DESCRIPTION AND OPERATION
INNIS01 NETWORK INTERFACE SLAVE MODULE
WBPEEUI250016A1
2 - 3
bytes. The control field contains time of origination, sequence,
source, size, circulation count, message type, destinations,
and checksum.
The NIS module increments the circulation count field of all
incoming messages. When a message count field exceeds 255,
the message is discarded. This is useful in keeping retry and
spurious message traffic to a minimum. The NIS module uses
the message type to determine how to process the message.
The checksum and cyclic redundancy check code fields verify
data integrity.
MESSAGE TRANSMISSION
Any NIS module can transmit a message at any time without
regard to the activities of any other NIS module on the system.
Each NIS module can transmit and receive messages simulta-
neously. NIS start-up/shutdown is local and requires no inter-
action with other NIS modules on the INFI-NET loop. Each NIS
module receives all incoming messages and transmits a new
stream of messages in a store and forward fashion to the next
node. When there are no messages for the NIS module to trans-
mit, the NIS module transmits flag characters (null packets) as
the loop synchronizing condition to keep the receivers in lock.
Data Integrity
There are three methods by which the NIS module insures data
integrity. They are retry logic, node status table, and polling.
RETRY LOGIC
If, on the first transmission of a message, the NIS module does
not receive positive acknowledgment from the destination
node, it retransmits the message 11 times. If after this series of
retries there is still no response, the destination node is
marked off-line.
NODE STATUS TABLE
The NIS module maintains an internal table of system wide
node status such as off-line and busy. The NIS module relays
node status changes to the NPM module. When the NIS module
periodically polls nodes, it updates this table accordingly.
POLLING
The NIS module uses the information in its status table for
polling purposes. As it scans the status table, it picks out des-
tinations targeted for multicast messages that have been
marked off-line or busy. After polling the destination, the NIS
module updates its table and forwards pertinent information to
the NPM module.
