Operations Manual

OPERATIONS 4-35
O&M of the MCC-545B PACKET DATA RADIO
12/2000
This paragraph explains how message accountability is used to guarantee that text messages get
delivered to their proper destinations within an allotted time. Data reports, position reports, and
remote commands/responses will not get this guarantee. They get only one chance to get
through the network.
Networks can have units go offline for various reasons; local noise can interfere with RF links,
congestion can slow throughput to a crawl, RF link bit errors can cause segments of a message to
get lost, etc. The more complex a network is, the more chances there are to be problems.
Messages entered at each source unit specify a time-to-live (TTL); this time is the maximum
time to attempt to deliver it. If it is not delivered in this time, the operator at the source unit is
informed so something can be done about it. The time-to-retry (TTR) is the number of minutes
between attempts to deliver the message. Once a message is sent, it goes through the network
one hop at a time and can get blocked at some point if the connectivity changes suddenly. The
retry attempts are separated to allow network changes to settle out and establish alternate routes.
When a message is received by a destination, an end-to-end-acknowledgement (ETE) is sent
from the destination back to the source to stop any more retries and let the operator know the
message was received.
The maximum message size is determined by the text length (TEXTL) setting. A message
packet can consist of up to 3570 characters and is further subdivided into segments. Each
message is uniquely identified so it can be tracked through the network and the ETE can be sent
for each individual message. The message ID consists of the originator ID (16 bits) and message
serial number (8 bits). Serial numbers range from 1 to 255 and are assigned in round-robin
order. Each message is then split into 14-byte segments which are in sequence from 0 to 255.
The segments allow the message to be transmitted a little at a time over short meteor bursts. The
segment sequence numbers are used by the RF link software to identify which ones are
acknowledged and to indicate where to resume on each burst.
The first segment (sequence number 0) is the message header and contains all the network
overhead (originator ID, message serial number, priority, I/O port entered on, message type,
number of destinations, number of segments, time to live, retry count, multi-packet message
serial number, packet sequence number, total number of packets and first destination ID code).
If the message has only one destination, segment 1 is the start of the actual text. If there are
multiple destinations, segment(s) 1, . . . n contain the remaining destination codes, 7 destination
codes per segment. The text starts on the next segment after the last destination code.
SNP,NUP Number of receptions for neighbor-up
SNP,NDOWN Number of minutes for Master neighbor down
SNP,RDOWN Number of minutes for Remote neighbor down
Network connectivity tables are automatically created and updated each time a Master detects a
new neighbor unit or times out an existing unit. The NUP parameter gives the number of
transmissions that must be received in one minute to declare a new neighbor. The NDOWN
parameter gives the number of minutes with no receptions to time out a Master neighbor. The