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Doug Crompton said...
>
> Anyone using commercial VOip? I was looking at the AT&T site. They claim
> they need 90Kb Up/down BW/line/call. That really sounds excessive. What is
> this super high fidelity phone connections! I would think they could get
> by with a lot less then that even with overhead. Given a 3K voice channel
> you should be able to sample at 12-16K, Add overhead and control.. so we
> double that... 32K ... what the heck is the rest? I think verizon breaks a
> T1 down into 23 voice channels / 1 control, each 48K. That always seemed
> like a waste of BW to me.
ISDN is 64k, the control/call setup is out-of-band (on the 24th channel).
there is no compression, this is an 8-bit sample * 8k samples/second. no IP.
g.711 is also 64k (g.723=6k and g.729=8k) not including tcp/ip header overhead.
That's where it gets confusing, since cisco defaults with a very poor payload
(framesize) to header (frame) ratio. you can tweak it so you get 10-12k per
call with compression. (you can squeeze a t-1 of voice into 256k of data.)
The problem isn't bandwidth, though; low latency and congestion are more
important to a good sounding call. Although the SIP/H.323 setup is TCP
(which gives you a better guarantee of orderly packet delivery), the voice
data is UDP (which has no flow control or retransmission facility) and if
the packets get lost or arrive out-of-order quality suffers. So what happens
is that you wind up with lots of little packets that require lots of management
to peacefully co-exist with other TCP/UDP data. This kills router CPU and slows
your download speed.
> Anyhow it might make sense to change a DSL lines analog voice number to
> absolute minimum service and get one of these all inclusive digital voice
> packages. You can get everything under the sun for $29 and there are many
> less expensive packages. The difference is that there are no hidden
> charges like wireline or wireless where they hit you for darn near 20% of
> your bill in hidden costs.
I would be very reluctant to give up my wireline service (I already have the
cheap-ass dial plan). It's nice to be able to make calls (especially 911) in
the event of an extended power failure. If you make a lot of long-distance
or intra-state toll calls, then the toll-bypass savings are attractive.
but most of my calls are around the neighborhood or into the city, so...
--
Ron Mansolino RMsolino@netaxs.com http://www.netaxs.com/~rmsolino/
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