Mike Bilow wrote:
>
> Thomas Sailer wrote in a message to Mike Bilow:
>
> TS> I've never understood the T2 waiting business anyway. If
> TS> only AX.25 and the channel access algorithm were coupled
> TS> more tightly.
>
> This is true, but AX.25 doesn't really have a channel access algorithm. Most
> of the textbook models do not take into account the hidden transmitter problem,
> and we have never really developed an effective method of dealing with that.
> What we run in practice tends to reduce to pure Aloha. Seen in that light, the
> importance of T2 is more sensible.
Don't mix things up please. T2 has _nothing_ to do with channel access.
It's solely for delayed ACKs and gives the chance to merge multiple ACKs
into one or even into an implicit ACK via an I-Frame. You won't be able
to cope with hidden stations anyway. You can achieve the same effect
with a proper slottime/persistence setting, without rapeing an AX.25
protocol element for MAC purposes ;-) Aloha generally means: No
throughput when many stations.
> TS> IMHO there's no need to do any waiting at all (or even use a
> TS> poll bit) in that situation. When the carrier goes off, you
> TS> can be quite sure that you are expected to send an ack, so
> TS> there's no point in waiting. This is more or less what
> TS> FlexNet does.
>
> This is, of course, completely true of a point-to-point link, or at least of
> any link generally where there are no hidden transmitters. However, as soon as
> hidden transmitters are introduced, the channel utilization starts to follow a
> classical Poisson distribution, so the introduction of strategic delays is
> actually the only thing that prevents degeneration into total chaos.
Correct. Thats what channel access algorithms deal with. But the
mathematical facts wipe all attempts to be clever, as long as you don't
really solve the problem of the hidden stations. In a contention
environment with competing stations and pure ALOHA you can at best
achieve something like 21% of the maximum possible throughput of a
channel. No matter how clever you try to be with persistence or T2
settings.
> I'm not really sure what sort of pattern results from ARQ, but I would assume
> that it tends to look more or less binormal. As long as it is something
> predictable, we should have no trouble adapting our rtt measurements.
Hm, it's not fully predictable. No ARQ scheme is. CSMA/CD isn't either,
you can have inifinite delay of delivery. No prediction scheme can deal
with this.
Gruss,
Matthias
