Wireless Standards Comparison Table

	802.11b	802.11g	802.11a	802.11n
IEEE Ratified	1999	2001	1999	2008
Frequency	2.4GHz	2.4GHz	5GHz	2.4GHz	5GHz
Non-overlapping Channels	3	3	12	3	12
Baseline Bandwidth Per Channel	11Mbps	54Mbps	54Mbps	65Mbps	65Mbps
Number of Spatial Streams	1	1	1	2, 3* or 4*	2, 3* or 4*
Channel Bonding	No	No	No	No	Yes
Max Bandwidth Per Channel	11Mbps	54Mbps	54Mbps	130Mbps	270Mbps

Common WiFi Connectors

These are some common connectors and pigtails used in the Wireless world. The pictures should help when choosing pigtails to connect a wireless card to an antenna. There are other connectors as well, so hopefully the page will grow to be a definitive guide.

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U.fl (Hirose)

The U.fl connector comes on most MiniPCI cards such as the senao NL-2511MP(prism 2.5 chipset), Senao NL-3054MP(PrismGT chipset), Intel ProWireless, Cisco, Toshiba etc. ( Also known as AMC)

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MC CARD (Lucent) to N-Male Pigtail

The Lucent connector comes on cards such as the Orinoco PCMCIA. It has a bad reputation as being easily broken. Connects to a antennas that have an N-Female such as the WaveGuide, Yagi, Cantenna and bigger Omni antennas.

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U.fl to N Female Bulkhead Pigtail

The U.fl connector comes on most MiniPCI cards such as the senao NL-2511MP(prism 2.5 chipset), Senao NL-3054MP(PrismGT chipset), Intel ProWireless, Cisco, Toshiba etc. When setting up a WRAP, this pigtail can go on the MiniPCI and the N Female Bulkhead is installed in the case.

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RP SMA Male to N Female Pigtail

The RP-SMA is commonly used on PCI cards and access points/routers that have a removeable antenna. Brands such as Netgear and D-Link have these. Hills Parabolic Dish antennas come with a short length of coax and a N-Male connector, so this pigtail can go between an AP and Hills.

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MC Card (Lucent) Connector

Commonly found on PCMCIA cards such as D-Link, Orinoco and Lucent based cards

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N Male Connector

Hills Parabolic Dish antennas, among others, come with a short length of coax and a N-Male connector. Connects to alot of anennas that have an N-Female such as the Yagi, Cantenna and bigger Omni antennas.

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RP SMA Female Connector

The RP-SMA Female is commonly used on PCI cards and access points/routers that have a removeable antenna. Brands such as Netgear and D-Link have these.

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RP SMA Male Connector

The RP-SMA Male is commonly used on Antennas that go onto PCI cards and access points/routers that have a removeable antenna. Brands such as Netgear and D-Link have these.

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SMA Female Connector

Normal SMA is not commonly used on wireless gear

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RP SMA Female PCB Jack

The RP SMA Female Straight PCB Jack is commonly used on access points/routers and wireless cards that use a removeable antenna. Brands such as Netgear, Mitsubishi, Edimax/UltraWAP,Minitar, and D-Link have these. It is available in a variety of configurations such as straight, right-angled, long straight and edge mount.

ACK Timeouts and the effects on distance links

All 802.11 a/b/g wireless devices use a number of time constants, defined by the IEEE specifications, to sense other carriers using the wireless media and avoid collisions (compared to 802.3 which only senses collisions) as well as for retransmission of lost frames. The important constants to consider are Slottime, CTS timeout and ACK timeout. Slottime is more important for collision avoidance when multiple stations are connected, or when one is trying to simulate full duplex communication, while ACK timeouts are more important for point to point communication. Other constants that may restrain the maximum link distance are SIFS (Short Inter Frame Spacing), DIFS (Distributed IFS) and PIFS (Point Coordination IFS.) DIFS is the amount of time a station must sense a clear radio before beginning a new transmission sequence. SIFS is the amount of time a station must wait before sending or beginning to receive a RTS, CTS or ACK frame. PIFS is the DIFS for the access point in a special access method known as Point Coordination Function. The times are defined such that the RTS, CTS and ACK frames are given a higher priority (ie once a packet transmission sequence has begun, the station holds onto the channel until it is finished)

In 802.11b, the constants are mandated by IEEE as follows:

• Slottime = 20 µs
• SIFS = 10 µs
• PIFS = SIFS + Slottime = 30 µs
• DIFS = SIFS + 2 x Slottime = 50 µs

In 802.11g:

• Slottime = 9 µs
• SIFS = 10 µs
• DIFS = SIFS + 2 x Slottime = 28 µs

In 802.11a:

• Slottime = 9 µs
• SIFS = 16 µs
• DIFS = SIFS + 2 x Slottime = 34 µs

Note: DIFS/SIFS/PIFS are used for physical layer carrier sensing while the MAC layer performs the collision detection using CTS and ACK timeouts. Default CTS and ACK timeouts vary between manufacturers.

Normal Transmission flow without RTS/CTS handshaking

Sender: Wait DIFS, Send Data, Wait SIFS, Listen for and receive ACK (until maximum ACK timeout), Repeat

Receiver: Listen for and receive Data,Wait SIFS, Send ACK, Wait DIFS

Distance Limitations in 802.11b

Optimum ACK Timeout, Slottime and DIFS all depend on Air Propagation Time in some way. The IEEE standard defines Air Propagation Time as 1 µs ±(10% SIFS.) Radio waves propagate at approximately 300 meters per µs. 

    ACK Timeout: Clearly if the ACK timeout is shorter than the time it takes for the end of the last data packet (+ SIFS) to propagate to the receiver + the start of the ACK for that packet to propagate back to the sender, then the sending MAC will assume that the packet has been lost and will unnecessarily retransmit the data packet. The retransmitted packet will end up colliding with the ACK that is on its way back, inducing the back-off part of the protocol thus reducing throughput. If, conversely, the ACK timeout is set too long, the transmitter waits unncesessarily long before retransmitting any lost frames and thus reduces the throughput of the link (more important as the bit error of the environment increases.) Similar conclusions can be drawn about the CTS timeout, however RTS/CTS is only for reducing collisions with hidden nodes and can be turned off in the majority of cases. Most implementations assume the IEEE recommended one-way Air Propagation Time of 1 µs, thus tune the ACK Timeout of their devices to 300 meters. This is the maximum distance that high throughput can be achieved between a local network of IEEE 802.11b compliant devices out of the box. The links will still work over longer distances, but throughput will drop as the distance increases.

    Example: With radio waves propagating at an approximate speed of 300 meters per microsecond, a 3000 m link would require an Air Propagation Time of 10 µs (9 µs more than usual.) The ACK Timeout must account for a round trip propagation, therefore for a 3000 m link to achieve maximum throughput, another 18 µs must be added to a standard ACK Timeout. This maximum throughput will be slightly less than can be achieved at close range, but will be much more than if the ACK Timeout is not changed.

    Slottime: A station is only allowed to transmit at the beginning of the slottime, so this time should not end before the signal reaches the destination. This gives a maximum distance of 20  µs x 300 m / µs = 6000 meters. Assuming that the ACK timeouts have been increased accordingly, this is the maximum distance between any station in a point-to-multipoint environment (assuming no hidden stations) without incurring heavy exponential backoff.

    DIFS: A station must wait DIFS before initiating a new transfer, in other words, for 802.11b, the stations have 50 µs to sense the channel to avoid collision with a frame sent from the furthest node. This means the maximum distance that the furthest node can be is 300 m / µs x 50 µs = 15000 meters. Assuming that the ACK timeouts have been increased accordingly, this is the maximum point-to-point distance before throughput drops dramatically without the slottime (DIFS is based on slottime) being changed.

Long distance links in 802.11g/a

The principles are the same, but the numbers are different. The default timings are much more strict, so for any reasonable distance link the ACK timeout and Slottime will probably need to be changed, even for point-to-point links. Many Ethernet bridge/gateway devices provide little or no way to change this, however certain miniPCI adapters such as the CM9, Senao and SuperRange cards have driver support in BSD and Linux to alter these settings. The possible throughput that can be achieved when properly tweaked, though not full theoretical rate, is much higher than 802.11b can achieve over the same distance and is worth the effort.

Changing ACK timeout and Slottime

For every further 300 meter increase in distance above 300 meters add 1 µs to the Slottime of your device, and 2 µs to the ACK timeout and CTS timeout.

Every station connected on the same channel should have the same time constants.

Use the furthest distance between any two nodes as the distance in your calculations.

For specific examples for popular operating systems and drivers please see http://www.air-stream.org/Change_ACK

Definitions

ACK Timeout = Air Propagation Time (max) + SIFS + Time to transmit 14 byte ACK frame [14 * 8 / bitrate in Mbps] + Air Propagation Time (max)

Slottime = MAC and PHY delays + Air Propagation Time (max)

DIFS = SIFS + 2 * Slottime

Summary

Moderately long distance 802.11b links may work well "out of the box" due to the lax timings, and even more lenient MAC implementations by some manufacturers. With the right wireless card and some tweaking, 802.11b can maintain almost maximum theoretical throughput over very long distances and thus is the distance king.

However 802.11a is still the throughput king, but specialised cards that allow tweaking are certainly required for any distance links. Tweaking 802.11g requires a card that can be locked into 11g mode so that the slottime doesn't keep reverting every time a 11b client tries to connect. 802.11a wins out over 802.11g because there are more non-overlapping channels in the 5.8 GHz spectrum, meaning most 11a links will more likely be able to maintain their high throughput.

- shadey

AMTA website posts safety fact sheets on Wi-Fi

AMTA has posted two new fact sheets on its website to explain the safety of Wi-Fi and how it works and the range of radio communications in the community.

See their website for more details.

Antenna Polarisation

Antenna polarization is a very important consideration when choosing and installing an antenna.

Most systems use either vertical, horizontal or circular polarization. Knowing the difference between polarizations and how to maximize their benefit is very important to users.

Polarization
An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The electric field plane determines the polarization or orientation of the radio wave. In general, most antennas radiate either linear or circular polarization.

A linear polarized antenna radiates wholly in one plane containing the direction of propagation. Where a circular polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. If the rotation is clockwise looking in the direction of propagation, the sense is called right-hand-circular (RHC). If the rotation is counter clockwise, the sense is called left-hand-circular (LHC).

An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the "whip" antenna on an automobile. Horizontally polarized (linear) antennas have their electric field parallel to the Earth's surface. Television transmissions use horizontal polarization.

Circular polarized wave radiates energy in both the horizontal and vertical planes and all planes in between. The difference, if any, between the maximum and the minimum peaks as the antenna is rotated through all angles, is called the axial ratio or elliptically and is usually specified in decibels (dB).

If the axial ratio is near 0 dB, the antenna is said to be circular polarized, when using a Helix Antenna. If the axial ratio is greater than 1-2 dB, the polarization is often referred to as elliptical, when using a crossed Yagi.

Important Considerations
Polarization is an important design consideration, as each antenna in a system should be properly aligned for maximum signal strength between stations. When choosing an antenna, it is an important consideration as to whether the polarization is linear or elliptical. If the polarization is linear, is it vertical or horizontal? If circular, is it RHC or LHC?

This is becomes a greater concern in Wireless Lan devices as line-of-sight (LOS) paths are required due to the low power levels involved, consequently the polarization of the antennas at both ends of the path must use the same polarization.

In a linearly polarized system, a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned and 90 degrees the attenuation can be more than 20 dB.

Likewise, in a circular polarized system, both antennas must have the same sense. If not, an additional loss of 20 dB or more will be incurred. Also note that linearly polarized antennas will work with circularly polarized antennas and vice versa. However, there will be up to a 3 dB loss in signal strength. In weak signal situations, this loss of signal will mean a great deal.

Cross polarization is another consideration. It happens when unwanted radiation is present from a polarization, which is different from the polarization in which the antenna was intended to radiate. For example, a vertical antenna may radiate some horizontal polarization and vice versa. However, this is seldom a problem unless there is noise or strong signals are nearby.

Typical Applications
Vertical polarization is most commonly used when it is desired to radiate a radio signal in all directions over a short to medium range.

Horizontal polarization is used over longer distances to reduce interference by vertically polarized equipment radiating other radio noise, which is often predominantly vertically polarized.

Nevertheless both horizontal and vertical polarization may be deployed over long distance if a reflector is deployed to focus the energy being emitted.

So consequently the decision is using the polarization, which offers the best rejection of local unwanted signal.

Circular polarization is most often used in satellite communications. This is particularly desired since the polarization of a linear polarized radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere.

Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary especially if the satellite appears to move with respect to the fixed Earth bound station. Circular polarization will keep the signal constant regardless of these anomalies.

These Antennas make very good point-to-point long run connections due to a combination of linear noise rejection and high gain. The two most common a crossed yagi or helix

When setting up an exclusive communications link, it may be wise to first test the link with vertical and then horizontal polarization to see which yields the best performance (if any).

If there are any reflections in the area, especially from structures or towers, one polarization may outperform the other. Further, if there are other RF signals in an area, using a polarization in the opposite predominant high level signals will give some isolation as discussed earlier.

On another note, when radio waves strike a smooth reflective surface, they may incur a 180 degree phase shift, a phenomenon known as specula or mirror image reflection. The reflected signal may then destructively or constructively affect the direct LOS signal.

Circular polarization has been used to an advantage in these situations since the reflected wave would have a different sense than the direct wave and block the fading from these reflections.

Diversity Antennas
Even if the polarizations are matched, other factors may affect the strength of the signal. The most common are long and short-term fading. Long term fading results from changes in the weather (such as barometric pressure or precipitation). Short term fading is often referred to as "multipath" fading since it results from reflected signals interfering with the LOS signal.

Some of these fading phenomenon can be decreased by the use of diversity reception. This type of system usually employs dual antennas with some kind of "voting" system to choose the busiest signal. This is commonly used in many 802.11 wireless network equipment.

However in theory for the best results when using external antennas they should be at least 20 wavelengths apart, so that the signals are no longer correlated, particularlly in medium and long-distance situations.

Bits and Bytes

    byte(s)
    bit(s)

Data Transmission conversion (kilobit):

In data communications, a kilobit is a thousand bits, or 1,000 (10^3) bits. It's commonly used for measuring the amount of data that is transferred in a second between two telecommunication points. Kilobits per second is usually shortened to Kbps.

Some sources define a kilobit to mean 1,024 (that is, 2^10) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two.

Easy dBs

Off the back of the meeting last night where we were talking about the power output of cards and
antenna gains. I'd thought I'd post how I remember the whacky world of dB gains.

Rule of Thumb: For power gain, every decade of dB adds a zero to the "10".

eg.
0dB = 1
10dB = 10
20dB = 100
30dB = 1000
40dB = 10000
etc.

Rule of Thumb #2: Every doubling of power is +3dB

eg.
100 = 20dB
200 = 23dB
400 = 26dB
etc.

This stems from a formula used to calculate gain in terms of dB,
Av(dB) = 10 log( Av )
Where Av(dB) is the gain in terms of dB, and Av is the raw gain which is determined by
dividing the output power by the input power,
Av=Po/Pi

Also as a side reminder
dBi - Gain of an antenna with respect to a unity-gain isotropic antenna.
dBm - Gain of an amplifier (power output of a transmitter) with respect to 1mW.

Edit: Those wanting to know the dBm of their card, just put the power figure in the formula I mentioned
above (ie. for a 350mW card, 10*log(350)= 25.4dBm)

Effective Isotropic Radiated Power (EIRP)

Effective Isotropic Radiated Power (EIRP) is the output power when a signal is concentrated into a smaller area by the Antenna.

An isotropic radiator radiates power equally in all directions, however a perfect isotropic radiator is only theoretical as even the simplest antennas will concentrate the signal in certain direction(s). E.g. a 1/2 wave dipole has a gain of 2.15 dBi.

The EIRP is calculated using this formula:

EIRP = Effective Isotropic Radiated Power

Pout = transmitter power output (dBm)

Ct = signal loss in cable (dB)

Gt = gain of the antenna (dBi) Pout - Ct + Gt = EIRP

When installing a wireless system with external antenna, your EIRP calculation should not exceed the class license limit. Other wise you must adjust either the transmitter power output, the length of cable and/or the choice of antenna.

Access Point Details

Power Output:		dBm
Cable Attenuation:		dB
Antenna Gain:		dBi
EIRP:		mW
		dBm
Note: this is for use as a guide only

Fresnel Zone

Here is scenario...

It’s early in the morning on a cool, clear day. The top of a proposed Air-Stream site is just visible through your binoculars and everything seems good

You go out and purchase and test the necessary wireless equipment, mounting an antenna on the rooftop, then fire it up, and… then nothing but problems.

The link does not seem to cooperate at all. It fades in and out, and the radio signal levels are low when there is any link at all. You check all the connections, twist and turn the antenna, and use your best language to get it to work… no improvement.

On first analysis it appears that some component in the wireless unit is malfunctioning, but the test worked properly and the units performed as expected.

Standing on the roof, you snap open your mobile and call another Air-Stream member. They listen to your story, and then they ask a question:

"Any chance that another signal is interfering with your signal?"

I checked, and can pick out a few other SSIDs but nothing on the same channel "Good", you reply

"Now, do you have clear Radio Line of Sight?" "Of course", is your response “I have some binoculars, and can see the other site from where I’m standing??”

They pauses and then ask "That’s good to know, but what about proper Fresnel Zone clearance?"

Fresnel Zone clearance????
At that moment, you begin to suspect that there are other factors other than a visual sight involved with establishing proper, usable Radio Line of Sight.

So what is going on here?
For two radios, or wireless units, to connect to each other, the radio signal must reach both units with adequate strength, and in a usable form.

For a wireless network spanning many kilometres these factors collectively are known as "Radio Line of Sight", and become very important indeed.

Visual and Radio line of sight: are they different?
Light and radio are both forms of electromagnetic radiation but operating at very different frequencies. Just because you can establish visual line of sight, or see a distant building with binoculars on a clear day, does not mean you can establish a wireless link between them.

What is the Fresnel Effect?
In the early 19th century French physicist, Augustin Fresnel (pronounced "Freh-Nel") made an important observation about the behaviour of light. Fresnel noted that a ray of light passing near a solid object is subject to diffraction, or bending.

This diffraction caused the intensity of the original light beam to increase or decrease depending on how near the object was to the beam. This characteristic of electromagnetic radiation is known as the Fresnel Effect.

Light and radio waves are subject to the same laws of physics, including the Fresnel effect. If an object like a mountain ridge or building is close to the radio signal path, it can affect the quality and strength of the signal.

Radio waves diffracted by such objects can affect the strength of the received signal. This happens even though the obstacle does not directly obscure the direct visual path.

This area, known as the "Fresnel Zone", and must be kept clear of all obstructions. That means the earth curvature, depth of the Fresnel Zone, and height of objects in the radio path must be added together to get the antenna mounting height. It’s usually adequate to use less than the full depth of the Fresnel Zone to calculate clearance.

How much Fresnel Zone clearance do I need?
60% of the Fresnel Zone (F) is the generally accepted portion which must be kept clear. This assumes that there are no buildings or other obstructions in the way. If such obstructions exist, their height must be added to the total antenna mounting height.

To see the distant end with binoculars, we need only elevate our eye high enough to clear the Earth’s curvature. Remember, you saw the top of the distant site. The problem was that you only confirmed visual line of sight, not radio line of sight.

When it came time for the radio signal to pass from site A to site B, the lack of adequate Fresnel Zone clearance caused signal diffraction, and degradation of the radio signal.

So what do you do?

• Use an antenna with a more narrow lobe pattern, usually a higher gain antenna will achieve this
• Raise the antenna mounting point on Site A and/or Site B
• Build a new structure, e.g. a tower tall enough to provide adequate clearance
• Increase the height of the existing antenna mounting point by installing a taller mast with stabilizing guy wires
• Locate a different mounting point, e.g. building or tower, for the antenna
• Remove the obstacle (such as taking a chain saw to that Tree)

LOS (Line of Sight)

802.11a and b wireless LAN equipment has proven to be a very useful and a low cost way to setup networks over distance and is commonly used by many individuals, community groups and small businesses.

However for most setting up a wireless network for the first time, one major hurdle that will almost always comes up is the problem of establishing good LOS between sites. LOS or Line of sight is a very important consideration, as without it, it would be impossible to establish a reliable network connection over a few hundred metres regardless of the antenna, equipment or mast deployed.

This is because the high frequency and low power of most standard wireless devices 802.11(a/b/g) is unable to pass through a solid object without a significant reduction and dissipation of the signal.

Where as good LOS it is not uncommon to see links established over 10kms with sustained data rates exceeding 18Mbps in 802.11a Unfortunately, in the real world good LOS between different locations is a rare coincidence for individuals working on their own and it is for this reason many community groups like Air-Stream Wireless have been formed.

By establishing a group, members can help each other by establishing a shared network where specific sites with good LOS become the relay points for others without good LOS consequently overcoming many topographical barriers that an individual would find difficult to overcome on their own.

MIMO and 802.11n

MIMO an acronym for multiple input, multiple output, and is a system which is deployed along with OFDM (Orthogonal frequency-division multiplexing) in the new 802.11n standard. This new standard offers many advantages over conventional standards such as 802.11g Wireless LAN equipment of which we have become familiar.

MIMO a system which exploits multiple transmitters and antennas to increase the bit rate in a wireless LAN link with no additional power or bandwidth consumption using a method called Spatial Multiplexing (SM). The benefits of MIMO over conventional 802.11g equipment is pronounced.

Benefits include:

• Improved signal to noise ratio through increased antenna array gain,
• Improved link reliability using phase nulling techniques
• Near and Non LOS performance enhancement
• Improved ability to ignore other signals which inhabit the same band
• Increased throughput due parallel data channelling.

Here is a simplified summary of how it all works

The input data stream is split into independent sub-streams which together occupy less bandwidth than is required to transmit the original stream on a single channel. These sub-streams are applied separately to individual transmitters and antennas on the same frequency, where the receivers at the other end recover each sub-stream and merg them back together.

Due to the presence of various scattering objects eg: buildings, walls, cars, trees, etc. signals experience a multi-path propagation and when it is captured by the receiver antenna these signals will arrive with random phase and amplitude. In conventional 802.11g equipment only the strongest signal is used and the other mulit-path signals are rejected as noise.

However in a MIMO device this is turned to an advantage as these deferent phases and amplitudes will have a specific spatial signature. The receivers can be viewed as a bank of superposed spatial weighting filters where every filter aims at extract one of the multiplexed sub-streams by spatially nulling the remaining ones. This not only allows the added benefit of array gain due to multiple antennas, but also diversity techniques which reduce signal fading.

The disadvantages of the MIMO system is mostly the need for multiple Antennas; the cost of the equipment compared to existing equipment available and limited open source driver support.

However as poeple become more aware of the posibilities the standard it is certain to become popular and with this the price should come down significantly. This I'm sure also will see more manufacturers making code available for the Open Source Community, which will enhancing their competitive advantage over other brands and boarder support by wireless network communities and enthusiasts.

Minimising Interference

Users of wireless LAN equipment over distance are increasingly being effected by rising levels of interference due to the ubiquitous nature of wireless usage in the home and office, further fueled by the low costs, easy setup and the potential benefits that the technology offers.

Also access to the radio bands used by this equipment is relatively unconstrained, called the 'public park' concept the planning objective of regulative authorities is for all users to be able to access a small portion of the total resource and to share that resource in a way that requires minimal regulatory intervention provided the power is kept well down.

The use of a 'public park' approach is administratively efficient and gives great freedom to users, but the price of this freedom is increased risk of interference. What is often miss-understood by many users of wireless LAN equipment is that this framework equally applies to all users regardless if you are a telecommunications carrier or an individual.

Clearly the very thing that has created an environment where communities can establish wide area networks like Air-Stream is the same environment which has created the interference problem.

Consequently, minimising the impact of interference through public awareness, and providing an open platform for users to share information to maximise the effectiveness of their equipment and minimising interference is important to all.

What is a dB?

Newcomers to wireless technology often find terms such as dB, dBm etc confusing. This document explains these terms.

We use certain types of antennas because they have “gain” – they increase signal strength. They don’t magically create extra energy though, they direct radio signals into a smaller beam in the same way a spot light does. The higher the gain the narrower the beam and the more concentrated power – in certain directions only though!

We often talk about this gain in terms of ratios, because the gain is independent of the power level – an antenna that doubles the signal strength of a one watt transmitter will also double the signal strength of a ten watt transmitter.

You can also have losses – a poor cable might lose one half (or more) of the signal power. This is also independent of the absolute power level – one watt becomes half a watt, ten watts becomes five, etc.

Ratios are messy to use though when using several of them together – imagine “I have a seventeen times antenna with a one third power loss in my cable and one tenth of my power lost in the connectors”. It’s hard to figure out just how much overall gain or loss that represents.

To make it easier, we work with deci-Bells (dB), which are defined as ten times the logarithm of the ratio. The beauty of this is that to multiply gains (or losses) we just add the dB numbers. (Gain will be a positive dB number, while loss will be negative, so subtracted) So now “I have a 15dB antenna, with 3dB of cable loss and 2dB of connector loss – a total of 10dB gain”. Much easier!

Here are some examples of dB ratios:
+3dB is doubling the power, -3dB is only a half.
+6dB is Quadrupling power, -6dB is only a quarter.
+10dB is 10 times the power, -10dB is only a tenth.
+20dB is 100 times the power, -20dB is only one percent.

(You can now see why the “ten times” factor is used – it avoids fractions in the log values for commonly encountered ratios)

All of the above deals with RATIOS, which are independent of power. So it’s meaningless to say “How much power will a 23dB antenna give me” – the antenna gets as much power as the transmitter feeds it, but focuses it to give the effect of a 23db stronger (200 times more powerful) transmitter – in the focus direction only. In fact, the effective width of the strengthened signal is specified as “degrees of beamwidth” and is the number of degrees between the points at which the gain falls off by 3dB.

We often talk about the “Effective Radiated Power” and that is the equivalent power level produced by the transmitter/antenna combination (in the highest gain direction). So a 1 watt transmitter with a 20dB (100 times) antenna is producing a ERP of 100 watts.

High gain antennas have narrow beamwidth, and so can be harder to align. It should be noted that gain can be achieved in either the horizontal or vertical plane (or both). Most antennas have quite different patterns of radiation in the vertical and horizontal planes, with different antennas chosen for the different pattern according to their function. Antennae also have a “polarisation” – either horizontal or vertical, which defines the way in which the radio waves are transmitted, but the main connection between signal strength and polarisation is that having different polarisations between transmitter and receiver gives another 20dB or more of loss!

Finally, another term encountered is dBm, and this one IS related to power. Zero dBm is defined as one milliwatt (one thousandth of a watt) and so transmitter powers are often expressed in terms of this reference. This is NOT the same as antenna gain – this is real power. Receiver sensitivity is also expressed in terms of how many dBm or what power level is required to receive a signal. So if I have a 20dBm transmitter, a 15dB gain antenna at each end, 110dB of path loss (signal attenuation in the atmosphere) and a –90dBm receiver, can I get a signal through?

Add them up: 20 plus 15 minus 110 plus 15 gives -60dBm, so I will theoretically have a signal of 30dB more than the minimum I need, so it should be a good path. Of course, this is only theoretical, trees and other obstructions can reduce the signal dramatically, but this is a good starting point before a real-world trial.

Steve Fraser

Channels for 802.11b

802.11b applies to wireless LANs and provides 11 Mbps transmission (with a
fallback to 5.5, 2 and 1 Mbps depending on Range and Signal Strength) in the
2.4 GHz band. 802.11b uses only DSSS (Acronym for direct-sequence spread spectrum.
DSSS is one of two types of spread spectrum radio.) 802.11b was a 1999 IEEE
ratification to the original 802.11 standard.

Channels for 802.11g

802.11g is a proposed standard, describing a wireless networking method for
a WLAN that operates in the 2.4 GHz radio band. By using OFDM (Orthogonal Frequency
Division Multiplexing) technology, 802.11g-based WLANs will be able to achieve
a maximum speed of 54 Mbps. 802.11g-compliant equipment, such as wireless Access
Points, will be able to provide simultaneous WLAN connectivity for both 802.11g
and 802.11b equipment.

Channels for 802.11a

Channel Separation Layout