Telco and ISP Dial Scenarios and Configurations
PPP Calls over X.25 Networks
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
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 This X.25 network is owned by an ISP or telco that is heavily invested in X.25 equipment, currently upgrading its outdated equipment, and creating separate networks for PPP connections.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
PC running Windows 95 
and browsing
the Internet
Modem
| Berlins PAD |
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Modem | Modem |
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| X.25 |
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| Service provider | X.25 | IP network |
| European X.25 | Eastern United | |
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| ||
| network |
| States |
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| Cisco 4500 | |
X.25 | X.25 | installed at | |
service provider | |||
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central site
S6551
Warsaw PAD | Milan PAD |
Modems | Modems |
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 IOS Dial Services Configuration Guide: Network Services