It is preferred to its predecessor, superframe, because it includes a cyclic redundancy check (CRC) and 4000 bit/s channel capacity for a data link channel (used to pass out-of-band data between equipment.) It requires less frequent synchronization than the earlier superframe format, and provides on-line, real-time monitoring of circuit capability and operating condition.
- All odd-numbered frames (1, 3, ..., 23) are used for the data link (totalling 4000 bits per second),
- Frames 2, 6, 10, 14, 18, and 22 are used to pass the CRC total of the previous extended superframe (all 4632 bits, framing and data), and
- Frames 4, 8, 12, 16, 20, and 24 are used to send the fixed framing pattern, 001011.
The CRC is computed using the polynomial x6+x+1 over all 24×193 = 4632 bits (framing and data) of the previous superframe, but with its framing bits forced to 1 for the purpose of CRC computation. The purpose of this small CRC is not to take any immediate action, but to keep statistics on the performance of the link.
Like the predecessor superframe, every sixth frame's least-significant data bit can be used for robbed-bit signaling of call supervision state. However, there are four such bits (ABCD) per channel per extended superframe, rather than the two bits (AB) provided per superframe. (Specifically, the robbed bits follow framing bits 6, 12, 18 and 24.)
Unlike the superframe, it is possible to avoid robbed-bit signalling and send call supervision over the data link instead.
- Yoon, Hee Byung (June 1991). The Error Performance Analysis Over Cyclic Redundancy Check Codes (PDF) (M.Sc. thesis). Naval Postgraduate School. pp. 8–10.
- This article incorporates public domain material from the General Services Administration document: "Federal Standard 1037C".
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