CAS in the E1 Frame
In an E1 system that uses CAS, TS16 is assigned to signaling. Signaling is control information for managing connections, such as dialing digits, busy signals, and ringing notifications. A single 8-bit timeslot per frame cannot carry signaling for all 30 user channels. The 16-frame CAS multiframe provides the required schedule.
The channel-associated 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.
TS16 in the MF16 CAS Multiframe
The MF16 CAS multiframe assigns a different signaling function to TS16 in each frame. The 8 bits of TS16 are therefore shared over time among all 30 user channels. This CAS multiframe in TS16 is separate from the CRC-4 multiframe carried in bit 1 of TS0.
- TS16 in Frame 0: Multiframe Alignment
The eight bits of TS16 in Frame 0 have the form
0000xyxx. The first four bits form the Multiframe Alignment Signal (MFAS),0000, and identify the beginning of the 16-frame CAS cycle. The x bits are spare and set to 1 when unused. The y bit carries the remote alarm indication, with 0 for normal operation and 1 for an alarm. The receiving equipment uses this position to interpret signaling in subsequent frames. - TS16 in Frames 1-15: User Channel Signaling
TS16 in the remaining 15 frames carries the signaling data. Its 8 bits are divided into two 4-bit . Each nibble carries signaling for a specific user channel according to a fixed schedule.
CAS Allocation Schedule
The TS16 allocation assigns each user channel a defined signaling position.
| 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 |
Each user channel receives the a, b, c and d signaling bits once per multiframe. Each signaling bit has a rate of 500 bit/s, so the four-bit group is updated every 2 milliseconds. The group has 16 possible combinations. G.704 recommends that channels 1-15 do not use 0000 and specifies unused b, c and d bits as 1, 0 and 1.
CAS in E1 and T1 Robbed-Bit Signaling
The E1 approach dedicates the full TS16 timeslot to CAS. Common North American T1 arrangements do not reserve a dedicated signaling timeslot.
They use robbed-bit signaling. In common SF and ESF arrangements, bit 8, the least significant bit (LSB) of a user timeslot, carries signaling in selected frames, usually every sixth frame. The signaling frames are 6 and 12 in a 12-frame multiframe, and 6, 12, 18 and 24 in a 24-frame multiframe. Bit 8 is unavailable to the user payload in those frames.
E1 (CAS) characteristic
Leaves the 64 kbit/s traffic timeslot intact; TS16 carries CAS and therefore consumes one E1 timeslot for signaling.T1 (robbed-bit) characteristic
Keeps all 24 DS0 timeslots available for payload, while the LSB used for signaling in selected frames reduces the payload precision in those frames.CAS ties each signaling field to its traffic channel and keeps the traffic timeslot at 64 kbit/s. The 2-millisecond update interval can delay call-state changes, TS16 remains reserved even when few calls are active, and the historical A/B/C/D signaling alphabet offers limited room for additional services.