[Gllug] Bank Holiday Monday type question

[Ian Norton]

On Mon, May 30, 2005 at 05:25:19PM +0100, Benedikt Heinen wrote:
> >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).

atm cells are 53 bytes, 48 payload, 5 header ( gfc, vpi, vci, pt, clp, hec ).

some ISPs recomend dropping the MTU of your ppp connection a little so that its
a multiple of 48 so as to not have wasted cells

afaik most adsl lines are setup with a 1500 byte mtu, for a max size datagram
this means you will waste the last 36 bytes of every 32nd atm cell.

which can add up, it also adds to the overhead of re-assembly of your packet.

dont forget tho, that your ppp actually runs over AAL5 of the atm connection so
you loose a few more bytes there ( 5 or so i think )

soo.

if my brain is working, you will get better efficiancy by setting your mtu to
be about 1483 bytes (or maybe 1531).

See:-
http://support.metronet.co.uk/adsl/general-opt.xhtml1
 
-- 
Ian Norton-Badrul

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