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Thursday 10 December 2020 Expected Duration: 90 minutes Start time: 14:00 Time Allowed: 3 hours
時間: 2021-08-16 11:33:35

Networked Systems 1. (a) State the name of the layer where the MAC address exists and indicate the protocol that can resolve the IP and the MAC address of a given device in a local area network. [2 marks ] (b) Describe how a TCP connection is established and explain the significance of the sequence number in TCP packets. [4 marks ] (c) Consider the following Python code of two server-side implementations for two different servers. #server1.py from socket import * serverPort = 13000 serverSocket = socket(AF_INET, SOCK_DGRAM) serverSocket.bind((’’, serverPort)) print("AKMServer...waiting...") while True: message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.decode().upper() serverSocket.sendto(modifiedMessage.encode(),clientAddress) #server2.py from socket import * serverPort = 13000 serverSocket = socket(AF_INET, SOCK_STREAM) serverSocket.bind((’’, serverPort)) serverSocket.listen(1) print(’AKMServer...waiting...’) while True: connectionSocket, addr = serverSocket.accept() phrase = connectionSocket.recv(1024).decode() capsPhrase = phrase.upper() December Diet 1 Continued connectionSocket.send(capsPhrase.encode()) connectionSocket.close() State on which protocol each server runs and indicate the segment of code justifying your answer in each one of the servers explaining the code definition. [4 marks ] (d) Now consider the code in server2.py and assume a hypothetical scenario having another host running a client.py script and creating a clientSocket object to connect with server2.py. 1. Write the code statement enabling the server socket to be attached. 2. Explain the purpose of socket.listen(1). 3. Explain the purpose of connectionSocket. 4. In which port is the server receiving data. 5. How many bytes are received by the server in every message sent from the client. [5 marks ] (e) Explain why server1.py requires one socket and server2.py requires two sockets. [2 marks ] (f) Explain how we can have multiple IP addressable devices in our home network when we are assigned only one single IPv4 address from our Internet Service Provider (ISP). [3 marks ] December Diet 2 Continued Security 2. (a) Explain what TLS is and describe the steps associated with the TLS handshake between a client and a server. [4 marks ] (b) Alice uses a public WiFi and wants to access her Gmail account and she originally connects to http://mail.google.com. Describe and state the steps by which her Gmail session could be hijacked if an attacker is in close proximity. [6 marks ] (c) You are a network security architect and you are asked to propose a solution in which demanding real-time network applications (e.g., teleconferencing) require network and transport layer end-to-end encryption for the whole IP packet. Discuss which security protocol you will use and justify your selection. Also indicate how you will ensure that the complete packet will be encrypted. [5 marks ] (d) You are asked to develop a cryptosystem. Discuss and indicate: 1. Cryptography categories to consider and how to configure the encryption scheme. 2. How you would satisfy the aspect of data integrity. [5 marks ] December Diet 3 Continued Operating Systems 3. (a) Describe the structure of the CPU register file on a 64-bit architecture. [2 marks ] (b) Consider the following set of processes: Process Arrival Time (ms) Burst Time (ms) P0 0 5 P1 3 9 P2 4 15 P3 6 10 P4 8 11 By considering a time quant of 5ms and a Round-Robin Scheduling scheme: ? State the order in which these processes will be executed. ? Provide the time intervals of execution for each process. ? Compute and provide the turnaround time for each process. [11 marks ] (c) Based on the previous table from question 3(b) also consider that a priority value is assigned to each process as follows: Process Priority P0 1

Tuesday 30 April 2019 9:30 am – 10:30 am (Duration: 1 hour) DEGREES of MSci, MEng, BEng, BSc, MA and MA (Social Sciences) Computing Science 1S (Answer All Questions) This examination paper is worth a total of 50 marks
時間: 2021-08-12 09:38:04

1. (a) Convert 1100 0011 to a decimal number, assuming binary representation. [2] (b) Convert 1100 0011 to a decimal number, assuming two’s complement representation. [3] (c) Translate the statement a := b*c – d/b into Sigma16 assembly language, assuming that a, b, c, and d are integer variables. You do not need to write a complete program, and you don’t need to write data statements for the variables. Just translate this one statement. [3] (d) In the following program, variables k, i and n are integers, and x is an array of signed integers containing n elements. Suppose the program is executed with these initial values: n = 7 x = [9, -2, 13, 0, 45, -8, 7] After the program executes, what are the values of k and x? k := 0; for i := 0 to n-1 do { if x[i] < 0 then { x[i] := -x[i]; } else { k := k+1; } } [2] (e) Translate the program in part (d) into low level language. You do not need to define the variables or array, just translate the program code. (The low level language contains assignment statements, goto statements, and statements of the form if b then goto label, where b is a Boolean expression.) [5] (f) Translate the low level program in part (e) into a complete program in Sigma16 assembly language. Use data statements to define the initial values specified in part (d). [10] Summer Diet -2- Continued Overleaf/ 2. (a) Describe the behavior of the 1-bit register circuit (reg1) with data input x, control input load, and output r. [3] (b) Design a circuit that takes two inputs a, b. It produces one output, x, which is defined by the truth table below. Implement the circuit using any of the standard logic gates (inv, and2, or2, xor2). You may specify the circuit using any of the following notations (just use one): a schematic diagram, Boolean algebra, or Hydra notation. a b x 0 0 1 0 1 1 1 0 0 1 1 1 [3] (c) Explain what could happen in a synchronous circuit if the clock speed is too fast. [3] Summer Diet -3- /END (d) The following program takes two inputs: an integer x and a pointer header to a list header. The list is represented by nodes. Each node is a record consisting of two words: the first word value is an integer, and the second word next is a pointer to the rest of the list. The last node in the list has nil in the next field (nil is represented by 0). The value field of the list header is unused, and its next field points to the first node with a value (if any). p := header; q := p; p := (*p).next; done := false; while not done do if p=nil || x<(*p).value then { new := avail; avail := (*avail).next; (*new).value := x; (*new).next := p; (*q).next := new; done := true; } else { q := p; p := (*p).next; } Suppose the program is given x=5 and header points to a header for a list containing the initial values [3, 8, 9]. After the program executes, what is the value of the list that the header points to? [4] (e) Translate the program in part (d) to Sigma16 assembly language. You don’t need to write out the low level language version, and you don’t need to define the variables or the linked list. Just write the instructions that implement the algorithm, but not the data statements. [8] (f) Define the term concurrent processes. Explain how concurrent processes are implemented, and how the system ensures that all process continue to run even if one of them goes into an infinite loop. [

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