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WLAN Primer

2005.06.14 09:59 by Tom Blakely
Tags: WLAN, Wireless, Wi-Fi, antenna, gain, dBi, FHSS, DSSS, ad-hoc, bridge


Historical Background
The International Telecommunication Union (ITU), a specific division of the UN, has set aside a range of frequencies for Industrial, Scientific and Medical use (Section S5.138 & S5.150). These ISM frequencies are unlicensed and unregulated. They fall in the following ranges:

  • 902-928 MHz
  • 2.4-2.4835 GHz
  • 5.725-5.850 GHz

Individual countries do have the option of creating their own rules for the ISM band use, though few have created restrictions beyond what the ITU set.

In 1985 The FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices, to allow the use of these spectrums for wireless data networking. The Institute of Electrical and Electronics Engineers then formed the 802.11 committee. This committee was to develop standards for using this spectrum for wireless data networking communications. In 1997 the IEEE 802.11 committee approved the 802.11 standard, which provided for wireless data links at up to 2mbps. In 1999 the 802.11b standard was added. This standard expanded carrying capability to 11mbps.

Since 1999, the use of wireless networks as an adjunct network path has grown almost exponentially, as much as 30% per year. The number of deployed Network Interface Cards (NICs) has gone from about 427 million to 627 million between 2002 and 2003 alone. Because the FCC allows users to operate wireless networks without obtaining FCC licenses, the products must meet certain requirements, such as operation maximum of no more than 1-watt transmitter output power. Because of the low cost and ease of implementation, the deployment of WLANs in corporate and government facilities has grown dramatically. This does not mean they are being deployed either effectively or securely, but the growth in usage is nonetheless taking place because of the ease of deployment and utility of a WLAN path. As well, in many developing nations WLANS are now used as a low cost semi-permanent alternative data path for backbone networks. Another emerging use of WLANs is in difficult terrain or for temporary, re-locatable networks.

There are two primary types of 802.11b RF networks available:

  • Frequency Hopping Spread Spectrum
  • Direct Sequence Spread Spectrum

Each of the above is discussed in greater detail in the following section. In sum, both divide the spectrum up into channels. FHSS divides itís data equally across all channels while DSSS concentrates more of the data in certain specific channels.


Theory of Operation: FHSS

As stated earlier, a 22MHz wide ISM signal is used. In a Frequency Hopping Spread Spectrum System, the bandwidth is broken up into 79 individual channels, and data is sent on each channel in timed, discrete increments. This can be a user defined pseudo random sequence, such as channel 1, channel 52, channel 23, channel 67, and so on. Without knowing both the frequency pattern and the dwell time that is spent on each channel, it is impossible for a non-participating station to receive and decode the data. To an unintended receiver, FHSS appears to be short-duration impulse noise as maximum dwell time must be under 400ms.

Click to expand
FHSS

Total throughput is measured in half-duplex amounts, and it is important, when considering full duplex issues, to cut the network speed in half. Thus an 11mbps wireless link effectively provides a 5.5mbps full duplex medium. In a FHSS system, the bandwidth is limited to 5mbps half-duplex or 2.5mbps full-duplex.

  

Theory of Operation: DSSS

In Direct Sequence Spread Spectrum, the same ISM bandwidth is broken into as many as fourteen channels. Channels overlap with a center frequency located at 5-MHz intervals. Usually these center frequencies are located at Channels 1, 6, and 11 because it allows for non-overlapping coverage. The data is simultaneously sent on all the channels, using either an 11-bit Barker Sequence or Complementary Code Keying (CCK). The receiver, using the same sequencing recombines the RF input to recover the original data and minimize impact from interference. Because of the use of Center Channels and Barker/CCK sequencing it is in fact possible to collocate up to three DSSS devices such as access points, creating an aggregate data path of up to 33mbps half-duplex. No one user will see an increase in speed, however the network link as a whole will increase in capacity. To an unintended receiver, DSSS appears as low-power wideband noise and is rejected (ignored) by most narrowband receivers.

Click to expand
DSSS

In the US, the standard is 11 channels. In Europe and parts of Asia more channels are available. Bandwidth is limited to 11mbps half-duplex or 5.5mbps full-duplex, thus it is important that an accurate site survey be conducted to place access points so as to divide up multiple users in any one area into discrete groups so that no one Access Point is overwhelmed by traffic. This is discussed in more detail in later sections.

The typical form factor is an extended PCMCIA Type II card (below), and can be inserted in any compliant laptop or mobile device slot. It allows communication at rates up to 11mbps used the 802.11b standard. Claimed ranges include 300ft indoors, and outdoors up to 1000ft along Line of Sight using the stock antennas. The example below shows an 802.11b compliant card mated to a PCI chassis for insertion into a standard motherboard. It also uses an external dipole antenna connection which means the small antenna may be removed and larger antennas utilized for transmission.

Click to expand

With an external Yagi Directional, or Parabolic antenna (see below), these distances can be increased significantly, as far as 40 miles in some cases. It should be stressed that point-to-point aiming beyond 25 miles becomes extremely difficult.

Click to expand
Yagi Directional Antenna
Click to expand
Parabolic Antenna

  

Applications

Note that because of the spectrum being unregulated you are subject to interference from other operators, therefore accurate antenna aiming, particularly in natural environments, is important. As well, there are power considerations in the ETSI area (Europe, Asia, Middle East, and Africa) that do affect use and range. There are antenna designs and power modifications that can be made to minimize the possibility of interference and any effect it may have. It is worth noting that as long as the ETSI EIRP regulations are followed there appear to be no additional Host Nation Approval required in most areas. This has the potential to significantly reduce deployment times and associated system costs. There are four primary architectures the systems are part of:

  • AD-HOC PEER-TO-PEER WLAN
  • INFRASTRUCTURE WLAN
  • WIRELESS BRIDGING
  • MASTER BRIDGING


Typical Local Deployment Architecture

 

AD-HOC PEER-TO-PEER WLAN

Click to expand
Ad-Hoc WLAN

In this environment each system uses the same WLAN Identifier and optional WEP (Wireless Encryption Protocol) Key to communicate directly between each unit. Any other System may enter the WLAN structure provided that unit has both the WLAN identifier and (if being utilized) proper WEP key. Typical range indoors is estimated at 300ft and outdoors up to 1000ft provided clear line of sight is available. There is no theoretical limit to the number of units that may participate, however, as a practical matter the limit is roughly between 6 - 20 units. The differential is based on the type of traffic being passed between the units. High bandwidth intensive applications like live streaming video significantly reduce the number of units that can be accommodated.

 

INFRASTRUCTURE WLAN

Click to expand
Wireless Access Point WLAN

Using a Wireless Access Point allows the mobile unit to interface with a LAN environment. This is the usual deployment within a building structure environment. It is important to implement an Access Point network correctly. Many times, the systems are simply plugged into the network to provide a wireless data path without proper attention to security and efficiency implications. Before deployment, a site survey should be done to take note of obstructed or shielded areas, as well as minimizing the broadcast range on the exterior of the building. WAP placement requires a pre-deployment strategy for implementation to make sure that security is maintained and that adequate bandwidth is available for the number of units to be serviced. This may require using multiple access points collocated on different center frequencies to increase the aggregate bandwidth available to the coverage area as a whole. It may also require placement in such a way so as to minimize crosstalk between units on different floors, as these devices maintain a three-dimensional broadcast area.

 

WIRELESS BRIDGING

Click to expand
Wireless Bridging WLAN

In this environment the Wireless service is used to extend a data path between two different LAN areas without use of copper or fiber cable runs. The link is transparent to the network as a whole. If desired, a router can be used on both sides to divide the different LANs into discrete segments. This can provide for traffic control and maximize bandwidth availability. It is worth noting that some testing has already been done using this type of architecture to extend a ship to shore data path at ranges up to 2mi (3km), using older equipment using 1watt signal boosters. The experiment included the streaming of video from a remotely piloted undersea vehicle to a shore facility. Other testing has included the use of remote security cameras and sensors streaming video and other sensor data to secured facilities, allowing permanent placement of equipment in difficult to reach or unattended areas.

 

MASTER BRIDGING

Click to expand
Wireless Bridge (Master Mode) WLAN

Master Bridging Mode combines both Wireless Bridging and Access Point services. This will allow networks to be extended quickly. This is usually for temporary LAN services extension to specific remote units. The number of clients than can be accommodated by this method is limited since the bandwidth on the Master Bridge unit is shared between both the local units accessing itís services, and itís own link with the other Bridging unit.

As previously stated, this spectrum is unlicensed/unregulated, and we have yet to find any documentation that restricts its use in any countries other than Spain, France, and Nigeria. There are no known restrictions on the use of 802.11b compliant equipment and as an example; Bahrain is a signatory to the ITU standard of implementation. There are already commercial products being sold in Bahrain and companies building WLANs in buildings and in the Bahrain International Airport.

For reference below there are some ranges available using both North American and European Specifications:

 

EXAMPLE NORTH AMERICAN DEPLOYMENT RANGES
North American specifications are limited by the FCC

NANZ Spec at 11mbps
Site 1 (& Site 2)
Power level: 50mW
Data Rate: 11mbps
Antenna: 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 46ft
Effective Isotropic Radiated Power: 35.976
Max Distance (w/ 10dB Fade Margin): 15.6 Miles / 25.2 Kilometers
Earth Bulge at above distance: 50 Feet / 15.2 Meters
Fresnel Zone clearance for above: 55 Feet / 16.7 Meters
Req Ant height above obstructions: 105 Feet / 31.9 Meters

NANZ Spec at 5.5mbps
Site 1 (& Site 2)
Power level: 50mW
Data Rate: 5.5mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 46ft
Effective Isotropic Radiated Power: 35.976
Max Distance (w/ 10dB Fade Margin): 24.8 Miles / 40 Kilometers
Earth Bulge at above distance: 78 Feet / 23.8 Meters
Fresnel Zone clearance for above: 69 Feet / 21 Meters
Req Ant height above obstructions: 147 Feet / 44.8 Meters

NANZ Spec at 2mbps
Site 1 (& Site 2)
Power level: 50mW
Data Rate: 2mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 46ft
Effective Isotropic Radiated Power: 35.976
Max Distance (w/ 10dB Fade Margin): 31.3 Miles / 50.3 Kilometers
Earth Bulge at above distance: 90 Feet / 27.4 Meters
Fresnel Zone clearance for above: 78 Feet / 23.6 Meters
Req Ant height above obstructions: 168 Feet / 51.0 Meters

NOTE: Distances above 25mi present aiming difficulties.

NANZ Spec at 1mbps
Site 1 (& Site 2)
Power level: 50mW
Data Rate: 1mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 46ft
Effective Isotropic Radiated Power: 35.976
Max Distance (w/ 10dB Fade Margin): 39.4Miles / 63.4 Kilometers
Earth Bulge at above distance: 90 Feet / 27.4 Meters
Fresnel Zone clearance for above: 87 Feet / 26.5 Meters
Req Ant height above obstructions: 177 Feet / 53.9 Meters

NOTE: Distances above 25mi present aiming difficulties.

 

 

EXAMPLE ETSI DEPLOYMENT RANGES
ETSI area specifications are limited by +20dBm EIRP requirement

ETSI Spec at 11mbps
Site 1 (& Site 2)
Power level: 1mW
Data Rate: 11mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 23ft
Effective Isotropic Radiated Power: 19.988
Max Distance (w/ 10dB Fade Margin): 3.5 Miles / 5.6 Kilometers
Earth Bulge at above distance: 5 Feet / 1.5 Meters
Fresnel Zone clearance for above: 26 Feet / 7.9 Meters
Req Ant height above obstructions: 31 Feet / 9.4 Meters

ETSI Spec at 5.5mbps
Site 1 (& Site 2)
Power level: 1mW
Data Rate: 5.5mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 23ft
Effective Isotropic Radiated Power: 19.988
Max Distance (w/ 10dB Fade Margin): 5.5 Miles / 8.9 Kilometers
Earth Bulge at above distance: 13 Feet / 4.0 Meters
Fresnel Zone clearance for above: 33 Feet / 9.9 Meters
Req Ant height above obstructions: 46 Feet / 13.9 Meters

ETSI Spec at 2mbps
Site 1 (& Site 2)
Power level: 1mW
Data Rate: 2mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 23ft
Effective Isotropic Radiated Power: 19.988
Max Distance (w/ 10dB Fade Margin): 7 Miles / 11.2 Kilometers
Earth Bulge at above distance: 13 Feet / 4.0 Meters
Fresnel Zone clearance for above: 37 Feet / 11.2 Meters
Req Ant height above obstructions: 50 Feet / 15.1 Meters

ETSI Spec at 1mbps
Site 1 (& Site 2)
Power level: 1mW
Data Rate: 1mbps
Antenna 21dBi Parabolic Dish
Cable Loss/100 ft : 4.4
Length: 23ft
Effective Isotropic Radiated Power: 19.988
Max Distance (w/ 10dB Fade Margin): 8.8 Miles / 14.2 Kilometers
Earth Bulge at above distance: 13 Feet / 4.0 Meters
Fresnel Zone clearance for above: 41 Feet / 12.5 Meters
Req Ant height above obstructions: 54 Feet / 16.5 Meters

 

 

INDEX


Channels/Frequencies allowed per regulatory domain

Regulatory Domains

Channel
#
Center
Frequency
NAM/ANZ
FCC
ETSI
France
All Others Singapore Japan Israel
1 2412 MHz X X** X - X -
2 2417 MHz X X** X - X -
3 2422 MHz X X** X - X X
4 2427 MHz X X** X - X X
5 2432 MHz X X** X - X X
6 2437 MHz X X** X - X X
7 2442 MHz X X** X - X X
8 2447 MHz X X** X - X X
9 2452 MHz X X** X - X X
10 2457 MHz X X X X X -
11 2462 MHz X X X X X -
12 2467 MHz - X X X X -
13 2472 MHz - X X X X -

 



Maximum power levels and antenna gains per regulatory domain

North American & ANZ (4 watts or 36 dBm Maximum EIRP)
Except in Point-to-Point Bridging Applications - with 20 dBm transmit maximum antenna gain allowed is 30)

Antenna Gain (dBi) Power Level (mW)
0 100
2.2 100
5.2 100
6 100
8.5 100
12 100
13.5 100
21 100

 

ETSI (except France)
(100 mW or 20 dBm Maximum EIRP)

Antenna Gain (dBi) Power Level (mW)
0 100
2.2 50
5.2 30
6 30
8.5 5
12 5
13.5 5
21 1

 

France - Channels 1-9
(10 mW or 10 dBm Maximum EIRP)

Antenna Gain (dBi) Power Level (mW)
0 5
2.2 5
5.2 1
6 1
8.5 1
12 1 (must have 2 dBi of loss in cable)
13.5 1 (must have 3.5 dBi of loss in cable)
21 1 Antenna cannot legally be used

 

Singapore & France - Channels 10-13
(100 mW or 20 dBm Maximum EIRP)

Antenna Gain (dBi) Power Level (mW)
0 100
2.2 50
5.2 30
6 30
8.5 5
12 5
13.5 5
21 1

 

Israel
(100 mW or 20 dBm Maximum EIRP)

Antenna Gain (dBi) Power Level (mW)
0 100
2.2 50
5.2 30
6 30
8.5 5
12 5
13.5 5
21 1

 

Japan
(10 mW/MHz or 00 dBm Maximum EIRP)

Antenna Gain (dBi) Power Level (mW)
0 50
2.2 30
5.2 30
6 30
8.5 N/A
12 N/A
13.5 5
21 N/A

 

___________________

North American -ANZ
United States, Canada, Mexico, America Samoa, Antigua and Barbuda, Argentina, Aruba, Ashmore and Cartier Islands, Australia, Bahamas, Baker Island, Barbados, Bermuda, Bolivia, Bouvet Island, Brazil, Cameroon, Central African Republic, Chad, Chile, China, Christmas Island, Clipperton Island, Cocos Island, Colombia, Cook Island, Coral Sea Islands, Costa Rica, Ecuador, El Salvador, Europa Island, Faroe Islands, Fiji, Glorioso Islands, Grenada, Guadeloupe, Guam, Guatemala, Guyana, Haiti, Heard Island, Honduras, Hong Kong, Jamaica, Kingman Reef, Malawi, Malaysia, Mali, Marshall Islands, Midway Islands, Navassa Island, New Caledonia, New Guinea, New Zealand, Nicaragua, Niger, Nigeria, Norfolk Island, Northern Mariana Islands, Palau, Palmyra Atoll, Panama, Papua New Guinea, Paracel Islands, Paraguay, Peru, Phillippines, Pitcairn Islands, Puerto Rico, Russia, Saint Kitts and Nevis, Saint Lucia, Saint Pierre and Miquelon, Saint Vincent and the Grenadines, Samoa, Saudi Arabia, Solomon Islands, South Korea, Spratly Islands, Taiwan, Togo, Tonga, Trinidad and Tobago, Tromelin Island, Turks and Caicos Islands, Uruguay, US Virgin Islands, Venezuela, Wake Island, Western Sahara.

ETSI
Afghanistan, Albania, Algeria, Andorra, Angola, Angullia, Armenia,Austria, Azerbaijan, Bahrain, Bangladesh, Bassas da India, Belarus,Belgium, Belize, Benin, Bhutan, Bosnia, Botswana, British Indian OceanTerritory, British Virgin Islands, Brunei, Bulgaria, Burkina Faso, Burma,Burundi, Cambodia, Cape Verde, Cayman Islands, Comoros, Cote díIvoire, Croatia, Cyprus, Czech Republic, Democratic Republic of the Congo, Denmark, Djibouti, Dominica, Egypt, Equatorial Guinea, Eritrea, Estonia, Ethiopia, Falkland Islands, Finland, France, French Guiana, French Polynesia, French Southern and Antarctic Lands, Gabon, Gambia, Georgia, Germany, Ghana, Gilbraltar, Greece, Greenland, Guernsey, Guinea, Guinea-Bissau, Hungary, Iceland, India, Indonesia, Ireland, Isle of Man, Israel, Italy, Ivory Coast, Jan Mayan, Jarvis Island, Jersey, Johnston Atoll, Jordan, Juan de Nova Island, Kazakhstan, Kenya, Kiribati, Kuwait, Kyrgyzstan, Laos, Latvia, Lebanon, Lesotho, Liberia, Liechtenstein, Lithuania, Luxembourg, Macau, Macedonia, Madagascar, Maldives, Malta, Martinique, Mauritania, Mauritius, Mayotte, Micronesia, Moldova, Monaco, Mongolia, Montserrat, Morocco, Mozambique, Namibia, Nauru, Nepal, Netherlands, Niue, Norway, Oman, Pakistan, Poland, Portugal, Qatar, Republic of the Congo, Reunion, Romania, Rwanda, Saint Helena, San Marino, Sao Tome and Principe, Senegal, Serbia, Seychelles, Sierra Leone, Singapore, Slovak Republic, Slovenia, Somalia, South Africa, South Georgia, Spain, Sri Lanka, Sudan, Suriname, Svalbard, Swaziland, Sweden, Switzerland, Syria, Tajikistan, Tanzania, Thailand, Tokelau, Tunisia, Turkey, Turkmenistan, Tuvalu, Uganda, Ukraine, United Arab Emirates, United Kingdom, Uzbekistan, Vanuatu, Vatican City, Vietnam, Wallis and Futuna, Yemen, Zaire, Zambia, Zimbabwe

Japan
Japan

 

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