Re: Extended sequence numbers for AX.25

[Phil Karn <karn@unix.ka9q.ampr.org>]

>> Your network tests are very interesting. You should publish them.  I
>> note that the usual rule of thumb for pure ARQ protocols (e.g., TCP)
>> is that the packet loss rate should not exceed 1% for good
>> performance.

>With TCP (which has no feedback about lost packets and always assumes
>lost packets indicate congestion, not loss by transmission error) this
>is very true.  That is a reason why most of our network uses VC mode for
>IP traffic.  (actually, the current software automatically switches between

The 1% rule is not just because of TCP's backoff algorithms. Any ARQ
protocol (TCP or LAPB) wastes valuable RF energy and channel time
whenever it discards a frame. A channel that loses half the packets
sent over it essentially wastes 3dB of transmitter power.

The point is that you'll often do better, power and timewise, by
adding FEC redundancy that acts to decrease the loss at the ARQ level.
ARQ protocols are good at taking a low bit error rate and reducing it
to zero (for all practical purposes), while FEC is good at taking a
high bit rate and lowering it (but not to zero). They work well
together.

Phil