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Broadband Wireless > 2017 Exam > Flashcards

2017 Exam Flashcards

1
Q

Where in the protocol stack should network QoS be evaluated?

A

Between the i/p of source appl and o/p to dest appl

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2
Q

Metrics used to evaluate network QoS (3)

A
  • Data rate / throughput
  • Delay (mean and jitter)
  • Loss/Error rate (PLR and PER)
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3
Q

PHY Parameters that a system can change + effects upon PHY performance (2)

A
  • System can adapt modulation in terms of bits per psymbol + its coding rate
  • Both of these will increase/decrease the raw rate of data tranmission at PHY whilst incr/decr BER + PER
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4
Q

Implications of changing the bits per symbol + coding rate on network QoS (3)

A
  • Changes in PHY affect the throughput + delay
  • Increase PHY rate will increase throughput, as long as not too many errors require retransmission
  • Increased PER results in more delay + jitter
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5
Q

Definition of CSMA (2)

A
  • ‘listen before talk’
  • A node must first check whether the channel is in use. If so, it must wait until its free
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6
Q

Definition of ARQ

A
  • all nodes required to ACK receipt of packets addressed to them, otherwise packet will be retransmitted
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7
Q

6What are the implications of the compression efect on the QoS requirements? (3)

A
  • Compression reduces throughput requirement
  • However channel errors usually have a greater impact upon the QoS perceived by the user after the src
  • Compression causes a requirement for a lower PER/PLR
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8
Q

Which layers of the ISO protocol are covered by 802 standards + how is functionaility of other layers of the protocol stack required? (2)

A
  • Controls functionality at layers 1 (PHY) + 2 (Data Link Control)
  • A convergence layer is also specified, enabling generic higher layer protocols (3+4) to operate on top of the specific 802 standard.
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9
Q

Why does compression effect in 802.11 occur?

A
  • Various overheads in the MAC protocol are of fixed time duration
  • Increasing PHY rate shortens the data frame but not the overheads. As PHY rate increases, the total time to transmit reduces but less is used for actual data.
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10
Q

Implications of the “compression effect” for effective throughput? (2)

A
  • Overall slower data rate meaning that throughput converges to a maximum irrespective of the PHY rate.
  • Compressive MAC eff. results in diminishing returns
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11
Q

Definition of ‘backhauled’

A

links between core and subnetworks

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12
Q

Impact of 802.11 BBS being backhauled (2)

A
  • Backhaul tech will limit the packet sizes arriving at the 802.11 AP
  • Smaller packets results in a lower MAC eff thererfore limit the throughput
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13
Q

Merits of operating 2x 20MHz BSS vs 1x 40MHz when providing service to multiple user (3)

A
  • Better to operate 2x 20MHz
  • Sum of lower rates is greater than rate achieved by 40MHz BSS
  • Only works for multiple users as a single user cannot connect to multiple networks
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14
Q

What is BSS (2)

A
  • Basis Service Set
  • Deinfe a network of nodes communicating ONLY via 802.11 protocols
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15
Q

Traffic Class Scheduling: UGS

A

(Unsolicited Grant Services)

BS automatically schedues a fixed fraction of a resource for this service without service having to continually request it, only requests once.

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16
Q

Traffic Class Scheduling: RTPS (2)

A

(Real Time Priority Service)

  • BS does not auto schedule resource, it schedules regular oopportunities in the UL phase for the SS to request resource for their RTPS service.
  • Creates more resource request overheads than UGS but is more adpatable to a services’ need.
17
Q

Traffic Class Scheduling: NRTPS (2)

A

(Non-real Time Priority Scheduling)

  • Similar method to RTPS but also allows nodes to contend to use TDMA slots for resource requests.
  • Enables BS to schedule fewer resources for the resource request but makes the availability of resource to NRTPS less reliable
18
Q

Traffic Class Scheduling: BES (3)

A

(Best Effort Service)

  • No resource requests are scheduled ny BS
  • SS may only obtain resource by contending to transmit resource in spare UL data frames
  • BES gets worst deal
19
Q

Order of Priority for Traffic Scheduling

A

UGS

RTPS

NRTPS
BES

20
Q

Implication of contention based MA srategies for network QoS as well as fixed/variable packet size with contention based MA?

How does this affect when scaled?

(5)

A
  • Contention based medium access requires nodes to compete for medium access
  • No guarantee which node will win
  • An ‘unlucky’ node may have to wait longer to access the medium, increasing delays
  • Variable packet sizes result in variable packet time occupation of the channel therefore exacerbates delays if node keeps loosing contention to nodes transmitting large packets
  • Exacerbated further when there are more nodes contending for access to the medium
21
Q

Fragmentation Threshold - 1500 bytes

RTS / CTS threshold - 700 bytes

Src node commences contention. Channel remains free until the src’s contention window reaches zero.

Next packet = 800 bytes. No errors occur during the transmission of any frames (5)

A
  • Src win contention, next packet exceeds the RTS/CTS threshold so src transmits an RTS
  • Dest responds w/ a CTS
  • Src transmits a data frame
  • Dest responds w/ ACK
  • Upon receiving ACK, src resets CW back to starting value + generates a new value
22
Q

Fragmentation Threshold - 1500 bytes

RTS / CTS threshold - 700 bytes

Src node commences contention. Channel remains free until the src’s contention window reaches zero.

Next packet = 800 bytes. Error occurs in first frame transmission (4)

A
  • Next packet exceeds threshold, src transmits RTS
  • Error occurs, dts CANNOT send CTS
  • Src fails to recieve CTS, src infers a collision
  • Src doubles CW + generates new value
23
Q

Sr node commences contention, but before contention value reaches 0, it sense another node transmitting (3)

A
  • Src shouldn’t transmit
  • Retain contention value it had when detected other transmission
  • Use again in the next contention opportunity
24
Q

Fragmentation Threshold - 1500 bytes

RTS / CTS threshold - 700 bytes

Src node commences contention. Channel remains free until the src’s contention window reaches zero.

Next packet = 500 bytes. No errors occur during the transmision of the data frames (3)

A
  • Below CTS / RTS threshold therefore transmits the data frame
  • Due to no errors, dst should transmit ACK
  • Src should reset contention range
25
Q

Fragmentation Threshold - 1500 bytes

RTS / CTS threshold - 700 bytes

Src node commences contention. Channel remains free until the src’s contention window reaches zero.

Next packet = 500 bytes. Errors occur during the transmision of the data frames (3)

A
  • Below CTS / RTS threshold therefore transmit the data frame
  • Due to error, dst does NOT respond w/ ACK
  • Src should double its CW range + generate new random value
26
Q

Block Diagram of ARQ Codec + FEC codec

A
  1. ARQ Coder/Decoder
  2. FEC Coder/Decoder
  3. FEC Codec, reduces: BER, probability of packet error
  4. ARQ Codec, eliminates residual packet errors

ARC encoder 1st

HARQ attempts to correct errors at reciever

27
Q

SDU definition

A

a unit of data above a particular layer

28
Q

PDU definition

A

a unit of data below a layer

29
Q

From perspective of the PHY layer @ dst, explain what would be the i/p a+ o/p + typical overheads (3)

A
  • PHY @ dst i/p is a PHY PDU, processed to a PPDU to produce a PSDU (PHY SDU), i/p to MAC layer
  • PHY removes overheads from the PPDU such as preamble + info on the modulation + coding used in the PPDU
  • Use these in equalisation, demodulation + decoding to help recover the correct PSDU content.
30
Q

Sub-carrier Choice: Arbitrary + Static

A

requires minimum signalling overhead as the BS will only inform SSs of their resource allocation once

31
Q

Sub-carrier Choice: Random (3)

A
  • mitigate intercell interference effects especially @ cell edge
  • SSs near edge average out the interference effects over time if random allocation changes
  • follows a pre-defined pttern to minimise signalling but this pattern MUST be different between cells
32
Q

Sub-carrier Choice: Channel Aware (2)

A
  • enables BS to exploit channel conditions to ‘best’ effort
  • greedy strategy to maximise system throughput, fair strategy or some comprimise between these such as proportional fair strategy
33
Q

Differences between UL + DL in BS (4)

A
  • DL - multiplex, makes little difference how resources are allocated in time + frequency
  • UL - multiple access. Frequency domain multiple access yields a processing gain
    • better to share resources in freq rather than time
    • simultanteously transmits using a fraction of the total b/w
34
Q

Why does scheduler not allocate resources in a way that maximises PHY performance (2)

A
  • Scheduler should allocate resources according to traffic needs + maximise PHY performance
  • If one SS requires more throughput, it should get more resources even if this lowers processing gain
35
Q
A

Broadband Wireless (5 decks)

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