1.4.2 RF Gain/Loss

RF Gain/Loss

WiFi uses RF to transmit data. The signals amplitude decreases as it travels away from the source. Think of the area of the wave increasing. As the area increases, the amount of signal that hits the receiving antenna is decreased (most of the signal spreads out in other directions). The rate of decrease is affected by the material the wave passes through. Even if the wave hits no obstructions, it will still decrease. This is called Free Space Path Loss. The free space path loss in dB can be calculated according to the following equation:

FSPL (dB) = 20log 10 (d) + 20log 10 (f) + 32.44

As you can see, the loss is proportional to both the distance and the frequency. The higher frequencies (of the 5GHz band) will be attenuated more than lower (2.4GHz) frequencies. This is one reason why 2.4GHz WiFi bands cover a larger area than 5GHz bands.

When an RF signal passes through other materials such as walls, windows and people, the annuation is greater than the FSPL.

Antennas are used to amplify the signal. Remember antennas amplify both the transmitted signal and the received signal. So an antenna on an AP will improve the reception of WiFi signals as well as the transmission.

The link budget is the combination of gains and losses between the transmitter and the receiver. It is the transmit power – cable loss + transmit antenna gain – path loss + receive antenna gain – cable loss.

1.6.1 IBSS

IBSS - Independent Basic Service Set

Also known as an ad-hoc network, an IBSS is when end devices communicate via WiFi without an AP. One device set up the IBSS – SSID name and other parameters. Other devices can join this IBSS if they have the authentication parameters and communicate directly. An IBSS does not scale well. There is no central control. Services such an IP addressing must be performed manually etc.

1.4.5 Reflection

Reflection The effect of an RF signal meeting a dense, reflective material, such that it is sent in a different direction

Reflection

If an RF signal traveling as a wave meets a dense reflective material, the signal can be reflected just like a light wave is reflected off of certain surfaces. In a wireless LAN scenario, the wave will reflect off metal surfaces such as filing cabinets etc or off windows. There would also be some level of reflection from walls and in outdoor deployment from the earth, buildings and water.

The reflection is a copy of the signal and if it is received along with the original signal (straight path) then it is likely to be out of phase. This causes the net received signal to be less. This is known as multipath. Prior to 802.11n, multipath was always detrimental. In 802.11n/ac, multipath has been harnessed to improve the performance of the transmission/reception train.

1.4.4 Refraction

Refraction

When an RF signal meets the boundary between media of two different densities, it can also be refracted. Think of reflection as bouncing off a surface and refraction as being bent while passing through a surface.

A refracted signal will have a different angle from the original. The speed of the wave can also be affected as it passes through the different materials. A signal can be refracted when it passes through layers of air having different densities or through building walls with different densities, for example.

1.4.3 EIRP

EIRP - effective isotropic radiated power (EIRP) The resulting signal power level, measured in dBm, of the combination of a transmitter, cable, and an antenna, as measured at the antenna

Once you know the complete combination of transmitter power level, the length of cable, and the antenna gain, you can figure out the actual power level that will be radiated from the antenna. This is known as the effective isotropic radiated power (EIRP), measured in dBm.

EIRP is a very important parameter because it is regulated by governmental agencies in most countries. In those cases, a system cannot radiate signals higher than a maximum allowable EIRP. To find the EIRP of a system, simply add the transmitter power level to the antenna gain and subtract the cable loss.

When you work with wireless LAN devices, the EIRP levels leaving the transmitter’s antenna normally range from 100 mW down to 1 mW. This corresponds to the range +20 dBm down to 0 dBm.

FCC 

Band	Allowed Use	Transmitter Max	EIRP Max
2.4Ghz ISM	Indoor or outdoor	30 dBm (1 W)	36 dBm
U-NII-1	Indoor only	17 dBm (50 mW)	23 dBm
U-NII-2	Indoor or outdoor	24 dBm (250 mW)	30 dBm
U-NII-2	Extended Indoor or outdoor	24 dBm (250 mW)	30 dBm
U-NII-3	Indoor or outdoor	30 dBm (1 W)	36 dBm

ETSI

Band	Allowed Use	EIRP Max
2.4 GHz ISM	Indoor or outdoor	20 dBm
U-NII-1	Indoor only	23 dBm
U-NII-2	Indoor only	23 dBm
U-NII-2	Extended Indoor or outdoor	30 dBm
U-NII-3	Licensed	N/A

1.2 Describe the impact of various wireless technologies

1.2.                      Describe the impact of various wireless technologies

802.11 wireless LANs use unlicenced spectrum and therefore the signals can be affected by other devices and technologies that also use this spectrum. Wireless communication is generally divided into a number of categories, wireless LAN (WLAN) being the one we are mostly interested in but the others are Wireless Personal Area Networks (WPAN), Wireless Metropolitan Area Networks (WMAN) and Wireless Wide Area Networks (WWAN).

The spectrum allocated to 802.11 is in two bands, in the 2.4GHz and 5GHz range.

WPAN - a WPAN uses low-powered transmitters to create a network with a very short range, usually 20 to 30 feet (7 to 10 meters). WPANs are based on the IEEE 802.15 standard and include technologies like Bluetooth and ZigBee, although ZigBee can have a greater range.

WMAN - A wireless service over a large geographic area, such as all or a portion of a city. One common example, WiMAX, is based on the IEEE 802.16 standard. Licensed frequencies are commonly used although the specification does include use of the ISM band. No commercial deployments of WiMAX use the ISM band and for this reason is not a source of interference for wireless LANs.

WWAN - A wireless data service for mobile phones that is offered over a very large geographic area (regional, national, and even global) by telecommunications carriers. Licensed frequencies are used. Examples are 2G, 3G and 4G (LTE) mobile carrier networks. WWANs do not use the same frequencies as 802.11 and therefore are not a source of interference.

                                 1.2.1.      Bluetooth

Bluetooth is a PAN technology, used mainly for telephony headsets and file transfer. Found on most laptops, tablets and mobile phones. Bluetooth has low power consumption, requires line of sight and has good security, making it a good choice for mobile, battery-powered devices.

Bluetooth developed by the Bluetooth Special Interest Group and was incorporated into the IEEE 802.15.1 standard, but that standard is no longer maintained.

Devices operate in the 2.4-GHz ISM band (2.402 to 2.480Ghz), but are not compatible with the 802.11 standard. Bluetooth uses a frequency hopping spread spectrum (FHSS) technique, with devices moving through a predefined sequence of 79 channels with a bandwidth of 1 MHz each.

Class 3 radios – have a range of up to 1 meter or 3 feet, power is 1mW

Class 2 radios – most commonly found in mobile devices – have a range of 10 meters or 33 feet, 2.5mW power

Class 1 radios – used primarily in industrial use cases – have a range of 100 meters or 300 feet, power is 100mW

Up to eight devices can be paired or linked into a PAN, with one device taking a master role and the others operating as slaves.

Bluetooth transmitters could potentially interfere with the majority of the 2.4-GHz band because their channels overlap with the three non-overlapping 802.11 channels but only at a close range because of their low transmit power. If there are many Bluetooth devices in an 802.11 cell, they can create a saturation effect.

                                 1.2.2.      WiMAX

Worldwide Interoperability for Microwave Access (WiMAX) is a wireless technology designed to provide “last mile” broadband access to consumers within a geographic area and defined in the IEEE 802.16 standard. WiMAX does not require line of sight and can provide connection up to 3 to 10-km.

WiMAX operates in several bands between 2 and 11 GHz and from 10 to 66 GHz and can possibly interfere with 802.11 devices, but such interference is highly unlikely. No widely deployed solutions use the ISM bands; the systems that are advertised for ISM are not supported by any major WiMAX players.

                                 1.2.3.      ZigBee

ZigBee is wireless mesh LAN technology that uses relatively low power consumption and low data rates (20 to 250 Kbps). As a result, it offers reliable communication. ZigBee is commonly used for energy management and home and building automation applications.

ZigBee is defined in the IEEE 802.15.4 standard. It allocates the 2.4-GHz ISM band into 16 channels of 5 MHz each. Even though ZigBee uses the same band as 802.11 devices, it has a low duty cycle.

                                 1.2.4.      Cordless phone

Cordless phones use several wireless technologies to connect remote handsets to a central base station, using TDMA and FDMA techniques. Phones operating in the 2.4- and 5.8-GHz bands can cause significant interference with nearby WLANs. Cordless phones can use one channel at a time, but can also change channels dynamically. As well, transmit power levels can rise up to 250 mW, overpowering an AP at maximum power however they typically do not use the ISM band.

1.4.1 Antenna types

There are a number of types of antenna used for wireless LANs. Generally they can be categorised as omnidirectional versus directional, indoor or outdoor. Many modern indoor APs have internal omnidirectional antennas. For point to point deployments, directional antennas are used. The provide high gain and are design for the outdoor environment. Examples of outdoor directional antennas are Yagi, patch, dish and mesh.

Omnidirectional

Omnidirectional antennas are design to transmit and receive signals in all directions equally (although less so along the axis of the antenna). The rubber-ducky antenna is a common example of an omnidirectional antenna.

Antennas are passive gain devices, in other words they concentrate a signal in certain directions but the overall power output is never more than the input power. Antenna gain is compare to a theoretical isotropic antenna which radiate signal in a perfect sphere. A dipole omnidirectional antenna will generally have a gain of about 2.15dBi.Its radiation pattern will be squashed in the vertical plane (usually desibed as doughnut shaped).

Directional Antennas

Directional antennas focus the energy and therefore have a higher gain than omnidirectional antennas. They are used for indoor areas such as hallways and in warehouses to direct the signal between racking.

Antennas are designated by their beamwidth. This describes how focused the energy is in the vertical plane and is calculated as the range between the half power points i.e. the points where the power output is half that of the maximum power.

Antennas - more than you need to know