Bulletins from the Pacific Packet Radio Society - page 154

Access algorithm:

Defer : Any node may transmit *Transmissions are not previous conflict *The channel is sensed 2 ms, corresponding to packet plus switching a packet if blocked as result of a to be free for at least the duration of an ACK times and path delays.

ACK : When a node receives a correct (BCS-check) 'F' - packet which contains the address of the receiving node, interpretation of IF' should result in the immediate transmission of an 'A' acknowledge packet (no channel busy check applies). The ACK should be transmitted with 2 ms after the reception of the 'F' - packet. If an incorrect packet or a packet with erroneous format word is received no acknowledgement should result. Broadcast packets, i.e. packets containing 'wildcard' address fields should not trigger the transmission of acknowledge, even if a match is obtained in the address check procedure.

Conflict : A node detecting a conflict by mission acknowledgement packets after 2 ms will block its transmissions for a random amount of time. This delay is rectangularly distributed over the interval (0,100 ms). The defer procedure applies also when attempting retransmission. This procedure is repeated the number of times specified by the user (See NODE environment for details) Packet handling procedure:

'F' - packets

Receive all incoming packets and check the packet BCS (checksum) field. If the checksum is correct, check LDID. if packet has correct destination and does not contain wildcard address fields, issue an A-packet, switch to the NODE environment and start interpretation of the NODE field (below). In case of conflict, retransmit an 'F' packet the number times specified in the NODE environment.

Always check if the LSID (see below) is known to the node. If not, update the NEIGHBOR list (see NODE environment for description).

In case that some known neighbor node does not acknowledge a transmitted IF' packet after the maximum number of retries, this neighbor is detected from the NEIGHBOR list.

Packet formats:

The two format words 'F' and 'A' also define a basic packet format as depicted below.

Click for Original Graphic Image of this page.