section overheads

8/8/2019 Section Overheads

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1. Section Overheads

The SDH standard was developed using a client/server layer approach. The overheadand transport functions are divided into layers. They are :

Regenerator Section

Multiplex Section

PathThe layers have a hierarchical relationship, with each layer building on the servicesprovided by all the lower layers. This section details the different SDH overheadinformation, specifically :

Regenerator Section Overhead

Multiplex Overhead

Path Overhead

1 2 3 4 5 6 7 8 9 10

R

S

O

H

1 A1 A1 A1 A2 A2 A2 J0 J1 H

O

2 B1 E1 F1 B3

3 D1 D2 D3 C2

M

S

O

H

4 H1 H2 H3 G1

P

O

H

5 B2 K1 K2 F2

6 D4 D5 D6 H4

7 D7 D8 D9 F3

8 D10 D11 D2 K3

9 S1 Z1 Z2 M1 E2 N1

Reserved for national use.

Media-dependent use (radio-link, satellite

Undefined


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1 .1 Regenera tor Sect ion Overhead

The Regenerator Section Overhead contains only the information required for the elements located at both ends

of a section. This might be two regenerators, a piece of line terminating equipment and a regenerator, or two

pieces of line terminating equipment.

The Regenerator Section Overhead is found in the first three rows of Columns 1 through 9 of the STM-1 frame.

1.1 .1 A1 and A2 - Framing

These two bytes indicate the beginning of the STM-1 frame and are unscrambled.

A1 has the binary value 11110110, and A2 has the binary value 00101000.

The frame alignment word of an STM-N frame is composed of (3 x N) A1 bytes followed by (3 x N)

A2 bytes.

1.1.2 J0 - Regenerator Section (RS) Trace message

Used to transmit a Section Access Point Identifier so that a section receiver can verify its continued

connection to the intended transmitter.

The coding of the J0 byte is the same as for J1 and J2 bytes.

This byte is defined only for STM-1 number 1 of an STM-N signal.

1.1.3 Z0 Reserved and Undefined

These bytes, which are located at positions S[1,6N+2] to S[1,7N] of an STM-N signal (N > 1), are reserved

for future international standardisation.

1.1.4 B1 - RS bit interleaved parity code (BIP-8)

A parity code (even parity), used to check for transmission errors over a regenerator section.

Its value is calculated over all bits of the previous STM-N frame after scrambling, then placed in the B1 byte

of STM-1 before scrambling. Therefore, this byte is defined only for STM-1 number 1 of an STM-N signal.

1.1 .5 E1 - RS Orderwire

Allocated to be used as a local orderwire channel for voice communication between regenerators, cross-

connects, and remote terminal locations.

1.1 .6 F1 - RS User Channel

Set aside for the user's purposes; it terminates at all section terminating equipment within a line; that is, it

can be read and/or written to at each section terminating equipment in that line.

1.1.7 D1, D2 and D3 - RS data communications channel (DCC)

Together, these three bytes form a 192 kbps message channel providing a message-based channel for

Operations, Administration and Maintenance (OAM) between pieces of section terminating equipment.

The channel can be used from a central location for alarms, control, monitoring, administration, and other

communication needs. It's available for internally generated, externally generated, or manufacturer-specific

messages.


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1 .2 Mult ip lex Sec tion Overhead

The Multiplex Section Overhead contains the information required between the multiplex section termination

equipment at each end of the Multiplex section (that is, between consecutive network elements excluding the

regenerators).

The Multiplex Section Overhead is found in Rows 5 to 9 of Columns 1 through 9 of the STM-1 frame.

1.2.1 B2 - Multiplex Section (MS) Bit Interleaved Parity (MS BIP-24)

This bit interleaved parity 24-bit code is used to determine if a transmission error has occurred over a

multiplex section. It's even parity, and is calculated over all bits of the MS Overhead and the STM-N frame

of the previous STM-N frame before scrambling.

The value is placed in the three B2 bytes of the MS Overhead before scrambling.

These bytes are provided for all STM-1 signals in an STM-N signal.

1.2.2 K1 and K2 - Automatic Protection Switching (APS channel)

Used for MSP (Multiplex Section Protection) signaling between multiplex level entities for bi-directional

automatic protection switching and for communicating Alarm Indication Signal (AIS) and Remote Defect

Indication (RDI) conditions.

1.2.3 D4 to D12 - MS Data Communications Channel (DCC)

These 9 bytes form a 576 kbps message channel from a central location for OAM information (alarms,

control, maintenance, remote provisioning, monitoring, administration and other communication needs).

This message channel is available for internally generated, externally generated, and manufacturer specific

messages.

1.2.4 S1 Synchronisation Status Message (SSMB)

Bits 1 to 4 of this S1 byte are used to carry the synchronisation messages. The following is the assignment

of bit patterns to the four synchronisation levels agreed to within ITU-T (other values are reserved) :

0000 Quality unknown (existing sync. network)0010 G.811 PRC

0100 G.812 transit

1000 G.812 local

1011 G.813 Synchronous Equipment Timing Source (SETS)

1111 Do not use for synchronisation.

This message may be emulated by equipment failures and will be emulated by a Multiplex

Section AIS signal.

1.2.5 M1 Remote Error Indicat ion (MS-REI)

The M1 byte of an STM-1 or the first STM-1 of an STM-N is used for a MS layer remote error indication

(MS-REI).

Bits 2 to 8 of the M1 byte are used to carry the error count of the interleaved bit blocks that the MS BIP-8 hasdetected to be in error at the far end of the section.

This value is truncated at 255 for STM-N >4.

1.2 .6 E2 - MS orderwire

Provides a 64 kbps channel between multiplex entities for an express orderwire. It's a voice channel for use

by maintenance staff and will be ignored as it passes through the regenerators.

1 .3 Higher-Order Path Overhead (VC-4/VC-3)

The Path Overhead is assigned to, and transported with the Virtual Container from the time it's created by path

terminating equipment until the payload is demultiplexed at the termination point in a piece of path terminating

equipment.The Path Overhead is found in Rows 1 to 9 of the first column of the VC-4 or VC-3.


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1.3.1 J1 - Higher-Order VC-N path trace

This user-programmable byte repetitively transmits a 15-byte, E.64 format string plus 1-byte CRC-7. A 64-

byte free-format string is also permitted.

This allows the receiving terminal in a path to verify its continued connection to the intended transmitting

terminal.

1.3.2 B3 - Path Bit Interleaved Parity (Path BIP-8)

An even parity code used to determine if a transmission error has occurred over a path.Its value is calculated over all the bits of the previous virtual container before scrambling.

1.3 .3 C2 - Path Signal Label

This byte specifies the mapping type in the VC-N. Standard binary values for C2 are :

0000 0000 Unequipped

0000 0001 Equipped, non-specific

0000 0010 TUG structure

0000 0011 Locked TU-n

0000 0100 34 Mbps or 45 Mbps into C-3 asynchronous

0001 0010 140 Mbps into C-4 asynchronous

0001 0011 ATM mapping

0001 0100 MAN (DQDB) mapping0001 0101 FDDI mapping

1111 1110 Test signal, O.181 specific mapping

1111 1111 VC-AIS

1.3 .4 G1 - Path Status

Used to convey the path terminating status and performance back to the originating path terminating

equipment. Therefore the bi-directional path in its entirety can be monitored, from either end of the path.

1.3 .5 F2 - Path User Channel

Used for user communication between path elements.

1.3.6 H4 - Tributary Unit (TU) Multiframe IndicatorProvides a generalized multiframe indicator for payload containers. At present, it's used only for tributaryunit structured payloads.

A 4-value incrementing count (0-3) is used to indicate the frame number within a 2 Mbps multiframe

structure.

1.3 .7 F3 - Path User Channel

Allocated for communication purposes between path elements and is payload dependent.

1.3 .8 K3 APS Signal l ing

Provided in K3 (b1-b4) bits, allocated for protection at the VC-4/3 path levels.

K3 (b5-b8) bits are allocated for future use. These bits have no defined value. The receiver is required to

ignore their content.

1.3.9 N1 - Network Operator

Allocated to provide a Higher-Order Tandem Connection Monitoring (HO-TCM) function


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1 .4 Lower-Order Path Overhead (VC-2/VC-1)

The bytes V5, J2, N2, and K4 are allocated to the VC-2/VC-1 POH.

V2 L

O

P

O

H

J2

N2

K4

1.4.1 V5 Error Checking, Signal label and Path Status

The first byte of the multiframe and its position is indicated by the TU-2/TU-1 pointer. It provides the

functions of error checking, signal label, and path status of the VC-2/VC-1 paths. The bit assignments for

the V5 byte are :

BIP-2 REI RFI Signal Label RDI

1 2 3 4 5 6 7 8

BIP Bit Interleaved Parity

REI Remote Error Indication

RFI Remote Failure IndicationRDI Remote Defect Indication

Bits 1 and 2 of the V5 byte are allocated for error performance monitoring. A Bit Interleaved Parity (BIP)

scheme is specified.

Bit 3 is a VC-2/VC-1 path Remote Error Indication (LP-REI) that is set to one and sent back towards a VC-

2/VC-1 path originator if one or more errors were detected by the BIP-2, and is otherwise set to zero.

Bit 4 is a VC-2/VC-1 path Remote Failure Indication (LP-RFI). This bit is set to one if a failure is declared,

otherwise it is set to zero. A failure is a defect that persists beyond the maximum time allocated to the

transmission system protection mechanisms.

Bits 5 through 7 provide a VC-2/VC-1 signal label :

b5 b6 b7 Description0 0 0 Unequipped or supervisory-unequipped

0 0 1 Equipped non-specific

0 1 0 Asynchronous

0 1 1 Bit synchronous

1 0 0 Byte synchronous

1 0 1 Reserved for future use

1 1 0 Test signal, O .181 specific mapping

1 1 1 VC-AIS

Virtual Container path REI coding : 0 = 0 errors 1 = 1 or more errors

Bit 8 is set to 1 to indicate a VC-2/VC-1 path Remote Defect Indication (LP-RDI), otherwise it is set to zero.

1.4 .2 J2 Path TraceUsed to transmit repetitively a Lower-Order Access Path Identifier so that a path receiving terminal can

verify its continued connection to the intended transmitter. A 16-byte frame is defined for the transmission

of Path Access Point Identifiers. This 16-byte frame is identical to the 16-byte frame of the J1 and J0 bytes.

1.4.3 N2 Tandem Connect ion

Allocated to provide a Lower-Order Tandem Connection Monitoring (LO-TCM) function.

1.4.4 K4 Automatic Protection Switching

Bits 1 through 4 are allocated for APS signalling for protection at the Lower-Order path level. Bits 5

through 7 are reserved for optional use.

section overheads

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