LAN e WLAN
Alguma Teoria
Self-help book for WLAN installers/providers
Introduction
To meet the needs of many people interested in building WLAN network, we have decided to collect in one article a bit of theory and practical information on quick and effective implementation of wireless networks working in 2.4 and 5 GHz bands (802.11).
WLAN (Wireless Local Area Network) means technology which allows to build wireless data networks with satisfactory parameters and quite large ranges of operation at a comparatively low cost. Additional advantage of this technology is short time needed for its implementation.
Potentiality of WLANs
- wireless access to local network in home, office, business etc.
- wireless access to the Net in public space, e.g. in airports, stations, cafes etc. (hot-spot)
- wireless point-to-point links (connecting LAN networks, telemetry, remote control, remote monitoring)
- wireless access to the Internet (both in cities and in the country)
- emergency communications link (duplication of wired network)
WLAN standards
We will describe some solutions compliant to the next three standards:
- 802.11a - in 5 GHz band: 5.150 - 5.350 GHz and 5.470 - 5.725 GHz, transfer rate up to 54 Mbps;
- 802.11b - in 2.4 GHz band: 2.4 - 2.483 GHz, transfer rate up to 11 Mbps;
- 802.11g - in 2.4 GHz band: 2.4 - 2.483 GHz, transfer rate up to 54 Mbps;
However, there are also used other standards:
- 802.11f - IAPP - Inter Access Point Protocol - for cooperation between access points;
- 802.11i - standard defining new security methods in wireless networks;
- 802.11n - standard for transmitting multimedia in homes using MIMO technology, up to 300 Mbps;
- 802.11e - standard defining QoS - support for high quality of services;
- 802.16 - WiMax standard for backbone networks of high capacity
Wireless network range
It should be realized that the range of a wireless network depends on many factors; we can have an influence on some of them and the rest is unknown. The range of wireless network depends on:
1. Factors related to the devices used:
- output power (it has been decided by the manufacturer),
- cable attenuation (depends on the cable and its length),
- gain of the antennas (given by the manufacturer),
- sensitivity of the devices (given by the manufacturer).
2. External factors:
- attenuation between antennas (can be estimated basing on FSL model);
- interferences from other devices (can't be predicted - some additional margin of safety needs to be provided to compensate these interferences),
- influence of physical barriers (walls, floors, trees etc.)
So, if we want to know what would be the effective range of our network we have to gather the information mentioned above and carry out simple calculations showed in the further part of this self-help book.
Propagation of radio waves
Fresnel zone
Fresnel zone is one of the most important conceptions connected with propagation of electromagnetic waves, which is indispensable to assess parameters of wireless link. It is the area actively participating in transmission of radio signal energy. Shape of this area is an ellipse in longitudinal section, and circle in cross-section. Radius of this circle is a function of the ratio of distances between antennas - it has the maximum value in the middle of the link. The importance of first Fresnel zone comes from the fact that almost all energy of the signal is conveyed via this space.
The shape of Fresnel zone. R1 is the radius of the I zone.
 | [m]; | where: |
- dkm = d1km+d2km, is the distance between masts in km
- d1km - distance from the first antenna in km
- d2km - distance from the second antenna in km
Wrongly made installation. The installer didn't secure mutual visibility of antennas. The radio link does not work.
Another example of wrongly made installation. Presence of barriers in the first Fresnel zone causes that radio link still doesn't work properly.
Installation made correctly.Visibility of antennas and no barriers in the first Fresnel zone. The link has been set up properly.
In practice, no obstacles in the central 60% of the I Fresnel zone guarantee quite minimal power loss.
Relationship of the I Fresnel zone radius in function of radio link length for systems working on 2.4 GHz and 5 GHz frequency
.
.
Radio link length [km] | 60% of the I Fresnel zone radius (0.6R1[m]) | |
| 2.4 GHz | 5 GHz | |
| 0.1 | 1.1 | 0.7 |
| 0.2 | 1.5 | 1.0 |
| 0.5 | 2.4 | 1.6 |
| 1 | 3.4 | 2.3 |
| 2 | 4.7 | 3.3 |
| 3 | 5.8 | 4.0 |
| 4 | 6.7 | 4.6 |
| 5 | 7.5 | 5.2 |
| 6 | 8.2 | 5.7 |
| 7 | 8.9 | 6.1 |
| 8 | 9.5 | 6.6 |
| 9 | 10.1 | 7.0 |
| 10 | 10.6 | 7.3 |
Curvature of the Earth
In the case of distances reaching a few kilometers and more, it is needed to include curvature of the ground. For the distance equal 5 km the height of barriers in the middle of link raises by 1m (let's represent the quantity as curvature factor) and for 10km distance - yet 4 m. Antennas should be situated at or slightly above the minimum height fulfilling the condition:
Height of antennas = elevation of the highest barrier on the route + 0.6 R1 + curvature factor
At longer distances, more precise calculations should be performed, based on hypsometric profile of terrain and methods including effects of beam refraction and multiple reflections.
Attenuation of gases and rain
These phenomena are well known and recognized as disadvantageous for proper work of radio systems; in practice they are harmless for 2.4 GHz and 5 GHz WLAN systems.
FSL model and attenuation in free space
The basic problem is to estimate attenuation between transmitter and receiver. When we design outdoor link we can use for this purpose FSL model. It is propagation model of free space, which assumes that:
- there is no barrier between transmitter and receiver,
- reflected waves don't influence the receiver,
- the fist Fresnel zone isn't covered,
- there aren't taken into consideration outer interferences and fading.
Attenuation of free space is defined as the loss of signal caused by spherical dispersion of radio waves in space.
FSL for the frequency of 2.4 GHz is determined by the pattern:
Lp [dB] = 100 + 20log10 D, where D - distance
FSL for frequency 5.4 GHz is determined by the pattern:
Lp [dB] = 106 + 20log10 D, where D - distance
Attenuation of free space and 6dB rule
Radio signal will weaken during its propagation in space, as it moves away from transmission antenna. Determination of attenuation of radio signal is the next step in the designing process.
Distance [km] | Attenuation [dB] | |
| 2.4 GHz | 5 GHz | |
| 0.1 | 80.4 | 86.4 |
| 0.2 | 86.4 | 92.4 |
| 0.5 | 94.4 | 100.4 |
| 1 | 100.4 | 106.4 |
| 2 | 106.4 | 112.4 |
| 3 | 109.9 | 116.0 |
| 4 | 112.4 | 118.5 |
| 5 | 114.4 | 120.4 |
| 6 | 116.0 | 122.0 |
| 7 | 117.3 | 123.3 |
| 8 | 118.5 | 124.5 |
| 9 | 119.5 | 125.5 |
| 10 | 120.4 | 126.4 |
6dB rule says that double increment in distance causes increase of signal attenuation by 6dB and double reduction of distance causes increase of signal level by 6dB. Simplicity of this rule allows for easy memorization of the relation. It is enough to remember that at distance of 1km in 2.4 GHz band the attenuation is 100dB.
So, using the 6dB rule, we will get for distances 2, 4, 8 km the next attenuation values: 106, 112, 118 dB. For distances 500, 250, 125 m, the attenuation will be: 94, 88, 82 dB adequately. 6dB rule is also applicable for 5 GHz band and other bands, however, the attenuation on 5 GHz band for 1km distance will be 106dB, so it means that the 6dB rule is also valid in frequency domain.
Other propagation models
For professional applications the engineers use highly sophisticated models, often developed for specific conditions and environment:
- propagation model with covered Fresnel zone
- propagation model including attenuation of walls inside buildings
It is not possible to use such models in amateur calculations.
RSL calculations
The basis for range calculation is creation of radio link balance and obtaining RSL value (received signal level).
Elements of energetic balance of a link:
- TSL[dBm] - transmitter signal level (transmitter output power)
- RSL[dBm] - received signal level
- FSL[dB] - free space loss
- GT[dBi] - transmitting antenna gain
- GR[dBi] - receiving antenna gain
- CLT[dB] - loss of transmitted signal in cable and connectors
- CLR[dB] - loss of received signal in cable and connectors
The transmitter sends high frequency signal with TSL[dBm] power level via cable link with CLT[dB] attenuation to transmitting antenna input. Then the antenna radiates the signal and simultaneously focuses it in half power angle, this way achieving amplification effect, given as the gain of the antenna GT[dBi]. Radio wave is attenuated by the value FSL[dB] after it passes distance d [km]. Receiving antenna changes electromagnetic wave into received signal that is increased thanks to the gain of receiving antenna GR[dBi]. After the signal passes through the cable link with CLR[dB] loss to the receiver, it finally has the RSL[dBm] level.
RSL[dBm]= TSL - CLT + GT - FSL + GR - CLR
To secure operation of wireless link against short power drops (or fluctuations), into the calculation there is put additional FM parameter (fade margin). Typical value of this parameter is equal 10dB.
FM = RSL- RSLFM
RSLFM means the minimum level of received signal (when a fading occurs). If we want to reach RSLFM = - 80dBm, it is required that the radio link has to obtain RSL = - 70dBm
Our goal is to choose such antennas and equipment that would guarantee the required level of signal (-80dBm) for most of the time. Most of wireless WLAN devices ensure highest possible speed then.
Selection of devices - an example
However, there are already available latest cables up to 6 GHz. These cables are recommended for use in new installations instead of above mentioned cables - Tri-Lan 240 (E1171) and Tri-Lan 400 WLL E1173.
More about cables used with WLAN equipment you can find in the article:
Use of coaxial cables in WLAN systems
More about cables used with WLAN equipment you can find in the article:
Use of coaxial cables in WLAN systems
In 5 GHz band, antennas reach energetic gain from 10dBi to 27dBi. So the gain is generally by 3dB higher in comparison to 2.4 GHz band. The cables used for this band are e.g. CNT-400 E1162 with attenuation 33dB/100m or, for shorter lengths, RF-240 E1181, compatible with connectors for H155.
As an example - we want to set up radio link over 2km distance and achieve the best possible parameters of the connection. We use devices with 18dBm output power. The length of the cable connecting antenna with WLAN device is 7m for both sides of the link. We can read from the table that for these parameters the sum of GT and GR gain shouldn't be less than 21.65dB. From the next table we know that we should use ATK8 A7120 antennas.
Caution. Some manufacturers, for marketing purposes, intentionally overestimate energetic gain of antennas. It may cause poor work of radio links using such antennas, drop in transmission speed and even momentary loss of connection. So, the best solution is to use antennas which have been tested in laboratory and which have relevant documents proving these tests. Besides that, existence of a number of neighbor wireless networks may cause degradation of our signal. Therefore it is sometimes better to increase the criterion for FM and assume FM=20dB.
Transmitter | Type | Length | Range of radio link [km] | ||||||||
| 0.5 | 1 | 2 | 3 | 4 | 6 | 8 | 10 | 15 | |||
| 16 | H-155 | 3 | 11.38 | 17.38 | 23.38 | 26.88 | 29.38 | 32.98 | 35.48 | 37.38 | 40.88 |
| 7 | 15.34 | 21.34 | 27.34 | 30.84 | 33.34 | 36.94 | 39.44 | 41.34 | 44.84 | ||
| 15 | 23.28 | 29.28 | 35.28 | 38.78 | 41.28 | 44.88 | 47.38 | 49.28 | 52.78 | ||
| H-1000 | 3 | 9.79 | 15.79 | 21.79 | 25.29 | 27.79 | 31.39 | 33.89 | 35.79 | 39.29 | |
| 7 | 11.65 | 17.65 | 23.65 | 27.15 | 29.65 | 33.25 | 35.75 | 37.65 | 41.15 | ||
| 15 | 15.36 | 21.36 | 27.36 | 30.86 | 33.36 | 36.96 | 39.46 | 41.36 | 44.86 | ||
| 18 | H-155 | 3 | 9.38 | 15.38 | 21.38 | 24.88 | 27.38 | 30.98 | 33.48 | 35.38 | 38.88 |
| 7 | 13.34 | 19.34 | 25.34 | 28.84 | 31.34 | 34.94 | 37.44 | 39.34 | 42.84 | ||
| 15 | 21.28 | 27.28 | 33.28 | 36.78 | 39.28 | 42.88 | 45.38 | 47.28 | 50.78 | ||
| H-1000 | 3 | 7.79 | 13.79 | 19.79 | 23.29 | 25.79 | 29.39 | 31.89 | 33.79 | 37.29 | |
| 7 | 9.65 | 15.65 | 21.65 | 25.15 | 27.65 | 31.25 | 33.75 | 35.65 | 39.15 | ||
| 15 | 13.36 | 19.36 | 25.36 | 28.86 | 31.36 | 34.96 | 37.46 | 39.36 | 42.86 | ||
| 20 | H-155 | 3 | 7.38 | 13.38 | 19.38 | 22.88 | 25.38 | 28.98 | 31.48 | 33.38 | 36.88 |
| 7 | 11.34 | 17.34 | 23.34 | 26.84 | 29.34 | 32.94 | 35.44 | 37.34 | 40.84 | ||
| 15 | 19.28 | 25.28 | 31.28 | 34.78 | 37.28 | 40.88 | 43.38 | 45.28 | 48.78 | ||
| H-1000 | 3 | 5.79 | 11.79 | 17.79 | 21.29 | 23.79 | 27.39 | 29.89 | 31.79 | 35.29 | |
| 7 | 7.65 | 13.65 | 19.65 | 23.15 | 25.65 | 29.25 | 31.75 | 33.65 | 37.15 | ||
| 15 | 11.36 | 17.36 | 23.36 | 26.86 | 29.36 | 32.96 | 35.46 | 37.36 | 40.86 | ||
Table indicating required gain of radio link when there are given: length of the link, transmitter power, type and total length of the used cable
Total required gain | Recommended type |
| 14 | ATK-P1 |
| 22 | ATK8 |
| 26 | ATK16 |
| 28 | TetraAnt 14 dB |
| 33 | Grid 16N |
| 48 | Andrew 26T |
Above values are theoretical rather, the practical range of links working in 2.4 GHz band wouldn't exceed 2km. The reason is limitation of radiated power, max. 100 mW EIRP (20 dBm), and usually congested band, which requires to adopt high FM value. As a rule, it is more advantageous to use lower power transmitter and antenna with higher gain than the other way round.
EIRP and choice of devices
Will we break the law using transmitting antenna with very high energetic gain? It should be remarked that regulations don't inform about limits of gain, which can't be exceeded. So how is it possible that one person can have an antenna with 15dBi gain, when another breaks the law installing antenna having 10dBi gain? Why some companies indicate, in compliance certificate, antenna with 15dBi gain, when others recommend antennas with10dBi gain?
The answer for this questions involves the regulations regarding maximal acceptable value of radiated power - EIRP. In many countries the maximum value of EIRP that can be radiated without a special license is 100 mW (20 dBm) in 2.4 GHz band, and 1 W (30 dBm) in 5.47 - 5.725 GHz band. But the same levels of EIRP can be achieved in many ways, according to the formulas:
EIRP[dB]2.4G = Transmitter power [dBm] - (attenuation of connectors [dB] + attenuation of cable [dB]) + gain of antenna [dBi] <= 20dBm
EIRP[dB]5G = Transmitter power [dBm] - (attenuation of connectors [dB] + attenuation of cable [dB]) + gain of antenna [dBi] <= 30dBm
In order not to exceed maximum permissible EIRP value, there have to be selected adequate parameters:
- transmitter power,
- type and length of cables
- gain of antenna.
It's worth stressing again that it is much more advantageous to use lower power transmitter and the antenna with higher gain than the other way round. Why? From the link balance we see that desired radiated power level can be achieved in any way, however the base station isn't only a transmitter, but also a receiver, and then, when it receives signal from a client, no matter the power has been transmitted, only sensitivity of the receiver and gain of the antenna is important. So the gain of antennas is important both during transmitting and during receiving.
The output power level is an important issue, too. Usually it seems that the higher power the better results. But it's not the truth. There is some optimal power level adjusted for the location of clients. Too high transmitting power means needless transmission of our signal beyond the desired area. We can interfere networks working far away from us. We will also be vulnerable to attacks on our network performed by people that are quite a long way from us and thus difficult to spot.
The gains of client stations should also be selected carefully. The client that uses a high gain antenna close to the base station, although receives strong signal, it may during transmission also interfere other, even distant networks. Besides that, it will "see" those networks and what it implies, they will cause additional noise (the higher noise the larger number of errors and lower transmission speed), or it will even share with them the transmission medium - which will also decrease speed. On the other hand, client stations with lower gain, optimal for the specific distance, will only see the base station and won't cause such problems.
Connectors
Most of WLAN devices are equipped with SMA-RP connectors, whilst outdoor antennas have N-type connectors. Using H-155 cable it is needed to terminate it with SMA RP connector on one side, and adequate male or female connector (depending on the antenna) on the other side. If we don't have crimping tool we should choose twist-on connectors. However, crimp-on connectors are preferred for their reliability.
Crimping tools for H-155 and Tri-Lan 240
Crimping tools for H-1000 and Tri-Lan 400
There is also needed soldering iron.
Crimping tools for H-1000 and Tri-Lan 400
There is also needed soldering iron.
The ways of terminating cables can be found here. After we prepared the cable, it is needed to solder inner wire, and then put on the central tip on, heating it with soldering iron.
Selection of radio channel
2.4 GHz band consists of 13 channels, from which only three are independent from one another. It means that only maximum three WLAN networks can work in particular area. The installer of a new network should check for free channels before he starts to build the WLAN system. In the case of free channels, he should choose the channel with the lowest level of noise/interference.
Arrangement of channels in 2.4 GHz band. Only 3 channels of total 13 do not overlay, i.e. 1, 7, 13 or 1,6,11
Practical tests show that mutual influence of two networks working in the same area depends on the chosen channels and decreases with increasing the space between the channels. When both networks use the same channel, they have half the maximum capacity. The worst case is when they use neighboring channels - their signals make mutually a high level noise that dramatically reduces the effective transfer rate to 20% of their capacity. 4-channel spacing allows for 70% efficiency. Unfortunately, theoretically independent channels also have certain mutual influence on each other.
Choice of polarization
There are two popular variants of polarization: circle and linear. Circle polarization means that the end of the vector of electricity field draws a circle in space. Circle polarization can be dextrorotary or levorotatory. Radio systems with dextrorotary polarization don't influent systems with levorotatory polarization and vice versa.
 |  |
Circle polarizations: dextrorotary and levorotatory
In the case of linear polarization, the electric field vector oscillates only in one plane. It is horizontal or vertical plane.
Radio systems with horizontal polarization don't affect systems with vertical polarization, and vice versa, as these polarizations are orthogonal. This feature allows doubling the number of radio systems in one place.
Caution. It is not allowed to use antennas with orthogonal polarization i.e. an antenna with horizontal polarization at one side of a link and with vertical polarization at the other side of the link. When it comes to cooperation of circle polarization antennas with linear polarization antennas - it is possible - however with 3dB power loss.
Noise
In practice, noise is the sum of undesired radio signals, i.e. interferences. Too big level of noise can spoil parameters of radio link or even make the link unserviceable. Even well balanced radio link may appear to be useless for the reason of high noise level. The designer has no influence on the level of ambient noise. So, how can we protect ourselves against interferences? The simplest way to defend our link against them is finding less congested radio channel. Another way is to select antennas with higher gain, to improve signal to noise ratio (S/N).
The speed of radio link depends on the power level of received signal and S/N ratio (It is marked as signal strength and signal quality on the drawing.) To reach maximal speed 11Mbps the indicator should be on green field (Excellent). If the level of noise increases, even high value of received signal won't protect us from bandwidth loss.
Effective transfer rate
Because WLAN system is based on CSMA/CA techniques and uses transmission with ACK confirmation, the end user connected to the network via e.g. 11Mbps link cannot reach real file transfer higher than half the value, i.e. about 5Mbps. Effective transfer rate of WLAN link is less than half of the declared capacity of wireless link.
Modes of operation of Access Points
Access Point may work in a few different modes. Each mode is characterized by capability (or not) of supporting specific devices and the features collected in the table:
AP mode | LAN support (number of supported computers) | Support for clients equipped with WLAN cards | Cooperation with APs |
| Wireless bridge | Yes | No | Wireless Bridge |
| Multiple bridge | Yes | No | Wireless Bridge |
| Transmitter | No | Yes | Access Point |
| Access Point | Yes | Yes | Transmitter, AP Client |
| AP Client | Yes ( up to 63) | No | Access Point |
Planning of WLAN cells and the service for clients
There are a few ways to cover an area with WLAN signal. It all depends on desired range and capability of the network.
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The ways of covering an area with radio signal - sector cells and omnidirectional cell
In the left case we have the terrain covered by using three APs and three sector antennas. Each AP uses different frequency. In the right example we have one AP with omnidirectional antenna. The first system covers 6 times bigger area than the second one and can have 3 times more subscribers. The cost of connecting a subscriber in both systems will be dependent on the distance from the subscriber to base station. Subscribers who are located closer to base station can be equipped with low-gain antennas, which means lower cost.
The size of a cell should be chosen taking into consideration all capabilities of base station, density of population in the area, and estimated degree of saturation of the market.
In practical solutions, the size of a cell is limited by shape of the land and the barriers, e.g. trees, chimneys, buildings etc.
Devices integrated with antennas
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Common problems with WLAN networks
| Reasons for no connection | Solution | |
| 1. | Barriers in the I Fersnel zone | Use higher masts. change locations of antennas |
| 2. | Wrongly calculated energetic balance of the link. wrongly chosen devices | Use cables of lower attenuation. e.g. instead of H-155 use H-1000; use antennas with higher gain |
| 3. | Wrong polarization of antennas | Align antennas to the same polarization |
| 4. | Wrong alignment of antennas | Use signal level meter during antennas' installation. Set antennas in positions in which signals have the highest power |
| 5. | High level of interferences or noise | Select radio channel with the lowest noise level. change polarization of the link to the opposite. use antennas with higher energetic gain. As a last resort - change antennas locations. |
Wrong operation of radio system | Diagnosis | Solution | |
| A. | Loss of connection and low bandwidth of radio link | Low value of S/N parameter | Points 1-5 of previous table |
| B. | Low transfer rate from base station with radio link working at maximal speed | Frequent collisions | Turn on RTS/CTS mechanism for clients |
We suggest getting familiar with the articles: WLAN in single-family house and WLAN - indoor installations.
SECCAO LIVRARIA
LIVRARIA NOVOS TITULOS
PARA SABER MAIS
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