* Frame overhead - 16kbps
* D channel - 16kbps
*  2B channels at 64 kbps - 128 kbps
The transmission field consists of three fields.
1) Synchronization word : It is used for frame alignment and physical layer synchronization consists of 18 bits.
2) Data : B- and D-channel information is sent in  12 groups, each having 8 bit of first and second B-channel  and 2 bits from D-channel.
3) Maintenance field :  It  consists of CRC information, power status indication, physical channel maintenance . It consists of 6 bits.

         The interleaving scheme for B- and D-channel is like  B1, B2, and D. The same sequence follows.  This scheme is different from  that of S/T reference point. The NT1 will do conversion between the   two frame formats. Data is transmitted in a superframe consisting of 8 240-bit frames for a total  of  1920 bits (240 octets). The sync field of the first frame in the superframe is inverted (i.e. -3 -3 +3 +3 +3 -3 +3 -3 +3).