Circuits and Signalling

T1 Cicruits:

Each T1 circuit consits of 24 channels. Each channel is capable of holding one voice call of 64kbps. If there are not enough voice calls then the gaps are padded with null values. The 24 channels are grouped in a frame. It depends whether it is SF (i.e. frame of 12 channels) or ESF (i.e. frame of 24 channels). SF stands for Super Frame and ESF is Extended SF.

E1 Circuits:

E1 is like T1 but with 32 channels. Only 30 channels are used for voice while other 2 are left for Signalling and framing.

CAS- T1:

Channel Associated signalling is where there is no seperate channel for Signalling. It is also called ‘Robbed-bit’ Signalling. Using 1-bit per channel in every 6th frame gives two 12-bit Signalling strings (known as A and B) per SuperFrame and four 24-bit Signalling strings (known as A, B, C and D) for ESF.

CAS- E1:

No bit is ‘robbed’ for Signalling. Bit 0 (timeslot 1) is for framing and bit 16 (timeslot 17) is for Signalling information. Timeslots 2-16 and 18-32 carry voice data.


Common Channel Signalling provides complete out-of-band signalling. In ISDN, the function of D channel is based on CCS. Full 64kbps is available for voice per channel. Instead of generating bits like A,B,C and D; proctocol called Q.931 is used for out-of-band signalling in a seperate channel.

ISDN PRI T1 = 23B+D = (23×64)+(64)= 1.544Mbps
ISND PRI E1= 30B+D= (30×64)+(64)= 2.048Mbps
ISDN BRI= 2B+D= (2×64)+(64)=192Kbps

Reference: Cisco Press Book

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