Services Router T1 interfaces use two types of framing: superframe (D4) and extended superframe (ESF). E1 interfaces use G.704 framing or G.704 with no CRC4 framing, or can be in unframed mode.
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] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx] [xxxxxxxxx]
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.
ESF extends the D4 superframe from 12 frames to 24 frames. By expanding the size of the superframe, ESF increases the number of bits in the superframe framing pattern from 12 to 24. The extra bits are used for frame synchronization, error detection, and maintenance communications through the facilities data link (FDL).
The ESF pattern for synchronization bits is 001011. Only the framing bits from frames 4, 8, 12, 16, 20, and 24 in the superframe sequence are used to create the synchronization pattern.
The framing bits from frames 2, 6, 10, 14, 18, and 22 are used to pass a CRC code for each superframe block. The CRC code verifies the integrity of the received superframe and detects bit errors with a CRC6 algorithm.
The framing bits for frames 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, and 23 are used for the data link channel. These 12 bits enable the operators at the network control center to query the remote equipment for information about the performance of the link.