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The options of Circuit Switch Voice Bearer (CSV) and Circuit Switch Data Bearer (CSD) are broad
categories of Bearer Services that the phone companies can provide. Different bearer services provide
different types of guarantees about the reliability and synchronization of the data. There are currently ten
different bearer services for circuit-mode, and three services for packet mode.

These 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. 64Kbps), transfer capability (basically, the
content, for instance speech, 7Khz audio, video, or unrestricted), and several other attributes that specify
protocols to use and other things.

The attributes of the bearer service are encoded into a Bearer Code, or BC, that is sent everytime a new
connection is being set up. In theory, this allows the switches to dynamically choose from a variety of
different switching paths techniques depending on requirements. In practice, the SPID is used to determine
what services are needed for switching, as this greatly simplifies things for the telephone companies. The
Bearer Code will not be completely ignored, however there are certain bearer services that will be
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 only sent to the
switch 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.

An organization called 

ISDN Solutions Group, made up of ISDN service providers and equipment

vendors, was formed to simplify the often complex task of ordering an ISDN line.  This 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 following ordering codes:  B1, Capability C, Capability R,
I2, J2, J3, Capability S, and M5, as described in the following sections.

This ISDN line ordering code supports:

•  1B Service,
•  Data only on 1 B channel, and
•  One directory number.

This ISDN line ordering code supports:

•  1B Service,
•  Alternate voice and data on one B channel, and
•  One directory number.

This ISDN ordering code supports:

•  2B Service,

•  Data only on each B channel, and

•  Two directory numbers.

Applications for Capability R include:

•  115.2K or 128K bps data only (no voice capability, and

•  Generic 115.2K/128K bps data transfer applications (including remote access and LAN/WAN
connectivity and telecommuting).