SCS PTC-IIex, Manual

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15 Basics
Chapter 15
15 Basics
15.1 Why PACTOR?
PACTOR ( Latin: the mediator ) is a modern radio teletype mode developed in Germany
by DF4KV and DL6MAA to improve on inefficient modes such as AMTOR and
PACKET-RADIO in weak short wave conditions.
The AX.25 PACKET protocol certainly has its advantages on VHF/UHF FM channels,
but gives a lot of problems on short wave:
•
The data rate of 300 baud combined with a large packet length used by many radio
amateurs is very susceptible on fading or multipath conditions and QRM.
•
The large protocol overhead dramatically reduces the amount of information
contained in a packet.
AMTOR had been developed specially for transferring text on an HF channel. Even weak
signals under distorted conditions, where a PACKET connect would never be possible,
could be copied. But AMTOR also has its disadvantages:
•
Using 5 bit code makes it impossible to transfer the whole ASCII character set or
binary data.
•
Detecting and correcting errors is insufficient for error free transmission of binary
data.
•
The effective data rate is only 35 baud.
PACTOR offers a much better error correction system, and a considerably higher data
transfer rate, than AMTOR. The synchronous transmission format, and the short packet
lengths of AMTOR, have been retained. These result in a protocol much more resistant to
interference than Packet-Radio under poor propagation conditions.
The PACTOR protocol, together with the
SCS
-PACTOR Controller, allows a much
higher throughput than AMTOR, with the efficient error correction and data transparency
of Packet-Radio.
One should not, however, be under the impression that PACTOR is just a combination of
Packet and AMTOR! Although essential parts of both systems have been included, such
as data integrity, by using a CRC from Packet, and the synchronous transmission format
and short block lengths (compared to Packet) of AMTOR, a fully new concept has also
been included from the very beginning. For the first time in amateur radio, online data
compression is used to markedly increase the effective transmission speed. Also the use
of memory ARQ in PACTOR is a milestone, although it has been known for a long time
in the commercial sector.
Previously it has been very difficult, or impossible, to apply this concept in amateur radio.
The use of memory ARQ is the main reason that PACTOR does not loose the link under
bad conditions. With memory ARQ, defectively received packets or blocks are not just
simply thrown away. They are stored and added to other defective packets, until enough
data is collected to reconstruct the original packet, and thus keep the link during
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