Timewave PK-232MBX, Руководство по эксплуатации, Страница: 55 / 231

PK-232 MBX – OPERATING MANUAL PACKET-RADIO
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4.5.3
FRACK and RETRY
When the PK-232 is connected and sends a packet to another station, it expects an ack-
nowledgment (ACK) packet from the other station to confirm that the packet was re-
ceived. 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 pack-
et size is exceeded. The maximum packet size is set by the PACLEN command which de-
faults 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.
The first attempt to avoid collisions was through the use of the DWAIT and RESPTIME
timers. DWAIT forced the TNC to delay the transmission of any packet except for digi-
peated frames by the time selected. This fixed timer helped, but Packet was still plagued
by collisions. The RESPTIME was added to help with large file transfers. Still, more
needed to be done to reduce collisions.
Another attempt to reduce collisions was the introduction of AX.25 version 2 protocol. On
VHF Packet, most everyone uses version 2 which is controlled by the AX25L2V2 command
(default ON). On VHF this helps, but some users on HF Packet are turning this command
OFF. An exponentially distributed random wait method was proposed by Phil Karn (KA9Q)
called P-persistent CSMA. When the command PPERSIST is ON (default) the PK-232 uses
the number set in PPERSIST and the time value set by the SLOTTIME command to more
randomly distribute the transmit wait time. This is more efficient than using the DWAIT
time.
As a further attempt to improve Packet performance, Eric Gustafson (N7CL) proposed giv-
ing priority to acknowledgment packets (ACKs). This protocol is controlled by the
ACKPRIOR command which currently defaults OFF. Check with the experienced Packet
users in your area and find out if they are using priority acknowledge or have changed
any other parameters.