CSCI 278 : COMPUTER NETWORKS Instructor : Seung Bae Im MIDTERM EXAM ----- March 1996 Closed book, closed notes. Print your name on each page of the answer sheets. Do not write on the back of a page(they will not be graded). State all assumptions. Majority of the questions require Brief answers! 1.(5 points) Draw the communication model used in ITU-T standards(DTEs, DCEs) and identify the interface points. 2.(15 points) Define/describe the following terms: (a) Nth Layer Protocol (b) OSI 7 layer model (c) Data Encapsulation/decapsulation (d) Main reasons for the digital encoding( D to D--Manchester/D-Manchester/Bipolar codes, ....) rather than using ordinary binary codes. 3.(10 points) (a) List the advantages of fiber optics. Any disadvantages? (b) Describe the 3 modes of fiber: multimode, multimode-graded-index, single-mode 4.(10 points) (a) What is Nyquist rate? (b) What is ShannonÕs Capacity? Why canÕt we achieve ShannonÕs capacity in practical systems? 5.(5 points) Say few words about IEEE 802. 6.(35 Points) Answer the questions. ( C = H log2( 1 + S/N ) ) (a) Why do we twist wires(UTP/STP)? (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. If you are given a line with 1MHz bandwidth, calculate the S/N in decibel. Exact value is not needed. (3) Does E1 use Òin-band-signalingÓ or Òout-of-band signalingÓ ? (c) What are the advantages of ISDN(Integrated Service Digital Network) compared to analog telephone systems? (d) What is ÒNational ISDN? 7.(20 points) (a) Describe the operation of Ònon-persistent CSMAÓ. (b) Describe why there is Òminimum packet lengthÓ in Ethernet. (c) Describe the function of ÒEthernet 10BaseT HubÓ. What are the advantages of the 10BaseT compared to 10Base5 or 10Base2?