Chapter 6: Wireless LANs

Explain the relationship between the radio frequency spectrum, service bands, and channels.

The radio frequency spectrum is the range of all possible frequencies from zero hertz to infinity. Regulators divide the frequency spectrum into contiguous spectrum ranges called service bands hat are dedicated to specific services. Service bands are subdivided further into smaller frequency ranges called channels. A different signal can be sent in each channel because signals in different channels do not interfere with one another.

Where did the term "Wi-Fi" come from?

The term Wi-Fi stems from the Wi-Fi Alliance, which is an industry consortium of 902.11 product vendors.

Which one has more capacity?

5GHz

What do we mean when we say 802.11n is "backwards compatible" with 802.11g?

The backward compatibility permits products meeting new standards to be installed gradually over time. An 802.11n access point must be able to work with an even older 802.11g client.

What is the relationship between the IEEE 802.11 committee and the Wi-Fi Alliance?

The dominant WLAN standards today are the 802.11 standards, which are created by the IEEE 802.11 Working Group. It is common to call 802.11 standards "Wi=Fi" standards. The term Wi-Fi stems from the Wi-Fi Alliance, which is an industry consortium of 802.11 product vendors. When the 802.11 Working Group creates standards, it often creates any options. The Wi-Fi Alliance creates subsets of 802.11 standards with selected options.

What do we mean when we say that Wi-Fi radios operate at a higher frequency than an FM radio station?

The lowest frequency of an FM channel is 89.0 MHz and the highest is 89.2 MHz.

Which one is more prone to interference?

2.4 GHz

What are the two primary frequency bands used by Wi-Fi?

2.4 GHz and 5 GHz

Summarize the key differences between 802.11b, 802.11g, 802.11n, and 802.11ac with respect to frequency band used; channel bandwidth; and speed/throughput.

802.11g: 2.4 GHz band, bandwidth 20MHz, only 3 channels 802.11a: 5 GHz band, Uses 20 MHz channels 802.11n: 2.4 and 5 GHz, bandwidth 40 MHz 802.11ac: 5 GHz, 80 MHz (mandatory), 160MHz (optional),

How does MIMO differ from MU-MIMO?

Beamforming also allows multiuser MIMO (MU-MO), in which the access point focuses on two wireless devices at the same time. IT can communicate with two or more devices simultaneously.

Compare and contrast the CSMA/CA+ACK media access control (MAC) mechanism used on Wi-Fi to the CSMA/CD media access protocol used on legacy Ethernet.

CSMA/CA+ACK- Carrier Sense Multiple Access with Collision Avoidance and Acknowledgement

How much bandwidth does a Wi-Fi channel use in comparison to an FM radio channel?

Channel bandwidth of FM radio is .2 MHz. 2.4 GHz has 40 MHz of bandwidth.

Provide an example of an environment where wireless mesh networking might be appropriate.

Corporations

Which one would you prefer if you were linking two building networks together using a wireless bridge?

Dish

How does spread spectrum radio transmission compare to traditional narrowband radio transmission?

In normal radio transmission channel bandwidths are limited to the speed requirements of the user's signal. In contrast, spread spectrum uses far wider channels than transmission speeds require. Spread spectrum transmission takes the original signal, called a baseband signal, and spreads the signal energy over a much broader channel than is required by the transmission speed.

Describe the following wireless propagation phenomena: free space attenuation (inverse square law); absorptive attenuation; dead zones; multipath interference; electromagnetic interference.

Inverse square law: the amount the signal strength weakens by 1/r^2. This is due to the signal spreading out as a sphere and so becoming weaker at each point on the sphere. Absorptive attenuation: the air molecules, plants, and other things the radio signal is absorbed by as it travels. This is signal loss through energy absorption. Dead zones: Sports where the receiver cannot get to the signal, due to a dense object (thick wall) blocking the direction path between the sender and receiver. Multipath interference: multiple signals traveling different paths will interfere. This may cause the signal to range from strong to nonexistent within a few centimeters. This is the most serious propagation problems at WAN frequencies. Electromagnetic interference: many devices produce EMI at frequencies used in wireless data communications. Placing access points so that they give good coverage without creating excessive mutual interference is difficult.

What is the difference between licensed and unlicensed radio bands?

Licensed radio bands: transceivers must have a government license to operate. Unlicensed radio bands: a company can add of drop access points any time it chooses, and can have as many hosts as it wishes. All 802.11 Wi-Fi Bands operate in unlicensed bands.

What is MIMO and why is it considered one of the most important innovations in radio of the past 100 years?

Multiple input/ Multiple output. The host or access points sends two or more spatial streams in the same channel between who ore more different antennas or access points and wireless hosts.

What is OFDM and why is it important to the study of WLANs?

OFDM- Orthogonal Frequency Division Multiplexing Each broadband channel is divided into many smaller sub channels. OFDM transmits part of a frame in each subcarrier and sends data redundancy across the subcarriers, so if here is an impairment in one all of the frame will usually still get through.

Which one would you prefer for a home network?

Omnidirectional

What is the difference between an omnidirectional antenna and a dish/directional antenna?

Omnidirectional antennas: transmit signals equally strongly in all directions and receive incoming signals equally well from all directions. The antenna does not need to point in the direction of the receiver, however only a small fraction of the energy transmitted reaches the receiver. Best for short distances (LAN or cellular telephone network). Dish antennas: point in a particular direction, which allows them to send stronger signals in that direction for the ska power and to receiver weaker incoming signals from that direction. Good for longer distances, but hard to use.

What is wireless radio signal propagation?

Propagation effects in wireless networks are complex and difficult to implement. Radio propagation is very unreliable. Radio signals bounce off obstinacies, fail to pass through walls and filing cabinets.

What is a Wi-Fi access point (AP)?

When a wireless host wishes to send a frame to a server, it transmits the frame to a wireless access point.