Certified Wireless Technology Administrator - Chapter 5 - Physical Layer Access Methods and Spread-Spectrum Technology

single input/single output or SISO

The most basic wireless antenna technology used in a wireless LAN system. One antenna is used at the transmitter to transmit data, and one antenna is used at the receiver to receive the data.

Channel 3 and channel 9 Channel 6 and channel 11 Channel 2 and channel 8

Which two channels could be used so that the access points do not interfere with each other in an 802.11b wireless network? Choose three. A. Channel 1 and channel 5 B. Channel 3 and channel 9 C. Channel 6 and channel 11 D. Channel 2 and channel 8 E. Channel 4 and channel 7

HT-OFDM

Which wireless LAN technology can be used to obtain the highest data transfer rate possible? A. DSSS B. Ethernet C. HT-OFDM D. OFDM

MIMO

Which wireless LAN technology in wireless networking uses the effects of multipath to provide data rates of 600 Mbps? A. OFDM B. HR/DSSS C. HT/DSSS D. MIMO

Hedy Lamarr

Who was the famous actress who patented frequency-hopping spread spectrum or FHSS?

14

Without any regulatory domain taken into consideration, the 2.4 GHz frequency range allows for how many channels using 802.11b? A. 3 B. 6 C. 11 D. 14

co-located or co-location

Multiple wireless LANs that can function in the same RF space.

54

OFDM that is used with 802.11a and 802.11g stations (STA) supports a maximum data rate of _______ Mbps. A. 11 B. 22 C. 33 D. 54

IEEE 802.11n High Throughput or HT

Ratified in September 2009, it allows extensibility of up to four spatial streams, using a channel width of 20 MHz. Also, transmission using one to four spatial streams is defined for operation in 20/40 MHz channel width mode. This technology is capable of supporting data rates up to 600 Mbps using four spatial streams with a 20/40 MHz channel.

dwell time

Refers to the duration of time used with frequency-hopping spread spectrum or FHSS where it remains on a frequency for a specified amount of time.

dynamic frequency selection or DFS

Required for the middle 5 GHz UNII band, this will allow an access point to change the radio frequency channel it is operating on in order to avoid interfering with certain type of radar systems.

Extended Rate Physical or ERP

The IEEE 802.11g amendment was released in 2003, allows for higher data rates up to 54 Mbps using orthogonal frequency division multiplexing or OFDM and still maintain backward compatibility with existing 802.11b technology and devices. Allows admin to choose "b only", "g only", or "mixed".

interframe spacing or IFS

The amount of time in microseconds required for a wireless interface to process a received frame and to respond with a response frame.

Carrier Sense Multiple Access/Collision Detection or CSMA/CD

A contention-based media access control method that Ethernet devices use to share the medium. This method allows only one device to transmit at any one time.

Point Coordination Function or PCF

A contention-free mode that works by polling stations and giving them an opportunity to send information without contending with other devices. This mode was optional and never implemented by any manufacturers.

Carrier Sense Multiple Access/Collision Avoidance or CSMA/CA

A media access control method used by wireless LAN devices that instead of detecting transmission collisions, uses mechanisms that attempt to avoid collisions through use of "countdown timers"

channel

A specific range of RF frequency as defined by its center frequency.

direct-sequence spread spectrum or DSSS

A spread-spectrum technology that uses special techniques to transmit digital data across the air using radio frequency (RF). This is accomplished by modulating or modifying the radio frequency characteristics such as phase, amplitude, and frequency. It uses technology known as a spreading code to provide redundancy of the digital data as it traverses through the air.

orthogonal frequency division multiplexing or OFDM

A technology designed to transmit many signals simultaneously over one transmission path in a shared medium and is used in wireless and other transmission systems. Every signal travels within its own unique frequency subcarrier, a separate signal carried on a main RF transmission.

exclusive OR or XOR

A way of combining binary data bits in digital electronics.

22 MHz

An HR/DSSS channel is how wide? A. 1 MHz B. 20 MHz C. 22 MHz D. 40 MHz

Hybrid Coordination Function or HCF

An access method used with wireless networking that was introduced with 802.11e for quality of service or QoS technology.

adaptive frequency hopping or AFH

An improvement on frequency hopping spread spectrum or FHSS, devices will try to avoid using the same frequencies, decreasing the chance of interference.

co-channel interference

Another term used for this type of interference is "co-channel cooperation." This is because the wireless devices are contending to use the medium rather than just being seen as radio frequency noise to each other. This type of interference will have an impact on the amount of actual throughput between devices over a wireless network.

1 MHz-wide subcarriers

FHSS uses which communication method to exchange data? A. 1 MHz-wide subcarriers B. 20 MHz-wide subcarriers C. 22 MHz-wide subcarriers D. 40 MHz-wide subcarriers

High power, narrow bandwidth

FM radio stations use what type of RF communication? A. High power, narrow bandwidth B. High power, wide bandwidth C. Low power, narrow bandwidth D. Low power, wide bandwidth

narrowband

Frequency ranges that are tuned to a specific radio frequency in the FM band. A signal at a high power of tens of thousands of watts in a very narrow frequency. Depending on the conditions, a receiver can hear the station for tens or even possibly hundreds of miles.

2.400 GHz ISM

HR/DSSS devices operate in which frequency range? A. 2.400 GHz ISM B. 5.250 GHz UNII C. 5.350 GHz UNII D. 5.750 GHz UNII E. 5.725 GHz ISM

Three

How many access points can be co-located without channel reuse in the same radio frequency area to maximize total system throughput while minimizing RF interference in an IEEE 802.11g network? A. Two B. Three C. Four D. Six

14 although depending on the country and location, all 14 channels may not be available.

How many channels are available in the 2.4 GHz industrial, scientific, and medical or ISM license free band?

23

How many channels are in the 5 GHz UNII band?

Frequency hopping spread spectrum or FHSS

Bluetooth devices use ___________, which can potentially cause interference with WLANs. A. Bluetooth spread spectrum (BTSS) B. Orthogonal frequency division multiplexing (OFDM) C. Direct-sequence spread spectrum (DSSS) D. Frequency hopping spread spectrum (FHSS)

52 subcarriers equally spaced apart, and 4 of the 52 subcarriers do not carry data and are used as pilot channels.

How many subcarriers does 802.11a and 802.11g OFDM distribute computer data over?

56 subcarriers of which 52 are usable for data with a 20 MHz wide channel and 114 subcarriers, of which 108 are usable for data with a 40 MHz wide channel.

How many subcarriers does 802.11n OFDM distribute computer data over?

5 MHz on center

How much is each channel in the 2.4 GHz ISM band is separated by?

Typically, the decrease in throughput is about 25 percent to 30 percent.

How much of a decrease in throughput will occur if a DSSS or HR/DSSS device is introduced in the radio range of the ERP-OFDM device?

All UNII band channels are 20 MHz wide and separated by 20 MHz from the center frequencies of each channel.

How wide are 5 GHz UNII band channels and what is the separation between them?

22 MHz

How wide is a direct-sequence spread spectrum or DSSS channel?

Half duplex

IEEE 802.11a/b/g/n devices use what type of communication? A. Half diplex B. Full diplex C. Half duplex D. Full duplex

ripple effect

If an 802.11g access point can "hear" another 802.11g access point on the same radio frequency channel that is in ERP protection mode, that access point will also enter a protection mode state. This is known as what some call the ___________ ___________.

Barker

DSSS uses which spreading code at 1 Mbps? A. Barker B. CCK C. DBPSK D. DQPSK

High Rate/Direct-Sequence Spread Spectrum or HR/DSSS

Defined in the IEEE 802.11b amendment, it introduced higher data rates of 5.5 and 11 Mbps. It uses one of fourteen 22 MHz wide channels to transmit and receive digital computer data. It uses a different spreading code or an encoding technique called complementary code keying or CCK.

Channel planning

Designing wireless networks so that overlapping RF cells are on different or nonoverlapping channels—for example, channels 1, 6, and 11 in the 2.4 GHz ISM band. This will help optimize performance and minimize degradation of throughput because of adjacent and co-channel interference.

802.11n

Devices compliant with which amendment to the IEEE standard use multiple radio chains and multiple antennas? A. 802.11a B. 802.11b C. 802.11g D. 802.11n

subcarriers

Smaller segments of the radio frequency channel that is in use.

diversity

Some SISO systems support this technology which uses two antennas with a single radio. It will help to lessen the effects of multipath, which is caused by reflections.

802.11b and 802.11g

Stations (STA) operating in which two IEEE 802.11 amendments are interoperable? A. 802.11 and 802.11a B. 802.11a and 802.11b C. 802.11a and 802.11g D. 802.11b and 802.11g

Transmit beamforming or sometimes abbreviated TxBF

Technology used with MIMO that improves speed - detects signals & sends feedback to transmitter Transmitter adjusts phase & amplitude at antennas

Maximal ratio combining or MRC

Technology used with MIMO where the weighted sum of signals received on all antennas is combined.

Spatial multiplexing or sometimes abbreviated SM

Technology used with MIMO where unique streams sent over the same channel.

network access methods

This allows devices connected to a common infrastructure to communicate and transmit data across a network medium from one device to another.

modulation and coding scheme or MCS

This is how the IEEE 802.11n amendment to the standard represents the data rates that are available.

Barker code

This is used as the spreading code for DSSS at the data rates of 1 and 2 Mbps. IEEE 802.11 wireless LANs can use this 11 "chip" spreading code for communications. Each digital data bit is combined with the set code through what is called an exclusive OR or XOR process. XOR is a way of combining binary data bits in digital electronics. The result then spreads the binary 0 or 1 over a 22 MHz wide channel, helping to make it resilient to radio frequency interference. Since both the transmitter and receiver understand the same code, they would be able to determine the information that was sent across the air.

distributed coordinated function or DCF

This method of access employs a contention period for devices competing to send data on the network. This collision avoidance mechanism is part of a detailed process requiring certain criteria to be met in order for a frame to be transmitted across the medium. To avoid collisions, the devices are required to: • Detect the RF energy of other devices transmitting, a technique known as Clear Channel Assessment or CCA. • Announce how much time is required for the frame exchange to occur allowing other stations read the duration field and set their Network Allocation Vector or NAV. • Wait for a predetermined period of time between frames, a technique known as interframe spacing. • Back off and retry if the medium is busy, a technique known as the random backoff timer via the contention window.

spreading code

This provides redundancy of the digital data as it traverses through the air. It transmits information on multiple subcarriers, and the redundancy helps the receiver detect transmission errors due to interference.

frequency-hopping spread spectrum or FHSS

This technology operates by sending small amounts of information such as digital data across the entire 2.4 GHz ISM band. It changes the frequency constantly in a specific sequence or hopping pattern and remains on a frequency for a specified amount of time known as the dwell time. A transmitter and receiver will be synchronized with the same hopping sequence, therefore allowing the devices to communicate.

False: The channel spacing is such that there is no overlap.

True or False: In the 5 GHz UNII band, there are several overlapping channels just like 2.4 GHz ISM.

False: A well-designed wireless network will account for a three-dimensional propagation. In other words, in a three-story building, the RF from an access point on the second floor building may pass through to the first and third floors; therefore, interference could be an issue if the network is not planned properly.

True or False: When designing a wireless network, you only have to worry about RF energy propagation on the floor you're working on.

adjacent-channel interference

Two or more different RF signals interacting with each other and causing a degradation of performance.

half duplex

Two-way communication that occurs in only one direction at a time.

5.25 GHz UNII range and 5.35 GHz UNII range

Which frequency ranges are used in an IEEE 802.11a-compliant wireless LAN? Choose two. A. 900 MHz ISM range B. 2.40 GHz ISM range C. 5.25 GHz UNII range D. 5.35 GHz UNII range

CSMA/CA

Which network access method attempts to avoid collisions? A. CSMA/CA B. CSMA/CD C. CSMA/CR D. CSMA/DSSS

Channels 1, 6, and 11.

Which of the 14 channels in the 2.4 GHz ISM band are 3 adjacent nonoverlapping channels?

Channels 1 through 11 are available for use in the Amercias.

Which of the available 14 channels in the 2.4 GHz ISM band can be used in the United States?

spread-spectrum

Uses low power over a wide range of frequency. Two examples include frequency-hopping spread spectrum or FHSS and direct-sequence spread spectrum or DSSS.

Multiple-input multiple-output technology or MIMO

Uses multiple radios or "radio chains" to transmit and receive radio signals. It actually uses the reflections to help enhance the performance and throughput using several radio chains in 802.11n.

spatial multiplexing (SM)

Uses several radio chains to transmit different pieces of the same information simultaneously, greatly increasing throughput.

FHSS

What IEEE 802.11 PHY technology specifies that frequencies change regularly while transmitting and receiving data? A. DSSS B. OFDM C. FHSS D. ERP-OFDM

• HT-greenfield format • Short guard interval (GI), 400 ns • Transmit beamforming (TxBF) • Space-time block coding (STBC)

What are some additional characteristics of IEEE 802.11n High Throughput or HT?

Throughput, reliability, and predictability: • Five times more throughput - Enhanced file transfer and download speeds for large files • Twice as reliable - Lower latency for mobile communications • Twice as predictable • More consistent coverage and throughput for mobile applications

What are some advantages of Multiple-input multiple-output technology or MIMO?

2.4 GHz industrial, scientific, and medical or ISM license free band.

What band does direct-sequence spread spectrum or DSSS and High Rate/Direct-Sequence Spread Spectrum or HR/DSSS operate in?

Data rates are what a station is capable of exchanging information, whereas throughput is the rate at which the information is actually moving.

What is the difference between data rate and throughput?

11 Mbps

What is the maximum data rate of HR/DSSS 802.11b devices? A. 5.5 Mbps B. 11 Mbps C. 24 Mbps D. 54 Mbps

1600 time per second across the entire 2.4 GHz band

What is the rate that Bluetooth was designed to hop frequencies per second?

Spread-spectrum

What technology is used to send WLAN data over a wireless medium using many subcarrier frequencies? A. Wireless broadband B. Narrowband C. Spread-spectrum D. Spectral masking E. Wideband

Although 40 MHz wide channels are allowed in the 2.4 GHz ISM band, best practices recommend against it. Using a 40 MHz channel in this band would only equate to a single channel without any channel overlap.

What would happen if you used 802.11n in the 2.4 GHz ISM band?

Wider channels mean more data can be transmitted over the RF medium simultaneously.

What's the advantage of the 40 MHz channel width over the 20 MHz channel width?

Japan

Which country uses channel 14 in the 2.4 GHz ISM band?