| (+8) |
(8%) 1. 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 a frame to B. Which of the following statements are correct for “hidden node problem” ? (多選)
 | |
| (a) These two frames will NOT collide with each other at B | ||
| (b) These two frames collide with each other at B | ||
| (c) Both A and C are aware of this collision | ||
| (d) These two frames collide with each other at A and C | ||
| (e) A and C are said to hidden nodes with respect to each other | ||
| (+8) |
(8%) 2. .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 assume C can reach B and D but not A. (多選)
 | |
| (a) A and D | ||
| (b) A and C | ||
| (c) C and D | ||
| (d) B and C | ||
| (e) B and D | ||
| (+10) |
(10%) 3. What are the purposes of RTS and CTS control frames used in IEEE 802.11 wireless LAN ? (多選)
| |
| (a) Reserve the wireless channel for a time period | ||
| (b) The sender is asking the receiver to send back a frame after a time period | ||
| (c) Informs all nearby nodes that a transmission is about to begin | ||
| (d) These two frames also contain data to transmit. | ||
| (e) The duration field in the RTS and CTS frames is used to specify the time period | ||
| (+10) |
(10%) 4. Assume that a sender A sends an RTS frame and the receiver B sends a CTS frame. Which of the following statements is correct for any node that sees a CTS frame ? (單選)
| |
| (a) It is close to the sender A and therefore cannot transmit for the period of time specified in the CTS frame. | ||
| (b) It is not close enough to the receiver B to interfere with it, so is free to transmit. | ||
| (c) It is close to the receiver B and therefore cannot transmit for the period of time specified in the CTS frame. | ||
| (d) It is not close enough to the sender A to interfere with it, so is free to transmit. | ||
| (e) It is close to the receiver B and therefore can transmit for the period of time specified in the CTS frame. | ||
| (+8) |
(8%) 5. For the “active scanning” steps of a node to select an AP in IEEE 802.11 wireless LAN, please fill in the number (1-8) in the following 4 steps. (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 _____ frame | ANS:3 | |
| Step 2. All APs within reach reply with _______ frame | ANS:4 | |
| Step 3. The node selects one of the APs and sends that AP ____ frame | ANS:5 | |
| Step 4. The AP replies with ______ frame | ANS:6 | |
| (+10) |
(10%) 6. Which of the following is correct for the “value” of the three priority levels: SIFS, PIFS, and DIFS, defined in IEEE 802.11 CSMA/CA protocol ? (數值的大小的比較)(單選)
| |
| (a)PIFS < SIFS < DIFS | ||
| (b)SIFS < DIFS < PIFS | ||
| (c)DIFS < PIFS < SIFS | ||
| (d)DIFS < SIFS < PIFS | ||
| (e)SIFS < PIFS < DIFS | ||
| (+12) |
(12%) 7. Which of the following statements about the CSMA/CA are correct ? (多選)
| |
| (a) A station with a pending data unit (MPDU) may transmit immediately when it detects the medium is free. | ||
| (b) A station with a pending data unit (MPDU) may transmit when it detects a free medium for greater than or equal to a DIFS time. | ||
| (c) If the medium is busy when a station desires to transmit data unit (MPDU), the station can transmit the MPDU when 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 data unit (MPDU), the Random Backoff Time algorithm shall be followed after 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 CA “collision avoidance” mechanism is used. | ||
| (+18) |
(18%) 8. 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 20us, 10us, 14us, and 6 us, respectively. Then based on the backoff procedure, please indicate the value for each of the following valuables: 每一個空格填入一個數字(整數)
 | |
| a = | ANS:20 | |
| b = | ANS:10 | |
| c = | ANS:6 | |
| d = | ANS:4 | |
| e = | ANS:10 | |
| f = | ANS:14 | |
| g = | ANS:4 | |
| h = | ANS:6 | |
| i = | ANS:2 | |
| (+8) |
(8%) 9. Which of the following statements are correct about the “Point Coordination Function (PCF)” ? (多選題)
| |
| (a) Each station must one by one become the Point Coordinator (PC), which generates the Superframe. | ||
| (b) The PCF provides contention free services. | ||
| (c) Not all stations must be capable of becoming the PC and transmitting PCF data frames. | ||
| (d) The length of a Superframe is a manageable parameter but the length of the contention free period is fixed on a per Superframe basis. | ||
| (e) The Superframe consists of a “Contention Free period” and a “Contention Period”. | ||
| (+8) |
(8%) 10. Which of the following are correct for the using of “Short IFS (SIFS)” in IEEE 802.11 CSMA/CA protocol ? (多選題)
| |
| (a) It is used for an ACK frame, | ||
| (b) It is used for a CTS frame | ||
| (c) It is used by the DCF to send frame after a node detects the medium free for the period SIFS. | ||
| (d) It is used only by the PCF to send any of the Contention Free Period frames | ||
| (e) It is used by a station responding to any polling | ||
沒有留言:
張貼留言