OSI Layer 2: Data-Link Layer Protocols and Applications
103
devices are known as NT devices. Type 1 (NT1) devices are treated as cus-
tomer premises equipment. Type 2 (NT2) devices perform Layer 2 and Layer
3 functions. The last type is a combination of the previous two types. It is
known as a Type 1/2 or NT1/2.
Frame Structure
Look at Figure 2.36 to get a picture of the ISDN frame. As you can see, this
frame is similar to the HDLC frame that you saw earlier. ISDN uses LAP on
the D channel for Layer 2 functions. Just like X.25, the ISDN frame is
bounded by Flag fields.
F I G U R E 2 . 3 6
ISDN frame format
Starting from the right and working to the left, we see the Address field.
The Address field contains several bits of key information:
SAPI
This field is the service access point identifier. It defines which ser-
vices are provided to Layer 3.
C/R
This field designates the frame as a command or a response.
EA
This is the last bit of the first byte of the Address field. This bit
defines the Address field as one or two bytes. If it is set to one byte, this
is the last field within the Address field. If it is set to two, then one more
field follows ending with another EA bit.
TEI
Terminal endpoint identifier. This is used to define a single terminal
or multiple terminals.
Copyright ©2000 SYBEX , Inc., Alameda, CA
www.sybex.com