7.1 X.25
7.1.7 X.25 virtual circuits
Virtual circuits (VCs) are used interchangeably with the terms virtual circuit number (VCN), logical channel number (LCN), and virtual channel identifier (VCI). A VC can be a permanent virtual circuit (PVC) or, more commonly, a switched virtual circuit (SVC). An SVC exists only for the duration of the session.

Three phases are associated with SVCs:

  • Call setup
  • Information transfer
  • Call clear

A PVC is similar to a leased line. Both the network provider and the attached X.25 subscriber must provision the VC. PVCs use no call setup or call clear that is apparent to the subscriber. Any provisioned PVCs are always present, even when no data traffic is being transferred.

The X.25 protocol offers simultaneous service to many hosts (for example, a multiplex connection service). An X.25 network can support any legal configuration of SVCs and PVCs over the same physical circuit attached to the X.25 interface. However, configuring of X.25 assumed service for time-sharing and terminal-to-host applications, not contemporary computer-to-computer applications. Cisco routers provide numbering for up to 4095 VCs per X.25 interface.

Throughput for encapsulating a specific protocol can be improved by using multiple SVCs. Multiple SVCs provide a larger effective window size, especially for protocols that offer their own higher-layer resequencing. SVCs may be combined to improve throughput for a particular protocol. This combination of SVCs does not benefit traditional X.25 applications, such as those available from a time-sharing host.

Single Protocol Virtual Circuits

The traditional encapsulation method for the Cisco router enables different protocols to transport their datagrams through an X.25 cloud because the router uses separate VCs. Each protocol is specified in an individual x25 map command that references the X.121 address used to reach the destination.

Multiprotocol Virtual Circuits

In Cisco IOS® Release 10.2 and later releases, a single VC to a host can carry traffic from multiple protocols. One X.25 map statement contains several protocol addresses, mapped to a single X.121 address associated with the destination host.

This capability uses the method described in RFC 1356. Each of the supported protocols can map to a destination host. Because routing multiple protocols over a VC generates higher traffic loads, combining SVCs, as described earlier in this chapter, may improve throughput.

X.25 Encapsulation

Two methods are available to encapsulate traffic in X.25: The long-available Cisco encapsulation method and the Internet Engineering Task Force’s (IETF's) standard method (defined in RFC 1356). The latter allows hosts to exchange several protocols over a single VC. The Cisco encapsulation method is the default (for backward compatibility), unless the interface configuration command specifies IETF.