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136 Chapter 6: Using ISDN and DDR Technologies
Q.921 institutes an addressing scheme similar to many other networking technologies. Just as
in LAN implementations, ISDN Layer 2 addressing is meant to provide physical addressing on
the network. Because multiple logical devices can exist in a single physical device, it is
necessary to correctly identify the source and/or destination process or logical entity when
transmitting or receiving data. In communication with the ISDN switch, an identifier must be
issued by the switch. This is known as a Terminal Endpoint Identifier (TEI).
The telco has the option of creating a specific profile for your implementation. Should this be
the case, the telco will assign a SPID for each of your bearer channels. The use of SPIDs is
optional.
Terminal Endpoint Identifier (TEIs)
A terminal endpoint can be any ISDN-capable device attached to an ISDN network. The TEI is
a number between 0 and 127, where 0­63 are used for static TEI assignment, 64­126 are used
for dynamic assignment, and 127 is used for group assignments. (0 is used only for PRI and is
discussed later.) The TEI provides the physical identifier, and the service access point identifier
(SAPI) carries the logical identifier.
The process of assigning TEIs differs slightly between North America and Europe. In North
America, Layer 1 and Layer 2 are activated at all times. In Europe, the activation does not occur
until the call setup is sent (known as "first call"). This delay conserves switch resources. In
Germany or Italy, as well as in other parts of the world, the procedure for TEI assignment can
change according to local practices.
In other countries, another key piece of information to obtain is the bus type. Supported types
are point-to-point or point-to-multipoint connection styles. In Europe, if you are not sure,
specify a point-to-multipoint connection, which will enable dynamic TEI addressing. This is
important if BRI connections are necessary because Cisco does not support BRI using TEI 0,
because it is reserved for PRI TEI address 0. If you see a TEI of 0 on a BRI, it means that a
dynamic assignment has not yet occurred, and the BRI may not be talking to the switch. In the
U.S., a BRI data line is implemented only in a point-to-point configuration.
Example 6-1 shows a typical ISDN Layer 2 negotiation.
Example 6-1
debug isdn q921 Output
RouterA#debug isdn q921
BRI0: TX -> IDREQ ri = 65279 ai = 127
BRI0: RX <- UI sapi = 0 tei = 127 i = 0x0801FF0504038090A218018896250101
BRI0: TX -> IDREQ ri = 61168 ai = 127
BRI0: RX <- IDASSN ri = 61168 ai =64
BRI0: TX -> SABMEp sapi = 0 tei = 64
BRI0: RX <- UAf sapi = 0 tei = 64
BRI0: TX -> INFOc sapi = 0 tei = 64 ns = 0 nr = 0 i = x08017F5A080280D1
BRI0: RX <- RRr sapi = 0 tei = 64 nr = 1
BRI0: RX <- INFOc sapi = 0 tei = 64 ns = 0 nr = 1 i = x08007B963902EF01
BRI0: TX -> RRr sapi = 0 tei = 64 nr = 1