User's Manual

545C Operations Manual

02/10/2012 Prerelease DCN 00001789-A

Message accountability guarantees that text messages get delivered to

their proper destinations within an allotted time. Data reports, position

reports, and remote commands/responses donot 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,