Timewave PK-232 MBX, Руководство по эксплуатации

Страница: 68 / 266

PK-232 OPERATING MANUAL PACKET RADIO
If you wish to see who is available in your local area, simply set your
UNPROTO path to QRA and send a packet. Within 1 to 16 seconds other-
stations should respond to your QRA request by sending an ID packet of
their own. This feature is compatible with TAPR's QRA feature
introduced in the 1.1.8 firmware release.
4.9 Seldom Used Commands
The following commands operate in Packet, but are seldom needed. They
are listed for reference and described in the Command Summary.
Afilter BBsmsgs ONPerm CPactime Dcdconn Flow HEAderln
MDMon MRpt MXmit
4.10 Packet Lite HF Packet Protocol Extension
Amateur radio needs a better communications mode for HF operation.
Baudot and ASCII have no provision for error detection. AMTOR FEC and ARQ
are more resistant to errors, but do not carry the full ASCII character
set. 300 baud AX.25 packet is undesirable because the long transmissions
are prone to bit errors, any one of which invalidate the whole frame.
Packet Lite is, as its name suggests, an abbreviated form of packet.
It is designed as a transparent extension to the AX.25 protocol that
reduces the “overhead" of all HF packet frames without digipeaters.
Packet Lite does not solve HF packets problems, but it should provide
some throughput improvement on HF where it is desperately needed.
Timewave’s engineering department is interested in hearing from Packet
Lite users with any comments or suggestions on improving the protocol.
A Brief Description of the Packet Lite Protocol
The main feature of Packet Lite that reduces overhead is that it uses
an address field of only 4 bytes. A standard AX.25 header without
digipeaters uses 14 bytes of addressing. Shortening the packet frame
header lessens the possibility of any given frame taking a hit.
Packet Lite reduces the length of an I-frame slightly, but its real
strength is the shortening of the acknowledgment frames, resulting in
fewer garbled acks and therefore fewer unnecessary retries. An ack
(RR, RNR or REJ frame) in standard AX.25 consists of 19 consecutive
bytes that must be copied with no hits; Packet Lite reduces the length
to 9 bytes, or 47% of the standard ack length.
A couple of restrictions are necessary to accomplish this.
First, Packet Lite works only between two stations connected directly,
with no digipeating allowed. If digipeaters are introduced to the
address field, the advantage of the reduced overhead disappears.
2/93 4-27