8-11
Configuring Demand Routing for Primary ISDN Modules
Overview of ISDN Connections
In the second octet, the first seven bits designate the connection’s TEI. TEIs
can be assigned statically or dynamically. A statically assigned TEI will have
a value between 0 to 63; dynamically assigned TEI range from 64 to 126. A
value of 127 designates a broadcast connection meant for all TEs. The eighth
bit is the second address extension bit and is always set to one.
Control Field. The third field of an LAPD frame is the control field, which
can be either one to two octets. This field identifies the type of frame and
contains sequence numbers, control features, and error tracking. The control
field identifies the frame as one of the following:
■ supervisory frame
■ unnumbered frame
■ information frame
Information Field. The fourth field of an LAPD frame varies in length and
contains the frame’s data payload and information. The information field often
contains encapsulated Q.931 packets.
FCS Field. The fifth field is the frame check sequence (FCS), which contains
a CRC checksum of the address, control, and payload fields.
Flag. The sixth field is a one-octet flag, which signals the end of the frame.
Q.931
The subscriber’s ISDN devices and the public carriers devices exchange Q.931
frames to establish, control, and terminate an ISDN call. Q.931 packets are
encapsulated in the LAPD frame in the information field.
Call Process
When an ISDN call is placed, the devices go through a procedure to ensure
that the connection is made. A basic knowledge of this procedure can help
you troubleshoot your ISDN connection. (See Figure 8-4).
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