Archive for the ‘Circuits & Signalling’ Category

Signalling and Call set-up

There are three types of Signalling:

1) Supervisory: On-hook, off-hook, Ringing
2) Address: Communicates the digits that were dialed like DTMF (Dual tome multi frequency) and Pulse dialing
3)Informational: Dial-tone, ring-back tone, recorder tone.

Call set-up:

1.The calling phone goes off-hook, closing the circuit to the local CO switch.
2. The local CO switch detects that current is flowing over the closed circuit and sends a dial tone to the calling phone.
3. Address signals (DTMF or pulse) are sent as the calling party dials the called number.
4. The local CO switch collects the digits and makes its routing decision; in this example, it uses an SS7 lookup tolocate the destination CO switch.
5. Supervisory signaling indicates to the far-end trunk that a call is inbound.
6. The PBX determines which internal line the call should go to and causes the connected phone to ring.
7. The ringback tone is heard at the calling party end.
8. The called party goes off-hook, and a voice circuit is established end-to-end.The fact that all this happens with very high reliability billions of times every day is pretty impressive. It also providessome insight into how complex it is to duplicate these functions in a VoIP system. More on that later

Reference: Cisco Press Book


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