2.4 GHz refers to a frequency spectrum in which much 802.11 transmitting and receiving equipment operates. It is the most common form of wifi equipment with the largest user base and the widest range of low-cost equipment. 2.4 GHz has been an unlicensed band since June 2003 in the UK and throughout Europe. Most commonly used for local wifi distribution as a replacement for end-user access to Broadband where ‘last mile’ telephone or cable companies have not provided it over their wires, and for remote surveillance.
Commercial operators in Band C must have a licence from Ofcom.
“Some of the existing users in the band will be provided protection through the use of geographical exclusion zones. Incoming operators will be required to endutre the integrity of these zones. Network planning and rollout plans will need to take into account the existence of these zones. To provide protection to existing users in the band and to enable the Agency [Ofcom] to record all terminals and their locations authorisation for use of the band will be granted through a Licence. All users will need to obtain prior approval for the installation and use of individual stations.” – Ofcom
Used in the 5.8 GHz frequency range (5.725 GHz to 5.850 GHz) with a maximum 54 Mbps data transfer rate. Less crowded and less interference than with 2.4 GHz because there are more channels available than with 802.11b.
Data rates up to 54Mbps using Orthogonal Frequency Division Multiplexing (OFDM). Eight non-overlapping 20MHz channels each carrying 52 sub-carriers nominally 300kHz wide in the unlicensed 5.15 – 5.825GHz ISM band.
802.11a has better tolerance to reflected signals and interference and has a smaller fresnel zone, so is more successful over greater distances than 802.11 b. It can be thought of as a Near Line of Sight (NLOS) system, i.e. it will produce results even if the line of sight is less than perfect.
As of February 2nd 2004, Ofcom requires that radios operating in 5.8 Ghz band comply with UK specifications and Band C operators must be Licenced.
2.4 GHz frequency range (2.4 GHz to 2.4835 GHz) with a maximum 11 Mbps. It is the commonest standard currently in use for wifi.
Dynamic rate shifting up to 11 Mbps using DSSS and Complimentary Code Keying (CCK). Over-lapping 22 MHz wide channels in the unlicensed 2.4GHz ISM band. Backwards compatible with 802.11-DSSS up to 2Mbps. This is also known as “WiFi”, for which interoperability is certified by the WiFi Alliance (formed by WECA)
This frequency is shared with microwave ovens, DECT cordless phones, medical equipment and Bluetooth and can be subject to interference from them.
2.4 GHz frequency range (2.4 GHz to 2.4835 GHz) with a maximum 54 Mbps. Uses different radio system to maximise bandwidth. Recent equipment offers 54 Mbs maximum throughput, 108 Mbs with compression. Manufacturers who were early to market may not comply fully with the g standard as define by IEEE, and their radios may fall back to the speed of the lowest 802.11n device found on the network, e.g. 11 Mbs. It is wise to check before purchase. Effectively a hybrid of .11a and .11b providing data rates up to 54Mbps in the 2.4GHz band, using OFDM for rates above 20Mbps, but with back-wards compatibility with 802.11b equipment.
54 and 108 Mbps speeds quoted are burst speeds; as a rule of thumb a 54 Mbps system on 80211.g will provide about 20 Mbps constant throughput, i.e. has about 20% of the speed of a typical wired 100 Mbps network found in offices.
802.11 e, f, i,h
These are MAC layer variants covering Grade of Service, Inter-Access Point roaming, and Encryption and Authentication respectively.
New standard, not yet ratified, will be backwards compatible with other 802.11 standards and with better encryption than WEP, but needs more processing power. 802.11h deals specifically with DFS and TPC for 5.8 GHz systems in Europe.
Three rival groups backed by different manufacturers have been working to submit proposals for the new 802.11n standard since 2005. Gn Sync and WWiSE, two of the three groups appear to be leading the progress towards a final standard intended to bring Wi-Fi speeds to 100 Mbps and eventually to 540Mbps.
WWiSE is backed by Texas Instruments, Broadcom, Conexant, STMicro, Airgo, Bermai,Motorola and Nokia. . TGn Sync’s is backed by Intel, Atheros, Agere, Infineon, Cisco, Qualcomm, Nortel, Mitsubishi, Sony, Panasonic, Philips and Samsung.
The standard defining WI-Max
The standard defining Mobile Wi-Max
An Access Point is a radio transmitter/receiver with an ethernet connection to the local wired network. It sends and receives data in its immediate vicinity. Any other radio device that can talk to it is connected to the network through it. Sometimes known as a ‘base station.’
An Access Point may also bridge with another access point (talk only to another unit set up in Bridge Mode). – see Bridge
Access points can have different levels of functionality and the general term includes multi-function devices such as wireless hubs, switches, bridges, routers and gateways. Wireless access points and hubs do not control bandwidth. What comes in goes out, and vice versa.
ACM (Adaptive Coding and Modulation)
Adaptive Coding and Modulation or Link adaption is the method of getting the best possible performance, reliability and capacity out of a radio link. This is done by the matching of the quality of a microwave radio link to the correct modulation and coding. For example with a poor signal quality (see CCQ) due to a poor pathloss profile and/or high interference, radio systems will use methods such as QPSK and as the radio link quality improves modulations such as APSK and QAM are used for higher capacity. The advantage of ACM is that it is adaptive, which means that it will constantly monitor the link quality and shift the modulation and coding methods accordingly for the best available performance at a given time.
In ad hoc mode, any wireless device connected to any computer can talk with any other device nearby. This is the equivalent of peer-to-peer in the traditional LAN.
Where each device talks to one central device the word Infrastructure is used to describe the process.
American for ‘aerial’ and now used everywhere to describe aerials for wifi. Antenna are passive devices, but by radiating the signal more precisely than just pumping it out everywhere, the data signal can be sent in a particular direction, thus they are said to have gain.
Different types of antenna radiate their signals in different patterns. See Choosing the right antenna
APSK (Amplitude and phase-shift keying)
Glossary definition available soon.
=reduction. Any object inbetween two wifi devices will reduce the strength of the signal that can pass between them. This reduction has the technical term ‘attenuation.’ Too much attenuation will reduce the rate at which data is transmitted or prevent data transmission completely, so it is important to know how much attenuation there will be between devices. Concrete, steel and foil-backed plasterboard attenuate the wireless signal strongly, so little signal will pass through them.
Sometimes it is necessary to reduce transmit power to stay within legal limits. One way to do this is to add an attenuator, or series of attenuators, in the antenna cable. Each attentuator reduces the signal by a known number of dB. Alternatively, the antenna cable can be made longer, or low quality lossy cable could be used because cables attenuate the signal that passes through them.
The central spine of a network, usually used to refer to a high speed link with a lot of bandwidth. Other sub-networks join onto it, so that any sub-network on the backbone can talk to any other sub-network.
How much data capacity the network can carry. This can vary, or be varied by the controller, to meet the needs of different client computers. Often considered as a pipe: a bigger ‘pipe’ has a bigger bandwidth. 80211b bandwidth is 11 Mbps. The .a and .g variants offer 54 Mbs.
– see Access Point
Beamwidth of an antenna is stated as the angle formed between two points on either side of the main radiation lobe where the points are at half the power of the main lobe (-3dB)
A Bridge is an access point that will only talk with one other bridge. It does this by knowing the MAC address of the other bridge it wants to talk to. Bridges are usually used as two ends of a long distance link. Both bridges must be using the same protocol.
There are a number of calculations that should be carried out when planning to install wireless equipment. The System Operating Margin calculation provides a good estimate of link viability.To ascertain the System Operating Margin – whether the proposed distance link will work satisfactorily or not – a small (18 kb) spreadsheet in Excel format is available for download. It includes convertors for mW/dB, kms/miles, metres/feet and calculates EIRP, Free Space Loss, Fresnel Zone Loss (ignoring earth curvature which is minimal), required antenna height and the link viability based on user input figures.
CCQ (Client Connection Quality)
Glossary entry available soon.
Wifi devices use the 2.4 GHZ spectrum, which is divided into 14 channels of slightly different frequency, or the 5.8 GHz spectrum, which has different numbers of channels depending on whether it is operating in Band A, B or C. In broad terms, devices send a quantity of data on one channel then hop onto another, then another, working through the channels available.A list of channels, their frequencies, and which ones are used in different parts of the world in the Reference Section.
Compex Systems Pte Ltd.
Owned by Powermatic Data System Ltd. Compex are a manufacturer of RF wireless systemboards, modules and antenna. Specialising in design and manufacture delivering OEM, ODM and JDM services in wireless communications. Website: www.compex.com.sg
Antenna cables use different types of connectors – see N, SMA, BNC, TNC, on our connectors page. Note that since the introduction of reverse versions of certain connectors it is no longer possible to refer to connectors as ‘male’ or ‘female’ in the traditional way. Connectors are now called ‘plug’ and ‘jack.’ A plug is the part with active components (a moving threaded outer ring for example) and a jack has no moving parts.
Customer Premises Equipment
Where connection to the backbone or ISP is being provided through long distance links to and local users connect to the end point of that link by smaller low power access points, the end user equipment is known as Customer Premises Equipment or CPE.
A logarithmic number used to represent power ratios. Doubling the power level adds 3 dB, while halving it subtracts 3 dB. A 3 metre antenna cable with a loss of 1 dB a metre would reduce the signal level by half.
db to mW
dB to mW Conversion table
dBd (dB dipole)
A figure used to compare the gain of an antenna with a basic type of antenna known as a ‘dipole’ because it consists at its most bais of of two pieces of wire. DBd figures are usually given for antenna designed for frequencies of 1 GHz and below.
dBi (dB isotropic)
Unit used to state the ratio of the radiation intensity in a particular direction compared to the gain if the antenna radiated equally in all directions. The higher the figure the greater the gain. Used for antenna designed for frequencies of above 1 GHz.
dBm (dB milliWatt)
A value which compares a radio signal level to a reference value and is used to measure power values. The reference value is 1 milliwatt and the measurement is made in dB: usually used to state radiated power.
This is one of four elements of the Ofcom specification for 5.8 GHz radios used in the UK; other countries may not have this requirement. The radio must be able to detect the appearance of an interfering signal such as a radar transmitter and switch to a different channel where there is no interference. This process, and re-establishing a link to other associated radios, can take over a minute. see 5.8 GHz
(Dynamic Host Configuration Protocol)
To connect to the internet, each PC needs a unique IP (internet Protocol) address. This can either be assigned manually or permit another device, usually a server, to automatically hand out unique IP addresses to machines connected to it. DHCP removes the work of manual configuration of IP addresses.
In a wireless network it is important to decide which device in a network is handing out IP addresses using DHCP: there should usually be only one source of DHCP in the network.
Directional antenna radiate a narrow beam to the front. Smaller side lobes are generated on each side of the main beam and often behind the antenna as well. Dish, Patch, Planar, Parabolic and Yagi antenna are directional.
Dish antenna consist of a solid dish with a transmitter mounted at the focal point. Dishes focus the transmitted signal into a very narrow beam to achieve long distance links.
– see Getting a Link
For ‘long distance’ links 1-2 kms is common, 4 kms frequent, 15-20 kms harder to achieve. 80 kms has been achieved in the US. How far transmission can be achieved depends on the equipment used and the throughput drops with increased distance. A radio quoted as performing at 11 Mbs at 100 metres will achieve much slower speed at a kilometre unless the antenna are changed.
Manufacturers claim distance figures that we have found to be wholly unrealistic. Typical factors are:
Presence and type of external antenna
Use of separate transmit/receive antennae
Good cable connections to antenna
Height of antenna above ground
Location of obstacles such as buildings and trees
Curvature of the earth
Adverse weather conditions – torrential rain or heavy snow can reduce link speed
When deciding if a proposed link will work, calculations should be made to establish the System Operating Margin.
Radio signals travel from the transmitter to the receiving antenna in a straight line but they also hit walls and furniture inside a room or buildings and hills when used outside. Some of their energy is absorbed by what they hit, but some is bounced off to travel to the antenna or hit other objects before reaching the antenna. Part of the signal is reflected so there are multipaths in all environments (except maybe the desert).
The reflected signals arrive a little while after the direct path signals (it takes time for them to travel further) and a signal that arrives 180 degrees out of phase with the direct signal (a kind of mirror image of it) will add together in the same way as +1 and -1 add together to give a result of zero.
Because of the short wavelengths used, having a second antenna just a few centimetres away is useful, because that second antenna will probably receive the reflected signal not so far out of phase with the direct signal, and may even add to it, so the sums become quite different: +1 added to -0.4, or even +1 add to +0.8. These are much more acceptable results because they are not zero
Diversity is the system whereby two antenna are fitted to a radio and the signal is switched very quickly between them. It results in a considerable improvement if the radio talking to the transmitter is in a laptop and likely to be used anywhere in the area, not one fixed position.
If the radio’s output is to be connected to an external antenna for a long-distance link, diversity is definitely not required, and should be switched off in the software settings, all the radio’s output beings sent to one antenna socket only.
DSSS radios use a single channel, in which the signal is spread-out by mixing it with a Pseudo-Noise (PN) code so the radio signal occupies a much wider band. Receivers that are not designed to separate the spectrum cannot distinguish the wanted signal from noise, so they ignore it. Interference between equipment working in close proximity to other sets is minimised or eliminated by using DSSS.
A long(ish) cable connecting an antenna with an access point. This is usually made of low loss co-axial cable such as type 195 or type 400, with type 600 cable increasingly in use for 5.8 GHz systems. See our Cable page for more information about cable types.
see also Pigtail
EIRP (Effective Isotropic
The power transmitted by a transmitter and antenna combination, measured or calculated at the antenna. European wireless networks must not exceed 100mW or 20 dBm EIRP in the 2.4 GHz band, though there are variations of this in France and Spain. See 5.8 GHz for the EIRP figures in that band.
A close estimate of EIRP is calculated by adding the transmitter output in dB to the gain of the antenna in dBi. A wifi device with 15 mW transmit power (11.5 dB) can be used with an antenna having 8.5 dB gain. This totals 20 dBm and does not exceed the maximum permitted power.
In practice the power reaching the antenna is reduced by about 0.5 dBm for every connector in the antenna cable and by n dB for every metre of cable used to feed the antenna. Losses of typical cables are on our cables page and a downloadable spreadsheet to make these calculations easy is available to download. See also RF power level and Maximum power levels by Region
There are several standards for encryption. Ultimately any wireless system is vulnerable to hacking since the data packets can be read by anyone in the vicinity with a wireless client and the will to run decryption software.
The first step towards maintaining a secure network is to design the radio components and antenna correctly. Driving a high-gain omni antenna with excessive signals is guaranteed to spray your data packets to everyone in the vicinity whether they want them or not, drowning their own networks in excessive unwanted signal, slowing their networks and yours down and ensuring a complaint to Ofcom which will result in your network being shut down. Ensuring that it is hard to receive them in places other than where they are intended to go is a really good start and has the merit of not intruding on the netwoks of those around you. Good neighbourliness and a professional approach to the design of your network or your wireless links go hand-in-hand with keeping yourself secure.
WEP (Wired Equivalent Privacy) was the first to be included in wifi devices and comes in 40-bit (also called 64-bit), or as 108-bit (also called 128-bit). Tools available on the web allow WEP-encryted systems to be cracked by a competent hacker in approximately 2 minutes.
WEP2 was an improvement to WEP. It takes a little longer to crack.
WPA WPA uses longer keys and uses a feature known as TKIP (Temporal Key Integrity Protocol) which changes the keys on the fly, as well as more sophisticated encryption methodology. Attempts to break the encryption results in temporary blocking of the attacking source. WPA requires an authentication server. WPA2 added a further Advanced Encryption Standard known as CCMP (Counter Mode with Cipher Block Chaining Message Authentication Code Protocol) to enhance security.
WPA and WPA2 require an authentication server. More often than not this is a RADIUS server which recognises the user and allows them access to the system.
PSK (Personal Key Mode) uses a passphrase stored in the PC and on the Access Point to which it talks and does not require an authentication server. Using industry techniques for creating passwords and assuming that passwords are kept secure by their users, PSK is very effective.
EAP (Extensible Authentication Protocol) is a framework defining a further encrytion method. Within the standard, multiple systems have been devised with variations on the EAP theme some of which are proprietary to a single manufacturer.
Encryption of data involves sharing a piece of private information, effectively a codeword or key, between devices that talk to each other. This is called an encryption key. Data is encrypted using an algorithm based on the key: a device that also knows the key can decrypt the data. Other devices cannot decrypt the data, so the data is (relatively) secure. Communication of the key is an automatic process.
(European Telecommunications Standards Institute)
The following is extracted from the home page of ETSI.
“ETSI is a not-for-profit organization whose mission is to produce the telecommunications standards that will be used for decades to come throughout Europe and beyond.”
Front-to-Back Ratio, stated in dB, is the ratio of the gain from the front of the antenna to the gain to the rear gain of an antenna. A directional antenna which is radiating as much behind it as in front could hardly be said to be directional, so the F to B ratio allows the difference to be quantified. F to B ratio should be at least 15 dB and a figure of 20 or above is very acceptable. The higher the figure, the more effectively directional the antenna is.
See System Operating Margin.
A system where the available frequency range is divided into a number of channels, and transmitters send short bursts of data on one channel before hopping to the next in a known or calculable sequence. See also Spread Spectrum and DSSS.
An appliance that sits between the outside world connection and the internal network. Firewalls prevent access to the internal network from the outside world while allowing internal users to communicate with the outside world. Firewalls can also set restrictions on outgoing data and isolate different parts of the internal network.
Free Space Loss (FSL)
The loss of signal over a distance in free air. This needs to be calculated when considering what equipment should be used for a point to point link. A calculator is available for download.
Frequency ranges are listed in detail in our Reference Section and more briefly here for 5.8 GHz..
Though Line of Sight between antennae for long distance links is essential, it is often assumed that this line is a straight optical line. In fact the ‘shape’ of the signal is more like a rugby ball: the line through the middle is the straight line, but the signal extends above and below this line and to each side as well. Objects that are in this wider area around the centre line, known as the Fresnel Zone, will degrade performance by attenuating the signal. It is not enough that objects are clear of the line of sight, they must be out of the Fresnel Zone as well. e.
A wireless gateway is an access point that offers more facilities such as firewall, DHCP and NAT routing, etc.
Stated in dBi, gain is a ratio of the radiation intensity in a particular direction compared to the gain if the antenna radiated equally in all directions. Antenna have gain by focussing the incoming signal into a a cone, semi-circle or circle of different sizes. The gain is a figure that represents how much the signal has been focussed as opposed to being allowed to travel outwards in all directions.
Even omni-directional antenna (which radiate the signal in a circle all around them) focus the signal: they radiate the signal with a shape closer to a plate than to a sphere, thereby focussing the signal into a much narrower vertical range.
An antenna in which the radiating element consists of a wire formed into a coil or helix, of specific length and spacing, to match the frequency range required. Helical antenna are very directional and often used in tunnels to focus the signal along its length.
Locations where wireless connections are available on a free or paid-for basis. Typically these are found in airport lounges, conference centres and major public buildings and provide web connectivity. Payment is by pre-paid voucher, credit card or subscriptions.
“The IEEE is a non-profit, technical professional association of more than 377,000 individual members in 150 countries. The full name is the Institute of Electrical and Electronics Engineers, Inc., although the organization is most popularly known and referred
to by the letters I-E-E-E.
Through its members, the IEEE is a leading authority in technical areas ranging from computer engineering, biomedical technology and telecommunications, to electric power, aerospace and consumer electronics, among others.
MISSION: The IEEE promotes the engineering process of creating, developing, integrating, sharing, and applying knowledge about electro and information technologies and sciences for the benefit of humanity and the profession.”
When a wifi device is set up in infrastructure mode it will talk to only one defined access point and not to any other local devices.
see also Ad Hoc and Bridge modes
Glossary entry available soon.
(Internet Protocol address)
An IP address is a unique number identity number for a device that enables it to be identified on a computer network and is the equivalent of a postal address for network-connected devices. The device could be a computer, a router, a printer or a VOIP phone.
IP addresses are currently in the form of xxx.xxx.xxx.xxx, so yours might be 123.456.789.007 or similar. You can find out by opening a Command prompt (start/run/command) and typing IPconfig. This will tell you your IP address , the subnet mask for your network and the IP address of any Gateway through which you connect to the Internet.
(Local Area Network)
Traditional wired network inside a building in which all PCs are connected to a hub, though they may sometimes be connected in peer-to-peer mode with no hub.
Leaky Feeder is a cable very similar to ordinary coaxial antenna cable, but it is designed to leak signal evenly along its length, thereby substituting for antenna. It is particularly useful in residential buildings which would need a large number of access points and antenna to provide full coverage.
Line of Sight (LOS)
External antenna fitted to access points for a long distance link must be able to ‘see’ other antennae with a clear view unobstructed by any objects. Merely having the target antenna visible is not enough: there is a difference between an optical line of sight and a Line of Sight for wifi antennae (sometimes called Clear Line of Sight) – see Fresnel Zone for a fuller explanation
LMR 195/WBC 195
Low loss coaxial cables suitable for wifi made by Times Microwave, and Commscope respectively. They are 5 mm in diameter, weatherproof, have losses of 0.65 dB per metre at 2.4 GHz and fit RG58 (5mm) size connectors. Very common in 2.4 GHz deployments.
LMR 400/WBC 400
Very low loss coaxial cables suitable for wifi made by Times Microwave, and Commscope respectively. They are 11 mm in diameter, weatherproof, have low losses of 0.22 dB per metre at 2.4 GHz and must be used with type 400 connectors: RG213 connectors will not fit the centre conductor. Common in 2.4 GHz deployments where losses must be minimised or cable lengths need to be longer.
LMR 600/WBC 600
Extremely low loss coaxial coaxial cables made by Times Microwave, and Commscope respectively. They are 15 mm in diameter, weatherproof, have low losses of 0.14 dB per metre at 2.4 GHz and 0.238 dB at 5.8 GHz and must be used with LMR 600 connectors: no other size will fit.
A miniature version of the PCI bus connector used with small cards often found in laptops. Many wireless cards are built on mPCI format and are fitted to wireless router boards or into PCI adaptors so they may be plugged into a PC PCI bus.
(Media Access Control address)
Every device capable of listening/talking on a network has a unique hardware-coded MAC address to identify it. The address is usually printed on the device and can be used in wireless connections to identify other wireless devices in the vicinity and/or ensure that the network talks only to specific devices whose MAC addresses it recognises.
A mesh box is usually a small dedicated PC fitted with a PCI wifi card and sometimes an external antenna. Its sole purpose is to befriend another similar unit working locally in ad hoc mode like itself and talk to it. If such units form a daisy chain and one of them is connected to the internet through an ADSL or leased line, web access can be shared through all the boxes running the system. There is little control of bandwidth but web access is cheap for owners.
Mesh networking is very popular and while it was not really suitable for commercial systems when it was first developed, reliability and robustness are now much highre. The best known is Locust Mesh.
MIMO is really an extension of the diversity principle with sophisticated time/frequency modulation techniques. Transmitters and receivers are fitted with multiple antennas and without increasing EIRO or bandwidth use, much more data can be be passed through the system than with single antenna.
MIMO techniques play a significant role in the new 802.11n high speed wireless networking. Ubiquiti ‘Airmax’ equipment with dual antenna offers 300 Mbps maximum throughput over many kilometres.
milliwatt to dB
mW to dB Conversion table
(Network Address Translation)
This is a system whereby one computer with access to an incoming IP address creates a further set of local IP addresses for other PCs on the same LAN. Only one address is seen by the outside world.
NLOS (Near Line of Sight)
Radio waves bounce off and are reflected by solid objects in different ways and are scattered. In some cases scattered signals can reach a remote antenna where a signal cannot reach it directly, i.e. where there is no direct Line of Sight. Higher frequencies scatter more than lower frequencies, so 5.8 GHz frequencies are more likely to achieve a connection even though there is no LOS.
NLOS (Near Line of Sight) is used to describe 5.8 GHz systems as they are more likely to achieve a link without a direct LOS. Bear in mind that 5.8 GHz systems usually have higher transmission powers as well.
see Fresnel Zone for more details
Ofcom is the UK Regulatory Body for radio communications and came into existence in January 2004. One of its first announcements was related to wireless networking – the making available of the 5.8 GHz frequency range for wireless communications, with particular reference to the distribution of Broadband to areas that cannot easily reached by conventional telephone lines.
An antenna where the radiation pattern is omni-directional. This can be thought of as spherical, like ripples radiating outwards across the surface of a pond, but in three dimensions rather than two, though the sphere is flattened top and bottom.
Fuller descriptions of omni and other antenna types are available at Choosing the right antenna
Parabolic Grid Antenna
A dish antenna where the dish is not solid but is made of wire grid which stands up to windy weather very well.
See also Dish and Choosing the right antenna
Small unobtrusive antennae for internal or outdoor mounting. Ideal for customer premises which access a local node.
See also Choosing the right antenna
Glossary entry available soon.
PCI refers to a data bus inside a PC. A wireless PCI card fits into a vacant PCI slot and has a small antenna built in. Quite often the PCI card is just a carrier for a PCMCIA or mPCI card which retains it and interfaces the wireless connectivity with the PCI bus in the PC. PCI cards are thus devices fitted inside a PC and the lid or outer casing must be removed to fit them: they are usually plug and pray devices.
A (normally short) cable which connects to the antenna at one end and the wifi device at the other. Often a downlead cable may be made in one type of cable to reduce losses and have a more flexible pigtail joined to it for the last few cms/metres to link the wireless device to the downlead. There is no defining length above which a pigtail becomes a downlead or vice versa.
Flat Panel antenna usually containing the electrical equivalent of four patch antenna: consequently they are larger and have higher gains and directivity.
In the UK, planning permission may be a factor in siting antenna in listed areas and on listed buildings, or in conservation areas. Loft mounting of antenna may be a good solution as may be small patches mounted under eaves. Often an antenna an be mounted unobtrusively on an existing TV antenna pole.
Dish size should not exceed 90 cms in the UK and dishes should be mounted on the least public face of the building. If in doubt, check with the Local Planning Authority. Buildings may have restrictions regarding the number and siting of any dishes that may be installed. As a general rule one dish may be installed without planning permission but it is sensible to check before making any installation and wherever possible mount so that the antenna is not easily seen from the front, or most public, face of the property.
(Power over Ethernet)
It is sometimes desirable to mount an access point close to an antenna, or it is not desirable to have a long co-ax downlead. The access point is likely to need a low voltage power supply which usually comes from a PSU brick that plugs into the mains, but the mains supply is a long way from the Access Point and running a mains supply to it is expensive.
One solution is to use four (spare) cores in the eight core ethernet cable that connects the AP to the LAN to carry the power supply to the access point. Normally this is done by means of two small boxes, a POE injector at the bottom of the cable which accepts a data cable from the LAN and the low voltage supply from the power brick and another injector at the top working in reverse to pick off the power supply off the cable and forward just the data to the wireless device.
In many cases, the ethernet socket on the wireless device can accept the end of the ethernet cable directly, having been designed to power the board it’s mounted on by picking power off the ethernet cores and separating out the data.
Ethernet cable introduces a voltage drop and beyond 10 metres on a 12v supply the power available to the wireless device may not be enough to make it work properly. For longer cable runs 24 and 48 volt supplies are used. The lower the voltage used the greater the voltage drop will be so with radios requiring only a 5v supply the length of the ethernet cable cannot be very long, probably in the region of 5 metres or less.
Some systems use a more sophisticated system with an IEEE standard that defines how it operates. An 802.11 ‘af’ device will accept more or less any voltage up to 48v, in any polarity. In the less sophisticated devices we are referring to above, care must be taken to ensure that polarisation and voltage are what the remote device expects them to be or the device can quickly become a brick.
Point to point
One wi-fi bridge talks to one other wi-fi bridge and other radio devices are ignored.
Point to multipoint
One master wi-fi bridge talks to several other wifi bridges, like a hub in a conventional wired LAN.
Polarisation refers to the plane in which the radiated wave is transmitted. This is typically vertical, horizontal or circular. If two antenna do not use the same polarisation, the signal received will be considerably weakened or no signal will be received at all.
QAM (Quadrature Amplitude Modulation)
Glossary definition available soon.
QPSK (Quadrature Phase-Shift Keying)
Glossary definition available soon.
Radiated power levels
How far will your wireless network reach? Most Wi-Fi systems will provide a range of a hundred feet or more. Depending on the environment and the type of antenna used, Wi-Fi signals can have a range of many kilometres linking together offices and buildings or providing Broadband connectivity for communities whose telephone exchanges are not DSL-enabled.Whether a long-distance link will be possible depends on several factors.
Wireless network links have two ends, and each end includes a transmitter and a receiver. It doesn’t matter how huge the signal that the transmitter radiates may be, if the receiver is deaf no data will pass through the link. It is important to know the sensitivity of the receiver. A very sensitive receiver will have a strong link even though the transmitted signal is quite poor, but an insensitive receiver may get a poor link even when the incoming signal is excellent. The most important trick in the wireless book is to make sure the receiver has good sensitivity.
Receive sensitivity is measured in dB, always with a minus sign in front. The higher the number, the greater the receive sensitivity and the better the radio will perform. ‘Higher’ in this instance means that -97 dB is ‘higher’ than -90 though it is actually lower, being a minus number. Because of the way that radios for 2.4 and 5.8 GHz are designed, sensitivity reduces with data throughput speed so at 1 Mbps the sensitivity is higher than at 11 MBps. This way a link may be established, buit the speed may be slow: this is regarded as better than trying for a fast link but getting no result at all.
If the receive sensitivity of the radio intended for use is less than- 84dB at the speed you are hoping for the results may be disappointing.
As for gateway, but in a home.
RF power level
Power is expressed in Watts (for wifi in thousandths of a watt, or milliWatt) but is conventionally quoted in dBm. To convert milliWatts to dBm use the formula: Power(dBm)= 10Log milliwatts
A cheap, lossy, type of coaxial cable not recommended for wireless antenna connection. Always be clear with your supplier if you are purchasing RG58 size cable that you are not purchasing RG58 cable. Both cables are 5 mm in diameter and ‘RG58’ is often used to describe the size, which can cause confusion.
A standard size (3 mm) for MMCX cable connectors and also a very cheap, very lossy, type of coaxial cable not recommended for wireless antenna connection.
(pronounced ‘roota’) [router pronounced ‘r-ow-ta’ is a woodworking tool]
A router routes directs packets of data to the correct destination. Each packet includes a destination address (similar to a postal address). Routers in a network know the IP addresses of PCs connected to other routers and work together to choose the shortest route across the network to send each packet. Packets may travel by different routes before being finally reassembled in the correct order.
An antenna usually having a restricted radiation pattern, e.g.the signal is radiated only in front of it, but in a beamwidth of between 80 and 180 degrees.
In addition to the principal beam, secondary beams known as side lobes are radiated by an antenna.
This polar diagram of an planar antenna shows the direction of radiation viewed from above. The main beam is the larger lobe extending upwardsand the side lobes (two on each side) can be clearly seen below the principal lobe.
Signal to Noise Ratio (SNR)
The number of decibels difference between the signal strength and unwanted background noise. The bigger the difference, the better the quality of the link. The number quoted is the figure for the noise below the wanted signal so it appears as a minus figure. -90 dB is much better than -70 dB.
A method of transmitting data across a band of frequencies. Developed for military applications originally as piecing together a complete signal is very difficult if bits of it are being transmitted on different frequencies. Interference is also reduced. Two types are in common use: see FHSS and DSSS.
The IEEE has produced standards for the operation of 80211x wifi devices and devices which comply with this specification can be described as Wi-Fi certified. (see also WECA)
see 802.xx above
System Operating Margin
The difference between the received signal level (in dBm) and the receiver sensitivity (in dBm) needed for error-free reception. Also known as Fade Margin. System Operating Margin should be calculated when designing a point to point link to be sure that it will work.
One requirement of the Ofcom UK specification for 5.8 GHz radios operating in Band C. The paired radios adjust themselves to the lowest power level required to achieve a working link, which allows higher gain antenna to be attached to enable longer distances to be achieved at lower transmission powers. see 5.8 GHz
USB wireless devices are available. They connect to the PC via a USB cable or as plug-in dongles and may or may not have an external antenna socket.
USB devices avoid having to open up a PC and install a PCI card and are therefore quick and easy to install, requiring only a driver to be loaded from CD. Normally they are USB 1.1 compatible.
Where a LAN is involved, ethernet-based devices give better results.
(Voice over IP)
Voice over IP is a system of using data packets streaming to and from the internet to carry digitally-sampled sound. Commonly found in VOIP phones and computer software such as Skype. Calls are carried internationally without a wired connection being made to a telephone exchange, so there is thu
Standing Wave Ratio)
An antenna will always reflect some of the signal fed into it back down the cable that feeds it. This is undesirable as it creates standing waves back down the cable which reduce the power of the signal coming up.
The VSWR is a figure that represents the tendency of the antenna to create these standing waves.
A ratio of 1:1 represents a perfect match with no standing waves and is almost impossible to achieve.
Any figure below 2:1 is acceptable and the lower the ratio the better.
We have taken the liberty of turning it into a pdf for download and acknowledge the copyright of the author, to whom all enquiries should be addressed directly.
War chalking, Wardriving
Logging on to a wireless hotspot without the knowledge of its owner or his/her permission. No matter how much defenders of this practice may try to rationalise it, this is theft and is no less so because what is being stolen is invisible. Those who do help themselves to access provided involuntarily by others may also deface local walls with symbols so that other warchalkers may also log on, following a similar system to that used by tramps for marking the generosity or otherwise of house owners. Curiously, adherents would recognise this as a form of theft if it involved making an illicit connection directly by wiring a cable into a system owned and paid for by someone else. It’s a sad development led by freeloaders and is morally indefensible. We await a test case with interest. If you want something, pay for it, don’t steal it.
( Wireless Distribution System)
WDS enables access points to link together wirelessly to form a chain, thereby eliminating network cabling and LANS.
The same shared encryption keys must be used on each AP and be set on the same channel. WDS is a mix of bridging and access points to form a chain, but the throughput decreases substantially with every repeater added to the chain.
(Worldwide Interoperability for
WiMAX is a standard, rather than a technology, but has already come to be used for the technology used to deliver it.
“WiMAX is not a technology, but rather a certification mark, or ‘stamp of approval’ given to equipment that meets certain conformity and interoperability tests for the IEEE 802.16 family of standards. A similar confusion surrounds the term Wi-Fi, which like WiMAX, is a certification mark for equipment based on a different set of IEEE standards from the 802.11 working group for wireless local area networks (WLAN). Neither WiMAX, nor Wi-Fi is a technology but their names have been adopted in popular usage to denote the technologies behind them. This is likely due to the difficulty of using terms like ‘IEEE 802.16’ in common speech and writing.” – Organisation for Economic Co-operation and Development.
Wi-Max uses central antenna in an area and is quoted as being able to provide connectivity of 70 Mbps over 70 miles. In urban areas it is more likely to achieve 10 Mbs over 2kms, i.e. similar to DSL speeds but by radio.
Customer Premises Equipment is far more expensive than for 2.4 and 5.8 GHz systems. Major investment is required to establish Wi-Max systems and the field is being led in the UK by licenced providers and telcos. It seems of particular interest to holders of 3G Licences for use in the 3.5 GHz band. Pipex, now, Urban Wimax and British Telecom are planning to conquer the UK market and worldwide most countries have a player planning to provide Wi-Max.
Wireless Ethernet Compatibility Alliance (WECA)
The Wi-Fi standards laboratory (WECA) tests, certifies and promotes the 802.11 Wi-Fi Standard
Cross-Polarization Interference Cancellation is a microwave communication system used to utilise the frequency spectrum by allowing the reuse of the same frequency channel by having two signals in the same frequency range in inverse polarisation to each other allowing interference to be compensated.
A low cost, unobtrusive medium gain directional external antenna which usually resembles a small UHF TV aerial. Planar antenna usually give better results.
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