CAS Signaling in PDH
Channel Associated Signaling distribution across multiframe.
The Signaling Challenge
A key feature of the E1 frame is its use of a dedicated timeslot, TS16, for signaling. Signaling refers to the control information used to manage connections, such as dialing digits, busy signals, or ringing notifications. However, a single 8-bit timeslot per frame is not enough to convey signaling for all 30 user channels. This presents a capacity problem that the E1 system solves using the Multiframe structure.
The method used in E1 is called . In this approach, signaling information for a user channel travels in a path that is logically tied to that user's channel.
Dissecting TS16 within the Multiframe
The MF16 Multiframe structure provides the solution by varying the function of TS16 from frame to frame. This allows the 8 bits of TS16 to be shared over time among all 30 user channels.
- TS16 in Frame 0: Multiframe Alignment
The TS16 of the very first frame in the sequence (Frame 0) does not carry user signaling. Instead, it carries the Multiframe Alignment Word (MFAW), typically the bit pattern `0000`. This special word acts as an anchor, allowing the receiving equipment to identify the beginning of the 16-frame cycle and correctly interpret the signaling in the subsequent frames.
- TS16 in Frames 1-15: User Channel Signaling
The TS16 of the remaining 15 frames is used to carry the actual signaling data. The 8 bits of TS16 are divided into two 4-bit . Each nibble is assigned to carry signaling for a specific user channel according to a fixed schedule.
CAS Allocation Schedule
The allocation of TS16 follows a precise pattern, ensuring every user channel gets a turn to transmit its signaling state.
| Frame Number in Multiframe | TS16 Bits 1-4 Assigned to | TS16 Bits 5-8 Assigned to |
|---|---|---|
| Frame 1 | User Channel 1 | User Channel 16 |
| Frame 2 | User Channel 2 | User Channel 17 |
| ... | ... | ... |
| Frame 15 | User Channel 15 | User Channel 30 |
This schedule means that each user channel receives a 4-bit signaling capacity once every multiframe (every 2 milliseconds). While slow, this was sufficient for basic telephone call control. The four bits (often named A, B, C, and D) could represent up to 16 different states for the channel.
Contrast with North American T1: Robbed-Bit Signaling
The European E1 approach of dedicating a full timeslot (TS16) to CAS is fundamentally different from the method used in the North American T1 system. T1 networks do not have a dedicated signaling timeslot.
Instead, they employ Robbed-Bit Signaling. In this method, the system "steals" the least significant bit (LSB) from the user data in every 6th frame. This stolen bit is then used to carry signaling information.