IP telephony

Dirk Koopman djk at tobit.co.uk
Wed Oct 18 18:34:35 BST 2006

On Wed, 2006-10-18 at 18:07 +0100, Toby Corkindale wrote:

> > * You need a line that does not have any other services (eg web, mail,
> > ftp) etc. Otherwise someone sending you a large email (for instance)
> > will completely wipe out any other incoming RTP (whilst letting them
> > still here you - most frustrating). 
> This is manageable with QoS, which I'm quite familar with.
> > * It also helps if people in your organisation don't use that line for
> > downloading huge volumes of stuff (otherwise the opposite problem occurs
> > - it will clobber outgoing RTP). Having the correct router would allow
> > you to use QoS to fix this.
> > 
> > * You need an ISP that understands the issues and is happy to let you do
> > it in the first place. More and more are doing their own
> > "triple"/"quadruple" play thing and are therefore reluctant / unable /
> > don't understand how to do useful things like QoS at their end (which
> > would mitigate the first issue). 
> Good point; we're going to be switching ISP providers shortly anyway, so I'll
> make sure they support QoS.

Sorry to labour this, as you have probably worked out, I have been (and
occasionally still am) bitten by this.

QoS on the public internet and by ISPs is not implemented particularly
well (and in many cases: at all). 

For instance a certain tier one ISP has frame relay "routers" that are,
in fact, simple frame relay multiplexers. These units are what your nice
shiny Cisco, doing outgoing QoS, is connected to. So most of^hall your
RTP packets will get routed before any other traffic *from your router*.
However, in the multiplexor at the ISP, they are simply mixed in with
all the lines' traffic connected to that unit and outgoing QoS is simply
thrown away. The units are (of course) *much* cheaper than Ciscos :-)

They have no QoS because that is not passed through to the frame relay
layers - in other words the multiplexors are completely dumb. 

On the incoming side (ie to you) it is even worse because there is no
QoS possible at all. Hence an incoming email with a 100Mb attachment can
completely destroy the incoming RTP stream for the length of time it
takes to transfer that email down your 2Mb pipe (remember only about
250KB/sec max transfer speed).  
You should question your new ISP very, very closely (and in writing)
before commiting...


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