4.5.1 Unconnected Packets
In order to allow amateurs to send message beacons and to call CQ, the
AX.25 protocol has the ability to send packets that are intended for
more than one specific packet station to see. Since all packets must
have a destination "callsign", the PK-232 sends Unprotocol packets TO
the callsign of CQ. This can be changed with the UNPROTO command, but
most people like this since it makes an easy way to call CQ.
4.5.2 Connected Packets
When you Connect to another station, the AX.25 packet protocol ensures
that the station to whom you are connected receives all the packets
that you send. Similarly, the protocol ensures you will receive all
the packets that the other station sends to you. The following
describes briefly how the protocol does this.
4.5.3 FRACK and RETRY
When the PK-232 is connected and sends a packet to another station, it
expects an acknowledgment (ACK) packet from the other station to
confirm that the packet was received. The AX.25 packet protocol will
automatically retransmit (retry) packets when an acknowledgment is not
received from the distant end of the link within a specified time.
The FRACK command (FRame ACKnowledge time) sets the time lapse allowed
before the originating station retransmits (retries) the packet.
The RETRY command sets the maximum number of retransmissions before
the sending station terminates the connection (DISCONNECTS).
The TRIES counter keeps track of the retries that have occurred on the
current packet.
4.5.4 PACLEN and MAXFRAME
Packets will be sent either when the <Enter> key is pressed or when
the maximum packet size is exceeded. The maximum packet size is set
by the PACLEN command which defaults to 128 characters. When large
amounts of data need to be sent, this value can be increased to 256.
When conditions are poor or the channel is crowded as on HF packet,
this value should be reduced to 64 or less.
The packet protocol allows more than one frame to be sent in a single
transmission. The default is set to 4 by the MAXFRAME command. When
conditions are good up to 7 frames can be sent to speed data transfer.
When conditions are poor or the channel is crowded, MAXFRAME should be
reduced to only 1 frame.
4.5.5 Reducing Errors through Collision Avoidance
If every packet station could hear every other station, there would be
very few "collisions" due to stations transmitting at the same time.
Since packet operates over the radio, there are usually many stations
operating on the same frequency that cannot hear each other. The
digipeaters and network nodes allow these stations to communicate with
each other, but this increases the chances of collisions.
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