I’ll get back to The Cloud soon, I have my next post mostly written but it’s a very complicated topic; and what’s not too hard to deliver as a presentation is proving far more difficult to put into a blog post. I’m going to start seeding these with diagrams soon, which should make things easier for me. For now, I’m going to write another post based again on a request – this time from the IT Manager of a major Sydney school (I’ll ask his permission before I publish his name) which is to explain how to get his VoIP working better over his Wireless network.
The short answer is that VoIP will never be ideal over WiFi (for the techies and purist out there, I know that it’s 802.11 networking and not WiFi but that’s what most people think of, and what most labels say). There are several reasons for this that I am going to cover in this post.
The main issue with VoIP over WiFi is that Wireless Networking (the 802.1<abgn> standard for the techies out there) is a half-duplex standard which means that a device can either transmit or receive at any given time (see http://en.wikipedia.org/wiki/Half-duplex#Half-duplex for a fuller explanation/definition). This provides a direct conflict with Voice conversations which tend to be bi-directional. The technology is kind of like trying to use a phone as a walky-talky where you have to hold the button down to talk and let go of it to listen. Wifi has sufficient bandwidth to cope with this somewhat, but it is far from an ideal medium (whereas all wired Ethernet networks other than the original 10Mbps are Full Duplex and hence better suited to Voice (even Wired networking has some issues with VoIP but that’s an easier topic I will cover in a future post).
Another issue is that the majority of Wireless telephony devices operate using legacy protocols that operate at very low data rates (as of 2009, commercial-grade 802.1g (54Mpbs) handsets were only starting to be introduced just as the 802.1n (up to 600Mbps) standard was approaching ratification). There are a few reasons for this. Firstly, VoIP needs very little bandwidth to operate. Most VoIP systems utilise SIP (Session Initiated Protocol) with G.711 as a streaming CODEC which consumes between 97-105Kbps depending on which VoIP vendor you are talking to. This is far less than the 1Mbps that the lowest level of 802.11b Wireless allows for an the rule for Wireless is that the lower the speed, the greater the range so WiFi phones tend to request/demand very low bandwidth from your Wireless Network. Far from being a good thing, this is a major problem as Wireless is a shared medium which means that bandwidth available (22MHz per channel) is all you get amongst everyone connected to that Access Point. That means that a WiFi phone connecting a 1Mbps consumes just as much of your Wireless Network as a laptop connected at 150Mbps with 802.11n. A Wireless Radio (NB: a Wireless Access Point can contain one or more radios; most APs contain only one radio although 2 or more radios in an AP is becoming more common) can only operate at a single channel or frequency at any given time. This means that most WiFi IP phone will not only starve your network of performance whilst they are connected, but they will also cause constant contention (since they are likely the only low-speed devices on the network).
Our next issue is frequency contention. Wireless Networks run at divisions of either the 2.4GHz or 5GHz spectra. Virtually every VoIP phone (and most other WiFi devices) use 2.4GhZ. There are three main reasons for this: Range, Price and Power. Lower frequency signals carry further than higher frequencies (which is why mobile phones tend to operate at as low as 850MHz since Cellular towers are kind of expensive) which makes 2.4GHz far less expensive to deploy (less radios/access points needed) than 5GHz. In order to achieve any decent range, 5GHz devices need to consume a lot more power which can take a rough toll on a WiFi Phone (or Laptop)’s battery. There are two big problems with the 2.4GHz spectrum. Firstly, the allowed spectrum goes only from 2.401 to 2.483MHz (Japan allows a bit more). Since Wifi consumes 22MHz per channel, that means that there are only 3 non-overlapping channels in the 2.4GHz band (or only 1 non-overlapping channel if you use 802.11n which consumes twice the bandwidth)! In other words, if you are running 802.11b Wireless phones, the maximum data rate you can provide to any given area is only 33Mbps (11Mbps x 3 channels) or actually half that once Wireless overheads are taken into account. That’s the bad news, here’s the worse news: since 2.4GHz is an unregulated band, anyone can use it and everyone does. Microwave ovens, baby monitors, cordless phones, cordless computer mice, wireless headphones and dozens of other devices all gleefully use this area of the spectrum and there’s nothing you can do about it. I once did a Wireless Survey at a University where the PIR sensors that turned the lights on in the lecture theaters were operating (and absolutely blasting) at 2.450GHz, completely locking out a big chunk of their Wireless just so someone doesn’t have to flick a light switch when they walk into the room. It doesn’t matter where you go, you will always get frequency contention with 2.4GHz. The 5GHz band is far less utilised but for reasons mentioned above is less suitable
Now we move on to the biggest bugbear of all VoIP (not just Wireless) which is Quality of Service. Nothing you run on your network demands more “real time” bandwidth than voice. Voice quality can be horribly distorted if any packet loss, delay or jitter is present on your network and that’s particularly bad news for WiFi (which is a slow, shared, contended media as described above). To deal with these issues, VoIP traffic needs to be granted absolute precedence on your Wireless network over any other type of traffic or it will be very poor. This is one of the reasons that VoIP over Wireless tends to starve your network’s performance (as described above). Until around 2007 (when the WME extensions to 802.11e were released) there was no standards-based method of providing QoS for Wifi which made deployments very problematic. One company – Spectralink – released a method of providing QoS for Voice over WiFi called SVP (Spectralink Voice Protocol) which depended on a dedicated appliance to govern the traffic in conjunction with compatible access points which many vendors (including Cisco, HP, Motorola, Proxim, etc.) subscribed to. Whilst Spectralink worked reasonably well, it was expensive, difficult to configure and allowed only the most basic (and easily hacked) encryption to be used on the network. With the release of standards for Wireless Media Extensions (or WMM for Wireless MultiMedia as its more commonly referred to), SVP became obsolete but many AP vendors still support it as there are still many older SVP-based IP handsets in use (mainly due to their high cost of replacement).
Those are the main issues with VoIP over WiFi, there are others but those are enough for you to see that it can be a challenging solution to deploy.
I’m sure that by now the gist of this post must seem to be don’t deploy VoIP over WiFi but nothing can be further from the truth. This technology is very useful and can be successfully deployed if the right strategies are used.
Before I get into how to do it right, I’m going to digress into the single biggest issue I encounter with not just VoIP over WiFi but Wireless Networking in general: More is better. I’m going use use bold, underline, italics and caps for the following: NEVER EVER BELIEVE SOMEONE THAT TELLS YOU THAT YOU CAN OVERCOME YOUR WIRELESS ISSUES BY FLOODING YOUR SITE WITH ACCESS POINTS OR RADIOS!!!
I use so much emphasis because I encounter this so often and it is upsetting to see organisations being ripped off by forcing them to buy needless amounts of equipment and spend huge amounts of money (it’s not just the APs, it’s the cabling and the data ports they consume along with their licensing and management that really costs money) with the result being an environment that’s worse than they started with. Yes, worse! As detailed above, there is very limited spectra available for use and the more APs you have in any given area, the more that spectrum gets carved up and the less efficient the overall network becomes. This is a particularly big problem for VoIP over WiFi since people tend to be moving when they are on the phone; not only does a site “flooded” with APs tend to cause the phone to “radio-hop” (i.e. jump from radio to radio or AP to AP) excessively, the results of that hopping can cause havoc on the rest of the network (also as detailed above). Each WiFi phone will be like a little ghost roaming around the premises causing all sorts of issues wherever it goes. It doesn’t even need to have a call active since the phone (like any other Wireless device) is always “on” the network (how else could it receive a call?) unless it is switched off. If you aren’t sure if you have too many radios or APs, download a program like Netstumbler (http://www.netstumbler.com/) and run it at several locations around your site. If you see full coverage of the three non-overlapping channels (i.e. 1,6 and 11) from 3 distinct radios and maybe a small amount of signal in those ranges from other radios then you probably have the right number of devices. If you see the same channels being strongly presented by multiple devices then you have a problem.
What can you do about it?
OK, enough with the doom and gloom. Let’s move on to steps you can take to ensure decent VoIP over WiFi coverage. These are only rough guidelines to help you; doing this right really needs a trained Wireless Networking professional with the right tools and knowledge but a lot of people claiming to be that really don’t know what they are doing so see if they are following these steps and if not, you might want to consider how qualified they really are.
Do a Wireless Survey. Whether you are planning a new deployment or are having issue with your existing environment, a proper Wireless Survey is a must to work out optimal radio placement and potential blackspots. Top of the line survey tools (like “AirMagnet”) can run to over $10k but they will provide very exacting information regarding what you need to do to get your Wireless Network deployed right. Note that a Wireless Survey of a large campus can take a week or longer so expect this to be a fairly expensive exercise but if you have a lot of problems, or if you can afford it, it is worth it when done properly.
A follow-on to a Wireless Survey is investing in decent Wireless Management tools for your environment. Most major vendors offer pretty decent self-branded tools that will allow you to easily prepare “heat maps” of your Wireless Network as well as to detect “rogue” devices (i.e. APs that don’t belong to you but are transmitting in your space) as well as blackspots (where there is no coverage), hotspots (where there is heavy congestion) or areas where there is too much congestion. These same tools can often also help you “tune” your network (see below). If your vendor doesn’t offer the right tools or you are running a multi-vendor environment (more on this below as well), there are excellent 3rd party solutions (like Aeroscout http://www.aeroscout.com/) you can use.
Tune your Wireless Network properly. I made strong mention of the issues of too many APs above, but how many is too many? The answer is based on the amount of channel overlap you have. A lot of environments (most notably secondary and tertiary schools, where every student has a Wireless laptop or similar) really do need a lot of coverage. It is not uncommon to even see two radios per classroom in some schools and this is fine, as long as they are tuned right. Avoid the temptation to turn the gain right up on the antennas. If anything – in high-density environments – you need to turn them right down in order to make sure their coverage is as localised as possible. Some vendors’ APs are “self-tuning” but my experience is that this is usually less effective than taking the time to set the antennas properly and lock them that way. Use the tools I mention above to do this and review it at least once a year (or whenever there is a problem, or the Wireless Network is being expanded).
Move high-bandwidth devices to the 5GHz band only. These days, most laptop and tablet vendors offer 5GHz WiFi in all but their cheapest models. Not only is the 5GHz band less cluttered, it also has many more channels to use for overlapping coverage. The number of channels differs by country (due to local regulations) but Australia – which is where I live and allows an average number of channels – has 21 non-overlapping 20MHz channels (or 10 for 802.11n). This firstly allows you to deploy far more APs/Radios than 2.4GHz (which allows you to provide far more data) but will also pull most of your contentious data off that spectrum leaving far more available for Voice. The only devices that should be on your 2.4GHz network should be Smartphones (which still mostly use 2.4GHz due to power constraints) and VoIP WiFi phones. This may mean investing in 5GHz dongles for some legacy laptops but these can be obtained for under $50 each. This will also mean getting rid of any 2.4GHz wireless printers (or print servers) you may have deployed. Given how much data is consumed by printing, these are likely causing issues anyway!
Consider reserving a channel just for VoIP. This is a fairly draconian but effective measure for maximum VoIP over WiFi performance. Choose one of the three non-overlapping 2.4GHz channels and use it exclusively for VoIP, allowing data devices to use only the other two channels. This will guarantee that your VoIP usually has decent bandwidth available (subject to interference from non WiFi devices as detailed above; use the cleanest of the three channels for VoIP). If the previous strategy is employed, this will only reduce the amount of data available to Smartphone devices.
Go with a single-vendor, managed WiFi solution. Wireless Networks come in two general flavors: un-managed and managed. Un-managed networks are where each Access Point is configured (and runs) individually. Managed networks use a central controller to administer and propagate policies to all the APs. Managed networks allow technologies like fast-roaming (i.e. quick handover from one AP to another during times of movement or congestion) and Layer 3 roaming (roaming between different network subnets) as well as better overall network management and congestion mitigation. But…
Avoid “Captive-Portal”-based managed WiFi solutions. There are two ways that managed Wireless Networks function. One way (and still the most common) is to tunnel all the traffic from the AP through the controller before sending it to its ultimate destination. The other way is for the AP to receive policy from the controller but to send the traffic is receives directly to its destination. The former solution is a major reason for poor VoIP over WiFi since the controller itself becomes a major bottleneck point. If you already have a Captive-Portal System…
Consider a dedicated Wireless Network just for VoIP. This isn’t as expensive as it may seem. Remember that the lower the data rate, the greater the range. Whereas 11Mbps 802.11b has a nominal transmission range of around 30m (indoors with typical walls), 1Mbps 802.11b has a range of around 90m! This means you can deploy far fewer APs to attain the same coverage for VoIP as you ordinarily would need to use for data. Further, performance could be greatly optimised since not only could you reserve a channel for VoIP, you could also lock the data-rate of the AP to match that of phones. This should be a last resort but if VoIP over WiFi is really important to your organisation it may be the only solution.
There you have it, my view of what issues you can expect to encounter with VoIP over WiFi and some of the ways you can deal with them. I must again stress that this is an area where good expert assistance will make all the difference to a successful deployment.
I would like to make one final note, and this is more around security than performance. Remember that many WiFi handsets only support very basic WiFi encryption which is very easy to hack so make sure that you isolate your “Voice” network from everything else using appropriate security policies on your APs, switches and routers/firewalls. If you consider the confidentiality of your VoIP traffic to be important then make sure that you invest in devices that support strong encryption (such as WPA2) if you don’t want people listening in on your conversations.