[Gllug] OT: Network connection for home to cloud backup

[James Courtier-Dutton]

On 18 September 2010 11:18, Richard Jones <rich at annexia.org> wrote:
> On Wed, Sep 15, 2010 at 10:02:42PM +0100, Alain Williams wrote:
>> At a lower level: I don't know if it includes the ATM overhead, or if the
>> measurement happens 'further inside' the ISP and so perhaps the ethernet
>> headers to whatever machine does the measurement.
>
> I'd be surprised if it included ATM overhead.  The only points that
> see ATM would be your local router and the DSLAM at the exchange
> (under control of BT?)  Back in the day when we used ATM for our local
> network, the so-called "155 Mbps" could only manage about 70-80 Mbps
> of measured throughput over a TCP connection because of all the ATM
> cell segmentation and header crap.
>
> I'm sure Jason will jump in and correct anything egregiously wrong in
> my answer above.
>

There are some DSLAMs that are IP only, (Most of the Cisco ones)
The DSL router would then not do any ATM and save on overhead.
A majority of DSLAMs in the UK are ATM based ones.
I have not tested recently, but one should be able to get more than
80Mbps on a 155 Mbps ATM link.
It does depend on whether you are using PPP compression options or not.
Most DSL routers use PPP compression over ATM AAL5, so the overhead is
less than without compression.
You can have very large packets in AAL5, something over 60000 bytes,
so one can, if the ISP permits, even have 1500 MTU over IPSEC without
having to fragment,
It also means it is ready for jumbo packets that are found in IPv6.
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug