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Primary Rate Interface (PRI) and T1 Feature
Description
Framing Format
To identify the DS0 channels, the DS1 signal is segmented into blocks of 193
bits called frames. A frame consists of 24 eight-bit words (one for
each channel) plus a framing bit at the beginning of each frame (24 words x
8 bits = 192 bits). Thus, a framing bit appears in every one hundred ninety-third
bit position of the 1.544-mbps DS1 signal.
Frames repeat at a rate of 8,000 per second. Each frame repeats DS0 channels
1 through 24 sequentially.
The following two methods of framing can be used by a 100D, 100R INA, or 100DCD
module, but the framing method chosen must match the framing at the far end:
- D4 Framing Format. The system is factory-set for D4 framing. A D4
frame consists of 24 eight-bit time slots and one framing bit. To perform
synchronization, the receiving equipment uses the framing information to identify
the start of each frame and to identify which frames contain signaling information.
The framing information repeats once every 12 frames; these 12 frames form
the D4 superframe.
- ESF Framing Format. The extended superframe (ESF) format extends
the 12-frame D4 superframe to a 24-frame superframe. The 24 framing bits include
a cyclic redundancy check (CRC) for the entire ESF and a facility data link
for maintenance. The ESF can detect more errors than D4 framing can. Use this
format for tandem trunks.
