Selecting Cisco Products
47
Integrated Services Digital Network (ISDN) Connections
Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) is
two B (Bearer) channels of 64k each and one D (Data) channel of 16k for sig-
naling and clocking.
ISDN BRI routers come with either a U interface or something known as
an S/T interface. The difference between the two is that the U interface is
already a two-wire ISDN convention that can plug right into the ISDN local
loop. Conversely, the S/T interface is a four-wire interface and needs a Net-
work Termination type 1 (NT 1) to convert from a four-wire to the two-wire
ISDN specification.
The U interface has a built-in NT 1 device. If your service provider uses an
NT 1 device, then you need to buy a router that has an S/T interface. Most
Cisco router BRI interfaces are marked with a U or an S/T. When in doubt,
ask Cisco or the salesperson you bought it from.
Primary Rate Interface (PRI) provides T-1 speeds (1.544Mbps) in the U.S. and
E-1 speeds (2.048) in Europe, but I'm not going to tell you all about it in this
book because it's not relevant for the CCNA course. See the book CCNP: Rout-
ing Study Guide (Sybex, 2001) for more information on PRI.
The ISDN BRI interface uses an RJ-45, Category 5, straight-through
cable. It is important to avoid plugging a console cable or other LAN cable
into a BRI interface on a router, because it will probably ruin the interface.
Cisco says it will ruin it, but I have students do it every week and haven't lost
one yet. (I probably shouldn't have said that--now I probably will lose one
next week.)
Selecting Cisco Products
T
o select the correct Cisco products for your network, start by gath-
ering information about where devices need to operate in the internetwork-
ing hierarchy, then consider issues like ease of installation, port-capacity
requirements, and other features.
If you have remote offices or other WAN needs, you need to first find out
what type of service is available. It won't do you any good to design a large
Frame Relay network only to discover that Frame Relay is only supported in
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