, mixed payloads, and the emerging broadband standards. Indeed,

SONET and SDH are compatible, but not identical, digital hierarchies.

Both hierarchies define similar sets of overheads and functions, however, there are differences in the usage of

the two overhead structures and pointer processing. Nevertheless, these differences are beyond the scope of this

document. For those differences, please see AT&T Communications Document “A Technical Report on A

Comparison of SDH and SONET.”

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Differences Between SDH and SONET Technologies (Continued)

The following are major differences between SONET and SDH:

1. The 51.84 Mbit/s Synchronous Transport Signal - Level 1 (STS-1) is the basic building block of SONET.

All lower-rate payloads are mapped into the STS-1, and all higher-rate signals are created by synchronously

multiplexing N STS-1s to form an STS-N. In contrast, the 155.52 Mbit/s Synchronous Transport Module -

Level 1 (STM-1) is the basic building block of SDH. All lower-rate payloads are mapped into the STM-1,

and all higher-rate signals are created by synchronously multiplexing N STM-1s to form an STM-N.

2. Eight different transmission rates have been defined for SONET: 51.84 Mbit/s, 155.52 Mbit/s, 466.56

Mbit/s, 622.08 Mbit/s, 933.12 Mbit/s, 1244.16 Mbit/s, 1866.24 Mbit/s and 2488.32 Mbit/s. In contrast,

only three different transmission rates have been defined for SDH: 155.52 Mbit/s, 622.08 Mbit/s and

2488.32 Mbit/s.

3. Unlike SONET, SDH allows different mappings for the same payload. All of the SDH payloads which can

be mapped into an AU-3 can also be mapped into an AU-4. SONET provides only one choice for the

defined payload mappings.

4. SONET and SDH differ in some of their payload mappings. See table below (compatible SONET/SDH

appear in brackets).

SONET/SDH Payload Mappings

Payload STS-1 STS-3c AU3 Based STM-1 AU4 Based STM-1

DS1E1DS1CDS2E3DS3E4ATMATMFDDIDQDB

1.5 Mbit/s2.048 Mbit/s3.152 Mbit/s6.312 Mbit/s

34.368 Mbit/s44.736 Mbit/s

139.264 Mbit/s149.760 Mbit/s599.040 Mbit/s125.000 Mbit/s149.760 Mbit/s

(VT1.5)(VT2)VT 3

(VT 6)None

(STS-1 SPE)NoneNoneNoneNoneNone

NoneNoneNoneNoneNoneNone

(STS-3c SPE)(STS-3c SPE)

VC4-4c(STS-3c SPE)(STS-3c SPE)

(VC11) or VC12*(VC12)None(VC2)VC3

(VC3)NoneNoneNoneNoneNone

VC11 or VC12*VC12NoneVC2VC3VC3

(VC4)(VC4)VC4

(VC4)(VC4)

( ) Compatible SONET/SDH mappings.* In SDH, a DS1 may be carried in a VC12 (2 Mbps)


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SDH Abbreviations

ACSE Association control service elementADM Add/Drop MultiplexerAITS Acknowledged information transfer serviceAIS Alarm indication signalALS Automatic laser shutdownANSI American National Standards InstituteAP Access PointAPDU Application protocol data unitASE Application service elementAPS Automatic protection switchingASN.1 Abstract syntax notation oneATM Asynchronous transfer modeAU Administrative unitAU-n Administrative unit of order nAUG Administrative unit groupBBER Background block error ratioBER Binary error rateBIP Bit interleaved parityBIP-8 Bit interleaved parity of order 8BIP-X Bit interleaved parity-XBITS Building integrated timing supplyB-ISDN Broadband integrated services digital networkC ContainerC-n Container of order nCAS Channel associated signalingCC Connect confirmCCITT The International Telegraph and Telephone Consultative CommitteeCCS Common channel signalingCEPT Committee European de Post et TelegraphCI Concatenation indicatorCLNP Connectionless network layer protocolCLNS Connectionless network layer serviceCM Connection matrixCMI Coded mark inversionCMIP Common management information protocolCMISE Common management information service elementCONP Connection oriented network-layer protocolCP Connection pointCR Connection requestCV Code violationDCC Data communication channelDCN Data communication networkDCS Digital crossconnect systemsDDF Digital distribution frameDIN Deutsche Industrie NormenausschussDPRing Dedicated protection ringDQDB Distributed queue dual bus protocolDS Degraded SecondDXC Digital crossconnectEA External access equipment

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EBU European Broadcasting UnionECC Embedded control channelEEC European Economic CommunityEOW Engineering order-wireES Errored secondESR Errored second ratioETSI European Telecommunications Standards InstituteEX Extinction ratioFAL Frame alignment lossFAS Flexible Access System, British Telecom’s fractional 2 Mbit/s service based upon ISDNFAW Frame alignment wordFBS Flexible Bandwidth System, another name for the Flexible Access SystemFDDI Fiber digital data interfaceFDM Frequency Division MultiplexingFEBE Far end block errorFERF Far end receiver failureFLS Frame loss secondFU Functional unitGNE Gateway network elementHDTV High Definition TelevisionHEC Header error controlHOP High order pathHPA Higher order path adaptationHPC Higher order path connectionHPT Higher order path terminationHVC Higher order virtual containerI/D Increment/decrementIDN Integrated Digital NetworkIEC International exchange carriersIEE British Institution of Electrical EngineersIEEE North American Institute of Electrical and Electronics EngineersIFU Interworking functional unitIP Interworking protocolIS Intermediate systemISDN Integrated services digital networkISO International Standards OrganizationITU-T The ITU Telecommunication Standardization SectorKILOSTREAM British Telecom’s 64 kbit/s service based upon X.25 circuitsKILOSTREAM+ British Telecom’s fractional 2 Mbit/s service based upon X.25 circuitsLAN Local Area NetworkLCN Local communications networkLEC Local Exchange CarriersLED Light emitting diodeLO Lower orderLOF Loss of frameLOM Loss of multiframeLOP Loss of PointerLOP Low order pathLOS Loss of signalLPA Lower order path adaptationLPC Lower order path connectionLPT Lower order path termination


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LVC Lower order virtual containersMAF Management applications functionMAN Metropolitan Area NetworkMCF Message communication functionMD Mediation deviceMEGASTREAM British Telecom’s 2 Mbit/s serviceMF Mediation functionMLM Multi-longitudinal modeMO Managed objectMOC Managed object classMRTIE Maximum relative time interval errorMS Multiplex sectionMS-AIS Multiplex section alarm indication signalMS-FERF Multiplex section far end receive failureMSOH Multiplex section overheadMSP Multiplex section protectionMSPG MS protection groupMST Multiplex section terminationMTG Multiplexer timing generatorMTIE Maximum time interval errorMTPI Multiplexer timing physical interfaceMTS Multiplexer timing sourceNA Not applicableNDF New data flagNE Network elementNEF Network element functionN-ISDN Narrowband integrated services digital networkNLR Network layer relayNNE Non-SDH network elementNNI Network node interfaceNOMC Network operators maintenance channelNPI Null Pointer indicationNPDU Network protocol data unitNRZ Nonreturn to zeroNSAP Network service access pointNU National useOAM&P Operations, administration, maintenance and provisioningOFS Out-of-frame secondOHA Overhead accessOOF Out of frameORL Optical return lossOS Operations systemOSF Operations system functionOSI Open systems interconnectionPDH Plesiochronous digital hierarchyPDU Protocol data unitPI Physical interfacePJE Pointer justification eventPJC Pointer justification countPOH Path overheadPPDU Presentation protocol data unitPS Protection Switch

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PSN Packet switched networkPSTN Public switched telephone networkPJE Pointer justification eventPOH Path overHeadPSTN Public Switched Telephone NetworkPTR PointerRACE Research into advanced communications for Europe programmeRMS Root-mean-squareROSE Remote operations service elementRS Regenerator sectionRSOH Regenerator section overheadRST Regenerator section terminationRZ return to zeroSA Section adaptationSAPI Service access point identifierSD Signal degradeSDCN SDH data communication networkSDH Synchronous digital hierarchySEMF Synchronous equipment management functionSES Severely errored secondSF Signal failSLM Single-longitudinal modeSM Synchronous multiplexerSMN SDH management networkSMS SDH management sub-networkSNDCF Sub-network dependent convergence functionSOH Section overheadSONET Synchronous optical networkSPDU Session protocol data unitSPI SDH physical interfaceSTM-N Synchronous transport module at level NSVC Switched virtual circuitTEI Terminal end-point identifierTMN Telecommunications management networkTPDU Transport protocol data unitTR TributaryTSAP Transport service access pointTU Tributary unitTU-n Tributary unit of order nTUG-n Tributary unit group of order nUAS Unavailable secondUAT UnAvailable timeUI Unit intervalUI Unnumbered informationUITS Unacknowledged information transfer serviceVC Virtual containerVC-n Virtual container of order nVC-n-Xc X time concatenated VC-n (n=2 or 4)VTG Virtual tributary groupWDM Wavelength-division multiplexing

sdh fundamentals

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