T1 - line coding, framing and signaling

T1 uses either AMI (Alternate Mark Inversion) or B8ZS (Binary 8 Zero Substitution) line coding. A CSU/DSU is the device that converts the input digital signal into T1 line code.

A T1 frame contains 24 8-bit time slots (also called channels) and a framing bit, which makes 193 bits in total, and transmit 8000 frames per second.

T1 framing is either D4 Super Frame (SF) or Extended Super Frame (ESF). The difference is the number of signaling bits. A D4 SF is the packaging of 12 frames, while an ESF has 24 frames and additional signaling bits means more control capabilities. Two mechanisms can be activated when using D4 SF or ESF: synchronization mechanism, which is always activated, and signaling mechanism, which is optional.

T1 can support voice, dedicated data or ISDN PRI:

On a dedicated data T1 circuit, the signaling is not needed and therefore is disabled. SF or ESF framing bits are only used for synchronization.

On a voice T1 circuit, however, signaling is required. SF/ESF framing bits are used for both synchronization and signaling. In-band signaling, CAS (channel associated signaling, also called robbed-bit signaling) is used because the voice quality will not be impacted by losing the least significant bit.

On a ISDN PRI circuit, a D channel is used for out-of-band signaling, which is also called CCS (common channel signaling).

CAS signaling can be either Ground/Loop or E&M Wink/Delay/Immediate.

When using the T1 interfaces several modes of operation are available. These modes are listed below:

1. Unframed (UNF): A stream of bits at 1544 Kbs. No channels are associated to any specific group of bits. None of the mechanism described above is used.
2. Superframe (SF): Data transferred using the SF format.
3. SF + CAS
4. ESF
5. ESF + CAS
6. ESF + FDL (Facility Data Link provides maintenance and supervisory control)
7. ESF + CAS/CRC/FDL (CRC provides error monitoring capability)
8. CCS: Can be used in each of the framed formats, by dedicating one channel (usually TS-24) for delivering the signaling messages, in a predetermined protocol.

More information can be found at:

Configuring Channelized E1 and T1

North American T1 Facilities

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