
Configuring Line Services
1-16
Figure 1-10. D4 Frame
ESF
The D4 format does not allow testing of a digital line while the line is in use. To
allow such testing, you can use the extended superframe (ESF).
An ESF comprises 24 D4 frames (F
igure 1-11). As each D4 frame contains a
framing bit, an ESF has 24 framing bits that it uses for the following purposes:
• Synchronization (6 bits)
• Error checking (6-bit cyclic redundancy check)
• Diagnostic data channel (12 bits)
Timeslot 24Timeslot 2Timeslot 1
Framing Bit
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