Layer 1 & Layer 2 Protocols • 7-7 •
The H.110 Basic Rate ISDN Board
negotiated between the TE and NT every time the TE powers up, restarts,
or otherwise needs a new TEI. The TEI assigned to a particular piece of
terminal equipment may be different each time assignment takes place.
Dynamically assigning TEI’s is the preferred method.
If a port is set as a TE, the board will automatically request a single TEI
upon power up or resetting unless the Layer 2 system option 1 is set to “1”
with the SS command. There will be a delay of a random interval of up to
15 seconds before this request is made. This delay is part of the Bellcore
specifications for an ISDN BRI terminal and the timer used is called
TWAIT. The purpose of this delay is to prevent too many terminals from
requesting TEI assignment at a time in cases such as the restoration after
a power outage. This delay can be eliminated by the application sending a
TEI assignment request command. Once this has been done, TWAIT is
no longer active. If needed, additional TEI’s up to seven can be requested
by the application. As each TEI assignment is made, a message informing
the application of the assignment is sent by the board.
If a port is set as an NT, it will grant TEI requests up to the limit of seven
TEI’s for the port. TEI values are granted sequentially beginning at 64 and
wrapping around at 126. Resetting the port or removing TEI’s does not
alter the sequence. Each time a TEI is assigned, a message is sent to the
application reporting the assignment. The application can request the
removal of a specific TEI or of all TEI’s if an error in TEI assignment is
suspected.
The command message to request a TEI assignment takes the form TAxx
where xx is the port number in hexadecimal notation. Thus, “TA0A” is a
request for a TEI assignment for the eleventh port (ports are numbered 00-
1F). TEI assignment requests can only be made for TE ports. If a TEI is
assigned, the board will send a message of the form TAxxtt where xx is the
port number and tt is the TEI assigned in hexadecimal. This message is
sent both by a TE port being granted a TEI and an NT port granting the
TEI. An example would be “TA0A40” which would indicate that a TEI of
64 has been assigned to or by the eleventh port.
A fixed TEI can be assigned to a port by using the fixed TEI command.
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