Layer 1 & Layer 2 Protocols • 7-3 •
The H.110 Basic Rate ISDN Board
in “EL” messages. In addition, the current state of each port is available to
the application in the dual-ported memory. For details on this, see Section
7.3.
7.2 Layer 2
Layer 2 deals with the exchange of messages between peer devices and is
referred to as the Data Link Layer. It is defined by the Q.921 specification.
The protocol for the establishment of data links and the exchange of
messages is called LAPD. On a Basic Rate Interface, LAPD messages or
frames are exchanged over the 16 kHz. D-channel.
More than one data link can be active on a D-channel at the same time. In
fact, in normal operation, at least three data links are required, one for
Layer 2 management, one for network broadcast messages, and one for a
user device. The data links are identified by a two part address called the
Data Link Connection Identifier or DLCI. The two parts are the Service
Access Point Identifier or SAPI, and the Terminal Endpoint Identifier or
TEI. SAPI values can have a range of 0-63 and TEI values can range from
0-127. However, for the purposes of the Basic Rate ISDN interfaces only
a limited number of possibilities are available. These are:
SAPI = 63, TEI = 127 Layer 2 management procedures
SAPI = 0, TEI = 127 Group TEI for broadcast messages
SAPI = 0, TEI = 0-126 Individual TEI’s for terminal equipment for call
control procedures
SAPI = 16, TEI = 0-126 Packet communication
LAPD messages or frames can take three formats. Information or I frames
are used to perform information transfers between Layer 3 entities.
Supervisory or S frames are used to perform data link supervisory control
functions such as the acknowledgment of I frames, requesting the
retransmission of I frames, or request the suspension or resumption of the
transmission of I frames. The third format is Unnumbered or U frames
which provide additional data link control functions and unnumbered
information transfers for unacknowledged information transfers.
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