When sending packets through an audio repeater you may require a
longer key-up delay than is normally needed for direct communications.
The AXDELAY command adds more key-up delay in your PK-900 so that
the repeater can stabilize. The AXHANG command sets the time your
PK-900 assumes is needed for the repeater to drop.
4.5 Packet Protocol Basics
Here we will talk a little about the AX.25 packet protocol. You do
not need to know the protocol to use packet, but it helps in
understanding the protocol parameters.
There are two modes of packet transmissions, Connected mode and
Unconnected mode. Usually you will converse with another packet
station in Connected mode. Still, the Unconnected or Unprotocol mode
comes in handy for beacon transmissions and roundtable conversations.
All packets have basically the same construction. Packets contain
source and destination callsigns (and any digipeaters if used), as
well as information identifying the type of packet. This packet
identification can be seen with the MONITOR command discussed earlier.
All packets contain an error check code called the CRC. This
virtually ensures that when a packet is received, it will not con-
tain a single error. The command PASSALL can disable the CRC error
check, but this should only be done for experimental purposes.
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-900 sends Unprotocol packets
TO the callsign of CQ. This can be changed with the UNPROTO command,
but most people like this since it makes calling CQ easy.
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-900 sends a packet to a Connected 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).
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