ENHANCING WLAN, Wireless Infrastructures -- Lecture 8 (Enhancing WLANs)

True

802.11a and 802.11g use the same MAC layer functions While they defer in physical layer. True/False?

1. Use of OFDM 2. A more efficient error-correction scheme

802.11a has more speed and flexibility over 802.11b due to:

802.11b: Had 3 channels which would send bits one by one. Uses 2.4 GHz ISM band. 802.11g: Has many more channels that send bits in parallel fashion at slower rate. Also uses 2.4 GHz ISM band.

802.11b vs 802.11g?

To slightly increase performance by transmitting the entire PLCP frame using OFDM while still preventing collisions.

802.11g amendment includes a change to the MAC layer for compatible devices:

802.11b

802.11g operates in the same frequency band as ______?

True

802.11n is backward compatible. True or false?

True

802.11n uses more bandwidth than other standards. True/False?

Antenna diversity

802.11n uses multiple radios and antennas for what?

True

A nomadic user moves frequently but does not use they quipment while in motion. True/False?

Frequency-hopping spread spectrum (FHSS)

Allows the participants in a communication to hop between predetermined frequencies. Security is enhanced because the participants can predict the next frequency to be used but a third party cannot easily predict the next frequency. This technique can also provision extra bandwidth by simultaneously using more than one frequency.

802.11ac multiple input

An IEEE wireless networking standard operating in the 5GHz range, with increased throughput compared to previous WiFi IEEE standards.

True

An all 802.11g network does not require the quiet time. True/False?

802.11n

Beamforming and Spatial Multiplexing are used by what standard?

1. By devices monitoring the pilot carriers. 2. By an exchange of information using sounding frames.

Beamforming capabilities can be learned by 2 ways. What are they?

True

Configurations such as 3x3 and 2x3 (transmitters and receivers) in a radio chain are also seen. True/False?

1. Transmits the preamble part using DSSS. 2. Transmits the data portion of the frame using OFDM. 3. Older devices can "understand" the beginning of the transmission as well as most control frames.

DSSS-OFDM transmission mode?

True

Demand for higher internet speed and significantly higher throughput of wired networks are two main drivers for IEEE 802.11g standard. True/False?

1 - When communicating only with 802.11a and 802.11g devices meaning when the communication is only backward compatible 2 - When used in mixed HT and legacy devices 3 - When communicating only with 802.11n devices (GREENFIELD MODE)

HT PHY layer supports how many frame formats?

SNR

Hardware designers can not increase the complexity of the modulation on the subcarriers beyond the maximum of 54 Mbps due to?

By sending the bits slowly enough that any delayed copies are late by a much smaller amount of time

How does OFDM avoid problems caused by multipath distortion?

By sending over parallel sub carriers

How is Radio interference from outside sources minimized?

1. Enhanced DCF (EDCF): Higher priority traffic waits less to transmit 2. Hybrid Coordination Function (HCF): Combination of DCF and point Coordination Function (PCF)

How many coordination functions are implemented by 802.11e?

4 and 4 max

How many maximum transmitters and receivers are used in radio chain of 802.11n?

3

How many non-overlapping channels are available in an 802.11g WLAN?

48

How many of the 52 subcarriers available in an 802.11a based network are utilized for data?

By combining Spatial Multiplexing and Beamforming

How our range and reliability of transmission increased?

True

IEEE 802.11g and 802.11a handle errors differently than 802.11b. True or false?

Multiple input multiple output (MIMO)

It uses multiple antennas for transmission and reception. These antennas do not interfere with one another, thanks to its use of spatial multiplexing, which encodes data based on the antenna from which the data will be transmitted. Both reliability and throughput can be increased with its simultaneous use of multiple antennas.

Direct-sequence spread spectrum (DSSS)

Modulates data over an entire range of frequencies using a series of symbols called chips. A chip is shorter in duration than a bit, meaning that chips are transmitted at a higher rate than the actual data. These chips not only represent encoded data to be transmitted, but also what appears to be random data. Because both parties involved in a this type of communication know which chips represent actual data and which chips do not, if a third-party intercepted this transmission, it would be difficult for that party to eavesdrop on the data because they would not easily know which chips represented valid bits. Note however that this moduclation technique is more subject to environmental factors, as opposed to FHSS and OFDM, because it uses an entire frequency spectrum.

Receiving device gets the signal from several different directions at different times

Multipath distortion

1. No more serializing data and modulating bits onto a single carrier using DSSS or CCK <--802.11b 2. Divides each of the 3 channels of 20 MHz bandwidth into 52 sub carriers and moderates the bits of data on to 48 of them. 3. Essentially splits a high speed digital signal in to several lower signals running in parallel 4. Send the transmission in parallel across several lower speed, narrower frequency channels. Slower data rate than 802.11 or 802.11b 5. OFDM uses 48 of the 52 sub carriers for data. Extra 4 are used to monitor the strength and quality of RF signal

OFDM facts (5)

802.11g

OFDM was introduced by which standard?

True

Overall performance in 802.11g is lower than that of 802.11a due to the added quiet time when 802.11b devices are present. True/False?

True

PLCP frame formats used in 802.11g are the same as for 802.11b? True or false?

True

Prior to 802.11n only TWO antennas were used for antenna diversity. True or false?

802.11b

Ratified in 1999, this standard supports speeds as high as 11 Mbps. However, 5.5 Mbps is another supported data rate. This standard uses the 2.4-GHz band and the DSSS transmission method.

802.11a

Ratified in 1999, this standard supports speeds as high as 54 Mbps. Other supported data rates (which can be used if conditions are not suitable for the 54-Mbps rate) include 6, 9, 12, 18, 24, 36, and 48 Mbps. This standard uses the 5-GHz band and the OFDM transmission method.

802.11g

Ratified in 2003, this standard supports speeds as high as 54 Mbps. It also supports data rates of 6, 9, 12, 18, 24, 36, and 48 Mbps. Further, it also operates in the 2.4-GHz band, which allows it to offer backward compatibility to legacy devices. It can use either the OFDM or DSSS transmission method.

802.11n

Ratified in 2009, this standard supports a variety of speeds, depending on its implementation. Although the speed of this network could approach 300 Mbps (through the use of channel bonding), many of the devices on the market have speed ratings in the 130 to 150-Mbps range. Interestingly, this standard of WLAN can operate in the 2.4-GHz band, the 5-GHz band, or both simultaneously. It uses the OFDM transmission method.

True

Reduced interframe space (RIFS) is only allowed in Greenfield mode. True or False?

The addition of quiet time

Short interframe space (SIFS) timing is affected by what?

True

Single channel transmission is faster while OFDM transmission is slower. True/False?

5 GHz

The Unlicensed National Information Infrastructure (UNII) band operates at the _____ frequency.

Guard interval

The delay at the end of every frame that allows reflected signals to arrive at the receiver is a __________ .

2 Mbps

The original IEEE 802.11 standard, established in 1997, was for WLANs operating at what maximum speed?

7

Total bandwidth available for IEEE 802.11a is almost how many times that of 802.11b?

False

True or False: 802.11 greenfield mode provides the capability of using reduced interframe space (RIFS), which can reduce the interframe space interval down to 9 microseconds.

True

True or False: An 802.11g based wireless network will still utilize DSSS for the preamble for backwards compatibility with 802.11b devices.

Short range and high speed wireless digital communications (802.11a)

U-NII is intended for?

802.11ac

What 802.11 standard under development uses up to 160 MHz bandwidth and can achieve data rates up to 7 Gbps?

1. @ 6Mbps -> PSK (1 bit/symbol) 2. @ 12 Mbps -> QPSK (2 bits/symbol) 3. @24 Mbps -> 16-QAM (4 bits/symbol) 4. @54 Mbps -> 64-QAM *QAM = Quadrature Amplitude Modulation

What are 802.11a PHY layer moderation techniques?

- Not compatible with non-HT devices - Most efficient way to communicate with other HT devices - Also not possible when there are legacy devices either present or within range of the WLAN

What are HT PHY layer frame 3 format characteristics?

1. Mode 0: Greenfield mode with ONLY HT devices 2. Mode 1: An HT mode but prevents interference from non-HT devices 3. Mode 2: Supports either 20 or 40 MHz channels 4. Mode 3: non-HT mixed mode. Supports either 20 or 40 MHz channels.

What are HT operation modes?

1. Same mode used by 802.11b that supports rates of 1, 2, 5.5, and 11 Mbps. 2. Mode that uses OFDM and only supports 802.11g.

What are TWO mandatory transmission modes?

1. PBCC (Packet Binary Convolutional Coding) that transmit at 22 or 33 Mbps using 8PSK. 2. DSSS-OFDM: DSSS transmits preamble of 802.11b and OFDM transmits the data portion.

What are TWO optional transmission modes?

1. Spatial Multiplexing Power Save (SMPS) mode: A MIMO device can shut down all but one of its radios. 2. Power Save Multi-Poll (PSMP) mode: Devices can turn off all but one radio.

What are TWO power management methods in HT MAC layer?

1. Approximately 2 GHz of spectrum available 2. Can support very wide channels

What are the advantages of 60 GHz band?

40 MHz

What channel does 802.11n use to get maximum throughput (300 to 600 Mbps)?

Beamforming

What do 802.11n devices employ to direct a transmission back to the device from which a frame was received?

Frame aggregation

What do we call the feature that is allowed in the 802.11n and 802.11ac amendments, where MAC (layer 2) frames can be combined in a data frame to further increase throughput on the WLAN?

DSSS and OFDM; Supports Channel bonding where two channels are combined to form a 40 MHz channel.

What does 802.11n support?

Feedback information

What does both Beamforming and Spatial Multiplexing rely on?

CTS-to-Self ("clear-to-send-to-self")

What is a coordination method used by 802.11g devices that prevents 802.11 and 802.11b devices that do not "understand" OFDM from attempting to initiate a transmission while the 802.11g device is transmitting data?

Antenna Diversity

What is a technique that uses two antennas to improve the range of 802.11 and transmits a signal through the antenna that received the strongest signal during the last transmission?

Enhanced DCF (EDCF)

What is an enhancement to the MAC protocol layer defined in 802.11e that enables prioritization of traffic, so that a station with higher priority frames, such as voice traffic, waits less time to transmit?

Directing transmissions to the device from which a frame was received.

What is beamforming?

Combining multiple MAC frames into one PHY frame to reduce overhead.

What is frame aggregation?

When a delay is required at the end of each frame to allow reflected signals to arrive in non HT transmissions.

What is guard interval?

A shorter 2 microsecond interframe space can be used instead of a 10 microsecond SIFS at the end of each transmitted frame

What is reduced Interframe space (RIFS)?

Frames are broken up and sent in multiple parts from different radios.

What is spatial multiplexing?

Shorter range

What is the disadvantage of an 802.11a WLAN compared to an 802.11b WLAN?

600 Mbps

What is the maximum speed of 802.11n?

To help prevent adjacent channel interference

What is the purpose of a guard band?

Avg: 6 feet Max: 33 feet with help of obstruction absorbing items.

What is the range of 802.11ad standard?

Both 2.4 GHz and 5 GHz bands. It is backward compatible with 802.11a.

What kind of band does 802.11n work in?

802.11n

What standard known as high throughput (HT)?

Man in the middle

What type of attack against a wireless network involves the use of an access point that attempts to spoof the parameters of another access point to gather information from connecting devices?

To expand the 802.11 standard to work in the 60 GHz portion of the ISM band.

What was the goal of 802.11ad amendment?

11 Mbps

What's the maximum throughput of 802.11b?

600 Mbps

What's the maximum throughput of 802.11n?

When device transmits at a higher rate than 802.11b.

When is a 6 μsecond quiet time of no transmission added at the end of the data portion of every frame?

Orthogonal frequency-division multiplexing (OFDM)

Whereas DSSS uses a high modulation rate for the symbols it sends, this technique uses a relatively slow modulation rate for symbols. This slower modulation rate, combined with the simultaneous transmission of data over 52 data streams, helps this technique support high data rates while resisting crosstalk between the various data streams.

Mode 3

Which HT operation mode provides the most compatibility for non-HT devices?

802.11e

Which IEEE standard supports QoS and enables prioritization of frames in DCF?

HT MAC layer

Which layer enhances throughput by frame aggregation and power management?

802.11e

Which standard allows the receiving device to acknowledge after receiving a burst of frames?

802.11 r

Which standard deals with security concerns regarding the handoff?

802.11e

Which standard defines enhancements to the MAC layer of 802.11?

802.11 r

Which standard designed to enhance the convergence of wireless voice, data, and video?

802.11e

Which standard enables prioritizaition of frames in distributed coordinated function (DCF) mode?

802.11n

Which standard specifies radio configurations using radio chains (multiple transmitters and receivers)?

802.11e

Which standard support traffic prioritization based on quality of service (QoS) and improves security features for mobile and nomadic users?

802.11 r

Which standard support voice over WLAN?

802.11a

Which standard uses Unlicensed National Information Infrastructure (U-NII) band?

To take advantage of multipath interference to increase SNR that results in increased range and reliability.

Why does 802.11n use multiple antennas along with multiple radios in each device?

To eliminate the need to retransmit when an error occurs which saves time and increases throughput.

Why does Forward Error Correction (FEC) transmit extra bits per byte of data?

To account for quiet time

Why is the SIFS time in the 802.11g standard effectively extended to 16 microseconds?

Channel bonding

With this technique, two wireless bands can be logically bonded together, forming a band with twice the bandwidth of an individual band. Some literature refers to it as 40-MHz mode, which refers to the bonding of two adjacent 20-MHz bands into a 40-MHz band.

Co-channel interference

____________________ can occur when two WLANs in the same vicinity share the same channel.

Quality of service

____________________ is a resource conservation mechanism that allows for the prioritization of traffic on a network.