Network

Optimising wireless access for the small business - tips for site surveying and analysis

Most people share the same idea of Office Hell. A plain, bland and featureless big room, with open desks that give no privacy or peace. At best, a maze of little cubicles with walls no higher than five feet eight, no door and no ceilings.

The good thing about Office Hell from the IT point of view is that it’s very easy to create working wireless networks there. There is nothing to impede the signal. Unfortunately for wireless networking (but fortunately for the people who work there!) real offices are often far more complicated. Offices used by small businesses are often in close proximity to other offices, occupied by companies with their own IT infrastructure. Or they’re in older buildings, with heavy walls. They tend to have multiple rooms and spaces needing wireless coverage. They may not have structured cabling for a LAN; put it all together and it means all sorts of problems for a wireless network.

Different floor plans create different problems when you’re setting up a wireless network. Old buildings, heavy walls, and internal subdivisions all mean the wireless signal won’t go as far. Other companies nearby mean you’re unlikely to find a channel free of interference for Wi-Fi. And without a well-cabled LAN you become more restricted in your choice of placement of wireless access points.

It may not be possible to get a wireless network configured perfectly under these circumstances. But with surveying, analysis and careful setting-up, you can definitely get a wireless network that’s configured as well as it can be for the site in question, which can mean much better bandwidth than putting an access point in the first place you think of.

Working in shared spaces
Shared space is convenient for small businesses, as it often includes some office services. The main problem is that very often, many or all of the wireless channels are already in use. This makes the choice of channel to somewhat complicated. However, understanding how wireless channels interact (See "The problems with channels") will allow you to pick the best from what’s available.

So the first thing to do is conduct a site survey. What you need to know from this is what other wireless networks are in the vicinity, what channels they are on, and their relative strengths. There are tools available to do this (see "Surveying the site:tools for the job" for recommendations. You will need to conduct the survey in all parts of the office: finding the sweet spot with minimum interference from other wireless networks in the vicinity gives you the optimum place to site access points. Once you have decided where interference from other wireless networks is at a minimum, you can to choose the channel for your network. What you’re looking for is the wireless channel that has the fewest number of overlapping channels in use — even if that channel is in use itself. There are two factors to bear in mind. Firstly, signals on overlapping channels count as noise to the signal on your channel. This causes poor signal to noise ratios, which are the main reason for an 802.11g connection dropping down from its nominal 54Mbps connection speed to something lower.

Don’t worry about choosing a wireless channel that’s already in use. The 802.11 standard takes care of this; two networks using the same channel operate a form of collision detection, very much in the same way that old 10Mbps Ethernet networks needed to before the invention of switched networks.

But the least busy channel is only a starting point and you’ll need to try it out; if the other users of that channel have many older 802.11b devices then your network will be dragged down to 802.11b speeds as well. The second-best channel may end up giving you better signal than your initial choice. Experimentation and testing is key here, including trying out different positions for the access points as well as the channels. Bear in mind that where many wireless channels are in use, other businesses will also be trying to get the best possible signal for their network. This may well result in impact on your network by interference or by channel sharing. Monitor network speed and bandwidth; you may find you have to come back to run another survey and consider moving to another channel.

There are some other tricks that are well worth trying. The placement of your wireless access point is important but you can give yourself more flexibility by upgrading the antennae. The standard antennae supplied with wireless access points are omnidirectional — they radiate power equally in all directions. This could be perfect if the office is the right size and shape and you can place the AP centrally. However, directional antennae are available too. They radiate all their power in a specific direction, which could boost signal strength where you need it — and also help nearby neighbours by not radiating signal in their directions, which improves security too. Make sure first that your access point has a removable antenna and a standard antenna connection.

It’s not too early to consider an 802.11n-based network too. Even though the standard may not be ratified for a year yet, it has several advantages compared to 802.11b and 802.11g. Firstly, 802.11n devices can operate on channels in the 5GHz frequency band rather than the 2.4GHz frequency band. This frequency band is not heavily used yet, which could solve your problems at a stroke. Secondly, it uses multiple antennae to enable multiple-input multiple output (MIMO). The greatest advantage of MIMO is that is compensates for signals returning to the access point by multiple routes. This gives greater effective range (up to 70m indoors, from up to 38m of 802.11b and 802.11g) as well as improving bandwidth.

If all the channels in the 2.4GHz band are effectively unusable at the site due to other networks, and your devices can use 802.11n (either natively, or with adaptors) then it’s a good option. You’ll probably want to plan for an 802.11b&g network for legacy devices that do not require maximum throughput, and a 802.11n-only network for n-capable devices. This would give the best of both worlds and a clear path to an entirely 802.11n-based network as legacy devices, and eventually the 802.11g network itself, are retired. Make sure that the 802.11n access points you specify come with a guaranteed update to make them compatible with the final standard, when it’s ratified.

Old and solid buildings
The other type of small business premises you’ll often have to deal with have been converted from an existing building. In towns and cities these are often old, solid brick or sandstone buildings. In more rural areas many farms and out-buildings have been converted into office space. These types of buildings have a different range of problems from high-density office space and the main one is the signal range. In one way, this is a good thing. There is much less chance of interference coming from other wireless networks in the area. However, the downside is that you’ll probably need more access points for the same size of office.

Again the first step is to set up a wireless access point temporarily, and to survey how far the signal travels. If range and signal strength are a problem start by moving the access point around to find a position that optimises coverage. Of course, you will be constrained in this by access to power and to the LAN, but small changes in position can sometimes make large changes in coverage. Also consider directional antennae again – there is no point radiating half your wireless signal out a window or into a wall if there is a way of projecting all the signal forwards into the office.

Managing multiple APs
If you’re installing multiple wireless access points, there are three ways you can use them to provide greater wireless network coverage. They can be configured as separate, individual networks, as multiple access points providing the same network, or they can be set up to use WDS (Wireless Distribution System) to extend the range of a network.

Using separate networks is rarely a good choice. It requires multiple channels and means each mobile device would need to be set up to work on all networks. The only time this is really useful is when the customer needs a private ‘company only’ network, and another that’s accessible to other users as well as the employees.

If you have a good-quality wired network where you need to install access points, then multiple access points providing the same network is a good choice. To do this, set each of your access points to the same SSID and assign the same WEP key — this will enable wireless devices to roam from one access point to the other. You will need to consider the wireless network channels in use though, and choose ones that do not cause interference between your access points as that reduces the range and throughput of each access point. As before, it’s perfectly possible to use the same channel for two access points, as they will detect network collisions between themselves, at the expense of a little throughput.

Wireless Distribution System comes into its own where you cannot attach an access point to the wired network directly and so need to extend range. WDS-capable devices allow access points to connect back to the AP that’s plugged in to the LAN, extending the wireless range by repeating the signal from one base station to another. Wireless clients can connect to any access point in WDS repeater mode. WDS also has a range-extending bridging mode, where clients are not allowed to connect; this may be useful to extend range in some circumstances, for example when you have two buildings that you can’t easily run a cable between.

WDS suffers from two implementation problems. Firstly, effective bandwidth is halved because when a wireless client is connected to a remote access point, that access point needs to receive from the wireless client and also transmit to the primary access point at the same time. This will get progressively worse as more wireless clients use the remote access point. Secondly, dynamically created WPA keys can’t be used; you have to set up pre-shared keys instead. The major problem with WDS is that it is not certified as a standard. Equipment that all comes from one manufacturer is likely to work together, equipment from several manufacturers is much less likely to co-operate.

Given these problems, WDS is a limited solution for very specific purposes. However, when there is no way of attaching an access point to the LAN when you need it to be, it’s the only available solution.

Again, 802.11n is a potential option for solid buildings. While range will be reduced when using the 5Ghz band (the higher the wireless frequency, the more the signal is attenuated by distance and by solid objects in the way), 802.11n also supports 2.4GHz and MIMO is very likely to provide some positive benefits in signal coherence, increasing the potential range of each access point under these challenging circumstances.

Should everything be wireless?
Now that every laptop and many peripherals have wireless built-in, it’s very tempting to do away with cables entirely, and set everything except the servers up to use wireless access.
You have fewer wires, less mess, more flexibility. There’s one obvious compromise; the speed is lower than wired Ethernet. But there’s another, less obvious compromise, because
of the way the Ethernet protocols work. This means that devices attached to a wireless network share the available bandwidth with each other, rather than having the full bandwidth to themselves, as in current LAN design.

To understand this, we need to step back in time to the days of thin and thick 10Mb Ethernet. Back then LAN design was something that needed to be done carefully, as bandwidth on any segment of the LAN was shared by all of the devices on that segment because of the Ethernet protocol itself. To send a packet over Ethernet, a device listens to see whether any other device is transmitting. If not, it sends the packet. However, another device could also have decided to send a packet at that instant. The packets collide, the devices realise this, and wait for a random time before listening to see if they can send again. All devices where their packets may collide with each other are said to be in a ‘collision domain’.
LAN switches alleviated this for wired Ethernet. Each port on a switch is isolated and therefore is in a collision domain of its own. Traffic on a port does not affect traffic on another. However, there is no equivalent of a switch inside a wireless access point. And that’s why a single wireless network is not suitable for a large number of devices. All of the devices attached to the access point are in the same collision domain, and heavy use of the wireless network by one device will impact the usability of the network for everyone else. You could mitigate the situation by using multiple wireless networks — but if it’s hard enough to set up one good wireless network because of others in the vicinity, it may be difficult or impossible to set up two.

For this reason, it’s still worth using wired Ethernet wherever possible. Certainly for servers, peripherals, user workstations that don’t move, and all similar devices. Wireless is great for guest access, and to provide an extra network for phones, laptops on the move and similar devices. However, a wireless network must still be seen as a useful bonus, not as the default way to set up a customer LAN, tempting though it is.

Visitors and security
Businesses often want to provide wireless access to visitors and guests. Obviously this wireless access needs to be carefully controlled; nobody wants guests, no matter how innocently, being able to access business data. This means that a guest-access wireless network should be on the outside of the company firewall.

This of course means another access point and another wireless network. But it won’t be heavily used, so could easily use the same channel you’ve chosen for your main network. It will need security, of course, but changing the WPA password from time to time will not be an inconvenience for visitors (or significantly more work for you). Configuring it to use a lower-power transmission mode will limit its range too, reducing the chance of accidental discovery (or people sitting parked outside to use it).

Getting a wireless network working as well as possible is going to take experimentation, trial and error and ongoing monitoring to make sure that everything stays optimised. It’s important to educate the customer about what wireless can and can’t do so they’re not caught out by relying on a good technology under difficult circumstances.

Insight
Surveying the site: tools for the job

You can perform a basic survey with no tools at all; the simplest way is to wander around your site with a laptop, looking at Windows’ signal strength meter in the bottom right of the start bar. However, there are tools that make the process much easier, showing which other networks are in the vicinity and their relative strengths and channels.

Professional tools include Airmagnet and Airsleuth Professional (for Windows XP or Windows Vista). Both tools are fully-featured spectrum analyzers and will give you all the information you need pretty much automatically. Neither are cheap and while the free tools available are certainly as powerful, they do give you the information you require in order to conduct informed analysis if you’re prepared to do more of the work yourself.

For Windows XP and Vista, a useful tool is InSSIDer, which shows nearby networks, information about them, and their relative strengths. Moving around the building whilst running InSSIDer lets you see how all of the networks nearby interact with your network. Various other free tools are available including Netstumbler (XP only) and Vistumbler (Vista only). These lack the graphical comparison between networks but are good tools in their own right; Netstumbler can play an audible tone that changes in pitch as the signal grows stronger or weaker, so you don’t need to keep your eyes fixed on the screen.

For the Mac, AP Grapher shows a table of available networks, their channels and relative strengths, and for the network you are on it shows a graph of signal, noise, ratio, and currently negotiated network speed. This is very useful indeed, making it very clear how the quality of the network is affected by your location in the site. iStumbler is also an handy tool, but doesn’t graphically show network speed.

InSSIDer

Vistumbler’s main window shows details of nearby networks.

Insight
The problems with channels
You would think thirteen wireless channels would be enough to go around. And they are; but at the expense of any of them working well when many channels are in use. This is because there are two fundamental problems in the way the channels and protocols are set up — and this limits performance when adjacent channels are in use.

The problem with channels is that each wireless channel uses a bandwidth of 25MHz, but is separated from its neighbours by only 5MHz. This means that there is a large degree of overlap between one channel and another.

In Europe, the only channels that do not overlap with each other are channels 1, 6 and 11. The second problem is that signal on a channel that overlaps yours is treated as noise, which affects the signal to noise ratio, which determines the transmission rate you obtain — simply put, less noise means better transmission rate. The noise increases too if the adjacent channel is being used with 802.11g devices.

This means it can be better to use an existing channel that has no overlapping channels in use, than to use an empty but overlapping channel. While noise reduces the signal to noise ratio for everyone, the collision detection mechanisms means two networks can often share a channel, affecting each other only in periods of heavy use.

Sharing 802.11b and 802.11g devices on the same channel also reduces throughput – as 802.11g devices drop down to 802.11b speeds when an 802.11b device is transmitting. Transmission rates of the proposed standard, 802.11n, are also affected by 802.11b devices, but are not significantly affected by 802.11g devices.

The above 2.5GHz spectrum that Wi-Fi uses is divided into 3 channels, each 25MHz wide. It’s easier to pick a channel once you understand the overlap between adjacent wireless channels; remember that signals on overlapping channels are noise to your channel.


 
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