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11-6 Internetwork Design Guide

Introduction

The AT&T 5ESS switch supports up to eight SPIDs per BRI. Because multiple SPIDs can be applied

to a single B channel, multiple services can be supported simultaneously. For example, the ﬁrst

B channel can be conﬁgured for data, and the second B channel can be conﬁgured for both voice

(using an ISDN telephone) and data.

DMS-100 and National ISDN-1 switches support only two SPIDs per BRI—one SPID for each

B channel. If both B channels will be used for data only, conﬁgure the router for both SPIDs (one

for each B channel). You cannot run data and voice over the same B channel simultaneously. The

absence or presence of a channel’s SPID in the router’s conﬁguration dictates whether the second

B channel can be used for data or voice.

Note There is no standard format for SPID numbers. As a result, SPID numbers vary depending on

the switch vendor and the carrier.

The isdn spid1 and isdn spid2 interface conﬁguration commands are used to assign SPID values for

a given BRI. These commands also allow the speciﬁcation of the local directory number (LDN),

which is a seven-digit number assigned by the service provider that is part of the incoming setup

message. The LDN is not necessary for establishing ISDN-based connections, but it must be

speciﬁed if you want to receive incoming calls on B channel 2. The LDN is required only when two

SPIDs are conﬁgured (for example, when connecting to a DMS or NI1 switch). Each SPID is

associated with an LDN. Conﬁguring the LDN causes incoming calls to B channel 2 to be answered

properly. If the LDN is not conﬁgured, incoming calls to B channel 2 will fail.

Signaling System 7

In the United States, Signaling System 7 allows central ofﬁce switches to communicate with other

central ofﬁce switches at 64 kbps. Some central ofﬁce switches in the United States have not been

upgraded to Signaling System 7, so they transfer data at 56 kbps. If your ISDN connection includes

a central ofﬁce switch that does not use Signaling System 7, conﬁgure the router conﬁgured with

dialer map commands that place the call at 56 kbps. You also need to apply the bandwidth interface

conﬁguration command to the ISDN interface to notify routing protocols that the line operates at

56 kbps.

Incoming Call Delivery

When originating calls are made at 56 kbps but delivered to the destination by the ISDN network at

64 kbps, the incoming data can be corrupted. However, on ISDN calls, if the router is informed that

the call is not ISDN end-to-end, it can set the line speed for the incoming call. This information is

delivered in the incoming ISDN setup message.

To set the speed for incoming calls recognized as not ISDN end-to-end, use the isdn not-end-to-end

interface conﬁguration command to set the line speed at which calls are answered.

Calling Line Identiﬁcation

Some central ofﬁce switches support calling line identiﬁcation (known as caller ID), which allows

the router to verify that an incoming call comes from an expected source. If your router connects to

a central ofﬁce switch that supports caller ID, you can conﬁgure the router to match the caller ID

delivered as part of the call setup against a conﬁgured value. If the values do not match, the router

rejects the call. For more information about this ISDN security feature, see the “Screening” section

later in this chapter.

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