User guide

August 2001 45
Understanding T1 Trunks
ESF Superframe
Extended superframes (ESF) consist of 24 frames. Unlike
D3/D4 frames, the F-bits are not all used for synchronization.
Only the F-bits for frames 3, 7, 11, 15, 19, and 23 carry
synchronization information. These follow the pattern: 001011.
For example, the F-bit for the fourth frame is a 0, and for the
eighth frame it is a 0, and for twelfth frame it is a 1, and so on
through the sequence, as shown in Figure 12.
The F-bit for frames 1, 5, 13, 17, and 21 carries error checking
information (CRC). For each superframe, the transmitting end
calculates a cyclic redundancy checksum for the data and places
the results in the following superframe. The remaining frames
carry network information.
As with D3/D4 superframes, ESF superframes rob the least
significant bit for each timeslot of every sixth frame for
signaling information. With 24 frames in a superframe, this
means that four bits per channel are used for signaling, as
shown in Figure 12. The first is bit A, the second bit B, the third
bit C, and the fourth is bit D. In most signaling protocols, C is
set to equal A, and D is set to equal B. However, in proprietary
protocols, these bits can be set to different values to indicate line
state.