Top Ten Wireless Network Misconceptions
1. Omni directional aerials are omni directional
Omni directional aerials radiate mainly around the horizontal axis – that is the signal is coming out the side. Aerial gain has to come from somewhere, so they achieve the gain by focusing the signal into a smaller area. The higher gain the aerial, the more focused the beam and so the beam width narrows down – in some cases to only +- 3 degrees from vertical. This is why you may find in a theatre, you can reach the stage from the stalls, but get nothing in the balcony. For a better explanation see understanding aerial plots
2. The more height the better/the more gain the better
This is really point one over again. Putting an omni directional aerial high up means that most of the signal passes straight over the top of the area you are trying to serve. Keep the height high enough to clear obstructions but low enough to mean that the other end of the link is falling in the vertical beam width of both aerials. In these situations where the height is mismatched, the worst performance will be achieved using two omni directional aerials as the beam from one will miss the sensitive region of the other. If you want to provide omni directional coverage, use patch antennas that can be tilted downwards.
The one area where height is good is when using long range point to point links with yagi directional aerials.
Gain is a two edge sword – it gives you more signal strength in the area it covers and then very rapidly ends up causing huge losses outside the coverage area. This is a good thing if you want to reduce the interference from areas outside those you are covering, but a common mistake is to choose a too high gain aerial , then fail to realise its beam width is so small that it doesn't quite cover the area properly – outside its coverage are it can introduce enough loss to reduce the links range in the area you are trying to cover to a few metres.
3. Microwaves don't go through trees
Well they do, but it depends on the type of tree. As I'm sitting here, my internet connection is traveling about 1.5 miles through six trees. However they are all large leaf trees and whilst a lot of signal is scattered, there is enough to provide me with an almost perfect link. If the trees are deciduous, it is a different matter, one wet leylandi would kill the link completely. A rough guide is that a tree will cost you 3-4db of signal strength – or about halve the working range.
4. 11mb lans work at 11mb, 54Mb lans work at 54mb
This depends on the manufacturer. In truth most 11Mb lans work at a real throughput of between 2 and 5 Mb. The rest is lost in transmission overheads.
5. There are 13 channels in the UK for 802.11b
In reality there are only three non overlapping channels, whilst there are 13 centre frequencies, spaced 5MHz apart, the signal uses 22MHz. So it is only possible to get three clean channels.
6. 802.11 can cope with interference as it listens before it sends and has collision avoidance.
In theory – yes, but that assumes that every station can hear every other station. It is a bit like a conversation: we all take turns at speaking, but if twelve people were in a room, and six people couldn't hear when the others were speaking, they would end up talking at the same time as everyone else and when they get no response, they would repeat themselves. This is what happens on a wireless LAN; as messages get lost they get repeated, but another station starts transmitting so both get repeated and pretty soon the actual data being passed becomes a fraction of the correct amount as all the bandwidth is used up with retransmissions. Whilst the standard has a mechanism for controlling this, not all manufacturers equipment actually utilises it and it only has a limited amount of success. see Hidden Nodes for a more detailed explanation.
7. 802.11 systems are Line of Sight – If you can see it you can connect to it
Whilst 802.11 systems are described as line of sight, this is somewhat misleading. On the positive side it does work through walls – on the negative side, there is an effect called the Fresnel zone. For a radio signal to travel a long distance between points, as well as the direct path it needs an extra area around the beam that looks like an ellipse – narrow at each end of the link, but wider in the middle. There is more information on the site on fresnel zones page, but to give you an idea, for a one mile long link, you need clearance of a radius of about 15 feet clear at the center point.
8. 22mb and 54mb systems are better than the 11mb systems
They are faster, but their ranges are less. The 22mb systems and 54Mb 802.11g require more signal to achieve a faster connection, the 54Mb 802.11a has a very short range and the 54mb 802.11a band C systems are much more expensive and not available as pcmcia or pci cards.
9. 802.11 links can work for 20/30/60… miles
There are sites on the internet which talk about being able to make links work over that distance, but what they don't always mention is that the operators hold amateur radio licenses – they have to pass a test and the link can only be used for non commercial purposes. With the right (and expensive) equipment, a reliable link of nine miles at a real throughput of 500Kbit per second is legally possible – but your aerials would probably have to be mounted at over a hundred foot high at both ends! Links of that length are not feasible in built up areas as the link will probably be interfered with by other stations in its path.
If you run more power than that, especially in a built up area, you are likely to annoy a lot of people and draw the attention of the radio communications agency. A calculator can be found in the useful tools section that will allow you to estimate the likely coverage ranges for a given set of equipment
10. The cable that came with the aerial is low loss.
You would be surprised. We have seen suppliers sell aerials where the loss in the cable is more than the gain of the aerial. Make sure that for a run of more than 10 feet the cable is times microwave LMR400. For a 100 foot run use times microwave LMR600 or preferably move the access point/bridge to the bottom of the aerial and use a smaller cable. For short runs use LMR195 or LMR240.
See the aerials topic for more information on cable losses.
…and an 11th for free
11. You can make a really good aerial with a pringle can
Yes, but you can make a better one even more easily with a large nescafe tin. Pringle cans go soggy when it rains too…
