Zhone Technologies 800, 900, 600 manual Introduction ISDN-PRI Card, Channel, Facility, Interface

Introduction ISDN-PRI Card

5.1.1.3B Channel

A B channel is a timeslot on any WAN link that is controlled by an ISDN D channel. B channel assignments to WAN links are used dynamically by the system as incoming and outgoing ISDN calls occur.

In the system, every timeslot is either a B channel or a DS0. Users can define which is which via the D channel configuration screen described later in this chapter.

5.1.1.4D Channel

A D channel carries signaling information for all B channels with which it is associated. Each D channel occupies an entire timeslot which is dedicated to ISDN signaling. The D channel can also be used to log into a remote system unit (see later in this chapter).

5.1.1.5Facility

A facility is another term for an individual T1 or E1 WAN link.

5.1.1.6Interface

The term “interface” is used interchangeably with facility when referring to an ISDN link.

5.1.1.7NFAS (Non-Facility Associated Signaling)

A basic ISDN PRI facility is a T1 link that consists of 23 B channels and 1 D channel (23B+D), or an E1 link that consists of 30 B channels and 1 D channel (30B+D). Note that one timeslot on any E1 link is reserved for maintenance use and is neither a B channel nor a D channel. The D channel provides signaling for all (23 or 30) of the B channels on the facility carrying the D channel.

However, many ISDN applications have relatively low call rates (i.e., the D channel is not very busy), but need more than 23 (or 30) B channels to carry user (bearer) traffic. In these cases, a D channel can be set up to perform signaling not only for the B channels on its own facility, but also for B channels on other facilities (i.e., other T1/E1 WAN links). When a D channel is so provisioned it is considered to be performing non-facility associated signaling (NFAS).

Some carriers’ implementations of NFAS allow one D channel to carry signaling for up to 20 facilities (i.e., 479 B channels and 1 D channel in a T1 environment). However, the system is limited to 8 WAN links. Thus, the system limit for NFAS is 191B+D in T1 environments (8 times 24 minus 1 D channel), and 239B+D in E1 environments (8 times 30 minus 1).

5-2 Server Cards