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Appendix
Bearer Service
Circuit Switch Voice bearer (CSV) and Circuit Switch Data bearer (CSD) are broad
categories of bearer service options that the phone companies can provide. Bearer
services provide different types of guarantees about the reliability and synchronization of
the data. Currently, there are ten different bearer services for circuit-mode and three
services for packet mode.
Bearer services are defined in terms of a number of attributes, which include mode
(circuit or packet), structure (bit-stream or octet-stream), transfer rate (e.g., 64 Kbps),
transfer capability (basically, the content, for instance speech, 7Khz audio, video, or
unrestricted), and other attributes that specify protocols to use along with other things.
The attributes of the bearer service are encoded into a bearer code (BC) that is sent
every time a new connection is set up. In theory, this allows the switches to choose from
a variety of different switching paths techniques, depending on requirements. In practice,
the SPID determines what services are needed for switching, as this simplifies things for
the telephone companies. The bearer code is not completely ignored; however, there are
certain bearer services that are unavailable on your B-channels, based on how they are
configured. It is important to note that the bearer code is sent to the switch every time a
connection is established. However, the SPID is sent to the switch only when you
physically attach your equipment to your phone line.
At this time the switch gives your device a terminal equipment identifier (TEI), which is
used from then on to identify all connection requests from that piece of equipment. This
allows the switch to look at the TEI and bearer code, determine the SPID, and see if the
bearer code and the SPID match up.
Using ISDN Ordering Codes (IOCs)
An organization called ISDN Solutions Group, made up of ISDN service providers and
equipment vendors, formed to simplify the often complex task of ordering an ISDN line.
Their method, supported by most (but not all) service providers, uses ISDN ordering
codes to define the ISDN line configuration. Your ISDN TA has been tested to operate
with the ordering codes B1, Capability C, Capability R, I2, J2, J3, Capability S, and M5.
B1 (previously
Generic Data B)
• 1B service
• Data only on 1 B channel
• One directory number
Capability C (previously
Generic Data C)
• 1B service
• Alternate voice and data on one B channel
• One directory number