Sample Solution for Exercise Communication Networks I

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Problem 3 - Layer Modela) Describe the Layers of the ISO-OSI Model and their function.Solution:LayerFunction1 sending bit 1 is also received as bit 1Physical (and not as bit 0):2 reliable data transfer between adjacent stations with framesData Link3 connection endsystem to endsystemNetwork4 connection end/source (application/process) to end/destinationTransport(application/process)5 support a “session” over a longer periodSession6 data presentation independent from the end systemPresentation7 application related servicesApplicationb) What are the differences of the ISO-OSI Model to the TCP/IP Model?Solution: ISO - OSI:- “A first step towards international standardization of protocols”- devised before protocols were inventedTCP/IP:- developed with the ARPANET, connect multiple networks- main idea: robust network (connectionless internetwork layer)(→ Tanenbaum)Exercise Communication Networks I - Summer Term 2013 Page 6 of 9
Problem 4 - TopologyIn topology I), 7 routers are interconnected by a binary tree. In topology II) they are interconnected in a ring, and intopology III) there is link between any two routers.12172172 336364 5 6 7I4 5II4 5IIIa) A message is sent via the shortest path. For each topology, what is the average number of hops, assumingthat all router pairs are equally likely? The router-host and router-router links are counted as one hop each.Solution:Total number router pairs: 6 + 5 + 4 + 3 + 2 + 1 = 21I/II/III 2 3 4 5 6 71 1/1/1 1/2/1 2/3/1 2/3/1 2/2/1 2/1/12 2/1/1 1/2/1 1/3/1 3/3/1 3/2/13 3/1/1 3/2/1 1/3/1 1/3/14 2/1/1 4/2/1 4/3/15 4/1/1 4/2/16 2/1/1I) cumulated # hops: 48 → n I = 48/21 = 2, 3II) cumulated # hops: 42 → n I I = 42/21 = 2III) 1b) The probability for a router being down is p. What is the probability that the path from router 4 to router 7exists, assuming that neither 4 nor 7 are down.Solution: Probabiliy for a router being up: (1 − p)I) P I = (1 − p) 3II) P I I = 1 − (1 − (1 − p) 2 )(1 − (1 − p) 3 )III) 1c) Discuss the advantages and disadvantages of each topology.Solution: Topology I) is very cheap to build. However, due to lack of redundancy, the failure of a router can causeproblems.Topology III) is the most expensive and highly redundant. Two routers can still communicate even if all others are down.Topology II) is somewhere in between. Some redundancy; not cost optimal.Exercise Communication Networks I - Summer Term 2013 Page 7 of 9
Page 1 and 2: Sample Solution for ExerciseCommuni
Page 3 and 4: V) Return to zeroSelect the correct
Page 5: e) Characterize a communication net
Page 9: I) SynchronousA1B1C1A2C2A1 B1 C1 A2

Sample Solution for Exercise Communication Networks I

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