PDH Frame Structure

This page of tutorial covers PDH frame structure. It covers 2Mbps, 8 Mbps, 34 Mbps and 140 Mbps PDH frames in detail with all the fields.

Frame alignment is very important in PDH which is done by inserting FAS word at the start and at the end. These words help in synchronization and to hence to find the beginning of the frame.

Frame synchronization for 2Mbps is done using FAS word at the beginning of each odd frame and at the beginning of all the frames for rest of the rate hierarchies. Fixed binary pattern '0011011' is used as FAS.

2.048 Mbps frame

Multi frame in 2Mbps rate is composed of 16 frames and each one has 32 bytes. The first time slot is for the control and 16th channel is used for signalling. The frame period is 125 micro-sec then 1 byte is 8 bit/125micro-sec. This gives 64 kbps channel. The transmission rate for 32 such channel is about 2.048 Mbps. The other parameters for this rate of PDH frames are as follows:

Basic Binary rate -2048kbps +/- 50 ppm
Line coding -HDB3
Frame length-256 bits
Available bits/slot -8 bits
Multiplexing type - Octet Interleaving
Frame Rate-8000 frames/sec
FAS bits rate-28000bit/sec (without supervision bits)

2Mbps frame is the most commonly used. All the European networks support this type of frame. Most of the narrow band networks are built over this type of frame. The most common networks are POTS, Frame Relay, N-ISDN, GSM, ATM and leased lines.

PDH Applications

PDH networks provide circuits to the public as well as private networks which include POTS,GSM,ISDN,FRL,video,audio and data. 2Mbps frame format is used to build synchronization network.

PDH is also used in test and measurement for in-service and out of service measurements.


PDH versus SDH
SONET SDH tutorial
SDH Frame structure