Zhone Technologies 800 - Manual (Page 68)

4-2 IMACS System Release 5.1.6 Server Cards
Model No. Running Head Introduction ACS-PRI/BRI Card
4.1.1.3 B 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.
4.1.1.4 D 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).
4.1.1.5 Facility
A facility is another term for an individual T1 or E1 WAN link.
4.1.1.6 Interface
The term “interface” is used interchangeably with facility when referring to an ISDN link.
4.1.1.7 NFAS (Non-Facility Associated Signaling)
A basic ACS-PRI/BRI 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).