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396 Chapter 11: Remote Access
Figure 11-5 shows several devices all accessing an ISDN channel from the central office. There
the PC is using the R interface to access an ISDN channel from the central office. It has a
terminal adapter or ISDN modem connected to its serial port on the PC.
The terminal adapter is connected to the NT1 device, which provides the access to the channel.
Some of the low-end routers do have NT1 built into them; the routers would have to be sold
with the U interface, which designates an internal NT1.
ISDN Basic Rate Interface (BRI)
ISDN uses time division multiplexing (TDM) to provide several logical channels on a single
physical wire. Under TDM, each logical channel is given exclusive use of the wire for a
duration of time. Think of three lanes of cars being squeezed into one lane. All the cars still get
to their destination, but they get there more slowly. The channels are swapped in and out of the
wire, which is sort of a weaving method. ISDN access interfaces comprise a D channel for
signaling and a number of B channels for user services.
·
The D channel operates at 16 Kbps and carries signaling information between the user and
the network using the LAPD protocol.
·
The B channel is 64 K to the central office and carries user services, including voice,
video, and data signals.
NOTE
ISDN B channels are guaranteed only 64 K between the local site and the central office.
Because some central offices use robbed bit signaling, ISDN is not guaranteed to be 64 K
between the central offices and might actually be 56 K.
ISDN Primary Rate Interface (PRI)
There are a number of different configurations of the PRI. In the United States and Japan, the
PRI consists of 23 to 64 K channels and one shared signaling channel.
In a PRI, the D channel operates at 64 Kbps, in contrast to the BRI, where it operates at 16 Kbps.
The North American PRI is based on the T1 carrier. It operates at a total bit rate of 1.544 Mbps,
of which 1.536 Mbps is user data. A 30B+D PRI is based on the E1 carrier. It operates at 2.048
Mbps, of which 1.984 Mbps is user data.
Because the PRI contains more channels than individual devices would need, it is typically used
for handling switching functions. For example, it might be used in a PBX or to connect a host
computer to remote terminals.
87200333.book Page 396 Wednesday, August 22, 2001 2:53 PM