Layer 1 & Layer 2 Protocols
• 7-10 •
The H.110 Basic Rate ISDN Board
a message of the form
TExxP
where xx is the port number. The link can
be disconnected by sending a message of the form
TDxxP.
Packet
messages are sent using the “LC” command as described in the next
section. The SAPI for these messages must be 16 (10h) and the TEI must
match that set using the “TP” command. The packet messages can be up
to 260 octets long.
7.6 Sending and Receiving Layer 3 Messages
When the protocol level for a port is set at Layer 2 using the “SL”
command, Layer 3 messages are sent and received using the auxiliary
mailboxes. To send a message, the application must first check to make
sure that the transmit flag is 0. The contents of the information field are
placed in the auxiliary transmit mailbox at an offset of 1402h bytes. The
length of the information field is placed at an offset of 1400h bytes. This
is a sixteen bit value with the low order byte first. An “LC” message of the
form
LCxxsstt
is then placed in the main transmit mailbox where xx is the
port number, ss is the SAPI, and tt is the TEI of the data link to be used.
The transmit flag is then set to 01h.
When a Layer 3 message is received, an “LC” message of the form
LCxxsstt
will be placed in the receive mailbox. This indicates that a Layer
3 message for the DLCI (ss,tt) on port xx is available in the auxiliary
receive mailbox. The length of the message will be at an offset of 1600h
and the contents of the information field will begin at 1602h.
There is no restriction on the contents of the Layer 3 message other than
a maximum length of 260 octets. Messages may be sent for any DLCI
with a SAPI of 0 and an assigned TEI or a SAPI of 16 and the Packet TEI,
as well as DLCI’s (0,127) and (63,127).
As an example to send a Q.931 CONNect message for a call reference of
1, the following bytes would be place in the auxiliary mailbox: