follow a set of rules for communicating. These rules are specified in the AX.25
protocol.
After a connection has been established between two stations, information
Packets are transmitted one-by-one (or in small batches), with a sequence number
as part of each packet, from the sending station to the receiving station. As the
communication proceeds, the receiving station keeps track of which packets it
has received error-free (as determined by using the check-sum that is part of each
packet) and uses this to tell the sending station which packet(s) to send next.
Using these rules of communication, a packet radio connection between a sender
and a receiver provides virtually error-free transmission of information, until the
communication is ended by the parties, or until the sender “times out” after trying
a number of times to get feedback and not succeeding.
+ Hint: In most cases, communication by packet radio is like mail, not the tele-
phone, in that it is not done in real-time, but via messages that are routed (with
delays) from one station, or “node,” to another.
How a Packet is Organized
Your packet radio station takes care of all the details of working with packets, so
you do not need to see the “inside” of packets. But it is useful to have at least a
basic idea of how they are organized.
All amateur radio packets (also called “frames”) are defined by the AX.25 proto
-
col, which is discussed in the next section. Packets used to carry messages, or
chunks of messages, as in the “HELLO” example, are called “information pack
-
ets.” The following diagram shows the basic building blocks of “connected in
-
formation” packets used in amateur packet radio:
Introduction Packet Communication
User’s Guide 30 KAM’98 v 8.3
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