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ISDN Protocols and Design 567
All that is necessary on the multipoint subinterfaces on Yosemite to enable FRF.9 compression
on just the VC to Albuquerque is one interface subcommand on each of the two routers. The
frame-relay map ip 10.1.7.251 901 payload-compress frf9 stac interface subcommand on
Yosemite tells the IOS for IP packets header out DLCI 901 to enable payload compression
and use the FRF.9 style of configuration. A similar command is used on Albuquerque--if
compression is disabled on only one end of the VC, or if a different type of compression is
enabled, then no packets will flow across the link.
If the Cisco STAC payload compression was desired instead, the frf9 stac keywords would
need to be replaced by the packet-by-packet keyword.
ISDN Protocols and Design
The goal of this section is to summarize the details and to clarify complex features of ISDN and
related IOS functions. The CCNA exam focuses on Basic Rate Interface (BRI) functions, and
the CCNP and CCIE cover all of ISDN, including PRI.
ISDN Channels
Two types of ISDN interfaces are focused on in IOS documentation: Basic Rate Interface (BRI)
and Primary Rate Interface (PRI). Both BRI and PRI provide multiple digital bearer channels
over which temporary connections can be made and data can be sent. The result is digital dial
access to multiple sites concurrently. Table 8-24 summarizes the features of BRI and PRI.
Bearer channels (B channels) are used to transport data. B channels are called bearer channels
because they bear the burden of transporting the data. B channels operate at up to 64kbps,
although the speed might be lower depending on the service provider. The section "ISDN
Configuration," later in the chapter, discusses how to configure the correct speed for the bearer
channels. D channels are used for signaling.
Table 8-24
BRI and PRI Features
Type of Interface
Number of Bearer
Channels (B Channels)
Number of Signaling
Channels (D Channels)
BRI
2
1 (16kbps)
PRI (T/1)
23
1 (64kbps)
PRI (E/1)
30
1 (64kbps)
ch08.fm Page 567 Monday, March 20, 2000 5:17 PM