[Gllug] Bank Holiday Monday type question

[Benedikt Heinen]

> But what's the physical reason for this? I can see that it takes about
> 20ms for a packet to cross the continental United States - which is
> acceptibly close to c.

There are a few to name:

   a) there is hardly any bandwidth competition on your local lan -
      i.e. you want to send a packet, and your lan is likely free.
      Whereas, once it leaves your home, it is on a shared medium,
      where the packet potentially gets stopped for a few ns before
      it can be forwarded.

   b) ADSL is based on ATM technology, that is - it's not routing
      frames or packets but cells of a fixed size (if my memory serves
      me correctly, it will break your IP packet down into payloads of
      40 bytes, with some 8 or so bytes overhead per cell), so there is
      some (minimal) extra overhead splitting (very minimal) the IP packet
      on one end, and (potentially a tad more costly - but still minimal)
      assembling the packet on the receiver's side.

   c) (Almost all) hops will handle your packet via store-and-forward, and
      it takes a little time to transmit the packet itself; then it will be
      evaluated, a routing decision done and finally the packet sent off
      again.
      (when I write almost all, it will be pretty much every net - I don't
       think there are significant numbers of cut-through switches around
       that would start relaying packets before having received them
       completely).


> But why does it takes that long for the packet to
> get out across BT's network? Is it really travelling through several
> thousand kms of wire and a lot of capacitors, or is it because of some
> hidden computational reason (ie a lot of switches storing it and then
> resending it?)

My guess is that it would be the latter...




   Benedikt

--
                     Gaudeo te illud de me rogavisse.
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