Polycom HDX series Reference Manual page 242

System Commands
Fractional PRI T1 or E1. Channel selection should be handled by your PRI
network administrator.
PRI E1 Channel Information. The PRI Status screen (for E1) shows 30
channels. However, E1 trunk lines have 32 timeslots, numbered 0 - 31.
Timeslot 0 is used for framing, and timeslot 16 is used for call signaling (the D
channel). The remaining 30 timeslots are used as bearer (data) channels. In call
signaling between our equipment and the switch, these channels are
numbered 1-15, 17-31. But the PRI Status screen numbers these channels
contiguously in the range 1-30. Therefore, on the PRI Status screen, channels
1-15 control the status of timeslots 1-15, and channels 16-30 control the status
of timeslots 17-31.
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