The ISDN-PRI interface uses out-of-band signaling as opposed to robbed-bit, in-band signaling to transmit control messages between two endpoints. User information channels carry digitized voice and digital data and are known as bearer channels (B-Channels). B-Channels are assigned to DS1 ISDN trunks or PRI endpoints. Call-control signaling for the B-Channels is combined and carried over the separate ISDN-PRI Signaling Link Port D-Channel.
Two types of DS1 interfaces exist:
24 DS0 channels on a 1.544-Mbps link
31 DS0 channels + 1 framing channel on a 2.048-Mbps link
On 24-channel interfaces, the B-Channels may use any of the first 23 ports. The signaling link is assigned to the 24th port.
On 32-channel interfaces, the DS1 ISDN Trunks (B-Channels) may use any of the ports 1 to 15 and 17-31. The signaling link is assigned to the 16th port. The 32nd channel (port 0) is used for framing.
In NFAS configurations, the 16th or 24th ports on some of the DS1 circuit packs may be used for B-Channels. Refer to ISDN-SGR (ISDN-PRI Signaling Group) for further details.
A problem with the ISDN-PLK will affect every associated B-Channel since no call control information can be conveyed to the far-end switch or terminal adapter. Stable calls may remain operational, but no new calls can be placed. In turn, the ISDN-PLK depends on the DS1 Interface Media Module that provides the link to the processor.
