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Broadband Wireless > 2013 Exam > Flashcards

2013 Exam Flashcards

1
Q

Star Topology (3)

A
  • Central node communicates w/ all other non-central nodes
  • Non central nodes do not communicate with each other
  • Possible in BT / WiFi
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2
Q

Mesh Topology (3)

A
  • No central node
  • Each node can communicate w/ all other nodes within range
  • WiFi ‘ad-hoc’ mode
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3
Q

Limitations of CSMA approach in 802.11 (2)

A
  • Two nodes to find medium free, both transmit + cause a collision
  • Hidden node problem, if node senses medium to be free + transmits, it may interfere w/ another device within its range which is currently receiving a transmission from a 3rd node out of its range
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4
Q

BEB Algorithm (6)

A
  • BEB adds a contention process
  • Takes place between a node sensing the medium is free + it being allowed to transmit
  • Node has to wait # time slots before it may transmit
  • Nuber chosen randomly (CW)
  • Makes it less likely (not impossible) for two nodes to start transmitting simultaneously
  • Nodes w/ higher # time slots wait to sense the other node starting first
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5
Q

Processing Gain in DL

A

post-processing gain SNR = pre-processing gain SNR

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6
Q

Centralised Control Meaning

A

1 node dictates medium access to all others in the network

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7
Q

Distribute Control Meaning

A

all noes equally share responsibility for decision as to which node may access the medium

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8
Q

Scheduling Meaning

A

task performed by the central controller to plan which resources will be allocated to each of the other devices

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9
Q

Contention Meaning

A

the process where nearby nodes in a distributed network compete for access to resources

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10
Q

Factors affecting performance under certain fading conditions? (3)

A
  • results are av. over a wide range of fading cond.
  • Actual instantaneous fading cond. may result in a different value to PER
    • causes packet delays, compromise QoS requirements
  • beneficial to take a conservative approach to LA + set a higher SNR requirement for the use of a given MCS to ensure reliability in a bad fading cond.
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11
Q

Infrastructure Networks

A

make use of a permanently sited device w/ access to a larger network such as a wired backbone network

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12
Q

Ad-hoc networks

A

operate w/o the support of infrastructure

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13
Q

Additional challenges of wireless compared to wired (4)

A

Collision Detection

  • not possible to ‘listen whilst talk’ on a wireless medium
  • can’t tell if another device is transmitting whilst it is transmitting, cannot detect collision

Errors

  • wireless medium introduces many more errors
  • MAC protocol should know the difference between errors from wireless medium and MAC

Signal Attenuation

  • wireless medium attenuates signal
  • affects comms rate between nodes
  • gives rise to hidden node problem

Power Eff.

  • operates from a limited power supply
  • MAC protocol must be eff. to use energy
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14
Q

Bluetooth + Interference Problems (3)

A
  • Operates in 2.4 GHz, unlicensed frequency band so potential for interference from other devices
  • No control of interfering signals, BT divides band into 79x 1MHz hop frequencies
  • Hops pseudo-randomly + quickly across the whole band to average out conditions over time
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15
Q

AFH Strategy (4)

A
  • avoids hop frequencies that result in error
  • algorithm learns which freq. are bad + avoid them for a certain time
  • slowly reintroduce bad frequencies to make sure diversity is maintained
  • halves hop rate ensuring each pair of packets uses same freq.
    • prevents loss of ACK due to bad freq channel
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16
Q

802.11 4-way handshake (incl RTS / CTS)

A
17
Q

Difference between Network + Appl QoS (2)

A

Network

  • capability of the comms network to provide for the needs of an appl

Appl.

  • the experience that the appl. provides to the user
18
Q

QoS Network Parameters (5)

A
  • Throughput / data rate - useful bit/s provide to appl
  • Throughput jitter - amount of variation of throughput over time
  • Delay - delay between a packet being generateed by src. appl. + its delivery via the netwrok to the dst. appl.
  • Delay jitter - amount of variation of delay over time
  • Packet Error / Loss Rate - prob of any given packet being delivered containing errors or not being delivered at all
19
Q

Concept of Multiuser Diversity (3)

A
  • Multiuser diversity arises in fading environments due to shadowing + multipath
  • Different users in different physical locations will percieve uncorrelated fading as a function of time/freq/space
  • DRA attempts to allocate different fractions of the common medium resource to user w/ best signal
    • Majority of fadding effects can be mitigated as even exploited to achieve an incr. in the received signal energy
20
Q

What does the black dotted line represent?

A

result of DRA to exploit multiuser diversity + has higher total receiver power than any individual user

21
Q

Definition of shadowing

A

the effect that the receieved signal power fluctuates due to objects obstructing the propagation path between transmitter + receiver

22
Q

Definition of Shadowing

A

variation / attenuation of a signal w/ various variables due to multipath induced fading or shadowing

23
Q

DRA can only be performed in the dimensions in which …

A

… MA is possible

Broadband Wireless (5 decks)

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