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QOS for VOIP on SRP 527W (help needed)

Hi

I'm very pleased with the SRP527W router, except I have not had much success with getting it's VOIP QOS features to work well for us.  We have an IP PBX on the LAN, for which we need to reserve a certain amount of bandwidth and prioritisation for it's VOIP (SIP/RTP) traffic to/from the WAN.

I've tried everything I can think of, but cannot get the SRP 527W to reliably achieve the QOS needed for the VOIP traffic.

I've enabled the QOS, set the upstream bandwidth, reserved enough for the VOIP streams in the "Strict High Priority", plugged in the PBX on a dedicated LAN port on the router which I've configured to use that Strict queue, and I've configured all other ports to use the Medium queue.

With no significant traffic on the other LAN ports, voice quality is fine.  From the QOS statistics in the Status pages, I can see that the VOIP traffic is being placed on the Strict queue, and the usage is well within the bandwidth limit set for that queue.

However, as soon as I introduce a significant amount of traffic on the other ports (I'm using a relatively low rate of BitTorrent traffic for this test), I can see that the traffic for other ports is going on to the Medium queue as hoped, but nonetheless the voice quality deteriorates straight away.  The statistics show that there are packets being dropped from the Strict queue, even though the usage is well within the bandwidth limit set for that queue.

That suggests to me that the QOS/traffic-shaping feature is either not working as it should, or else I've configured it incorrectly.

Has any one else had success in configuring this router for effective VOIP traffic shaping?

Unfortunately, documentation for this feature of the router is extremely light.  Could anyone recommend a configuration for traffic shaping on the SRP 527W that they know would work?

We are currently looking for a reliable low-cost ADSL modem-router to recommend for use with an IP PBX product that our company develops for the SME market.  The feature set and price-point of the SRP527W seems to fit our needs quite nicely.  But effective QOS/traffic-shaping for VOIP traffic is a crucial feature for us, so we'll have to look for an alternative solution if this one doesn't work.

Many thanks,

-Dan

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