Technical data

102 Theoretical and general applications www.westermo.com
SAPI
The Service Access Point Identifier (SAPI) is a 6-bits field that allows the specification
of up to 64 different service functions that layer 2 supplies to layer 3.
TEI
Terminal Endpoint Identifiers (TEI) is a unique ID that is allocated to each TA/TE on the ISDN
S/T bus. The identifier can be allocated dynamically when the device is activated or statically dur-
ing installation.
The figure above gives a view of
usage of the SAPI field, where
SAPI = 0 is used for switch con-
trol and SAPI = 16 is used for
package routing when X.31,
X.25 over D-channel is used.
SAPI value Related layer 3 or management entity
0 Call control procedures
1–11 Reserved for future standardization
12 Teleaction communication
13–15 Reserved for future standardization
16 Packet communication conforming to X.25 level 3 procedures
17–31 Reserved for future standardization
63 Layer 2 management procedures
All others Not available for Q.921 procedures
TEI Value User Type
0–63 Non-automatic TEI assignment user equipment
64–126 Automatic TEI assignment user equipment
127 Broadcast to all devices
TE TEET PH
Package data via
D-channel
Package data via
B-channel
SAPI-16
Switch control
SAPI-0
B A C K