1. (15%) Consider a computer system with three users: Alice, Bob, and Cyndy. Alice owns the file a, and Bob and Cyndy can read it. Bob owns the file b, and Cyndy can read and write the file b, but Alice can only read it. Cyndy owns the file c, but neither Alice and Bob can read or write it. If a user owns a file, he/she can also execute the file.(a) Create the ACM (access control matrix) of the system.(b) Show the ACL of the ACM and the CL of the ACM.(c) Cyndy gives Alice permission to read c, and Alice removes Bob's ability to read a. Show the ACM after these changes.

Consider a slotted aloha system, where the time slot equals the fixed duration of each packet. assume that there are 4 stations a,b,c,d sharing the medium. (a) stations a,b,c,d receive one packet each from higher layers at times 1.3, 1.5, 2.6,5.7 respectively. show which transmissions take place when, according to the slottedaloha protocol; describe all transmissions until all four packets have been successful.when needed, each station has access to the following sequence of random number, provided by a random number generator and drawn uniformly between 0 and 1: (1) station a draws numbers: 0.31, 0.27, 0.78, 0.9, 0.9, 0.11, 0. (2) station b draws numbers: 0.45, 0.28, 0.11, 0.83, 0.37, 0.22, 0. (3)station c draws numbers: 0.1, 0.2, 0.3, 0.4, 0. (4) station d draws numbers: 0.36, 0.77, 0.9, 0.1, 0.1, 0.1, 0.1, 0. (b) in slotted aloha, a station transmits in each time slot with a given probability. what probabilities would you assign to each of the four stations so as to: (i) maximize the efficiency of the protocol? (ii) maximize fairness among the four stations? (c) will the efficiency increase or decrease if we modify slotted aloha as follows: (i) get rid of slots and allow stations to transmit immediately? (ii) implement carrier sensing? (iii) implement collision detection? (iv) implement collision avoidance?

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Match the feature to the network architecture. expensive to set up useful for a small organization easy to track files has a central server inexpensive to set up difficult to track files useful for a large organization does not have a central server client- server network peer-to-peer network