ISDN is available in many configurations, or line options. In this section, you will learn about
two of the most common: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). These
flavors of ISDN vary according to the type and number of channels that carry data. Each option
has two or more DS0s, or
B (bearer) channels,
and a
D (data) channel
. ISDN is characterized
by the presence of a D channel, which carries control and signaling information, freeing up the
B channels exclusively for voice and data transport.
Each DS0 is capable of carrying 64,000 bits per second of either voice or data. Telephone
companies (telcos) can provide ISDN on their current infrastructure with little additional work.
Table 26.1 shows the relationship between the DS level, speed, designations, and number of DS0s
per circuit. Only the DS1 level is associated with ISDN, which is the transport that a PRI circuit uses.
TABLE 2 6 . 1
North American Digital Hierarchy
Digital Signal Level Speed Designation Channel(s)
DS0 64K None 1
DS1 1.544Mbps T-1 24
DS2 6.312Mbps T-2 96
DS3 44.736Mbps T-3 672
DS4 274.176Mbps T-4 4,032
Different standards, called Synchronous Optical Network (SONET) and Synchronous
Digital Hierarchy (SDH), were developed for Fiber Optics Transmission
Systems (FOTS). These standards are not covered in this book.
Another ISDN element is the
service profile identifier (SPID)
. A SPID identifies the characteristics
of your ISDN line. SPIDs might or might not be needed, depending on the type of switch
your service provider uses. ISDN National-1 and DMS-100 switches require a SPID for each
B channel, whereas a SPID is optional with an AT&T 5ESS switch type. Please consult your
ISDN provider if you are not sure whether you need a SPID. The format of a SPID is usually the
10-digit phone number, plus a prefix and possibly a suffix. For example, say that your telephone
number is 949-555-1234. Now add a prefix of 01 and a suffix of 0100. This gives you a SPID
of 0194955512340100.
SPIDs are only used in the U.S.
To place an ISDN call, you will also need a
directory number
, or DN. A DN is the actual
number you would call to reach that B channel. In the example from the previous paragraph,
the DN would be 9495551234 or 5551231. Knowing the SPID, switch type, and DN will speed
up the configuration of your router. Your service provider should provide you with this information.
Other than the directory number, the rest might be automatically detected.