Describe how different WAN technologies can be used to
provide remote access to a network, including asynchronous
dial-in, Frame Relay, ISDN, cable modem, and DSL.
Explain the operation of remote network access control
methods.
Identify PPP components, and explain the use of PPP as an
access and encapsulation method.
Configure an ISDN solution for remote access.
Plan a Cisco ISDN solution for remote access or primary
link backup.
Troubleshoot nonfunctional remote access systems.
Integrated Services Digital Network (ISDN)
has gained quite
a following over the past few years. It offers a switched highspeed
data connection that you can also use to support voice,
video, or fax calls, making it an excellent choice for small office/home office (SOHO) users.
However, digital subscriber line (DSL) will probably replace ISDN completely within the
next few years because DSL is cheaper and faster—which means it must be better, right?
Maybe. Just like ISDN, DSL can also provide data, voice, and fax services to end users.
Cable modems have also been around for a few years. They provide a large amount of bandwidth
for a neighborhood to access the Internet, but cable modems are really just composed
of a large Thinnet network in which all your neighbors share the same bandwidth. Thinnet
is the type of wiring used for 10Base2 Ethernet networks, which was popular before the
10BaseT standard. It runs over a thin coaxial cable similar to RG-6 wiring used by cable
providers, hence the term
Thinnet network
.
Now, you might be thinking, “Hey, I thought this was an ISDN chapter; what’s with DSL
taking over the discussion?” It is an ISDN chapter, and you do need to know about the topic.
ISDN won’t be replaced overnight, and although DSL will probably replace it, it is possible that
it won’t. Remember about six or seven years ago when everyone was saying that ATM was
going to take over the world? Pretty glad we didn’t buy stock in that rumor. ATM is a contender,
but the expense and difficult technical administration make it unpopular compared to
Gigabit Ethernet for the LAN and to DSL for the WAN. In defense of ISDN, it does have a few
benefits over DSL and cable modems that we will describe in this chapter.
ISDN is still a good choice for WAN services because of its high speed (Cisco calls ISDN high
speed). It can run anywhere from 56K to T-1 speeds (1.544Mbps). 128Kbps is the most common,
though. Although 128Kbps is not high speed to most people, compared to a 33Kbps dialup
analog modem, it is.
Outside of the U.S., the maximum speed of ISDN is 2.048 Mbps (E-1 standard).
Unlike a modem (which is analog), ISDN is digital from end to end. Analog modems translate
from digital on the computer, to analog between modems, and then back to digital on the
remote end. ISDN is more efficient and faster, and it also has a faster setup connection speed
than an analog modem.
In this chapter, you will learn about ISDN, beginning with the Physical layer and working
up. Topics covered in this chapter include ISDN device types, layer 2 (Q.921) and layer 3
(Q.931) specifications, ISDN reference points (R, S, T, U, and V), configuring dial backup and
bandwidth on Demand configurations, and commonly used ISDN commands.