SDH Frame Structure

This page of SDH tutorial covers SDH frame structure and explains STS-1 SONET/SDH frame including interleaving concept, transport overhead and payload overhead part of the SDH frame in depth.

SDH SONET frame structure

The figure describes basic SONET STS-1 frame consisting of 9 rows and 90 columns. SONET frame is composed of 810 octets (bytes). Transmission is carried out row wise from left to right and from top to bottom. Bits are transmitted serially.

The STS-1 frame of SDH is composed of section overhead, transport overhead, payload overhead and data part. The frame starts with fixed A1/A2 bit pattern of 0xf628 used for bit/octet synchronization. SONET/SDH is referred as octet synchronous. The first three columns of SONET frame is referred as transport overhead. The next 87 columns of the frame are referred as Synchronous payload envelope (SPE). Payload overhead is part of SPE.

STS-1 data rate is about 51.84 Mbps. Let us examine how this has been achieved. Every SONET/SDH frame repeats once every 125 micro-sec. 90 columns in 9 rows and 8000 times per second and 8 bits per octet give us data rate of 51.84 Mbps. STS is the abbreviation of Synchronous Transport Signal. STS-1 is referred as OC-1(Optical Carrier) after scrambling is done on STS-1.

SDH/SONET Digital rate hierarchy table

SONET Rate Name SDH name Line Rate (Mbps) Synchronous payload envelope rate(Mbps) Transport Overhead rate(Mbps)
STS-1 None 51.84 50.112 1.728
STS-3 STM-1 155.52 150.336 5.184
STS-12 STM-4 622.08 601.344 20.736
STS-48 STM-16 2488.32 2405.376 84.672
STS-192 STM-64 9953.28 9621.504 331.776
STS-768 STM-256 39813.12 38486.016 1327.104

Interleaving in SONET/SDH

STS-3 frame is formed using three STS-1 frames with the help of interleaving technique. The interleaving is octet type i.e. A1 octet from 1st,2nd and 3rd STS-1 frame is taken first then A2 octet from all these three frames are taken and transmitted.

Transport Overhead

Framing octets (A1,A2) :
These two octets are used to determine start of the SDH frame. A1 is 0xf6 and A2 is 0x28 hexadecimal values.

Section Trace (J0) :
It is used allow connected sections to verify whether the connection is still alive and with the right terminations or not.

Pariry (B1) :
This parity octet is used by the receiver for bit error rate estimation. As this is of 8 bits, 8 parities are calculated.

Order Wire (E1) :
This is not used today. This was used by technicians to test the system while installation.

Section User Channel (F1) :
This is used by the network service provider. The octet is carried over from section to section within the line.

Section Data Communication Channel (D1,D2,D3) :
These octets form a communication channel to send administrative messages. These are considered as single 192 kbps message. Used for maintenance, control, alarm, monitor, administration and the other need of communication between section terminating equipments.

Pointers and Pointer Action (H1, H2, H3) :
These are used to point to the payload (SPE). They provide flags to indicate about changes to payload location and provide location for the data.

Line Parity (B2) :
B2 Octet is used for bit error rate estimation.

Automatic Protection Switching (APS) channel (K1,K2) :
These two octets are used for APS signalling between line level entities. APS stands for Automatic Protection Switching.

Line Data Communications Channel (D4-D12) :
D4 to D12 octets form communication channel to send administrative messages. Used for line data communication and consider as single 576kbps message based channel. Used for maintenance, control, monitoring, administration, alarms as well as communication need between line terminating entities.

Synchronization messaging (S1) :
It is used for transporting synchronization status messages and defined for STS-1 of the STS-N signal. Bits 5 to 8 are used for this purpose.

STS-1 REI(M0) :
This octet sends no. of errors detected by B octets back to the transmitter. This helps in knowing line status as well as receiver status.

STS-N REI (M1) :
The function is same as listed in M0 above.

OrderWire (E2) :
The function is same as listed above for E1.

Payload Overhead

The first column in synchronous payload envelope (SPE) is referred as Payload overhead (POH). It consists of Path Trace (J1), Path BIP-8(B3), STS Path Signal Label (C2), Path Status (G1), Path user channel (F2), Multi-frame indicator (H4), growth octets(Z3,Z4) and N1 fields.

Path Trace (J1):
It helps two ends to verify the connection status (live or not) and check whether it is connected with right terminations. It is used to transmit STS path Access point identifier repetitively, Hence path receiving terminal can verify its continued connection with intended transmitter. 64 byte frame is used for the purpose.

Path BIP-8(B3):
It is used by the receiver for BER estimation. It is calculated over all the bits of previous STS SPE before the scrambling process.

STS Path Signal Label (C2):
This indicates type of traffic carried in the payload part of the SDH frame.

Path Status (G1):
It is used to convey path terminating status/performance back to the transmitter (Originating STS PTE). PTE stands for Path Terminating Equipment.

Path user channel (F2):
It is used for user communication similar to F1 octet in transport overhead.

Multi-frame indicator (H4):
It provides generalized multi-frame indicator for the payloads. The first purpose of this indicator is for VT structured payload. The second purpose is for support of virtual concatenation of STS-1 SPEs.

Growth octets (Z3,Z4):
Reserved for future use.

N1 fields:
This octet is used to allocate support for tandem connection maintenance and tandem connection data link.

Frame structure of wireless standards, technologies