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4.5.1 Unconnected Packets<br />
In order to allow amateurs to send message beacons and to call CQ, the<br />
AX.25 protocol has the ability to send packets that are intended for<br />
more than one specific packet station to see. Since all packets must<br />
have a destination "callsign", the <strong>PK</strong>-<strong>232</strong> sends Unprotocol packets TO<br />
the callsign of CQ. This can be changed with the UNPROTO command, but<br />
most people like this since it makes an easy way to call CQ.<br />
4.5.2 Connected Packets<br />
When you Connect to another station, the AX.25 packet protocol ensures<br />
that the station to whom you are connected receives all the packets<br />
that you send. Similarly, the protocol ensures you will receive all<br />
the packets that the other station sends to you. The following<br />
describes briefly how the protocol does this.<br />
4.5.3 FRACK and RETRY<br />
When the <strong>PK</strong>-<strong>232</strong> is connected and sends a packet to another station, it<br />
expects an acknowledgment (ACK) packet from the other station to<br />
confirm that the packet was received. The AX.25 packet protocol will<br />
automatically retransmit (retry) packets when an acknowledgment is not<br />
received from the distant end of the link within a specified time.<br />
The FRACK command (FRame ACKnowledge time) sets the time lapse allowed<br />
before the originating station retransmits (retries) the packet.<br />
The RETRY command sets the maximum number of retransmissions before<br />
the sending station terminates the connection (DISCONNECTS).<br />
The TRIES counter keeps track of the retries that have occurred on the<br />
current packet.<br />
4.5.4 PACLEN and MAXFRAME<br />
Packets will be sent either when the key is pressed or when<br />
the maximum packet size is exceeded. The maximum packet size is set<br />
by the PACLEN command which defaults to 128 characters. When large<br />
amounts of data need to be sent, this value can be increased to 256.<br />
When conditions are poor or the channel is crowded as on HF packet,<br />
this value should be reduced to 64 or less.<br />
The packet protocol allows more than one frame to be sent in a single<br />
transmission. The default is set to 4 by the MAXFRAME command. When<br />
conditions are good up to 7 frames can be sent to speed data transfer.<br />
When conditions are poor or the channel is crowded, MAXFRAME should be<br />
reduced to only 1 frame.<br />
4.5.5 Reducing Errors through Collision Avoidance<br />
If every packet station could hear every other station, there would be<br />
very few "collisions" due to stations transmitting at the same time.<br />
Since packet operates over the radio, there are usually many stations<br />
operating on the same frequency that cannot hear each other. The<br />
digipeaters and network nodes allow these stations to communicate with<br />
each other, but this increases the chances of collisions.<br />
4/91 4-20<br />
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