PBX Networking: CBCSS

CBCSS is a PBX network feature that allows a single ISDN PRI services trunk group to carry calls to multiple network carrier services or facilities or carry calls using different interexchange carriers. CBCSS uses the same routing tables as those used by ARS/AAR. Without CBCSS, each trunk group must be dedicated to a specific carrier service or facility. Implementing the CBCSS feature allows a variety of services to use a single trunk group. The services are specified on a call-by-call basis. This optimizes trunking efficiency because traffic is distributed fully over the total number of available trunks, regardless of peak time period service requirements. Examples of services typically requiring dedicated trunk groups without using CBCSS are in-bound and out-bound WATS; direct long distance dialing; VPN; digital data services including ATM, frame relay, and IP; digital video services; presubscribed common carrier operator international 800 calls; and other user-defined services.

CBCSS allows services to share the same trunks to reduce total trunk requirements. The probability of denied feature and service access due to blocked trunk access is also reduced. Network engineering is simplified because trunk engineering analysis can use total traffic data instead of analysis by a per-service basis. By dynamically changing the mix of trunk circuits accessing different services and facilities, the system can function more efficiently.

CBCSS customer programming tools allow administrators to dynamically assign and reassign individual trunk members in a trunk group access to different services and facilities based on pre-set schedules (time of day, day of week), real-time traffic loads, or on demand.

The NFAS feature allows an ISDN PRI DS1 interface D-channel (signaling channel) to transmit signaling information for B-channels on ISDN PRI DS1 facilities other than the one containing the D-channel. A single D-channel can carry signaling information for numerous B-channels on different DS1 carrier facilities, thus providing a more economical interface between the PBX system and the ISDN network. This means that a customer can configure a single D-channel to support more than the standard 23 B-channels available on a facility-associated signaling ISDN PRI DS1 carrier circuit. A single D-channel can therefore provide signaling for 50, 100, or more B-channels based on the software programming limitations of a PBX system supporting the NFAS feature.

PBX systems implementing NFAS also can support D-channel backup. If a D-channel signaling link fails, a backup D-channel transports the signaling. The feature requires that one D-channel be administered as the primary D-channel and a second be administered as the secondary D-channel. When a transition from one D-channel to the other occurs, all stable calls (calls already answered) are preserved. Some messages may be lost, resulting in a loss of call-related information, but the calls themselves will be maintained.