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ISDN Addressing
ISDN PRI and BRI circuits have two numbered addresses associated with them: the
ISDN address (or service profile identifier) and a regional-compliant NPA subscriber
address. In most cases, with BRI circuits, the SPID and the NPA subscriber address
are the same, except for a two- or four-digit subaddress, which is at the end of the
ISDN address. This subaddress is used by the ISDN device and the LD for channel
identification. To establish a voice/data call with an ISDN device, the NPA subscriber
number is used. Depending on the BRI provisioning, the ISDN circuit might have
one or two (one for each B channel) ISDN/NPA subscriber addresses. The subscriber
address can be used to access only one of the B channels or both using a "rollover"
configuration. In ISDN BRI configurations with two NPA/SPID assignments, each B
channel is assigned its own primary NPA and SPID. Like the single NPA/SPID
configuration, the subscriber address can be used to access only one of the B
channels or both. This same approach is followed with ISDN PRI circuits, using a
slightly different NPA/ISDN address format. Figure 5.14 illustrates the NPA/ISDN
addressing schemes.
Figure 5.14. BRI/PRI address formats.
PRIs, which are commonly used with PBXs or digital modem banks, are assigned a
single NPA address/ISDN number. This number is used to identify the ISDN switch
device (for example, a PBX). The ISDN device is the root subscriber ID for the ISDN