QoS over wireless bridge


I’m planning a Asterisk-based PBX for our office. We want to use the PBX in two buildings near each other (about 150 meters). The buildings are already connected by a logical Ethernet (using a special Linksys WRT54GS+OpenWRT wireless router setup). I am planning on upgrading the network for QoS, but I am not sure how QoS works. Also, there are different types of QoS.

Say it looks like this (ignore line wrapping):

If the wireless components do not support QoS, would the wired switches on both sides still prioritize the traffic properly that travels through the wireless bridge?

Current plans are Linksys SPA942 hard phones, Linksys SRW248G4 hard-wired switches, and Linksys SRW224G4 hard-wired switches. The wireless system may be Linksys WAP4400N supports some QoS, but the WAP4400N specs are vague.

We are a non-profit, so we cannot spend much on this system. :smile: The current budget is $12,000 which includes an Asterisk PBX product from a local company, 35 phones, and the new network devices.

for wireless qos I believe there is a thing called WMM extensions designed to do just that… as i recall DD-WRT supports it. The most important QoS that you need though is on your WAN link if you use a VoIP service provider.

I know that some of the Zyxel wireless access points support QoS as do their wireless phones. QoS should ideally be end to end because any break in this will risk qualify degradation. Having said that if you are using a ToS bit for QoS then this setting should be forwarded by a device that does not support QoS and therefore does not recognise this setting. As IronHelix said it is the Internet/WAN conneciton where it is the must important place to implement QoS or at the very least prioritisation of packets. This is not always easy because the Internet itself does not support QoS.

I do not understand the point of WMM. The Linksys WAP4400 supports WMM, but it does not claim 802.1p, DSCP, etc support. So, how does it know which packets (VoIP vs. HTTP) to prioritize?

I checked out Zyxel, but I could not find a useful AP. (I am not looking for wireless phones, though. Everything but the wireless bridge is hard wired.)

The Internet/WAN connection here is not the issue. I am afraid the wireless link between the two buildings will become overloaded and destroy VoIP quality.

The Linksys WAP54GP supports 802.1p, but it does not mention voice applications. The WAP4400N advertises voice applications, but it does not mention support for protocols like 802.1p, DSCP, etc.

I suppose I could try to setup two WRT54GS with OpenWRT and QoS, but that sounds like a lot of work which I’d like to avoid if possible.

I have played with this at one of my previous companies before i sold it off :wink: the offices were about 100+ meters apart, we has at the time WRT54G-V2 Routers, so we flashed them and installed Open-WRT on then, we did this so we could install Asterisk on them as well (Yeah i know a little Geeky but hey it was fun).

I am not sure of your Linksys Model but the WRT54G with its original firmware supported QoS a very basic form of it, but it still supported it, Open-WRT had some improvements on it that gave slightly more options then the Original Linksys firmware.

I am not sure why you would want to purchase Asterisk from one of the local Stores, not that i want to deprive any business of money, but since your a Not-For-profit organization why don’t you install Open-WRT (I think you already have) and then install Asterisk on them, Your version of the router has way more processing power over the ones we were using so it can handle quite a few calls concurrently using the G.711a/u/LAW CoDec’s or even even some of the other ones that don’t require huge amounts of processing power or Bandwidth.

To take it one step further, with Open-WRT you can even safley overlock the process in the router, the system will tell you if it wont handle it through the SSH session into the router, it will tell you if you have gone too far, I.E yours i think is either a 200Mhz or 250Mhz Processor, you should be able to safely over clock it too 300Mhz.

However with that solution there is a downside, you cant store voice mail messages, you will have to work out a way to forward that part of it to another machine some where else on your network. but doing it this way you have End-2-End control over the voice traffic as you have Asterisk residing on both ends.

But i caution you on this level, you will not get full 54Mbit over your the Wireless link, especially over that distance using WEP/WPA, even if you have an 18DB Antenna on both ends it will still struggle a little bit due to Air traffic that will be passing by (Also Known as Air wave junk), so no amount of QoS you do over the Wireless link is going to avoid Packet Loss, you can only try to minimize its effects via Jitter buffer and the highest possible quality CoDec you can use. Then i add on top of that, over the Wireless link you have to keep in mind that the link will hit Saturation point at about 32Mbits (Assuming you can even truly get that which i highly doubt it), once the link hits Saturation point your going to start to loose a lot of packets and no amount of Jitter Buffering you do, or even the best quality CoDec you use is going to resolve that problem for you.

You are going to have to plan this out very very carefully, work out how many calls you expect to move over the link, preferably at the same time, what CoDec are you comfortable using (Obviously the higher Bandwidth ones at the better ones), and what are you plans for future scalability.

My only fear is you will blow the 12K and end up finding out you have a network that can not go past the point it is at now, do not let any sales person pull the wool over your eyes, i see it happen all the time, sales people are good with the technical jargon and are happy to take your money, but not there when you need them after the sale is done.

I hope you make the right decisions for this project of yours.



As this is a business deployment and not a test lab I will absolutely advise against voip over wireless links. Routing between buildings hardwired over the internet or physical link will give you better quality and peace of mind than wireless links (remember wireless is only 54Mbps and wired can easily give you 1Gbps). Because if anything falters (and it will more often than not with wifi) think about who’s head will be on the chopping board, YOU, senor. Leave Open-WRT for test labs and hobbyists, they are not business critical solutions (at least not yet). Its no joke when things go wrong especially with business phone systems.

You can check out techworxs.com/voip.htm . My boss has used them for our business for a while now with no problem. They have a small business asterisk-based VoIP package way more affordable than you are being charged. It comes with 5 IP Phones and you can just buy additional pre-configured IP phones (at a discount) and you are set. You even get a 24-port Gigabit and PoE switch to both power the phones and network them to the PBX.

I looked into many bridges. Only the Cisco and 3Com may do what I want, but the Cisco is $1000/each and the 3Com drops packets without a VLAN tag.

I e-mailed D-Link which has a WMM wireless access point/bridge. It seems WMM is useless and dumb in the sense it has no way of knowing which packets (voice vs. bulk) to prioritize. The guy from D-Link told me to get a QoS D-Link hard switch to put on both sides of the D-Link wireless bridge. :frowning: I am not a guru, but I am worried the QoS will get lost in the wireless bridge’s buffers.

Also, I found out that our phone system is so cheap because the phone vendor is counting on getting a CBeyond phone service reseller sales commission.