2013 Exam Flashcards
Star Topology (3)
- Central node communicates w/ all other non-central nodes
- Non central nodes do not communicate with each other
- Possible in BT / WiFi
Mesh Topology (3)
- No central node
- Each node can communicate w/ all other nodes within range
- WiFi ‘ad-hoc’ mode
Limitations of CSMA approach in 802.11 (2)
- 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
BEB Algorithm (6)
- 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
Processing Gain in DL
post-processing gain SNR = pre-processing gain SNR
Centralised Control Meaning
1 node dictates medium access to all others in the network
Distribute Control Meaning
all noes equally share responsibility for decision as to which node may access the medium
Scheduling Meaning
task performed by the central controller to plan which resources will be allocated to each of the other devices
Contention Meaning
the process where nearby nodes in a distributed network compete for access to resources
Factors affecting performance under certain fading conditions? (3)
- 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.
Infrastructure Networks
make use of a permanently sited device w/ access to a larger network such as a wired backbone network
Ad-hoc networks
operate w/o the support of infrastructure
Additional challenges of wireless compared to wired (4)
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
Bluetooth + Interference Problems (3)
- 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
AFH Strategy (4)
- 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
802.11 4-way handshake (incl RTS / CTS)
Difference between Network + Appl QoS (2)
Network
- capability of the comms network to provide for the needs of an appl
Appl.
- the experience that the appl. provides to the user
QoS Network Parameters (5)
- 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
Concept of Multiuser Diversity (3)
- 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
What does the black dotted line represent?
result of DRA to exploit multiuser diversity + has higher total receiver power than any individual user
Definition of shadowing
the effect that the receieved signal power fluctuates due to objects obstructing the propagation path between transmitter + receiver
Definition of Shadowing
variation / attenuation of a signal w/ various variables due to multipath induced fading or shadowing
DRA can only be performed in the dimensions in which …
… MA is possible
