Kantronics KPC–3 Plus User Manual page 23

the series of 1's and 0's in the incoming bit-stream, and identifies the burst as
an AX.25 packet of information containing the message, ―HELLO.‖
Also, and most significantly, NØGRG registers that the packet designates
NØGRG as the first (and only) intermediate station on a path between a
source, WØXI, and a destination, KBØNYK. As a fully functioning AX.25
packet station, NØGRG knows what to do: it sends the message back out
over the air (i.e., digipeats it), where it may or may not be received by its
intended destination, KBØNYK. Before doing so, it marked its call in the
packet to indicate to others that it had seen the packet and had sent it on its
way.
Note: Since a ―connection‖ has been established, we expect the message to
get through, but each packet is processed separately, and each must make it
on its own.
Step 13. As we know from the ―connection,‖ already established, KBØNYK, an AX.25
packet radio station, is on the air, close enough to receive the RF burst sent
by NØGRG, and currently set to display messages on a computer screen. So
the ―HELLO‖ message re-transmitted by NØGRG arrives at KBØNYK's TNC
(e.g., a KPC-3 Plus), where it is processed and recognized as an AX.25
packet addressed to itself. Then KBØNYK's TNC sends the message,
―HELLO,‖ to the computer screen for KBØNYK to read. At the same time,
KBØNYK's TNC creates an acknowledgment packet to send back to WØXI's
TNC, reversing the address path it found in the ―HELLO‖ packet. When
KBØNYK's TNC detects that the radio channel is clear, it keys the transmitter
and sends the packet back over the air.
Step 14. NØGRG's TNC now recognizes another packet which has its callsign in the
address path, requesting it to be digipeated. NØGRG's TNC does so as soon
as it can.
Step 15. WØXI's TNC now hears a packet addressed to it. Since the digipeated packet
originally came from KBØNYK, to whom he is connected, and since the
packet contains an acknowledgment for data that he had recently sent,
WØXI's TNC can rest easy knowing that the data he sent was properly
received.
This concludes our close-up look at a single packet communication event, showing how
the various parts of packet radio work with each other. To simplify and focus this
example, various details and possible complications were ignored, but this still shows
the basic steps involved in establishing a connection and sending the message
―HELLO‖ from one person/station to another person/station.
This example covers one of many uses of packet radio. As covered in other sections of
this manual, there are many ways to communicate with other stations.
Now that you have followed one particular message from one station to another, it is
useful to learn about the ―packets‖ in packet radio.
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