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X.25 343
Figure 10-10 shows the routers doing local X.25 switching, so the router is switching an SVC
call between two PADs.
Figure 10-10
Cisco Routers as X.25 Packet Switches
X.25 Over ISDN
It is possible to use X.25 over the Intergrated Services Digital Network (ISDN) (D Channel) to
provide 9.6K connectivity. Because the ISDN D channel is "always on" and available, you can
use it for X.25 purposes. This is known as AO/DI (Always On/Dynamic ISDN). Figure 10-11
shows how ISDN and X.25 work.
AO/DI offers ISDN telecommuting cost savings. Low-speed D channel services are typically
more cost-efficient than the time-based tariffs applied to the B channels, which usually carry
user data.
AO/DI is an on-demand service that is designed to make use of an existing Integrated Services
Digital Network (ISDN) signaling channel (D channel) to transport X.25 traffic. This provides
connectivity basically for free. Because the D channel is always up, why not use it? Depending
on the region, some phone companies provide continuous service to locally connected sites
working out of one central office. The X.25 D channel call is placed from the subscriber to the
packet data service provider.
Notice that the use of PPP allows protocols to be encapsulated within the X.25 logical circuit
carried by the D channel, not for authentication, for which PPP is also used. The bearer channels
(B channels) use the multilink protocol without the standard Q.922 and X.25 encapsulations
and invoke additional bandwidth as needed. You can also use the Bandwidth Allocation Control
Protocol (BACP) to negotiate bandwidth allocation as required.
x.121 address
x.121 address
x.121 address
Serial 1
Serial 1
Serial 0
Serial 0
X.25 switch
X.25 switch
Cisco router
Cisco router
Calls to and from the
PADS are switched by
the Cisco routers
PAD
PAD
87200333.book Page 343 Wednesday, August 22, 2001 2:53 PM