Look at Figure 36.12 to get a picture of the ISDN frame. As you can see, this frame is similar
to the HDLC frame that you studied earlier (Figure 36.11). ISDN uses LAP (Link Access Procedure)
on the D channel for layer 2 functions. Unlike the HDLC frame, the ISDN frame is
bounded by Flag fields.
After the Flag field, again going from right to 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 services 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 bytes, then one more field follows, ending with another EA bit.
TEI This is the terminal end point identifier, the layer 2 address used to identify individual
devices connecting to an ISDN network.