清華大學 計算機網路 第3週課程 IEEE 802.11 無線區域網路Wireless LAN

Video Part1
https://www.youtube.com/watch?v=E-Y-owGxRy4

3.1 In IEEE 802.11 Wireless LAN, the signals are propagated through which media ?
(a) Coaxial cable
(b) Optical fiber
(c) Twisted pair
(d) Space (air)
(e) Power Line

a b c d
hidden node (a,c聽不到對方，同時送封包給b會發生碰撞)
expose node problem (b->a, c->d  b,c可聽到對方，但b,c同時送封包不一定會發生碰撞)

CSMA/CA collision avoidance


3.2  Assume each of four nodes is able to send and receive signals that reach 

just the nodes to its immediate left and right. Suppose both A and C want to 

communicate with B and so they each send it a frame. Which of the following 

statements are correct for “hidden node problem” ?
(a) These two frames collide with each other at B
(b) These two frames will NOT collide with each other at B
(c) Neither A nor C is aware of this collision
(d) Both A and C are aware of this collision
(e) A and C are said to hidden nodes with respect to each other



3.3 For the following wireless configuration, which two nodes are hidden from each other ? Assume B can exchange frames with A and C, but it cannot reach D. And C can reach B and D but not A



(a) A and C

(b) A and D
(c) B and D
(d) B and C
(e) C and D



3.4 Assume each of four nodes is able to send and receive signals that reach just the nodes to its immediate left and right. Suppose B is sending a frame to A. Which of the following statements are correct for “exposed node problem” ?



(a) Although B and C are exposed to each other's signals, there is no interference if B transmits to A while C transmits to D.

(b) C cannot transmit to anyone because it can hear B's transmission.

(c) C is able to transmit to node D
(d) D is able to transmit to node C
(e) A is able to transmit to node B


3.5 For the following wireless configuration, which two nodes are exposed to each other ? Assume B can exchange frames with A and C, but it cannot reach D. And C can reach B and D but not A



(a) A and B

(b) A and C

(c) B and C
(d) B and D
(e) C and D

 Video Part3

control frame : Request to Send RTS/ Clear to Send CTS 解決hidden node的問題
聽到RTS可以送給B嗎?
送出RTS沒等到CTS表示Collision，等待一段時間再重送

802.11 using ACK frame in CSMA/CA



3.6 What are the purposes of RTS and CTS control frames used in IEEE 802.11 wireless LAN ?

(a) Reserve the wireless channel for a time period

(b) Informs all nearby nodes that a transmission is about to begin

(c) These two frames also contain data to transmit.

(d) The sender is asking the receiver to send back a frame after a time period
(e) The duration field in the RTS and CTS frames is used to specify the time period



3.7 For IEEE 802.11 CSMA/CA protocol, which of the following statements are correct for any node that sees a CTS frame ?

(a) It is close to the sender and therefore it can transmit for the period of time specified in the CTS frame.

(b) It is close to the receiver and therefore cannot transmit for the period of time specified in the CTS frame.

(c) It is not close enough to the receiver to interfere with it, so is free to transmit.

(d) It is not close enough to the sender to interfere with it, so is free to transmit.
(e) It is close to the receiver and therefore can transmit for the period of time specified in the CTS frame.

IEEE 802.11 Distribution system
scanning : send a probe frame
當手機送出probe frame，可能會收到多個Probe Response，挑一個AP送出Association Request跟AP連結，AP回應Association Response。


3.8 For IEEE 802.11 CSMA/CA protocol, which of the following statements are correct for any node that sees an RTS frame ?

(a) It is close to the sender and therefore cannot transmit for the period of time specified in the RTS frame.

(b) It is close to the sender and therefore can transmit for the period of time specified in the RTS frame.

(c) It is not close enough to the receiver to interfere with it, so is free to transmit.

(d) It is not close enough to the sender to interfere with it, so is free to transmit.
(e) It is close to the receiver and therefore can transmit for the period of time specified in the RTS frame.

3.9 For IEEE 802.11 CSMA/CA protocol, what happens if nodes A and B detect an idle link and transmit an RTS frame at the same time ? Assume that nodes A and B are located within the transmission range of each other.
(a) Both the RTS frames will be transmitted successfully without collision.
(b) These two RTS frames will collide with each other
(c) The senders realize the collision has happened when they do not receive the CTS frame after a period of time
(d) The senders realize the collision has happened by using a collision detection algorithm like the Ethernet.
(e) Collison will happen, and each sender waits a random amount of time before trying again, where the amount of time is defined like the exponential backoff algorithm used in the Ethernet CSMA/CD protocol.

Active scanning 由手機發起Probe frame。
Passive scanning 由基地台週期性發出Beacon frame，手機回Association Request要求連線，AP再回Association Response建立連線。


3.10  Please fill in the number for the “active scanning” steps of a node to select an AP in IEEE 802.11 wireless LAN. (1) a Beacon (2) a Beacon response (3) a probe (4) a probe response (5) an association request (6) an association response (7) a poll (8) a poll response

Step1. The node sends ___1___ frame

Step2. All APs within reach reply with ____2___ frame 

Step3. The node selects one of the APs and sends that AP __3__ frame

Step4. The AP replies with  ___4___ frame 
(1) 3
(2) 4
(3) 5
(4) 6


3.11Please fill in the number for the “passive scanning” steps of a node to select an AP in IEEE 802.11 wireless LAN. (1) a Beacon (2) a Beacon response (3) a probe (4) a probe response (5) an association request (6) an association response (7) a poll (8) a poll response

Step1. The APs periodically send ___1__ frame 

Step2. A node can change to an AP by sending it __2__ frame back to the AP 

(1) 1

(2) 5
3.12 At most how many MAC addresses may be contained in an IEEE 802.11 wireless LAN frame ?
(a) 2
(b) 3
(c) 4
(d) 5
(e) 6

Video Part7
https://www.youtube.com/watch?v=TNM88ppQbjs

DCF : Distributed Coordination Function
競爭式服務
Base on CSMA/CA, Implement in all stations and APs
Used within : Ad hoc, Infrastructured

PCF: Point Coordination Function
非競爭式服務
Implement on top of DCF
由point coordinator(通常是AP)進行polling，一個一個問有沒有人要送封包。
透過Inter-Frame Space當作priority控制參數

Point IFS PIFS < Distributed DIFS 表示PIFS等待的時間較短

在Superframe: PCF與DCF並存，先進入免競爭模式一個一個問有沒有封包要送，問完再進入競爭模式。

每個Superframe的時間可以調整，PCF與DCF所佔時間也可以調整。

3.13 Which of the following statements are correct for the “super-frame” structure in IEEE 802.11 ?
(a) All super frames have the same size.
(b) Super frames may have different sizes.
(c) A contention-free burst occurs at the beginning, followed by a contention period.
(d) A contention period occurs at the beginning, followed by a contention-free burst.
(e) The length of the contention-free burst in each super frame is fixed.

Video Part8
https://www.youtube.com/watch?v=EjZVm_9rFdE

Virtual Carrier Sense mechanism 是透過RTS與CTS的duration field來達成，duration記錄在Net Allocation Vector (NAV)。
長封包用RTS/CTS
短封包直接送

Physical Carrier Sense 透過PHY

一個成功送出的封包，必有回一個ACK，而且時間短於Short-IFS SIFS。
需要ACK的封包類型: Data、Poll、Request、Response

Backoff Time = INT(CW*RANDOM())*Slot Time
CW 可能為 7, 15, 31, 63, 127, 255



3.14 Which of the following statements are correct for the “positive acknowledgement” scheme used in IEEE 802.11 CSMA/CA protocol ?
(a) To allow detection of a lost frame an ACK frame shall be returned immediately following a successfully received frame
(b) The gap between the received frame and ACK frame shall be SIFS.
(c) The gap between the received frame and ACK frame shall be PIFS
(d) The gap between the received frame and ACK frame shall be DIFS
(e) The lack of an ACK frame means that an error has occurred


3.15 In IEEE 802.11 CSMA/CA protocol, which of the following frame types should be acknowledged with an ACK frame ?
(a) Beacon
(b) Data
(c) Poll
(d) Request
(e) Response


3.16 Which of the following are correct for the using of “Short IFS (SIFS)” in IEEE 802.11 CSMA/CA protocol ?
(a) It is used for an ACK frame
(b) It is used for a Beacon frame
(c) It is used for a CTS frame
(d) It is used by a station responding to any polling
(e) It is used only by the PCF to send any of the Contention Free Period frames


3.17 Which of the following are correct for the using of “PCF-IFS (PIFS)” in IEEE 802.11 CSMA/CA protocol ?
(a) It is used for an ACK frame
(b) It is used only by the PCF to send any of the Contention Free Period frames
(c) It is used only by the PCF to send any of the Contention Period frames
(d) The PCF shall be allowed to transmit after it detects the medium free for the period PIFS
(e) The PCF shall be allowed to transmit after it detects the medium free for the period SIFS


3.18 Which of the following are correct for the using of “DCF-IFS (DIFS)” in IEEE 802.11 CSMA/CA protocol ?
(a) It is used for an ACK frame
(b) It is used for a Beacon frame
(c) It is used for a CTS frame
(d) Used by the DCF to transmit asynchronous MPDUs
(e) A node using the DCF is allowed to transmit after it detects the medium free for the period DIFS, as long as it is not in a backoff period.

聽到free等DIFS
聽到busy，等到free，再加上DIFS以及Random backoff時間

3.19 Which of the following statements about the CSMA/CA are correct ?
(a) A station with a pending MPDU may transmit when it detects a free medium for greater than or equal to a DIFS time.
(b) A station with a pending MPDU may transmit immediately when it detects the medium is free.
(c) If the medium is busy when a station desires to transmit an MPDU, the Random Backoff Time algorithm shall be followed after it detects a free medium for greater than or equal to a DIFS time.
(d) If the medium is busy when a station desires to transmit an MPDU, the station can transmit the MPDU when it detects a free medium for greater than or equal to a DIFS time.

(e) In CSMA/CA protocol, no packet collision will happen as the “collision avoidance” mechanism is used.

3.20 Consider the random backoff procedure in the CSMA/CA protocol. For the following figure, assume the red arrow indicates the time of a station wishing to send a frame, and the backoff time for stations B,C,D,E, are 19us, 10us, 15us, and 7 us, respectively. Then based on the backoff procedure, please indicate the value for each of the following valuables: 


a = 19
b =  9
c =  4
d =  2
e =  10
f =  15
g =  5
h =  7
i =  2

Video Part11
透過RTS_Threshold來決定是否要送RTS/CTS

3.21 Which of the following statements are correct about the “Point Coordination Function (PCF)” ?
(a) The PCF provides contention free services.
(b) Each station must one by one become the Point Coordinator (PC), which generates the Superframe.
(c) Not all stations must be capable of becoming the PC and transmitting PCF data frames.
(d) The Superframe consists of a Contention Free period and a Contention Period.
(e) The length of a Superframe is a manageable parameter but that of the contention free period is fixed on a per Superframe basis.

3.22 Which of the following relationship is correct for the values of the three priority levels: SIFS, PIFS, and DIFS, defined in IEEE 802.11 CSMA/CA protocol ?
(a) PIFS < SIFS < DIFS
(b) SIFS < PIFS < DIFS
(c) DIFS < PIFS < SIFS
(d) DIFS < SIFS < PIFS
(e) SIFS < DIFS < PIFS



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