The advertised datarate (8mb/448k) are the speeds at which the circuit
between the customer and the central office is clocked and has no
relationship with *effective* throughput. At the central office are
*shared* facilities than connects each DSL connection with the network,
and over subscription to these shared facilities cause congestion.
Also, there is no QoS on the Internet, and congestion anywhere between
the end points will cause poor call quality.
Disclaimer: The following information is several months old--I've since
moved my customers away from Qwest DSL.
Here in Denver we have Qwest DSL "service" from a central office where
the effective throughput drops to dialup speeds during the day. Regular
web/email users don't usually notice packet loss because dropped packets
are recovered by the TCP protocol. For VoIP on UDP, however, the call
quality suffers to the point of being unusable (clicking, popping, and
dropouts).
Furthermore, Qwest doesn't have Denver peering with the "rest" of the
Internet. To leave the Qwest network, connections typically go to DAL,
LAX, or SFO on congested circuits.
So beware of VoIP over DSL. Your users need to be aware of the
tradeoffs between the cost of DSL vs. T1 and the effect on call quality.
Chris, if your customers are in the western US then please contact me
about dedicated circuits.
Chris Bagnall wrote:
Greetings list,
We have an issue with call quality at 2 sites where the users (4 Elmeg
IP290s at one site, 2 SPA942s at the other) do not have an asterisk box
on-site. Each site has an 8mb down/448k up ADSL connection and the phones
connect via SIP to an asterisk box in a datacentre using g729.
The asterisk box in the datacentre connects to our other asterisk boxes
providing pstn connectivity via IAX2. Latency between these boxes is between
1 and 2ms. The ADSL connections to the client sites are all consistently
delivering latencies of sub-25ms to the datacentre and there is traffic
shaping on that connection to give priority to any traffic from the phones'
IPs.
Comments from the users at these sites are as follows:
"call sounded like a dalek and I couldn't make out anything the caller was
saying"
"the phone on my desk is breaking up so badly it's virtually unusable"
"calls sound like they're breaking up with metallic background noises"
We have quite a few customers with asterisk boxes on-site (with phones
connected to them via the LAN) using ADSL connections from the same
supplier, and are not having these issues with them.
canreinvite=no and nat=yes are set on all these devices, since they are
behind NAT. Each device re-registers with asterisk every 5 minutes to
prevent any possible NAT state timeouts.
Any pointers/places to look for potential problems would be much
appreciated.
Regards,
Chris
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