Kantronics KPC–3 Plus User Manual page 24

Packets: Dividing Messages into Segments
An efficient way to send a message from one place to another is to break the
information into small segments, called packets or frames, and send each packet
separately.
Note: The correct technical term is ―data frame‖, not ―packet‖ or ―frame,‖ but we will use
the term ―packet,‖ since that is familiar to most people and is common in the field.
Your packet radio station automatically divides your communication (e.g., a message or
file) into small segments (see the PACLEN command for details) and puts each
segment into its own ―packet‖. Each packet includes information used for addressing
and error correction.
Unconnected Packets
Packets do not need to be addressed to any particular station. Packets transmitted
without any established connection between sender and receiver are ―unconnected‖
packets. ―Unconnected‖ packets are just sent out without any back-and-forth
coordination with any receiving station. Stations receiving these packets can check to
see if the packets are correct (i.e., the same bit pattern is received as when they were
sent), but they cannot request any retransmission of faulty packets. Nor can they
request re-transmission of any packets in a series that are not received at all. This is
because ―unconnected‖ packets do not include sequence information that could be used
to track them in order and also because a TNC sending or receiving ―unconnected‖
packets is not set up to do any communicating about the success or failure of its packet
transmissions.
Note: Packets sent as ―unconnected‖ are also called UNPROTO (non-protocol),
because the sender expects no response from the receiving station.
Connected Packets
Packet radio communication is often done between two stations that are connected
together. ―Connected‖ communication using packet radio assures transmission with
virtually 100% accuracy by having the sender and receiver follow a set of rules for
communicating. These rules are specified in the AX.25 protocol.
The following is an overview of what happens during a packet radio communication
between two connected stations.
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 what 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
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