CSCI 278 : COMPUTER NETWORKS Instructor : Seung Bae Im MIDTERM EXAM ----- March 1993 Closed book, closed notes. Print your name on each page of the answer sheets. State all assumptions, if necessary. Show all your work. Be neat! Majority of the questions require Brief answers! 1.(4 points) What is the main purpose of the OSI model? 2.(10 points) Discuss "layered architecture of protocols" including following terms ; "Layer services", "Service user", "Nth layer protocol", "Interface", "Virtual communication". 3.(6 points) Explain briefly : "Protocol", "Data Encapsulation/Decapsulation" 4.(5 points) Discuss why standards are necessary and the main dilemma of "standardization". 5.(20 Points) Answer the questions regarding transmission media. (a) What is Delay Distortion? What do you do to reduce it? (b) Given a channel of bandwidth 3000HZ and the Signal-to-Noise ratio of 40 db, calculate the channel capacity( C = H log2 ( 1+ S/N) ). You don't need to come up with exact value. Can our data rate(over a given channel bandwidth) ever exceed or equal the C ? Explain. (c) Point out the differences between "Baseband signaling" and "Broadband signaling". (d) Major Advantages of optical fibers over copper cable. 6.(8 points) The CCITT V.32 uses 2400 Baud and can achieve maximum 9600 bps. (a) Explain how it is possible(you do not need to describe/know the actual modulation scheme used in V.32). Discuss the difference between BAUD and BPS. (b) V.42bis is a CCITT standard for modem data compression. Using the standard we can achieve up to a 4-to-1 compression ratio. Now what is the maximum data rate achievable on V.32 modem combined with V.42bis? 7.(15 points) Assume a voice signal has the bandwidth of 4000HZ. (a) What is the minimum sampling rate to digitize the voice? (b) We like to use TDM(Time Division Multiplexing) to put 32 channels on a transmission line. We use the sampling rate that you derived in (a). Each sample is digitized into 256 discrete levels and 2 channels(among 32) will be used as control channels(signaling, synchronization, etc.). What is the total data rate of the system? (c) In systems like the above, we use "BNZS(Binary N Zero Substitution)". Why? 8.(4 points) What is the main advantage of the codes that are used in FDDI compared to the codes used in other LANs(Manchester, Differential Manchester). 9.(5 points) The bandwidth of a telephone voice is limited to about 3100HZ. But we know that human voice can produce frequency components up to 20KHZ. What is the major reason(s) that they can limit voice bandwidth to 3100HZ? Why do want to limit the bandwidth of a voice? 10.(4 points) Among the alternatives of Physical Layer in Ethernet, the 10BaseT became most popular recently. Discuss possible reason(s) for the popularity. 11.(10 points) Compare the two LANs(Ethernet & Token-ring) in terms of (a) Medium access contrtol mechanisms(Describe each briefly) (b) Network Traffic Loads and Performances. 12.(9 points) The current LANs(Ethernet, Token-ring) are adequate for many classes/kinds of user demands(e.g. terminal accesses, file transfers, e-mails, ...) except the transmission of real time signals such as voice, video. Discuss why the current LANs are not adequate for the real time signals. Suggest modifications to the current LANs to accommodate both worlds(real time & non-real time signals).