Hi everybody
Please have a look at the following link.
http://routingandswitching.blogspot.com/2004/10/t1-line-coding-framing-and-signaling.html
From the above link
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.
1) Suppose we are using ESF , and all the " F" " framing bits are only providing synchronization i.e no signaling information is being sent using " A , B, C, D" bits. ( A, B, C , D bits are borrowed from 6,12,18 and 24 frames to carry signaling information in ESF) .. Since signalling is not needed in our example so what happens to these bits ? will they be used to carry " user data" as no signaling information is being transmitted?
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From the link:
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.
Let say we configure T1 interface for ESF + CCS
We know that A,B,C,D bits in ESF carry signaling information. But above we are using CCS, so all the signaling information is transmitted using one channel out of 24 available channels. In such case what happen to these A, B, C, D bits? Will they be used to carry " User data " now ?
Thanks and have a great weekend.
