Quiz 3 (Answers)

ECTE 967 (Quiz 3)

Student Name : __________________________

Student Number : ___________________________

Part A (20 Marks)

Mark your answers clearly with an ‘X’

A B C D A B C D

Q1 Q11

Q2 Q12

Q3 Q13

Q4 Q14

Q5 Q15

Q6 Q16

Q7 Q17

Q8 Q18

Q9 Q19

Q10 Q20

Question 1

Binary Exponential Backoff (BEB) is carried out by a station in what scenario(s)?

A. Collision

B. After every transmission

C. After a busy channel

D. All of the above.

Question 2

Which of the following methods can be used by a station to increase its channel access priority?

A. Increase its DIFS

B. Increase the value of CW min

C. Increase its contention window size by a factor α after each collision, where α is a small value.

D. None of the above.

Question 3

In 802.11, stations use what protocol in the contention period?

A. PCF

B. DCF

C. BEB

D. NAPman.

Question 4

In 802.11e, the Direct Link Protocol (DLP) overcomes what problem?

A. The poor channel condition between stations in a Basic Service Set

B. The need to forward all packets to the Access Point (AP)

C. The high energy consumption of stations

D. Poor channel access experienced by stations and APs.

Question 5

In IEEE 802.11e, contention free burst is advantageous because …

A. It allows the transmission of multiple ACKs

B. Stations do need to perform any channel access

C. It allows the transmission of an infinite number of data packets

D. Stations only need to perform channel access once.

Question 6

Fair queuing ensures each station …

A. Receives similar number of transmission errors

B. Receives similar throughput

C. Has equal access to an outgoing link

D. Option B and C.

Question 7

Consider an AP using proportional fairness. The AP always select the station that has …

A. The highest data rate, and has the highest aggregated throughput amongst all stations

B. The highest data rate, and has received minimum service in the past

C. The lowest data rate, and has received minimum service in the past

D. The lowest data rate, and has the highest aggregated throughput amongst all stations

Question 8

Time fairness is important because …

A. It is the only way to ensure throughput fairness

B. It protects high data stations from low data rate stations

C. It improves the throughput of low data rate stations

D. It improves the channel condition experienced by high data rate stations.

Question 9

During handoff, current IEEE 802.11 devices use what metric to decide the best AP to associate with?

A. End-to-end delay to https://www.360docs.net/doc/6513744876.html,

B. Packet error rate

C. Number of stations

D. Signal strength.

Question 10

Which of the following is not a functionality of IEEE 802.11f?

A. It updates the forwarding table of APs and switches

B. It allows a station to inform its new AP the address of its previous AP

C. It enables a station to pre-install association information at APs

D. None of the above.

Question 11

The three phases of a handoff process include

A. Probe, authentication and re-association

B. Authentication, handoff, retransmission

C. Retransmission, probe, re-association

D. Transfer state, re-associate, and probe.

Question 12

How does channel mask help a station reduce handoff delay?

A. It represents the bandwidth of each channel

B. It identifies APs with the least load

C. It identifies APs with superior signal strength

D. It identifies the channels which have at least one AP.

Question 13

Load balancing in WLANs is important because …

A. It reduces collisions

B. It reduces congestion at APs

C. It reduces contention access delay

D. All of the above.

Question 14

Cell breathing is a strategy that reduces …

A. The channel assignment of APs

B. The data rate of APs

C. The transmission power of an AP

D. The carrier sense threshold of all APs and stations

Question 15

IEEE 802.11b has how many orthogonal or non-overlapping channels?

A. 3

B. 4

C. 5

D. 6.

Question 16

A conflict graph models …

A. The data rate between APs

B. APs that interfere with one another

C. The number of associated stations

D. The distance between APs

Question 17 (No Answer)

What problem do you foresee when an AP uses Least Congested Channel Search to assign itself a channel?

A. Multiple APs may select the same channel at the same time

B. It does not take into account stations that are in the interfering range of neighboring APs.

C. It does not consider the load of neighboring APs

D. Option A and B

Question 18

A weighted conflict graph captures …

A. The number of APs in a WLAN

B. The impact or implication of assigning two conflicting APs the same channel

C. The number of colors that can be assigned to an AP

D. None of the above.

Question 19

In a client driven conflict graph, the range set models …

A. The APs within a station’s transmission range

B. The signal strength to each AP

C. The number of interfering basic service sets

D. The distance of each AP.

Question 20

The controller in a WLAN is responsible for …

A. Assigning channels to APs

B. Monitoring the load of all APs

C. Controlling the association of stations

D. All of the above.

Part B

Question 1 (10 Marks)

Provide a short answer to each of the following questions:

1. A legacy IEEE 80

2.11b station is currently using a contention window size of 64. If its current

transmission fails, the station adjusts its contention window to what size?

Doubles the CW. In this question, it’s 128

2. A key problem in IEEE 802.11b is that an AP may have to delay its beacon transmission if the

channel is busy. How does IEEE 802.11e address this problem?

By bounding the transmission time of stations to TxOP time.

3. Explain the performance anomaly problem.

This is when a station with low data rate takes a significant amount of time to transmit its packet as compared to a station with high data rate. As a result, the transmission time received by the high data rate station will be a lot less than the low data rate station.

4. Fair queuing schedules packets according to what criterion?

Earliest finish time.

5. How does an AP determine whether the service received by a station is leading, in-sync, or

lagging?

By running a fair queuing and determining whether it has received more, same, less service/throughput than a channel without errors.

6. An engineer decides to set CW min to five to improve channel access delay. What problem do you

foresee?

When the number of stations increases, there will be lots of collisions.

7. What key feature does Multi-Scan use to reduce AP discovery time?

An additional radio.

8. What does it mean when we say channel 1 and 6 are non-overlapping?

There causes no interference to each other. I.e., their spectrum mask does not overlap.

9. A conflict graph has three cliques with size 2, 4 and 8. The graph needs a minimum of how

many colors or channels?

8

10. What is the key difference between coordinated and uncoordinated WLANs?

The existence of a central controller.

I n a different question, the controller is responsible for management of AP channels and transmission

power, station associations and security.

Question 2a (5 Marks)

Determine the minimum number of colors needed to color the following conflict graph?

Ans = 3. If the number of vertices is even, then you only need two.

Question 2b (5 Marks)

Consider the following weighted graph where the link weight corresponds to the number of stations if two APs are assigned the same channel. Assign a channel to each node/AP such that the sum of interference is minimum. The amount of channel interference between channel a and b is computed as follows: ChannelInterf = 1 – MIN(|C a , C b |, 5)*0.2. Show your workings.

Assign channel 1 to Node ‐1 and Node ‐4. Node ‐2 and 3 get channel 6 and 11 respectively.

Question 3 (5 Marks)

Consider the following WLAN . The data rate, in Mbps, of link L1 to L3 is also shown in the following figure.

(a) Calculate the throughput of each station at t 1 if the packet size is 1024 bytes.

TxTime(S1) = 1365 us. TxTime(S2) = TxTime(S3) = 151 us. Throughput S1 = S2 = S3 = 4.9 Mbps

(b) How much time should be given to each station in beacon period t 1 to ensure fairness?

1365us

Question 4 (5 Marks)

For each of the following scenarios, determine the best AP for station S 1.

Best AP = _AP 2__

Best AP = __AP 3_

In a different quiz set, the answer is AP 3 and AP 1

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AP 1

AP 1/CH=1

AP 2/CH=6 AP 3/CH=11 AP 1

/CH=1AP 2/CH=2 AP 3/CH=11