QoS VOIP Cisco switches 3550

OK thank you for that. Do you have any advice as to whether the rest of the config will work?

Thanks.

Rest of the config looks good. Where is your call-manager type server located ? If it is not on the local LAN, you will need to apply QOS on the router WAN links etc to ensure end-to-end QOS.
You can capture voip packets from ip phones close to the call-manager type server and check to see if the packets have the DSCP markings.

The PBX is located at the other end of the Wireless bridge, no routing involved. We should squeeze a full 54mb out of the wireless bridge but the half duplex is a bit of a concern. Do youhave any experience of putting VOIP over a wireless bridge?

VoIP and wireless is often problematic.

802.11g 54Mbps is realy only ~25Mbps at best and latency and jitter can be a difficult problem to solve.  you are correct that it is half duplex, but also it is subject to many forms of interference.

VoIP requires predictable and solid layer2 performance and wireless bridging is not really in that category 100% of the time.  It certainly can be made to work, but will require a good bit of effort and testing.  You can be successful, just be prepared to invest some time.

The bridge will need VoIP QoS configuration as well to ensure that the VoIP payload packets are always transmitted first as well as the switch settings you are showing.  Anytime there is a point of potential congestion in the LAN/WAN, QoS is required to ensure the packets exit the port in the right order.

What kinds of wireless bridge hardware are you using?

What kind of signal strengths are you seeing and how stable is the wireless now?

When you test the current wireless bridge link, what kind of throughput and latency do you see currently.  Latency and packet loss are the big killers for most Layer-2 VoIP quality problems.

Hi CoreyMac, thanks for your help. The wireless bridge is a bit of a nasty beast because we couldn't get direct line of sight, so we are going for a two-legged bridge. From both offices we can get line of sight to a pole which conveniently has power to it. We will place a 1240 in weather proof casing at the pole and have two directional antennas; one pointing at each office. One of the antennas will run on 5Ghz and the other on 2.4Ghz. The offices will then have a directional antenna, on the appropriate range, pointing at the pole. This will allow us to squeeze 54mb down each leg, rather than halving the bandwidth by sharing one particular range. We will turn off all unneccessary traffic (BPDUguard/filter) to conserver bandwidth.

Because the wireless bridge only has one feed at each end (a 3550) with QoS enabled on it, wont the bridge be QoS'd by deafault because its being fed packets in an already prioritised order?

That will be a fairly unique solution as you acknowledge.  :-)  An innovative use of the radios.  Using two 5Ghz channels would be preferable, but alas that is still not supported even on the latest 1252 series.  Have you been able to take a look at the RF spectrum in the area to see if there is any unusual interference?

On the surface, the QoS scenario would be as you describe, but especially with the uncertain latency added by two radio hops, I would suggest that the 1240 will need to also prioritize. It's buffers will contain both kinds of traffic when the regular clients are moving larger amounts of data, or when the signal quality degrades due to RF noise or environmental factors. You will always want to ensure that the VoIP payload packets are the first to exit every buffer along the way and they do not get caught behind other less time-sensitive traffic.

Fortunately the 1240/1250 series can support this type of QoS.  So long as the latency across the link for the voice traffic stays under 50msec and the jitter stays under 15-20msec, you should not have any trouble as long as the wireless links are stable.

If the numbers get much higher than than you will just need to keep an eye on things, but even at double those numbers it should still give good performance.  You should get better voice quliaty from G.711 even though it needs more bandwidth, the wireless link you describe should not have any problem with mutliple 90Kbps calls as long as the RF-side of the house is happy...

We took your advice and looked into QoS across the wireless bridge, and now we're  in need of some asprin!

When looking at QoS from a wired LAN perspective it appeared that as long as you could classify VOIP traffic as DSCP EF and CoS 5, and using Auto QoS then you were about set. We have a couple of 1131s bridged in a test rig and trying to get QoS across them, it appears that Voice has been bumped to CoS 6 and 7 and Video takes up 4 and 5. Would you be able to shed some light on this discepency between wired and wireless QoS? If you have any sample configs of VOIP QoS on a bridge, those would be very appreciated too :)