Bulletins from the Pacific Packet Radio Society - page 165

what use is a 512 byte packet when we only want to s end a carriage return character? We have already solved the problem of data transparency as Hank has pointed out. I don't want to review the entire issue here but I will make the following statement albeit unsubstantiated in this note: There are both disadvantages and advantages to both fixed and variable length packets but for the general use Amateur Packet Radio network the advantages of variable length packets outweigh those of fixed length packets.

2. The use of Forward Error Correction (FEC). There are three types of communication paths that Amateurs will likely be using for packet communication in the future, namely VHF/UHF, HF and Satellite. My best guess is that as the future unfolds the VHF/UHF terrestrial usage will predominate.

A. VHF/UHF paths are characterized usually by having either a good signal or no usable signal at all. In this environment FEC requires additional overhead a nd provides no benefit whatsoever. A retransmission scheme for error correction is being used now which has proved to be efficient. The chips and equipment that we are using now do not have any provision for FEC in any case.

B. HF paths used by Amateurs are so subject to disruption that I expect that they will never be widely used for communications requiring data integrity. It is true that commercially, HF channels are used for this purpose but the techniques used commercially are normally not available for the Amateur for both technical reasons and also because Amateur regulations do not permit their use. In addition to the unreliable channels, Amateur channels are not protected and so even when conditions are optimal, communications may be disrupted by QRM. Commercial usage is usually not troubled by this latter problem. In this environment FEC would only be useful when the propagation conditions were right and there was no QRM on the channel. As Karl says FEC doubles or triples the overhead and so it increases the chance of the packet being hit by QRM and Rayleigh fading. It is also only effective when a maximum of about 2 to 3 percent of the bits are lost. Note that it always has to be backed up by a retransmission system in any case to recover packets that the FEC system didn't handle. Let's look at the situations when FEC is of no use:

When signals are so weak that no communication is possible.

When a packet comes through perfectly with no errors.

When fading wipes out more than 3% of the packet.

When QRM wipes out more than 3% of the packet.

With a system using retransmissions alone, the same packet could be transmitted 2 or 3 times in the period that it would take the FEC packet. I would suggest using shorter packets for the HF channels rather than FEC. The AMTOR system uses this method on HF. When the channel is clear, the retransmission system will outperform the FEC system.

C. Satellite paths are more like the VHF /UHF situation where there is a solid signal or no signal, but the signal is not

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