companies to use permanently dialed ("nailed-up") ISDN connections instead of leased lines.
ISDN lines can provide access at 128kbps, using both B channels. Compression can increase
throughput, potentially getting 500kbps of throughput through the line. xDSL technology is an
emerging competitor to ISDN for dial-in ISPs.
for occasional connection. Backup is self-explanatory. Occasional connections would include
traffic for sites that do not use online applications or video conferencing, and cases in which
additional bandwidth between sites is desired. Most of the configuration needed for these
occasional connections is related to a topic called dial-on-demand routing (DDR), which is
covered in a later section of this chapter. Figure 8-25 shows some of the typical network
topologies when using ISDN.
when the traffic that needs to get to another site is sent by the user.
established between the same two routers.
to replace a Frame Relay access link or a failed VC.
network, replacing a failed VC or access link with a VC running over an ISDN connection
to the Frame Relay switch.
preferred choice in a dial environment. HDLC and PPP overhead per-data frame is identical;
however, PPP provides LQM, as well as CHAP and PAP authentication and Layer 3 address
assignment through several of the control protocols. Each of these features is particularly
important in a dial environment.
(CHAP) are used to authenticate (verify) that the endpoints on a dial connection are allowed to
connect. CHAP is the preferred method today because the identifying codes flowing over the
link are created using a Message Digest 5 (MD5) one-way hash, which is more secure than the
clear-text passwords sent by PAP.