The T1 Frame
T1 frames are 193 bits and are sent at 8KHz. Each of the 24 channels (virtual DS0s) carries 8 bits of data plus a single framing bit ([24 * 8] + 1 is 193). A dozen frames make up a superframe. The framing bits line up in a particular pattern (100011011100). The recipient uses this known pattern to discern which 8 bits belong to which channel. This is known as frame synchronization.
Extended superframe also provides error detection and a separate data channel. It's twice the size of an ordinary superframe, 24 193-bit frames. The framing synchronization is provided by every fourth bit.
The following are the 24 framing bits:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 - c - s - c - s - c - s - c - s - c - s - c - s
s bits provide synchronization. c bits provide 6-bit CRC made up of the data in the superframe. Error checking, therefore, is continuous. – bits make up a 4Kbps auxiliary data channel used for monitoring and supervising data transmission. Check out Figure 4 to see an example of a frame.
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Figure 4 Twenty-four channels (across) of 8 data bits (up), to which is added a single one framing bit—192 data bits + 1 = a 193-bit frame. The first vertical row of 8 bits is sent first, followed by the second, and on down the line. After the last set of 8 bits is sent, a single framing bit follows, and then comes the next frame.
T1 consists of 24 channels or "time slots," each 64Kbps. (It's possible that only 56Kbps will be available if "clear channel" is not supported. Clear channel is any signal encoding that does not require bit stuffing, so all 8 bits per channel are available for data payload.) Each time slot/channel can be used for either voice or data.
Synchronization is required so that the receiving end can determine the beginning of each frame.
The frame synchronization sequence is odd so that the number won't be accidentally produced by a test tone.
When T1s first became available in the 1960s, they implemented a type of framing known as "D1," in which the frame bits were in a repetitive pattern of 10101010101010. This was confused by the telco's 1KHz test tone, which frequently generated this series of numbers. For this reason, the phone company adopted a 1004Hz test tone. D1 framing itself was eventually discarded in favor of D4 (superframe) and finally extend superframe (ESF). The complex frame sequence of ESF ensures that the sequence will not likely be reproduced accidentally.
The back channel can be used to see if telephones are on- or off-hook by reporting line voltages or other supervisory tasks.
For digitized voice over T1 trunks, line supervision is achieved through bit stealing, where on 6th and 12th frames a single bit is retasked to supervision.