Ch.6: Wireless LANs 1

Which band dominated use on 802.11 initially?

2.4 Ghz - 2.485 Ghz

In what two service bands does 802.11 operate?

2.4 Ghz and 5 Ghz

How many non-overlapping 20Mhz channels does the 2.4 Ghz band support? Why is this a problem?

3 channels. They are 1,6 & 8. This limited channels with smaller bandwidths causes co-channel interference of nearby access points if two signals are sharing the same channel.

At what range of frequencies do most wireless systems operate?

500 MHz to 10 GHz

What is the dominant WLAN standard?

802.11

What is the difference between 802.11 and Wi-Fi

802.11 is the basic IEEE standard Wi-fi is set by the Wi-Fi alliance which takes a subset of 802.11 standards with select options.

What is the difference between 802.3 and 802.11 standards?

802.3 - ethernet 802.11 - wireless

Under what circumstances would you use an omnidirectional antenna? A dish antenna?

A wireless network When you needed to focus a signal is a specific direction

What two propagation problems become worse as frequency increases and why?

Absorptive Attenuation - higher frequencies are absorbed faster by air moisture. Dead Zone - higher the frequency, the less signals can bend around objects.

Dead zone

An area in which a receiver cannot get a signal because it is blocked by a dense object in its direct path.

What is the range of frequencies a signal will operate at?

Bandwidth

What is BSS.

Basic service set. It consists of a wireless access point and the host it serves.

Why is Multipath interference sensitive to location?

Because Multipath interference may cause signal to range from strong to nonexistent within a few centimeters. Lap time between the direct and reflected signal may mean that the reflected signal could interfere with the next direct signal

How many non-overlapping channels does the 5Ghz band support?

Between 11 and 24 non-overlapping 20Mhz

In radio, how do you send multiple signals without signal interference?

By subdividing service bands into channels to prevent interference.

What two propagation problems become worse as frequency increases?

Dead zone and absorptive attenuation.

How are dead zones created?

Dense object blocks the path of a signal.

Electromagnetic interference

Devices that send out electromagnetic waves can interfere with wireless frequency propagation.

What kind of antenna is best for long-range transreception?

Dish Antenna

Distinguish among frequency spectrum, service bands and channels.

Freq spectrum - range of all possible frequencies from zero hertz to infinity. Service bands- contiguous spectrum ranges of frequencies. Channels - service bands are subdivided into even smaller frequencies ranges. Signals sent in different channels do not interfere with each other.

Is wireless radio transmission usually expressed in terms of wavelength or frequency?

Frequency. Optical fiber is expressed in terms of waves

What are the two forms of attenuation?

Inverse Square Law Att. Absorptive Att.

What are the six wireless propagation problems?

Inverse square law attenuation Absorptive attenuation Dead zoned Multipath interference Electromagnetic interference

What is the benefit of spread spectrum transmission?

It reduces wave propagation problems.

At what layers do wireless LANs operate?

L1 L2

What is a wireless LAN?

Local area network that uses radio waves for physical layer data transmission.

What is the most series propagation problem at WLAN frequencies?

Multi-path Attenuation.

What is the most serious propagation problems in WLANs?

Multipath interference.

In 802.11 Wi-fi networks, can simple installation rules usually reduce propagation effects to nonissues?

No. Wireless installations are far more complex and b/c wireless networks use radio waves to transmit data they are more open to the influence of propagation effects.

Do wireless LAN comes from OSI or TCP/IP?

OSI standards

Dead zones

Occurs when a dense object blocks a signals path between the sender and receiver.

Multipath Interference

Occurs when a direct and reflected signal interact to create an amplitude of zero causing the signal to range from strong to nonexistent within a few centimeters.

What kind of antenna is best for short-range transreception?

Omni-Directional

What type of antenna is normally used in WLANs?

Omni-directional antennas.

What type of antenna normally is used in WLANs?

Omnidirectional

Distinguish between omnidirectional and dish antennas in terms of operation.

Omnidirectional - transmits and receives signals to and from all directions. Dish - transmits and receives signals to and from a particular direction.

What is hertz?

One frequency cycle per second.

What spread spectrum transmission standards dominates 802.11 today?

Orthogonal Frequency division multiplexing. Uses subcarriers to divide broadband channels into smaller subchannels. Transmits frame redundantly to compensate for any impairement.

Broadband channels

Radio channels with very large bandwidth.

Absorptive Attenuation

Radio signals are partially absorbed by the air molecules, plants and other objects it passes through as it travels.

Multipath interference

Radio waves bounce off walls, floors, ceilings, and others objects which means that a receiver may receive two or more signals. Most serious propagation problem at WLAN frequencies.

What is a transceiver?

Radios for data transmission; it both transmits and receives

Frequency spectrum

Ranges from 0hz to infinity

service bands

Regulators that divide the frequency spectrum into contiguous spectrum ranges. Usually dedicated to specific purpose.

What is an SSID?

Service set identifier. Identifies the access point and wireless hosts must know the SSID to associate with the access point.

Inverse Square Law Attenutation

Signal energy/strength is reduced as signal spreads equally over the area of a sphere.

EMI

Signal interference from electromagnetism produced from other sources.

Absorptive Attenuation

Signal is partially absorbed by molecules, plants, and other physical objects it passes through.

Inverse Square Law Attenuation

Signal spreads out over the area of a sphere and the signal weakens by 1/r^2

In wi-fi service bands what type of transmission method is required by regulators?

Spread spectrum transmission which is transmission that uses far wider channels than transmission speeds require.

Does a signal usually travel at a single frequency or does it spread over a range?

Spreads over a range.

channels

Subdivision of service bands into smaller frequency ranges.

What can produce EMI?

Telephones, microwaves, and nearby access points.

Why is a large channel bandwidth so desirable?

The larger a bandwidth, the faster you can theoretically send data over a signal frequency. Frequency signals transmit up to Nx as much channel bandwidth.

If you triple bandwidth what happens to channels in the service band?

The wider the channel bandwidth, the fewer channels on the service band.

What is a wireless access point?

This device connects the wireless devices to the firm's main Ethernet LAN and directs radio transmissions btwnitself and wireless clients.

What does layer 2 do in wireless LANs?

This layer organizes packet/frame

What does layer 1 do in wireless LANs?

Transmission of radio waves.

Dish Antenna

Transmits/receives signals in a narrow range and signals can travel longer distances.

Omni-directional Antenna

Transmits/receives signals in all directions and has rapid signal attenuation.

Does wi-fi use licensed or unlicensed radio bands? What are the benefits and what are the downsides?

Unlicensed bands. Benefit - companies can implement however many 802.11 channels they want. Downside - companies have to put up with interference from others.

What is a hybrid/switched single network?

Wireless clients communicate wirelessly to an 802.11 WAP which connects them to the wired Ethernet switched LAN.