CSCI 278 : COMPUTER NETWORKS Instructor : Seung Bae Im MIDTERM EXAM ----- April 1995 Closed book, closed notes. Print your name on each page of the answer sheets. Do not write on the back of a page. State all assumptions. Majority of the questions require Brief answers! 1.(5 points) One of the main advantages of digital communication compared to analog communication is ÒHigher S/N ratioÓ. Discuss how it is achieved. 2.(15 points) Describe Layered Architecture of Protocols(for example: OSI) including the following terms: Layer services, service user, Service Access Points(SAP), Virtual communication, Layer interface, Nth Layer Protocol. Underline when you use the terms. 3.(5 points) Say few words about IEEE 802. 4.(35 Points) Answer the questions. ( C = H log2( 1 + S/N ) ) (a) What is Crosswalk? What do you do to reduce it? (b) The European version of T1 carrier is called E1. E1 uses 8 bits per sample, 4khz bandwidth for each voice signal. It multiplexes 30 voice channels and use 2 extra channels for signaling and synchronization. (1) Calculate the total rate(bps) of E1. (2) Now, we like to carry an E1 on a transmission line. Assuming 20 dB signal-to-noise ratio(for the line), calculate the required bandwidth of the line. Exact value is not needed. (c) What is ISDN(Integrated Service Digital Network)? Describe it in few words. (d) What is Ògeosynchronous satelliteÓ? (e) Describe the 3 kinds of fibers: 1)Multimode 2)Multimode-Graded-Index 3)Singlemode. 5.(20 points) (a) Describe the function of ÒEthernet 10BaseT HubÓ. What are the advantages of the 10BaseT compared to 10Base5 or 10Base2? (b) Describe the difference(s) between 100BaseVGAnyLan and 100BaseT. 6.(10 points) (a) In Stop_And_Wait protocol, sending of a long frame may improve utilization(efficient use of channel). What could be the problem with a long frame? (b) Suggest a solution to the above problem. 7.(10 points) Errors in communication channels are unavoidable. List and describe(briefly) the schemes to handle errors.