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Chapter 10
Wide Area Networking Protocols
Basic Rate Interface (BRI)
ISDN Basic Rate Interface (BRI) service, also known as 2B
+1D, provides
two B channels and one D channel. The BRI B-channel service operates
at 64Kbps and carries data, while the BRI D-channel service operates at
16Kbps and usually carries control and signaling information. The total
bandwidth for ISDN BRI is then 144K (64
+ 64 + 16 = 144).
The D-channel signaling protocol spans the OSI reference model's Physical,
Data Link, and Network layers. The D channel carries signaling information
to set up and control calls. The D channel can also be used for other func-
tions like an alarm system for a building, or anything that doesn't need much
bandwidth, since it is only a whopping 16K. D channels work with LAPD at
the Data Link layer for reliable connections.
When configuring ISDN BRI, you will need to obtain service profile iden-
tifiers (SPIDs), and you should have one SPID for each B channel. SPIDs can
be thought of as the telephone number of each B channel. The ISDN device
gives the SPID to the ISDN switch, which then allows the device to access the
network for BRI or PRI service. Without a SPID, many ISDN switches don't
allow an ISDN device to place a call on the network.
To set up a BRI call, four events must take place:
1.
The D channel between the router and the local ISDN switch comes up.
2.
The ISDN switch uses the SS7 signaling technique to set up a path to
a remote switch.
3.
The remote switch sets up the D-channel link to the remote router.
4.
The B channels are then connected end-to-end.
Primary Rate Interface (PRI)
In North America and Japan, the ISDN Primary Rate Interface (PRI, also
known as 23B+D1) service delivers 23 64Kbps B channels and one 64Kbps
D channel, for a total bit rate of up to 1.544Mbps.
In Europe, Australia, and other parts of the world, ISDN provides 30
64Kbps B channels and one 64Kbps D channel, for a total bit rate of up to
2.048Mbps.
ISDN with Cisco Routers
Accessing ISDN with a Cisco router means that you will need to purchase
either a router with a built-in NT1 (U reference point) or an ISDN modem
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