3201 Computer Networks (Final Exam) (Model Answer)

Transcription

1 El-Shorouk Academy Acad. Year : 2011/ 2012 Higher Institute for Computer & Term : Second Information Technology Year : 3 rd Department of Information System No. Questions : Computer Networks (Final Exam) (Model Answer) Question #1: a) A client-server system uses a Satellite network, with the satellite at height of 40,000 Km. Calculate the best-case delay in response to a Request? (Note: Light speed C=300,000 km/sec.) Satellite delay: Time = The request has to go up and come down, and the response has to go up and come down. The total path length traversed is thus (40,000 *4 = 160,000 km). The speed of light in air and vacuum is 300,000 km/sec, so the propagation delay alone is 160,000/300,000 sec or about 533 msec. b) Which of the OSI layers handles each of the following Dividing the transmitted bit stream into frames. Determining which route through the subnet to use. Data Link Layer Network Layer c) List two ways in which the OSI reference model and TCP/IP reference model are the same. Now list two ways in which they differ. 1

2 Similarities between TCP/IP and OSI reference Model: Both models are based on layered protocols Both have a network, transport and application layer In both models, the transport service can provide a reliable end-to-end byte stream On the other hand, they differ in several ways: The numbers of layers are different TCP/IP does not have session or presentation layers OSI does not support internetworking and OSI has both connection oriented and connectionless service in the network layer d) What is the essential difference between message switching and packet switching? Packet Switching: packets are stored and forwarded from primary storage (RAM) has a maximum packet size. Any message longer than that is split up into multiple packets. Message switching: message are stored and relayed from secondary storage (disk). a node receives a message stores it until the appropriate route is free, then sends it along. sends data units that can be of any length 2

3 Question #2: In the IT environments you might have a network engineer role so what is the best solution for the following company requirements: 20 employees should have workstations to work on. You should provide the network users with a printing network device. 2 manager should have an internet connection Company work nature is heavy printing environment. Workers should work at the same time on some kind of projects so that is better to being a server based network. a) Draw a conceptual design for that small size network environment. b) Write in details the recommended specification of the network server and show its type. c) Write down the network architecture and briefly describe the medium used to connect clients together. d) If they need to secure the network against external threats which kind of network security devices should be added to that network and why? a) Internet Client1 Client2 Client3 Client20... router switch Manager1 Manager2 Printer Server 3

4 b) Server type File and Print Server Server specification : CPU any RAM 4 GB HD 200 GB Network 1/10 Gbit Ethernet c) Network architecture 1 Gigabit Ethernet or 10BaseTX Medium Twisted Pair (UTP) d) Firewall or Intrusion Detection system (IDS) Because they provide higher level of security 4

5 Question #3: a) The following Character encoding is used in a data link protocol: A: ; B: ; FLAG: ; ESC: Show the bit sequence transmitted (In Binary) for the four character frame: A B ESC FLAG when each of the following framing methods are used: 1. Character Count 2. Flag bytes with byte stuffing. 3. Starting and ending flag bytes, with bit stuffing. o Character count o Flag bytes with byte stuffing Flag A B Esc (byte stuffed) Esc Esc (byte stuffed) flag Flag o Starting and ending flag bytes, with bit stuffing flag A B Esc flag Flag b) Write a pseudo code for the piggybacking protocol. S1 : send seq =0 Rec seq = 0 S2 : From host (Buffer) S3 : generate Frame to be sent having S4 : F(S) S.Info= Buffer S.seq=sendseq S.ack=1- recseq S5 : start timer (S.seq) 5

6 S6 : wait ( event ={ Frame arrival ChkSumErr Timeout}) S7 : event <> frame Arrival S12 S4 S8 : get F( r ) S9 : if r.seq= rec.seq then ( tohost ( r.info); rec.seq=rec.seq ) S10 : if r.ack=send.seq then ( fromhost ( Buffer); sendseq=sendseq ) S11 : S.info = buffer S.seq=send.seq S13 : starttimer ( s.seq ) S14 : goto S6 c) Write a part of a normal scenario where you have two terminals A and B interchange packets between them and A starts first, stop when a gets 4 successful packets. A sends (0, 1, A0) A gets(0, 0, B0)* A sends(1, 0, A1) A gets(1, 1, B1)* A sends(0, 1, A2) A gets(0, 0, B2)* A sends(1, 0, A3) A gets(1, 1, B3)* B gets(0,1, A0)* B sends (0, 0, Bo) B gets(1, 0, A1)* B sends(1, 1, B1) B gets(0, 1, A2)* B sends (0, 0, B2) B gets(1, 0, A3)* B sends (1, 1, B3) 6

7 d) Write the abnormal scenario where both sides A & B simultaneously send an initial packet. Stop after each side gets three successful packets. A gets(0,1,b2) A sends(0,1,a2) B sends(1,0,b3) B gets(0,1,a2) 7

8 Question #4 write down the law to determine the Maximum bit rate to transmit binary data for noiseless and noise channels. For noiseless channel Maximum data rate = For noisy channel Maximum number of bits/sec = ) a) Calculate the maximum bit rate for a noiseless 6-KHz channel and a noise 3-KHz channel with a signal to noise ratio of 30 db. Maximum bit rate for a noiseless 6-KHz channel: for 6KHz then for noiseless channel using Shannon theorem, sampling rate will be 6K samples/sec. So with 2 bit encoding, 2 bits are sent per sample. So the data rates is samples / sec = (2 * 6000 ) = 12000bps. Maximum bit rate for a noisy 3-KHz channel: for 3KHz then for noisy channel using Shannon theorem, sampling rate will be 3K samples/sec. SNR (db) = 10 log10(s/n) 30 db = 10 log10(s/n) 3 = log10(s/n) 10 3 = (S/N) = 1000 Maximum bit rate= 3 KHz log2( ) = 3 * log2(1001) = 3000 (ln (1001)/ln(2))= 29,901 bps 30,000 bps 8

9 a) Explain the design issues taken into consideration during the design of physical and data link layers. Physical layer: The design issues must take in consideration: Sending 1 bit must be received 1 not 0. How many volts to represent one & zero 5 volt represent 1 for positive logic. How many microseconds or nanoseconds a bit lasts. How many pins the network connector has and what each pin is used for. Data link layer: The design issues must take in consideration: How bits of physical layer are grouped into frames Attaching special bit patterns to the beginning & the end of the frame when creating and recognizing frames boundaries. Problem of flooding slow receivers from fast senders must be taken into consideration. Problem of transmission on both directions must be solved. Dealing with transmission errors b) Modulation is used to let signals travel far away, explain this statement. Explain the different modulation techniques. Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals. A continuous tone in the 1000 to 2000-Hz range, called a sine wave carrier, is introduced. Its amplitude, frequency, or phase can be modulated to transmit information Amplitude modulation: two different amplitudes are used to represent 0 and 1, respectively. Frequency modulation: also known as frequency shift keying, two (or more) different tones are used. Phase modulation: the carrier wave is systematically shifted 0 or 180 degrees at uniformly spaced intervals. 9

10 c) What is meant by TDMA and FDMA? Explain each of them and write their delay equations. TDMA: Time Division with Multiple Access Time is divisible into slots of time. Each user is assigned a fixed number of predefined channel time slots. The user has access to entire channel bandwidth but only during its allocated slots. FDMA: Frequency Division with Multiple Access Bandwidth is divided by the number of users. Each user is assigned fraction where by the assigned buffer it can be seen that the system can be modeled as sub channel, each of them viewed as an queuing system with a constant service time for the packet transmission time for each sub channel. 10

11 Question #5: a) Define the term, a, S, G, and T and write the equation to calculate and a. end to end propagation delay the amount of time it takes for the head of the signal to travel from the sender to the receiver. a: normalized propagation delay S: throughput the amount of data moved successfully from one place to another in a given time period. Or the average rate of successful message delivery over a communication channel. G: input rate the amount of data transmitted in a given time period T: time of packet transmission the time the packet takes to be transmitted from source to destination : end to end propagation delay, = ( ) a : normalized end to end propagation delay, a= T: time of packet transmission, T = ( ) b) Define the term Channel capacity. Draw the channel capacity for PURE ALOHA, SLOTTED ALOHA, and CSMA/CD. Channel capacity: the maximum data rate that can be attained over a given channel. (maximum Throughput) 11

12 c) Explain the protocol CSMA/CD and draw its S versus G, also draw its delay versus input rate G. CSMA / CD Protocol: if two stations sense the channel to be idle and begin transmitting simultaneously, they will both detect the collision almost immediately. Rather than finish transmitting their frames, which are irretrievably garbled anyway, they should abruptly stop transmitting as soon as the collision is detected. Quickly terminating damaged frames saves time and bandwidth. At the point marked t0, a station has finished transmitting its frame. Any other station having a frame to send may now attempt to do so. If two or more stations decide to transmit simultaneously, there will be a collision. Collisions can be detected by looking at the power or pulse width of the received signal and comparing it to the transmitted signal. After a station detects a collision, it aborts its transmission, waits a random period of time, and then tries again, assuming that no other station has started transmitting in the meantime. 12

13 Delay d) Calculate the vulnerable period in each of the following protocols PURE ALOHA, SLOTTED ALOHA, CSMA, and CSMA/CD (write this value in terms of T and a). Vulnerable period of: Pure ALOHA = 2T Slotted ALOHA = T CSMA = T+2 CSMA/CD = 2 13