[MLB-WIRELESS] Speed degradation over multiple hops

[Donovan Baarda]

G'day,

----- Original Message ----- 
From: "Mark Aitken" <vk3jma at net2000.com.au>
[...]
> It only stands to reason that each frame has to be totally received before
> it can be re-transmitted at each hop. Therefore there is a finite amount
of
> time that is required for each system to receive and re-transmit the
> frame.  This can drastically affect the through put, especially if for
what
> ever reason a frame is lost in transit and needs to timeout before it is
> re-transmitted.

actually, this contributes to latency, not reduced throughput.

Provided the incoming link can receive at the same time as the outgoing link
can transmit, both should be able to go at full speed, with outgoing packets
being continuously transmitted as the next is being received. The "hop"
should only introduce a single packet transmission time worth of latency,
with no affect on throughput.

Smart routing hardware can start forwarding packets even before they have
been fully received, as only the packet header is needed for routing
decisions. This lowers the latency to only the header reception time. This
explains why in mtr/traceroute you sometimes see hosts that have a longer
RTT than the next host after them... the "hop" latency is less for
forwarding than for for responding, partly because responding requires
receiving the whole packet (also because routers often prioritise forwarding
of packets over responding to them).

Note that the key here is "can transmit at the same time as receiving". If
the transmit and receive path's interfere with each other you can get some
nasty effects, which is what I would suspect. Particularly when you have ACK
packets coming back from the other end to further clog things up.

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Donovan Baarda                http://minkirri.apana.org.au/~abo/
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