Channel Associated Signaling (CAS)

T1 CAS

Because T1 CAS steals bits from the voice channel to transfer signaling information, it is often called robbed bit signaling (RBS).

The 24 channels of the digital T1 circuit carry only voice data for the first five frames that they send. On the sixth frame (marked with an S), the eighth bit (also called the least significant bit) is stolen for the voice devices to transmit signaling information. This process occurs for every sixth frame after this (12th, 18th, 24th, and so on). The stolen bit relays the signaling information for each respective DS0 channel. For example, the bits stolen from the third DS0 channel relay the signaling information only for that channel.

Notice that at the end of each frame sent for the 24 DS0 channels is an F, signifyinng the T1 framing bit. When a T1 digital line sends voice data, it does so by sending all 24 of the smaller DS0 frames in one big T1 frame. So each T1 frame is 193 bits in length. Here’s the math:

Each DS0 frame = 8 bits T1 sends 24 DS0 frame at once (8 bits * 24 frames) = 192 bits Each T1 frame has a framing bit (1 bit + 192 bit) = 193 bits Digital T1 lines send 8000 of these 193-bit frames every second (because Dr. Nyquist’s voice model requires 8000 samples to be sent each second to accurately reconstruct voice), THis is why T1 lines runs at 1.544Mbps: (193 bits per frame * 8000 frames per second) = 1,544,000 bps or 1.544 Mbps

Super Frame (SF) sends groups of 12 T1 frames at a time. When using SF, all 12 of the T1 framing bits are used to keep the T1 equipment synchronized with the other side. This means that all 8000 T1 framing bits sent every second are dedicated to synchronization.

The newer standard, Extended Super Frame (ESF), sends groups of 24 T1 frames at a time. The newer ESF standard uses the framing bit more intelligently than the older SF standard. Of the 8000 bits every second, ESF is able to use 2000 bits for synchronization, 2000 bits for error checking, and 4000 bits as supervisor channels, which is able to send control functions and perform error reporting.

Note: All modern T1 service providers use ESF.

E1 CAS

E1 lines have 32 channels, which break down as follows:

• E1 DS0 1: Used for E1 framing information
• E1 DS0 2-16: Dedicated use for voice (no signaling)
• E1 DS0 17: Used for voice signaling information for channels 2-16 and 18-32
• E1 DS0 18-32: Dedicated use for voice (no signaling)

Note: It might seem odd to have channel associated sighnaling that is sent in a separate channel (channel 17) in E1 deployments, but that is how E1 CAS operates. It is still considered CAS because the signaling sent in time solt 17 uses the same system of ABCD bits as T1 CAS.