Kantronics KPC–3 Plus User Manual page 25

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 telephone,
in that it is not done in real-time, but via messages that are routed 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 protocol,
which is discussed in the next section. Packets used to carry messages, or chunks of
messages, as in the ―HELLO‖ example, are called ―information packets.‖
The following diagram shows the basic building blocks of ―connected information‖
packets used in amateur packet radio:
Single character for
"beginning of packet"
Destination station, Source station,
And up to 8 intermediate stations
Flag Address
Kinds of Packets
As shown in the above diagram, the ―control‖ field in an AX.25 packet includes a code
telling what kind of packet the current packet is. In addition to connected information
packets, the AX.25 protocol also defines two other kinds of packets, each of which is
nearly identical in structure to the connected information packet: control packets (of
Kind of packet, packet number,
and other control information
Optional Data with protocol ID
Data may have 1 to 256 bytes
Control PID
25
16 bits derived from this
packet used to check for
errors in transmission
Data Checksum
Single character for
"end of packet"
Flag