Introduction to Wireless Networking

Introduction to 2.4GHz WiFi Click here

Introduction to 5 GHz WiFi Click here

Introduction to 11N WiFi Click here

Wireless networking is, potentially, a quick, easy and economical alternative to running wires around your home or office. It also opens up possibilities for connecting buildings which are up to several kilometres apart. There are currently three standards upon which wireless networking devices are built. The table summarises some of the features for each

Standard Phi Rate Frequency Comment

802.11a

54 Mbps

5.1-5.8GHz

Becoming very popular for outdoor links due to isolation from all the 2.4GHz band rubbish. Still waiting to take off for indoor use but the large number of non-overlapping channels compared with the 2.4GHz band might end up making 5GHz more popular for 11n WiFi. Uses OFDM over all speeds so theoretically better non-line-of-site capabilities.

802.11b

11 Mbps

2.4GHz

The first system to appear at mass-market pricing. Suitable for both internal and inter-building applications though poor penetration and scatter of 2.4GHz radio can reduce effectiveness for both indoor and remote bridging applications. Not seen much nowadays.

802.11g

54 Mbps

2.4GHz

OFDM 2.4GHz standard gives much the same functionality as 802.11b but at higher data rates. The OFDM standard is supposed to give improvements over the older 11b products for indoor (non-line-of-site) use though this is offset, to some extent, by the OFDM function only operating at speeds above 20Meg. Most WiFi nowadays uses 11g.

11n

Up to 300Mbps (or even 600Mbps)

Available for 2.4Ghz and 5GHz frequencies

Uses 40MHz wide channels (though devices normally have the option to use single stream 20MHz WiFi channels – so-called ‘half-n’) and several other packet efficiency tricks to improve data throughput – typically three or four times the throughput of ‘normal’ 54Meg WiFi but theoretically can go can even higher than that. On paper it’s a lot better for non-line-of-site operation either indoors or outdoors but in practice the jury seems to still be out on this point. Due to pressures from the WiFi chip manufacturers 11n is looking like becoming the de-facto standard in the next few years. You can read more about 11n ********here*********

Range

You will see specifications for different brands of wireless networking devices quoting wildly different ranges. Take these claims of huge range with a pinch of salt. Unless the manufacturers have got something very wrong, or are operating at illegal power output levels, then products from different sources will behave much the same within similar parameters. Any variations in the true range between products, assuming the comparison is done with products running at similar power levels, comes down to factors such as antenna gain (different gain alters beam spread and also alters the affective receive sensitivity), receive sensitivity, and (particularly for indoor use) the ability to cope with multi-path reflection affects.

There are two types of application in which wireless networking is used: internal and inter-building.

Indoors
Radio waves travel in straight lines and at 2.4GHz do not penetrate obstacles very well. Some surfaces reflect the signals quite well whilst others tend to absorb them. Water, which comprises most of you , is particularly good at absorbing the energy, so you will find that putting your hand over an antenna can reduce the signal substantially. (Your hand won't warm up because output power is limited to 100mW in Europe - well below the power output of your mobile phone!). 5GHz wireless suffers from similar problems BUT, with better penetration and scatter, it can give improved non-line-of-site (NLOS) capabilities over 2.4GHz devices.

As a general rule 802.11b/g devices will usually cover a house quite well but there are no guarantees. You might also find, particularly for 11g, that the connection speeds will drop in order to get a reliable link. In fact, often an 11g product with up to 54meg potential will give similar performance to an old 11b device even though 11b is only up to 11meg speed. The signal passes better through wooden floors and ceilings than through brick walls, and has no chance at all through concrete or stone. The use of an access point in the loft connected to a directional antenna pointing down from the rafters has proved an effective way to get full coverage in a typical house. For a more restrictive range the built-in antennas often work very well.

Your choice of wireless network adapter may be significant. If your mini tower PC stands on the floor with it's back to a radiator, you can't a expect built-in adapter with integral aerial to work very well. Or if you tend to sit with your notebooks aerial sticking out of the left hand side and your access point is down the corridor to your right, then the computer itself will screen the signal. Try an adapter with an external aerial that can see over your keyboard.
See our 'Wireless Around the Home' article for a more in depth discussion on using 2.4GHz wireless in buildings

802.11a/5GHz radio, although costing a little more than 2.4GHz products, has much better penetration and scatter making it better for indoors operation where it needs to reach remote rooms. In our opinion, especially with 11n technology, 5GHz is the future for wireless networking in buildings (mind you we’ve been saying that for years now and still waiting for it to take over from 2.4GHz products!)

You can read more information on 5GHz wireless in our '5GHz Frequency Bands' Article.


Outdoors
The following table is a guide to the distances you might expect to achieve using 'off-the-shelf' wireless devices a standard 100mW access point with a given antenna gain at both ends . A receiver sensitivity of about -83dB is assumed.

Effective Gain Line of sight range for 2.4GHz
(20db UK power limit)
Line of sight range for 5.4GHz
(30db UK power limit)

0dB

200m

280m

4dB

440m

620m

7dB

620m

868m

10dB

1.25Km

1.75Km

13dB

2.8Km

5Km

16dB

5Km

7Km

20dB

12.5Km

17.5Km

24dB

31Km

43Km

 

  • Effective gain takes into account the antenna gain and also any losses in cabling. For instance if you have a 12dB antenna but lose 5dB in the cable to it, then you have an effective gain of 7dB.
  • The table should only be used as a rough guide, we are not promising that you will achieve these numbers, nor that the figures represent maximum or minimum limits.
  • Please also see our article about line-of-sight signal propagation.
  • You should ensure that you do not exceed any legal power density limits which apply in your region.
  • Metal external antennas can make good lightning conductors! Consider what equipment you are putting at risk if you choose not to invest in a lightning arrester.
  • The distance figures for 5GHz operation may not look that much better when compared to 2.4GHz but it's worth remembering that the permitted power output levels for 5GHz are 10 times those for 2.4GHz wireless (100mW vs 1W)! That means, if you stick to the legal power limits, then the practical range for 5GHz devices is further than the equivalent 2.4GHz equipment. Unfortunately, in the real world very few people stick to legal RF power limits so you will often find people using 2.4GHz outdoor units over huge distances.

Antenna Gain
Q. How does an antenna produce gain?
A. By focusing the available radio energy in one direction.
Q. OK, so how can an 'omni-directional' antenna have gain?
A. Because it radiates around itself in a disc pattern, stealing power from above and below.
The above Q&As should help you get a feel for the radiation pattern for different types of antennas. A 0dB antenna radiates equally over a complete sphere. Bearing in mind that 3dB represents a doubling of radiated power, you could imagine a 3dB directional antenna radiating its signal into one half of that sphere. A 3dB omni-directional antenna would have a radiation pattern in the shape of a sphere with a cone removed from the top and bottom.

Diversity Receivers
Wireless signals (both 2.4GHz and, to a much greater extent, 5GHz) reflect readily off many surfaces, there will often be a pattern of patches around the room where reflected signals cancel out the direct signals leading to 'dead zones'. If your antenna is in such a dead zone you get no signal. However, for every dead zone there will be a 'double-power' zone. Due to the shortness of the radio waves at these high frequencies then the distance between a good patch and a bad patch can be only a few centimetres Wouldn't it be nice if you could have a second antenna, just a few centimetres away? Then, there would be a good chance of this second antenna sitting in a better reception zone.
A system with two aerials in this arrangement is called a diversity receiver. Many wireless devices use diversity receivers to try and improve NLOS operation. Of course, 11n technology with multiple antenna and data streams is really a super version of diversity – this is one of the reasons why 11n is theoretically better for indoor/non-line-of-site operation.
More Articles

More in depth discussions on using wireless devices can be found on the following articles:

Line of Site and building linking

Download Wireless Around the Home

Download Wireless Networking Prima

Download Linking Buildings using Wireless

Download Wireless on Van Sites

 

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