Solutions to Chapter 4 - Communication Networks

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Communication Networks (2 nd Edition) Chapter 4 Solutions which is 4x 1/3 x 80% STS-3 or 16/15 STS-3. Adding the other 20% of the traffic which is destined to the higher tier link and has to traverse some of the lower tier links in both directions, the average link capacity in the lower tier links should be 16/15 STS-3 + 2 x 20% STS-3 or 22/15 STS-3, which is almost 1.5 STS-3 on average. On the higher tier ring, the traffic from each subring will be routed equally to each other subring. Thus the bandwidth required on each link will be four times the average traffic between two subrings or 4 x 1/3 of the total traffic from each subring. Because each ring produces 4×0.2 × STS-3 amount of traffic, the bandwidth required on each link of the higher tier ring is on the order of 4/3 × 0.8 × STS-3 ≈ STS-3. b. Discuss the bandwidth requirements that are possible if 80% of the traffic generated by each site is destined to sites in other rings. If 20% of the traffic is to stay within the same ring, then the required bandwidth on the higher tier ring will be increased. The required bandwidth on the lower tier rings will not change significantly, as each source still produces and receives STS-3 of traffic. The higher tier ring will now support 4 times more traffic. Thus, the bandwidth requirement is around STS-12. 15. Compare the efficiency of BLSR and UPSR rings in the following two cases: a. All traffic originating at the nodes in the ring are destined for a given central node. Considering that the total capacity of working and protection lines in both BLSR and UPSR is the same, the total bandwidth utilized in both BLSR and UPSR in this scenario is the same. The difference is the distribution of the traffic and unused capacity across the ring in the two cases. As an example we consider the ring shown in the figure below, consisting of four nodes. We first consider the UPSR scenario. The last hop before the central node is the bottleneck in this case limiting the traffic in the other hops on the working ring shown with solid lines. Assuming equal distribution of bandwidth among the nodes, if the last hop is fully utilized the hop prior to the last hop will be utilized up to 2/3 of its capacity and the one before that will be utilized up to 1/3 of its capacity. The remaining hop will not be utilized at all. The lines on the protection path will be utilized similarly but in the opposite order, shown with dotted lines. As a result the sum of working and protection lines together will be utilized only to half of the total capacity all across the ring. UPSR BLSR Leon-Garcia/Widjaja 8
Communication Networks (2 nd Edition) Chapter 4 Solutions Now we consider the BLSR scenario. In this case the working traffic can be sent in both directions but the protection traffic will be sent in the opposite direction and as a result as seen in the figure the total of working and protection traffic all across the ring will be the same as the other case as well as the unused capacity of the links. b. Each node originates an equal amount of traffic to all other nodes. In both BLSR and UPSR lines can be shared by traffic from other nodes. The more nodes share the same line the less the line can be utilized by the node attached to the line. In the given scenario each line is shared by the traffic from twice the number of nodes in UPSR compared to the BLSR. As a result BLSR is more efficient in this case. Working ring in UPSR Working ring in BLSR 16. Consider the operation of the dual gateways for interconnecting two bidirectional SONET rings shown in Figure 4.29. The primary gateway transmits the desired signal to the other ring and simultaneously transmits the signal to the secondary gateway which also routes the signal across the ring and then to the primary gateway. A service selector switch at the primary gateway selects between the primary and secondary signals. Explain how this setup recovers from failures in the link between the primary gateways. Solution: Two logical connections are maintained between the primary gateways. The first is a direct physical connection via the primary gateway link. The second is connected indirect via the secondary gateway as described in the question. This redundancy provides a backup in case of a link failure. If the primary gateway link fails, the service selector switch can select signals for the alternate indirect path until the primary gateway link is restored. Leon-Garcia/Widjaja 9
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Solutions to Chapter 4 - Communication Networks

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