A channelbank using the ESF-B8ZS frame format does not require a "D" channel. In this case, line status (on/off-hook, & ringing) is signaled using the least significant bits each frame interval (these are known as the ABCD bits). actual digit transmission over this type of circuit is via tone & modem.
Jim at gremlin-systems dot com
In channel bank
(Thu 21 of Oct, 2010 (21:20))crypticlabs
Can someone please explain to me why the connection between a channel bank and an ip-pbx server can use all 24 channels? Doesn't it need a d channel for signaling like the connection between an ip-pbx server and the phone company (over a PRI line)?