[Asterisk-Users] QoS General Question

[Rich Adamson]

> >How much of an impact can/does local network traffic have on call quality?
> >Would opening large files on local servers affect call quality? We are
> >running QoS on the router but that will only prioritize traffic in/out of
> >the network. 
> 
> Sure it can. If you have a network segment that's fully saturated and
> you're also pushing VOIP data over that segment you'll have problems.
> In practice most networks are not that busy, but it can happen. If your
> phones, switch and NICs are VLAN capable you can setup a dedicated VLAN
> for the voice traffic and ensure that it gets priority.

A vlan won't fix anything other then a minor step towards improving
security. (And, it really is a minor step.)

We do a lot of network performance assessments throughout the US, and I can't
begin to count the number of corporations/institutions that don't have a
clue how many packets are dropped by their layer-2 switches simply because
they don't monitor the "key" snmp oid. The key is watching for discarded
packets on outbound ports. (The majority of network managers believe their
layer-2 switches have buffers just like layer-3 boxes, and the majority do
not have buffers.

The most simple example is two PC's attached to the same switch sending
multiple packets at 100 meg, and the outbound (trunk) port running at 100
meg. The 200 meg of inbound data (to the switch) will frequently congest
the outbound port causing the switch to drop (discard) packets. In real
time, that can be as few as 5 or 10 packets from each PC, if they happen
at the same time. (Note: many of the newer switches on the market today
do have some amount of buffering, but the majority of the two to five year
old switches do not.)

For those that would really like to argue that point, take the covers off
your switch, identify the chip set, and read the techie detail in the spec
sheets. Or, do some simple tests by trying to overload an outbound port
and see what happens.

Essentially, if a switch supports QoS properly, it _will_ have some amount
of buffering. QoS will help, but if the outbound load is to great, the
traffic is still going to cause the switch to run out of buffer space and
drop the packets.