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Telco and ISP Dial Scenarios and Configurations
PPP Calls over X.25 Networks
DNC-331
Cisco IOS Dial Services Configuration Guide: Network Services
Overview
Many cities throughout the world have large installed bases of PCs interfacing with older modems,
PADs, and X.25 networks. These remote PCs or terminals dial in to PADs and make X.25 PAD calls or
terminal connections to mainframe computers or other devices, which run the X.25 protocol.
Unfortunately, the user interface is only a regular text-based screen in character mode (as opposed to
packet mode). Therefore, many ISPs and telcos that have large investments in X.25 networks are
upgrading their outdated equipment and creating separate networks for PPP connections. Because this
upgrade process takes substantial time and money to complete, using a Cisco router to allow PPP
connections over an X.25 network is a good interim solution for a dead-end dial case.
Remote PC Browsing Network Topology
Figure 57 shows a remote PC browsing the Internet through an X.25 PAD call and a Cisco 4500 router.
This X.25 network is owned by an ISP or telco that is heavily invested in X.25 equipment, currently
upgrading its outdated equipment, and creating separate networks for PPP connections. In this topology,
the Cisco 4500 router performs protocol translation between the protocols X.25 and PPP. The router is
configured to accept an incoming X.25 PAD call, run and unpack PPP packets over the call, and enable
the remote PC to function as if it were on the IP network.
Figure 57 Remote PC Browsing the Internet Through an X.25 PAD Call and a Cisco 4500 Router
For more information about configuring protocol translation, see the chapter “Configuring Protocol
Translation and Virtual Asynchronous Devices” in the Cisco IOS Dial Services Configuration Guide:
Terminal Services publication.
Cisco 4500
installed at
service provider
central site
PC running
Windows 95
and browsing
the Internet
Berlins PAD
Warsaw PAD
Modem
Modem Modem
Modems
X.25
X.25
X.25
X.25
Service provider
European X.25
network
Milan PAD
Modems
IP network
Eastern United
States
S6551