COMPUTER NETWORKS REVIEW QUESTIONS AND PROBLEMS

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1 COMPUTER NETWORKS REVIEW QUESTIONS AND PROBLEMS. INTRODUCTION TO COMPUTER NETWORKS. What is a computer network? Briefly define..2 Briefly define the client-server model..3 What is scalability? Briefly define..4 Classify computer networks with respect to their transmission technologies..5 Classify computer networks with respect to their physical sizes..6 Write the names of LAN topology types and draw their figures..7 Draw the bus topology types and write their names..8 Briefly explain the access methods used in broadcast networks. (BA-İ-4/ /AS/2a).9 Write the type (broadcast or point-to-point) and topology (bus, ring or star) of computer networks that use terminating resistors. What does proper termination mean? What problem is encountered if proper termination is not provided? (CSE-5/3.0.05/FS/).0 Briefly explain why the transmission time of a frame in LANs using CSMA/CD protocol should be greater than the round-trip propagation time (2τ).. Consider building a CSMA/CD network running at 0 Mbps with a network cable of 2500 meter (consisting of five network segments each having a length of 500 meter and connected by using four repeaters). Considering the delays in the repeaters, the average signal speed in the network cable is m/μsec. Calculate the minimum frame length in bytes, so that when a collision occurs, computers can determine that the collision is related to the frame being sent ( byte=8 bits). Answer: 64 bytes. (BA-EE-/ /AS/4).2 Briefly answer the following questions about WANs: a) Specify the physical sizes of WANs. b) Write the name of the transmission method used in WANs. c) Write the name of the switching technique that is generally used in WANs. d) What are the components of a subnet? Write their names. e) What is the difference between a WAN and a subnet? f) Write the name of the first network that used the TCP/IP protocol. (e,f: BA-İ-4/ /BS/d).3 (Extra Qestion) Consider building a CSMA/CD network running at Gbps with a frame size of 625 bytes. The signal speed in the network cable is km/sec. No repeaters are used in the network. What is the maximum cable length? (Answer: 500 m). (CSE-2/ /MT/4).4 (Extra Question) Briefly answer the following questions about terminating resistor. a) Which type (transmission type) and which topology of computer networks use terminating resistors? b) How is the value of the terminating resistor chosen for proper termination? c) What problem is encountered if proper termination is not provided? /8

2 2. BASICS OF NETWORK ARCHITECTURE 2. What is network architecture? Briefly define. (BA-Ö-22/ /ASM/a) 2.2 Briefly define protocol and peer protocol. Write the names of the peer protocols in the OSI reference model. (BA-Ö-/ /AS/2b) 2.3 What is service data unit (SDU)? Briefly define. (BA-Ö-23/ /FS/e) 2.4 What is protocol data unit (PDU)? Briefly define. (BA-Ö-22/ /ASM/b) 2.5 Briefly define the connection-oriented service and the connectionless service ; write their advantages and disadvantages. (BA-Ö-/ /AS/b) 2.6 By using only three of the proper terms among Connection-oriented, connectionless, acknowledged, unacknowledged, communication channel, circuit switching service and packet switching service, define the following: a) Virtual circuit d) Datagram service (BA-İ-4/ /BS/c) 2.7 Write the names of the service primitives, and explain the transfer of these primitives by drawing a figure. (BA-Ö-/ /AS/2a) 2.8 Which service primitives are used in confirmed and unconfirmed services? Write their names. (BA-Ö-4/ /AS/b) 2.9 Which service type (confirmed service/unconfirmed service/confirmed or unconfirmed service/none of these services) can be used for each of the following phases of a connection-oriented communication service? a) Circuit establishment phase b) Data transfer phase c) Circuit release (termination) phase (BA-İ-5/ /AS/b) 2.0 Briefly explain why acknowledged datagram service is not used real-time digital voice or digital video communication. (BA-İ-5/ /AS/c) 2. Draw the OSI and TCP/IP models side by side, and write the layer numbers and names. 2.2 Write the name of the 2 nd layer of the OSI reference model and write six of its tasks. (BA-Ö-3/ /BS/a) 2.3 Write the name of the 3 rd layer of the OSI reference model and write its tasks. (BA-Ö-/ /AS/a) 2.4 Network devices generally functions at the lower three layer of the OSI reference model. Briefly explain the reason. 2.5 Write three advantages of network standardization and mention about two aspects that can be seen as disadvantages of network standardization. (BA-Ö-6/ /AS/2a) 2.6 Briefly answer the following questions about ITU-T: a) Write the full name of ITU-T and its field of interest. b) Write the classes of ITU-T s memberships. c) Is it a must for the manufacturers to comply with the ITU-T s recommendations? What can be the consequences if these recommendations are not fulfilled? (BA-Ö-2/ /AS/3) 2.7 Write the full name and memberships of ISO. (BA-İ-2/ /3a) 2/8

3 3. BASICS OF DATA TRANSMISSION 3. The binary signal at the output of a computer is shown in Figure 3.(a). A character consisting of eight bits is transmitted in 00 µsec. The frequency characteristics of the transmission medium and the amplitude spectrum of the transmitted binary signal are given in Figure 3. (b) and (c) respectively. Using the amplitude and frequency values given in Figure 3., a) Find the fundamental frequency ( st harmonic frequency) of the binary signal. b) Find the bandwidth (B t ) of the transmitted binary signal by considering the frequency components with amplitudes greater than or equal to /0 of the amplitude of the st harmonic. c) Find the transmission rate (R) of the binary signal. d) Find the 3-dB bandwidth of the transmission medium. e) Find the bandwidth (B r ) of the received signal. f) Which harmonic is the highest frequency component of the binary signal arrived at the receiver? f(t) H(f) 3 db T = 00 μs t (μs) f(khz) (a) Transmitted signal. (b) Characteristics of the transmission medium. C n f (khz) (c) Amplitude spectrum of the transmitted signal. Figure 3. (BA-Ö-2/ /FS/) Answer: (a) 0 khz; (b) 90 khz; (c) 80 kbps; (d) 60 khz; (e) 50 khz; (f) 5 th harmonic. 3.2 A digital communication is performed by using a four-symbol alphabet (a four-symbol signal). Each symbol is transmitted in symbol duration of 0 µsec. The probabilities the symbols to be sent are given as follows: P =/2 ; P 2 =/4 ; P 3 =P 4 =/8 a) Find the symbol rate of the signal. b) Find the average information content of the signal per symbol interval. c) Find the minimum bandwidth and minimum channel capacity required for the transmission medium in order to send this message with small probability of error. d) Consider that, by using a binary encoder, the four-symbol signal is converted into a binary signal and transmitted as a binary signal. Repeat part (c) for this case. M Auxiliary information: H = Pk log 2 ( ) bits/interval k = Pk (BA-Ö-2/ /AS/4) Answer: (a) symbols/sec; (b).75 bits/symbols; (c) 00 khz, 75 kbps; (d) 200 khz, 75 kbps. 3.3 In a facsimile transmission, 420,000 picture elements are needed for one page to provide proper picture resolution. 6 brightness levels are specified for each picture element for good reproduction. Consider that the modem circuit of the facsimile machine operates at the rate of 9600 bps. a) Find the maximum information content of one page of fax message. b) How long does it take to send one page of fax message? (BA-EE-/ /AS/4) Answer: (a) bits/page; (b) 2.92 minutes (or 2 minutes 55 seconds). 3/8

4 3.4 An analog voice signal band limited to 4 khz is sampled at Nyquist rate and then quantized at 256 levels. a) Find the sampling rate f s and the sampling interval T s. b) Assuming the equally likely case (each level appears with equal probability), find the information content (H) per interval (sample interval). c) The output of the 256-level quantizer is applied to a binary encoder to obtain a PCM signal. Find the information rate of the PCM signal. d) What must be the minimum value of the channel capacity in order to transmit the PCM signal with arbitrarily small frequency of errors? (BA-Ö-4/ /.AS/3) Answer: (a) 8 khz, 25 µsec; (b) 8 bits/interval; (c) 64 kbps; (d) 64 kbps. 3.5 In a digital communication system a 32 symbol signal is used in transmission. Each symbol is transmitted in a 5 μsec time slot with equal probability. a) Calculate the symbol rate of the 32 symbol signal. b) Calculate the information content of a symbol. c) Calculate the required bandwidth (in khz) and the capacity (in kbps) of the transmission medium to transmit the 32 symbol signal. d) Consider that the 32 symbol signal is applied to a binary encoder and a binary signal is obtained at the output of the encoder. Calculate the required bandwidth (in khz) and the capacity (in kbps) of the transmission medium to transmit the binary signal. (BA-Ö-3/ /BS/2) Answer: (a) symbols/sec; (b) 5 bits/symbol; (c) 200 khz, Mbps; (d) MHz, Mbps. 3.6 In a digital communication system a 64 symbol signal is used in transmission. Each symbol is transmitted in a 50 µsec time slot with equal probability. a) Calculate the symbol rate of the signal. b) Calculate the information content of a symbol. c) Calculate the required bandwidth (in khz) and the capacity (in kbps) of the transmission medium to transmit the signal. (BA-Ö-7/ /FS/2) Answer: (a) symbols/sec; (b) 6 bits/symbol; (c) 20 khz, 20 kbps. 3.7 Compare serial and parallel transmission with respect to the following attributes: ) Transmission speed (fast/slow) 2) Length of transmission cable (long/short) 3) Cost of transmission cable (high/low) 4) Cost of transmission hardware (high/low) (BA-İ-3/ /FS/e) 3.8 (Extra Question): Briefly compare the synchronous and asynchronous transfer modes in the following aspects: a) Which one is suitable for constant bandwidth channels and which one for variable (or dynamic) bandwidth channels? b) Which one is subjected to waste of bandwidth and which one uses the bandwidth efficiently? (CN/S-2/ /FS/d) 3.9 (Extra Question): A channel is given with a bandwidth of 30 khz and a Signal-to-Noise Ratio (SNR) of 40 db. For error free transmission the Shannon s channel capacity formula is known as C=Blog 2 (+SNR). Calculate the channel capacity. (CSE-2/ /MTE/3) Answer: 400 kbps 4/8

5 3.0 (Extra Question): A signal band-limited to 3.3 khz is sampled. a) Find the Nyquist sampling rate and the Nyquist sampling interval. b) Find the minimum sampling rate and sampling interval when a guard band of.4-khz is used in the sampling. (CSE-/22..04/MT/3) Answer: (a) 6.6 khz; 5.5 μsec; (b) 8 khz; 25 μsec. 4. DATA COMMUNICATION SERVICES 4. For each of below items, specify which feature belongs to SMDS (Switched Multimegabit Data Service). ) Connection-oriented / Connectionless. 2) Circuit-switched / Packet-switched. 3) Is SMDS is suitable for busty traffic? Yes or no. 4) Is the mean transmission rate is greater than the burst mode rate? Yes or no. 5) Is SMDS is cheaper than leased lines? Yes or no. 6) Can subscribers access SMDS over MANs? Yes or no. 7) Can subscribers access SMDS over leased lines? Yes or no. 8) Does SMDS have multicasting feature? Yes or no. 9) Does SMDS use permanent virtual circuits? Yes or no. 0) If an SMDS subscriber has 500 byte credit in his counter and he sends a 2000-byte packet, does SMDS discard the whole packet? Yes or no. (BA-Ö-2/ /AS/2) 4.2 In an SMDS network with a channel capacity of 45 Mbps, consider a subscriber with a counter increment rate of one byte in 0 μsec. a) For continuous data transmission, a) Calculate the average data transmission rate, a2) Calculate the transmission time of a 5625-byte packet. b) Consider that the subscriber doesn t send any data for a while, and thus 5625-byte credit is accumulated in its counter. At this moment the subscriber sends a 5625-byte packet. b) Calculate the transmission time of the packet that is sent in burst mode. b2) Compare the results obtained in (a) and (b). How many times transmission rate is increased when the packet is sent in burst mode. b3) How many bytes of credit are left on the counter just after the packet is sent? (BA-Ö-6/ /.AS/3) Answer: (a) 800 kbps; (a2) msec; (b) msec; (b2) times; (b3) Zero credit is left. 4.3 In an SMDS network with a channel capacity of 45 Mbps, consider a subscriber with a counterincrement rate of bytes/sec. a) In order to transmit a 5000-byte data block in burst mode, how long the subscriber should stay in idle mode? If the subscriber sends a 5000-byte packet before the end of this duration, what operation does the router make on this packet? b) If the subscriber does not send data for 45 msec, how many bytes of data is the subscriber allowed to send in burst mode? How long does it take to send this data in burst mode? (BA-Ö-22/ /ASM/3) Answer: (a) 00 msec; packet is discarded; (b) 2250 byte; 400 µsec. 4.4 Compare Narrowband-ISDN and Broadband-ISDN for the following features: a) Transmission capacity (give values in terms of kbps and Mbps), b) Switching type (circuit-switching, packet-switching), c) Transmission type (synchronous, asynchronous). (BA-Ö-23/ /FS/d) 5/8

6 4.5 For the basic access ISDN, a) Write the names and the capacities of the channels used for communication and for signaling. b) At the maximum, how many ISDN terminal equipments can be connected to the S interface? How many of them can make communication at the same time? Specify the channels for the type of communication service (telephone, telefax, PC, short message, etc). (BA-Ö-5/ /2.AS/2c) 4.6 Briefly define bearer services, teleservices and supplementary services in ISDN. (BA-Ö-/ AS/4) 4.7 Briefly answer the following questions about ISDN: a) For a basic access ISDN subscriber, draw a block diagram showing the access of ISDN terminal units and non-isdn terminal units to the ISDN exchange. Show the S and U interfaces on your block diagram. Write the number of wires used at these interfaces. b) For a basic access ISDN subscriber, what is the maximum access rate (in kbps) of a PC with an ISDN communication card (ISDN modem) to the Internet? Which ISDN channels are used for this access? c) Which layers of OSI reference model is included in an ISDN in the network? (BA-Ö-0-/ /BS/2) 4.8 In a basic access ISDN, a data frame in the data link layer contains 6 bits for B channel, 6 bits for B2 channel, 4 bits for D channel and 2 bits for framing and control. The time required to transmit one frame is 250 μs. By using these figures, a) Calculate the channel capacities of B, B2 and D channels in terms of kbps. Show how they are calculated. Don t write the results by heart. b) Calculate the transmission rate (in terms of kbps) and bandwidth (in term of khz) of this signal. c) This binary signal is applied to an M-ary encoder with M=8 (that is, three successive bits are encoded as one symbol). Find the symbol rate and the bandwidth of the signal at the output of the M-ary encoder. (BA-İ-3/ BS/2) Answer: (a) 64 kbps for B and B2; 6 kbps for D; (b) 92 kbps, 92 khz; (c) 64 ksymbols/sec, 64 khz. 4.9 A basic access ISDN subscriber has the following equipment: 3 ISDN telephone sets ISDN fax machine PCs (Personal Computers) with ISDN card installed 3 non-isdn Terminal Equipment (non-isdn telephone or non-isdn data terminal) 3 ISDN Terminal Adaptors (TA) Network Termination Unit of type (NT) Draw a block diagram showing the connection of this equipment to an ISDN exchange. Show the reference points R, T (or S) and U on the block diagram. (BA-Ö-6/ /.AS/4a) 4.0 Define briefly adaptation and internetworking in ISDN. (BB-İ-2/ /AS/2b) 4. Briefly answer the following questions about the Frame Relay (FR) technique. a) Define the FR technique by specifying its service type and in important feature. b) Compare an FR network with an X.25 network from the aspects of error detection, error correction and performance. c) Draw a block diagram showing the access of a PC to a FR network. (BA-Ö-9/ /FS/) 6/8

7 4.2 Which of the following properties belongs to a Frame Relay service? Specify for each item. ) Acknowledged / Unacknowledged. 2) Connected-oriented / Connectionless. 3) Circuit-switched / Packet-switched. 4) Slow / Fast. 5) Virtual circuits are used / Virtual circuits are not used. 6) Composed of OSI layers and 2 / Composed of OSI layers, 2 and 3. 7) Performs frame routing in the network / Performs packet routing in the network. 8) The network performs error detection (at NNI) / The network does not perform error detection (at NNI). 9) The terminal equipments perform only error detection (at UNI) / The terminal equipments perform both error detection and error correction (at UNI). 0) Cheaper than leased lines / More expensive than leased lines. (BA-EE-/ /AS/4) 4.3 Briefly answer the following questions about ATM (Asynchronous Transfer Mode) technique. a) Describe the ATM technique in one sentence by specifying the following properties: ) Service type: Connection-oriented / Connectionless. 2) Transmission type: Synchronous / Asynchronous. 3) Transmission speed: Low speed / High speed. 4) Switching type: Circuit switched / Packet switched. b) Draw an ATM cell showing the header and the data field and their lengths in terms of bytes. (BA-Ö-4/ /.AS/2) 4.4 (Extra question): Consider an SMDS (Switched Multimegabit Data Service) network operating at 00 Mbps. The counter of an SMDS subscriber, which resides in the router to which the subscriber is connected, increments one byte in 0 μsec. a) If the subscriber transmits data continuously, () Calculate the average data transmission rate. (2) Calculate the required time to transmit a 4000-byte packet. b) Suppose that the line is idle for a period of time, and the user sends a data of 4000 bytes at the end of this period. Calculate the transmission time for the 4000-byte data in burst mode. Compare the results obtained in (a) and (b). How many times the burst mode transmission is faster than the continuous mode transmission? (BA-Ö-4/ /.AS/4) Answer: (a) 800 kbps; (a2) 40 msec; (b) 320 μsec; 25 times faster. 4.5 (Extra Question): An SMDS (Switched Multimegabit Data Service) network operating at 45 Mbps is used for LAN-to- LAN traffic. A subscriber of this SMDS network having a contract for an average transmission rate of bytes/sec wants to send data. If the line is idle for a period, a credit (in terms of bytes) will be accumulated on the subscribers counter at the router and the subscriber will be allowed to send an amount of data equal to or less than the counter reading in burst mode. a) Calculate the transmission time required to transfer a 4500 byte data to the transmission medium for continuous transmission case. b) Suppose that the line is idle for a period of 500 msec, and the user sends a data of 4500 bytes at the end of this period. Calculate the transmission time for the 4500 byte data in burst mode. c) Compare the results obtained in (a) and (b). How many times the burst mode transmission is faster than the continuous mode transmission? d) Briefly explain what happens if the user sends a data of 5500 bytes at the end of a 500 msec idle period. (CN/S-3/ /MT/4) Answer: (a) 450 msec; 0.8 msec; (c) times faster; (d) Data is discarded by the router. 7/8

8 4.6 (Extra Question): Which of the following teleservices may be offered over the B, H and D channels in narrowband ISDN? Specify. () Packet data communication (64 kbps-2 Mbps) (8) Alarms (2) Teleconference (9) Facsimile (3) Packet data communication (at 9.6 kbps) (0) Packet data communication (at 64 kbps) (4) Telemetering () Signaling (5) Videophone (2) User info (short user messages) (6) Videoconference (3) Remote control (7) Telephone (CSE-3/ /2MT/2) 4.7 (Extra Question) Which of the following properties belongs to an ATM (Asynchronous Transfer Mode) service? Specify for each item. () Service type: Connection-oriented service / Connectionless service / Both. (2) Switching type: Circuit switching / Packet switching / Both. (3) Transmission rate is suitable for: Only wide-band data transmission / Only narrow-band data transmission / Both. (4) Cell length: Constant / Variable depending on transmission rate / Variable depending on the length of data field. (5) Error correction on the data field is performed by: The ATM layer / The TC (Transmission Convergence) sublayer / The upper layers. (6) Header error control is performed by: The ATM layer / The TC sublayer / The AAL (ATM Adaptation Layer). (7) Congestion control is performed by: The AAL / The ATM layer / The upper layers. (8) Flow control is performed by: The AAL / The ATM layer / The TC sublayer. (9) Virtual path and virtual channel management is performed by: The ATM layer / The AAL / The TC sublayer. (0) Cell rate adaptation is performed by: The CS (Convergence Sublayer) / The ATM layer / The TC sublayer. (CSE-3/ /2MT/3) 5. PHYSICAL LAYER 5. Briefly answer the following questions about the RS-232-C interface. a) Draw a block diagram showing the place of RS-232-C interface in the communication of two computers over the telephone network. Show the DTE and DCE units on your block diagram and briefly define these units. b) For a 25-pin RS-232-C interface, specify which unit (DTE or DCE) activates the following pins: Pin Nr. Pin Name Abbreviation 20 Data Terminal Ready DTR 6 Data Set Ready DSR 4 Request to Send RTS 5 Clear to Send CTS 22 Ring Indicator RI c) Compare the balanced and unbalanced transmission methods with regard to the type of signal grounds and performances of the interfaces. (BA-İ-9/ /AS/2) 5.2 What is null modem? Briefly define. Between which units the null modem is used? Which type of transmission (series or parallel; synchronous or asynchronous) does it use? Which type of connector does it use? 5.3 (Extra question) Put into order the following transmission mediums with regard to increasing probability of creating transmission errors: Coaxial cable, fiber optic cable, the atmosphere (for radio waves), twisted pair cable. 8/8

9 6. DATA LINK LAYER 6. In a data link layer, character-oriented framing and character stuffing method is applied. In this method, FLAG characters are used as the starting and ending delimiter. a) At the sender, if the character string delivered by the network layer to the data link layer is given as follows, obtain the character string at the data field of the sender s data link layer frame. A B FLAG ESC C ESC D E F FLAG G b) At the receiver, if the character string at the data field of the receiver s data link layer frame is given as follows, obtain the character string that will be delivered by the data link layer to the network layer. ESC FLAG A ESC ESC B C ESC FLAG ESC ESC Caution: Note that (a) and (b) are independent. 6.2 In the data link layer of a computer network, bit-oriented framing and bit stuffing method is applied. Assuming that 00 bit string is used as the starting and ending delimiter, a) At the sender, if the bit string delivered by the network layer to the data link layer is given as follows, obtain the bit string at the data field of the sender s data link layer frame b) At the receiver, if the bit string delivered by layer to layer 2 is as follows, obtain the bit string before bit stuffing Caution: Note that (a) and (b) are independent. (BA-Ö-/ /AS/3) 6.3 In the data link layer of a computer network, bit-oriented framing and bit stuffing method is applied. Assuming that 00 bit string is used as the starting and ending delimiter, a) At the sender, if the bit string delivered by the network layer to the data link layer is given as follows, obtain the bit string at the data field of the sender s data link layer frame b) At the receiver, if the bit string at the data field of the receiver s data link layer frame is given as follows, obtain the bit string that will be delivered by the data link layer to the network layer. Caution: Note that (a) and (b) are independent. (BA-Ö-8/ /2MS/2) Write the link utilization formula for the stop-and-wait protocol in terms of T f (frame transmission time) and T p (propagation time). Briefly define these parameters. (BA-İ-/ /FS/d) 6.5 Write the Link Utilization formula for the sliding window protocol with N frames per window in terms of T f (frame transmission time) and T p (propagation time). Briefly define these parameters. 6.6 Two computers are communicating over a km satellite link by using 4000-bit frames at a transmission rate of 00 kbps. Assuming errorless transmission, and taking the signal speed as m/sec, a) Calculate the time required to transmit a frame (time for the transmitter to send out all of the bits of the frame). b) Calculate the link utilization for the stop-and-wait protocol. c) Calculate the window size (in terms of frames) for the sliding window protocol in order to have 00% link utilization? (BA-Ö-8/ /2MS/3) Answer: (a) 40 msec; (b) 6.25 %; 6 frames: 9/8

10 6.7 Two computers are communicating over a 0 km fiber optic link by using 500-byte frames at a transmission rate of 00 Mbps. The propagation speed as m/sec. Assuming errorless transmission, a) Calculate the transmission time required to transfer one frame to the transmission medium. b) Calculate the link utilization (U) for the stop-and-wait protocol. c) Calculate the link utilization (U) for the sliding window protocol for a window size of 7 frames. (BA-Ö-7/ /2.AS/3) Answer: (a) 5 μsec; (b) 4.76 %; (c) 33.3 %. 6.8 Two computers are communicating over a 3000 km fiber optic link by using 500-byte frames at a transmission rate of Mbps. The propagation speed as m/sec. Assuming errorless transmission, a) Calculate the transmission time required to transfer one frame to the transmission medium. b) Calculate the link utilization (U) for the stop-and-wait protocol. c) Calculate the window size (in terms of frames) for the sliding window protocol in order to have 00% link utilization? (BA-Ö-5/ /2AS/3) Answer: (a).5 msec; (b) 4.76 %; (c) 2 frames. 6.9 Two computers are communicating over a km satellite link by using 920-bit frames at a transmission rate of 64 kbps. The propagation speed as m/sec. Assuming that no error occurs in transmission, a) Calculate the transmission time required to transfer one frame to the transmission medium. b) Calculate link utilization (U) for stop-and-wait protocol. c) Calculate the window size for the sliding window protocol in order to have %00 link utilization. (BA-Ö-23/ /FS/2) Answer: (a) 30 msec; (b) 7. %; (c) 4 frames. 6.0 In the figure shown below, frames are generated at node A and sent to node C through node B. Determine the minimum transmission rate required between nodes B and C so that the buffers of B are not flooded, based on the following assumptions: Assumptions:. The data rate between A and B is 00 kbps. 2. The propagation delay is 0 μsec/km for both lines. 3. There are full-duplex lines between the nodes. 4. All data frames are 000 bits long; ACK frames are separate frames of negligible length. 5. Between A and B, a sliding window protocol with a window size of 5 is used. 6. Between B and C, stop-and-wait is used. 7. There are no transmission errors. A 2000 km 250 km B C Hint: In order not to flood the buffers of B, the average number of frames entering and leaving B must be the same over a long interval. (BA-Ö-9/ /FS/4) Answer: R BC = 200 kbps. 0/8

11 6. Go-back-N sliding window protocol is used between the data link layers of machines A and B with a window size of 7 frames at both sides. The figure below shows the a0 and b0 states of the sliding windows of machines A and B, respectively, just before Frame 2 is sent by A. The same figure also shows the sequence numbers of the frames sent by A and acknowledgements sent by B. a) Indicate the frame numbers that should be contained in the sliding windows and buffer memories at the sender and receiver for the states a, b, a2, b2, a3, b3, a4, b4,a5, b5, and b6. (a0) (a) (a2) (a3) (a4) Machine A Machine B Ç2 Ç3 Ç4 Ç ACK Ç6 Ç NAK b) Write a formula showing the relation between the window size (N, in terms of number of frames) and the size of the sequence number field (k, in terms of bits). Answer: (a) Content of the following table must be shown as a figure; (b) N=2 k -. (b0) (b) (b2) (b3) (b4) Machine A Machine B State Place of the Buffer Line Frames in the Sliding Window State Place of the Buffer Line Frames in the Sliding Window a Between -2 6, 7, 0 b Between -2 6, 7, 0 a2 Between 5-6 6, 7, 0,, 2, 3, 4 b2 Between 5-6 6, 7, 0,, 2, 3, 4 a3 Between 5-6 0,, 2, 3, 4 b3 Between 5-6 0,, 2, 3, 4 a4 Between 6-7 7, 0,, 2, 3, 4, 5 b4 Between 6-7 7, 0,, 2, 3, 4, 5 (BA-Ö-8/ /2.MS/4) 6.2 Sliding window protocol with window size of 7 frames are used between the data link layers of machines A and B. Assume that machine B sends an ACK (acknowledgement) for every frame it receives without error, and it sends a NAK (negative acknowledgement) for every frame it receives with error. This corresponds to a receiver window size of frame. Assume also that no frames are lost in the transmission medium. For the scenario shown in the figure on the right hand side, write the numbers of the data frames indicated by a, b, c, d, e, f, g, and write the numbers of the ACKs indicated by h, i, j, k, l, m, n, and also explain what kind of process does the receiver perform on the frames indicated by p for each of the following cases: a) For the Go-back-N Automatic Repeat Request method. b) For the Selective Reject Automatic Repeat Request method. (BA-İ-6/ /2.AS/4) A F0 F F2 F3 F4 F5 F6 a b c d e f g B ACK ACK 2 NAK 2 p h (ACK) i (ACK) j (ACK) k (ACK) l (ACK) m (ACK) n (ACK) Answer: (a) a: F2 h: ACK 3 (b) a: F2 h: ACK 7 p: Saved in the buffer memory and put into order. b: F3 i: ACK 4 b: F7 i: ACK 0 ACK 2, 3, 4, 5, 6 may not be sent. c: F4 j: ACK 5 c: F0 j: ACK d: F5 k: ACK 6 d: F k: ACK 2 e: F6 l: ACK 7 e: F2 l: ACK 3 f: F7 m: ACK 0 f: F3 m: ACK 4 g: F0 n: ACK g: F4 n: ACK 5 p: Discarded /8

12 6.3 Briefly define the piggybacking method used in Layer 2 (data link layer) protocol, and write its advantage. (BA-Ö-3/ /BS/b) 6.4 Briefly define the pipelining method used in Layer 2 (data link layer) protocol, and write its advantage. (BA-İ-/ /FS/a) 6.5 Briefly answer the following questions about the balanced configuration used in the HDLC protocol. () What name is given to the stations communicating in balanced mode? (2) How many stations can simultaneously communicate on the same link? (3) Which type (or types) of transmission is used among the following? Simplex, half-duplex, fullduplex. (4) Which type (or types) of connection are used among the following? Point-to-point, point-tomultipoint. (BA-Ö-23/ /FS/g) 6.6 In a data link connection where CRC (Cyclic Redundancy Check) is used, an 8-bit information is given as The generating function used in the CRC is G(x) = x 3 + x +. a) Calculate the CRC code. b) Write the T(x) polynomial corresponding to the bit string that will be sent, and show which terms belong to data and which terms belong to the CRC code. c) Write the bit string that will be sent, and show which bits belong to data and which bits belong to the CRC code. (BA-Ö-2/ /FS/2) Answer: Data bits CRC 6.7 In a data link connection where CRC (Cyclic Redundancy Check) is used, a 0-bit information is given as The generating function used in the CRC is G(x) = x 4 + x 3 + x +. a) Calculate the CRC code. b) Write the T(x) polynomial corresponding to the bit string that will be sent, and show which terms belong to data and which terms belong to the CRC code. c) Write the bit string that will be sent, and show which bits belong to data and which bits belong to the CRC code. (BA-Ö-7/ /FS/2AS/) Answer: Data bits CRC 6.8 In a communication using CRC (Cyclic Redundancy Check) error detection method, The information bits and CRC code arrived at the destination machine is given as follows: Information bits CRC code The generating function used in CRC is given as G(x) = x 4 + x 2 + x +. a) Perform the mathematical operation done by the data link layer of the destination machine and decide whether the received bit stream has error or not and give reason. b) Calculate the coding efficiency of the above coding. c) How many bits of burst error can be detected by the above given generating function? (BA-FB-2/ /FS/2) Answer: (a) Residue: R(x)=x Therefore there is error; (b) % 7.4; (c) N e =4. 2/8

13 6.9 The transmitted and received bit streams in a transmission with Hamming coding are given below. Assuming that there is only one bit error in transmission, show the mathematical procedure how the receiver decides that bit I 4 is incorrectly received. I I 2 I 3 I 4 C C 2 C 3 Transmitted bit stream : Received bit stream : (BA-İ-2/ /FS/2) Transmission error 6.20 In a computer network Hamming coding is used for error detection and correction. a) By using m control bits, one error bit can be corrected in a data block of n information bits. Express n in terms of m and calculate n for m=4. b) Write the control bits C, C 2, C 3, C 4 as functions of information bits (I s) by considering the combinations C s and modulo-2 addition. c) The data block of 6 information bits and four control bits sent by computer A has arrived at the computer B with one information bit changed in transmission. The bit stream consisting of the data block and the control bits received by computer B is as follows: I I 2 I 3 I 4 I 5 I 6 C C 2 C 3 C Assume that both the source machine and the destination machine use the following functions to calculate the control bits: C =I I 2 I 3 ; C 2 =I I 4 I 5 ; C 3 =I 2 I 4 I 6 ; C 4 =I 3 I 5 I 6 Find out which information bit is changed in transmission by assuming that control bits are not destroyed and only one information bit is changed. (BA-Ö-3/ /BS/4) 6.2 Answer the following questions related to Hamming coding. a) Calculate the maximum number information bits in a block of data that can be controlled by using Hamming code of five control bits (C, C 2, C 3, C 4, C 5 ) to correct one bit error? b) Obtain the mathematical expressions used in the calculation of the control bits (C, C 2, C 3, C 4, C 5 ) in terms of information bits (I m ). c) A code of 8 code words is given on the right-hand side. () Find the Hamming distance of the code and explain your method. (2) How many bits of error in a code word can be detected by this code? Calculate. (3) How many bits of error in a code word can be corrected by this code? Calculate. (BA-Ö-9/ /FS/3) Answer: (a) 26 data bits; (c) d=3; (c2) 2 bit; (c3) bit. Given code (Extra question) Go-back-N sliding window protocol is used between the data link layers of machines A and B with a window size of 7 frames at both sides. The figure below shows the a0 and b0 states of the sliding windows of machines A and B, respectively, just before Frame 0 is sent by A. The same figure also shows the sequence numbers of the frames sent by A and acknowledgements sent by B. a) Indicate the frame numbers that should be contained in the sliding windows and buffer memories at the sender and receiver for the states a, b, a2, b2, a3, b3, a4, and b4. 3/8

14 A Makinesi B Makinesi (a0) (a) (a2) (a3) (a4) Ç0 Ç Ç2 Ç3 Ç ACK Ç5 Ç6 Ç NAK (b0) (b) (b2) (b3) (b4) b) Write a formula showing the relation between the window size (N, in terms of number of frames) and the size of the sequence number field (k, in terms of bits). (BA-Ö-7/ /FS/3) Answer: (a) Content of the following table must be shown as a figure; (b) N=2 k -. Machine A Machine B State Place of the Buffer Line Frames in the Sliding Window State Place of the Buffer Line Frames in the Sliding Window a At the left of 0 5, 6 b At the left of 0 5, 6 a2 Between 4-5 5, 6, 7, 0,, 2, 3 b2 Between 4-5 5, 6, 7, 0,, 2, 3 a3 Between 4-5 0,, 2, 3 b3 Between 4-5 0,, 2, 3 a4 Between 5-6 6, 7, 0,, 2, 3 b4 Between 5-6 6, 7, 0,, 2, 3 7. MEDIUM ACCESS CONTROL SUBLAYER 7. Compare the delay of pure ALOHA versus slotted ALOHA at low and high traffic loads. Which method causes less delay for low and high traffic loads? 7.2 A group of N stations share a 56-kbps pure ALOHA channels. Each station outputs a 000-bit frame on an average of every 00 sec. Considering that the maximum efficiency of the pure ALOHA is 8.4 %, calculate the maximum number of N. Answer: 030 stations. 7.3 A -km long, 0-Mbps CSMA/CD LAN has propagation speed of 200 m/μsec. Data frames are 66 bytes long, including 26 bytes header, checksum and other overhead. The first bit slot after a successful transmission is reserved for the receiver to capture the channel to send a 32-bit acknowledgement frame. What is the effective data rate (TRIB: Transmission Rate of Information Bits), assuming that there are no collisions. Hint: Take the time required to capture the channel as 2τ. This is the round trip propagation time for the electrical signal. Answer: 3.72 Mbps 7.4 Eight stations with station numbers 0 through 7 use the basic bit map protocol. The figure below shows the bits transmitted by these stations in the contention slots. Write the number of the stations that send the frames a, b, c, d and e shown in the figure. Contention slots Frames sent Contention slots Frames sent a b c d e bit d bits 4/8

15 7.5 Four stations using the binary count down protocol are transmitting their addresses at the same time to get a shared channel. The addresses of the stations are 00, 0, 00 and 000. Explain the algorithm by indicating which station gives up in each bit interval and which stations continue to compete for the channel, and determine which station wins and transmits its frame. 7.6 Stations A, B, C, D, E, F, G, H, J and K using the binary count down protocol with virtual station numbers have the virtual station numbers 9, 8, 7, 6, 5, 4, 3, 2,, and 0 at a certain instant. The next three stations to send are E, C, and A, in that order. What are the new virtual station numbers after all three have finished their transmissions? Answer: 0, 9,, 8, 2, 7, 6, 5, 4, and stations (0 through 5) are using the adaptive tree walk protocol to access a shared channel. In the contention slot 0, stations 2, 3, 5, 7,, and 3 suddenly become ready at the same time to transmit frames, and start contending for the channel. a) Draw the binary tree defining the tree walk. b) Starting from the contention slot 0, determine which stations will compete under which node, and which station will transmit a frame. c) How many contention slots of one bit are needed to resolve contention? Answer: (c) contention slots. 8. NETWORK DEVICES 8. Briefly explain the operation principle of transparent bridges. (BA-Ö-5/ /2AS/a) 8.2 Briefly define the problem that may appear in the operation of transparent bridges during the flooding phase, and explain how we can eliminate this problem. (BA-Ö-9/ /FS/2b) 8.3 Answer the following questions about the spanning tree algorithm: () What type of bridges use this algorithm? (2) For which problem this algorithm is used, and in which phase? (3) How does this algorithm bring a solution for the problem? (BA-FB-2/ /FS/b) 8.4 Briefly explain the operation principle of source routing bridges. What is frame explosion that may appear in the operation of this type of bridges? (BA-İ-0/ /2AS/b) 8.5 Draw an illustrative block diagram that explains the operation principle of a translator bridge connected between an Ethernet LAN (CSMA/CD: IEEE 802.3) and a Token bus LAN (IEEE 802.4). Show the headers and trailers appended by each layer and sublayer to a packet sent by the network layer of computer A on the Ethernet LAN and addressed for the network layer of computer B on the Token Ring LAN. Show also the layers and sublayers that the packet passes while it travels from A to B. (BA-İ-7/ /FS/2) 8.6 Briefly define filtering rate and forwarding rate which are known as the performance criteria of bridges. ((BA-Ö-5/ /2AS/c) 8.7 Explain the differences between a router that gives connection-oriented service and a transparent bridge in the following aspects: () Layers they operate; (2) Addresses maintained in their tables; (3) Packets whose destination address are read; (4) Procedure they apply in forwarding the packets. (BA-Ö-9/ /FS/2a) 8.8 What is tunneling? Briefly explain and draw an illustrative figure. Specify the MAC addresses and network addresses of the frames on the LANs and those for the packets through the WAN. (BA-İ-3/ /FS/b) 5/8

16 8.9 In the computer network given below, delays between the routers are given in milliseconds. a) Apply Dijkstra s algorithm for Router A. In applying the algorithm, establish a table with the labels showing the minimum delay value and the corresponding path from Router A to all other routers for each iteration step. b) Briefly explain at which iteration and why the algorithm ends. c) Indicate which labels will be stored in the routing table of Router A. B 3 C A 3 E 6 2 F 2 2 D 2 3 G 4 H (AB-Ö-3/ /BS/3) Answer: (a) B(, A); C(4, A-B-C); D(5, A-E-D); E(3, A-E); F(3, A-G-F); G(2, A-G); H(5, A-B-C-H). 8.0 In the computer network given below, delays between the routers are given in milliseconds. a) Apply Dijkstra s algorithm for Router. In applying the algorithm, establish a table with the labels showing the minimum delay value and the corresponding path from Router to all other routers for each iteration step. b) Briefly explain at which iteration and why the algorithm ends. c) Indicate which labels will be stored in the routing table of Router (BA-Ö-7/ /FS/4) Answer: (a) D 2 (, -2); D 3 (3, ); D 4 (3, ); D 5 (2, -2-5); D 6 (5, ). 8. In the network given below, delays between the routers are given in milliseconds. a) Apply Bellman-Ford algorithm for the Router. In applying the algorithm, establish a table in which the maximum number of hops is started from zero and incremented by one for each iteration (h=0,, 2, 3, ). In the table you will establish, include the label values consisting of minimum delay and path definition from Router to other routers for each value of h. b) Briefly explain how the iteration ends. c) Indicate which labels will be stored in the routing table of Router at the end of the algorithm (BA-Ö-23/ /FS/3) Answer: (a) D 2 (2, -2); D 3 (3, ); D 4 (2, -5-4); D 5 (, -5); D 6 (2, -5-6). 6/8

17 8.2 Distance vector routing algorithm is used in the subnet shown below. The delay vectors that have just come in to router C from its neighbor routers and the measured delays to the neighbors of C are given below. Find the new routing table of C. Give both the outgoing line to use and the expected delay. Delay vectors received from C s neighbors: To B D F A 2 B C D E F G Delay measurements made by C: CB CD CF (BA-İ-3/ /BS/3) Answer: To New estimated Path delay from C A 4 Over B B 3 Directly to B C 0 - D 7 Directly to D E 6 Over D F 6 Over B G 2 Over B 8.3 Distance vector routing algorithm is used in the subnet shown below. The following delay vectors have just come in to router A: A, B, C, D, E, F From B: (6, 0, 5, 0, 6, 3) From C: (2, 7, 0, 3, 8, ) B C From D: (4, 3, 6, 0, 5, 7) The measured delays to B, C, and D are 3, 9, and 7 msec, A F respectively. Find the new routing table of A. Give both the outgoing line to use and the expected delay. D E (BA-Ö-8/ /BS/4) Answer: To Delay from A Path A 0 - B 0 Over D C 9 Over C D 7 Over D E 2 Over D F 4 Over D A B C D F G E 8.4 (Extra question) Distance vector routing algorithm is used in the subnet shown below. The following delay vectors have just come in to router C: From B: (5, 0, 8, 2, 6, 2) From D: (6, 2, 6, 0, 9, 0) From E: ( 7, 6, 3, 9, 0, 4) The measured delays to B, D, and E are 6, 3, and 5 msec, respectively. Find the new routing table of C. Give both the outgoing line to use and the expected delay. 7/8

18 B C A D (BA-Ö-2/ /FS/4) Answer: To Delay from C Path A Over B B 6 Over B C 0 - D 3 Over D E 5 Over E F 8 Over B 8.5 (Extra question) Distance vector routing algorithm is used in the subnet shown below. The delay vectors that have just come in to router A from its neighbor routers and the measured delays to the neighbors of A are given below. Find the new routing table of A. Give both the outgoing line to use and the expected delay. Delay vectors received by A: Delay measurements made by A: To Delay from B (ms) Delay from C (ms) Delay from D (ms) To Delay from A (ms) A B 5 B C A C D 4 D E (BA-FB-2/ /FS/3) Answer: To Delay from A Path A 0 Over A B 4 Over C C Over C D 4 Over D E 8 Over B E F B C D E 8/8

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