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Answers of Exercise 4 1. Prove that the total number of links in a point-to-point network with N computers is (N2-N)/2 Proof: 1st computer 2nd computer … (N-1)th computer Nth computer links N-1 N-2 1 0 2 Total = (N-1) + (N-2) + … +2+1= (N - N)/2 2. For each of bus, star and ring topologies, discuss the consequences if a station has broken. Answer: Ring topology can’t work if a station is broken since it can not pass data to the next. No affect to bus and star topologies. 3. In most technologies, a sending station can choose the amount of data in a frame, but the frame header is a fixed size. Calculate the percentage of bits in a frame devoted to the header, trailer and preamble for the largest and smallest Ethernet frame. Answer: Ethernet frame Preamble + Header + trailer = 8+6+6+2+4 = 26 Bytes For the largest frame: 26/(26+1500) = 1.7% For the smallest frame: 26/(26+46) = 36.1% Answers of Exercise 4 4. Assume a one megabyte file must be send from one computer to another (both are connected to a same 10Base-T Ethernet). What is the minimum time to send the file across the network? What is the minimum time across a Fast Ethernet? Across a gigabit Ethernet? Answer: 1 Mbytes 1024X1024=1048576bytes The maximum data can be sent in one Ethernet packet is 1500 bytes. 1048576bytes/1500 = 698 (1500_byte_packets) + 1 (576_byte_packet) Therefore at least 699 packets are needed to send the 1 Mbytes file. The length of preamble, header and trailer in an Ethernet packet is 26 bytes. Total bits to be sent for the file are 1048576 + 699x26 = 1066650 bytes = 8.533 X 106 bits 10Base-T: 0.853 sec, Fast Ethernet: 0.0853 sec, Gigabit Ethernet: 0.00853 sec 5. Give the definitions and examples of unicast, broadcast and multicast, respectively. Answer: Unicast: sending data to a single destination. Examples: modem, telephone, accessing a Web site, sending an email to an individual Broadcast: sending data to all destinations. Examples: radio and TV broadcast, email from network administrator to all users Multicast: sending data to partial destinations. Examples: news group, audiovisual conferencing