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Superframe (D4) Framing for T1

A D4 frame consists of 192 data bits: 24 8-bit channels and a single framing bit. The single framing bit is part of a 12-bit framing sequence. The 193rd bit in each T1 frame is set to a value, and every 12 consecutive frames are examined to determine the framing bit pattern for the 12-bit superframe.

The following sample 12-frame sequence shows the framing pattern for D4 framing:

[data bits][framing bit]
[xxxxxxxxx][1]
[xxxxxxxxx][0]
[xxxxxxxxx][0]
[xxxxxxxxx][0]
[xxxxxxxxx][1]
[xxxxxxxxx][1]
[xxxxxxxxx][0]
[xxxxxxxxx][1]
[xxxxxxxxx][1]
[xxxxxxxxx][1]
[xxxxxxxxx][0]
[xxxxxxxxx][0]

The 100011011100 12-bit pattern is repeated in each successive superframe. The receiving device detects these bits to synchronize with the incoming data stream and determine when the framing pattern begins and ends.

D4 framing requires the 8th bit of every byte (of every channel) within the frame to be set to 1, a process known as bit robbing. The bit-robbing requirement ensures that the 1s density requirements are met, regardless of the data contents of the frames, but it reduces the bandwidth on the T1 link by an eighth.

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