8-10
Configuring Demand Routing for Primary ISDN Modules
Overview of ISDN Connections
ISDN also supports the following B-channel Data Link Layer protocols:
■ Point-to-Point (PPP)
■ High-Level Data Link Control (HDLC)
■ Frame Relay
LAPD
LAPD establishes the ISDN connection between two endpoints. Exchanged
over the D channel, LAPD frames provide the addressing for the dial-up
connection, including the service access point identifier (SAPI) and the ter-
minal endpoint identifier (TEI). The SAPI identifies the ISDN service associ-
ated with the signaling frame, and the TEI identifies the TE on the subscriber’s
ISDN line. In addition, LAPD provides error checking and call control.
LAPD frames consist of six main fields. (See Figure 8-3).
Figure 8-3. LAPD Frame Format
Flag. The flag field is one octet and always has a value of 0x7E.
Address Field. The address field is two octets: In the first octet, the first six
bits define the SAPI. The seventh bit is the Command/Response bit (C/R),
which designates a command frame or a response frame. The LAPD frame is
a command frame:
■ when the LAPD frame is from the user and the C/R bit is set to one
■ when the frame is from the network and the C/R bit is set to zero,
Other values designate a response frame. The eighth bit is the first address
extension bit and is always set to zero.
LAPD frame structure
Flag
Control field Information FCS FlagAddress field
SAPI
8 7 6 5 4 3 2 1
TEI
C/R EA1
EA2
To Next Page
Loading...
Need help?
General
