7. The “collision window” of the CSMA/CD protocol is the time required to detect a collision. Which of the following are correct for collision window ? Assume the one way propagation time of the network is “a”. (多選)
A. (a) The collision window is equal to a
B. (b) The collision window is equal to 2a
C. (c) The worst case scenario to detect a collision happens when the two hosts are close to each other.
D. (d) The best case scenario to detect a collision happens when the two hosts are close to each other.
E. (e) The worst case scenario to detect a collision happens when the two hosts are at opposite ends of the Ethernet
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The minimum length of a frame for the Ethernet is 64 bytes. Which of the following is correct when a node receives a frame shorter than 64 bytes ?
A. (a) It is impossible for a node to receive a frame shorter than 64 bytes.
B. (b) Frame shorter than 64 bytes can still be accepted as some of the fields such as MAC addresses may be correct.
C. (c) Frame shorter than 64 bytes will be treated as “collided” frame and will be dropped directly.
D. (d) Frame shorter than 64 bytes can still be accepted as long as it is a “broadcast” frame to all nodes.
E. (e) A jamming sequence will be sent to enforce the collision.
Which of the following are correct for traditional Ethernet CSMA/CD performance ?
A. (a) Ethernets work best under lightly loaded conditions.
B. (b) Ethernets work very well under heavy loads as all collisions will be detected.
C. (c) It’s a fair network that all nodes may share the bandwidth very equally.
D. (d) Under heavy loads, too much of the network’s capacity is wasted by collisions.
E. (e) The throughput of an Ethernet depends on the traffic load, heavy load will have better throughput.
4.1 Assume a bridge receives a frame from port x, with destination MAC address D and source MAC address S. Which of the following statements about frame forwarding are NOT correct ?
A. (a) The bridge will broadcast the frame to all ports if D is not found in the Filtering database of the bridge.
B. (b) The bridge will broadcast the frame to all ports, except port x, if D is not found in the Filtering database.
C. (c) The bridge will filter the frame if D is found in the Filtering database and D belongs to port x.
D. (d) The bridge will forward the frame to port y, if D is found in the Filtering database and D belongs to port y.
4.2 Assume a bridge receives a frame from port x, with destination MAC address D and source MAC address S. Let R = (address, port, timer) be a record in the Filtering database, where timer (in seconds) is the duration this record existed (initially, timer = 0). Which of the following statements about address learning are correct ?
A. (a) The bridge will add a new entry of R = (S,x,0) to the Filtering database if S is not found in the Filtering database.
B. (b) The bridge will new entry of R = (D,x,0) to the Filtering database if D is not found in the Filtering database.
C. (c) If an entry R = (S, y, 20) already existed in the Filtering database, then the bridge will update the entry to (S, x, 0).
D. (d) If an entry R = (S, x, 20) already existed in the Filtering database, then the bridge will update the entry to (S, x, 0).
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