[Asterisk-biz] Forklift a 2000 phone PBX

[Herman Webley]

On Sun, 2005-03-27 at 16:34 -0500, steve szmidt wrote:
> On Sunday 27 March 2005 15:14, Reid Forrest wrote:
> > > This seems like a really bad idea. Why lock down into a not only old and
> > > outdated technology, but for 2000 users?!!
> > >
> > > You want to keep the network operating efficiently. Modern switches does
> > > a lot
> > > better job than yesterdays technology. If you have 250 people on the
> > > phone at
> > > the same time, and you are using g.729, you have used 10Mb right there.
> > > Never
> > > mind any other protocol.
> >
> > Steve, I agree with you that trying to run voice over Cat3 is not the best
> > idea. But, I think your math may be off in the calculation above. 250
> > people on the phone using g.729, even compensating for protocol overhead of
> > 25% is only 2.5Mbit/sec. 
> 
> Sorry but we are not talking about a one way conversation. It requires shy of 
> 20K for a two way conversation.
> 
> > In addition, you are assuming that the voice 
> > network is completely FLAT, not switched. A fully switched voice network
> > would reduce bandwidth consumption at every station. 
> 
> No it doesn't. Switched does not lower the needs of a g.729 connection. The 
> only savings are in not having collisions, and retransmissions, as with a 
> hub. 

You're partially correct. Layer 2 switches (equivalent to ethernet
bridges in this case) separate collision domains. In short, switches
transmit addressed packets _only_ through the interface that it has
detected the host with that address. This is the fundamental difference
between a hub and a switch. As such only packets originating or destined
for the host at the other end of the ethernet cable. It is this process
of filtering that reduces collision frequency.

(Collisions occur when multiple host transmit in the same collision
domain [ethernet segment] at the same time. Since the data is
segregated, the possibility of a collision is dramatically reduced)

In short only data packets from or to the phone at the end of the cable
will be using the 10Mb/s capability of the cat3 cabling. More than
enough for G.729 + IP overhead. In a hub extended network, all the data
would be carried throughout the network as there would be no separation
of collision domains. In this case the network would need to carry
2000x(g.729+IP overhead). So in a switched network the data passing each
host (phone station in this case) would be _significantly_ less.  I
believe this is what the previous poster meant.

> > A 2000 user switched 
> > voice network over Cat3 is viable, as long as you keep data off of it.
> 
> Well, it turned out that he has Cat 3 from the previous phone system 
> available, and that is what he is talking about. I missed that point.
> 
> As for a dedicated Cat 3 to run a voice leg to a switch that would obviously 
> be fully workable. However, if you were to integrate it to a 100Mb network 
> then it will switch down the 100 for each 10 packet entering that leg. Of 
> course that might still be faster than running a 10Mb network.
> 

I am not sure what you mean. 100Mb/s switches (any 100 switch, not just
fancy ones) conform to the fast ethernet standard. Each port on the
switch will perform autonegotiation (unless configured in some obscure
way) with the interface on the other end (the phone). Assuming the
interfaces correcty detect the capabilities of the medium (cat3 opposed
to cat5/cat5e) the connection will be set to 10Mb/s. However, this will
only be on that interface. The 'backbone' connection will still
negotiate to the full capapilities of that link. (gigabit according to
the poster).

So the topology will look like this

____1 Gbit___ [switch] _______10mbit_____ [phone]
                       |______10mbit_____ [phone]
                       |______10mbit_____ [phone]
                       |______10mbit_____ [phone]
                       |______10mbit_____ [phone]
                       |______10mbit_____ [phone]

Everybody will be happy. Each phone gets an ample 10Mb/s with an also
ample gigabit link carrying the aggregated traffic.
(24 x [ G.729 + IP overhead]?)

*NB Just be sure that you don't go past the 100 metre specification of
10base-t over cat3 is adhered to. As you approach it the possibility of
collisions on the segment will increase due to the increased propagation
delay of that segment.

Last mile connection for low bandwidth application? Switched Cat 3 is
fine.

Best regards,
Herman Webley,
Blitz Management