1.12 Exercises

In Example 1.1, an analogy was drawn between a network's architecture and design and a home's architecture and design. Provide a similar analogy, using a computer's architecture and design.
Hierarchy and interconnectivity are a fundamental trade-off in networks. Given the network hierarchy shown in Figure 1.30, with costs assigned to each link, show how interconnectivity would improve the performance of traffic flowing between Joe's computer and Sandy's computer. Costs are shown as numbers but could represent the capacity of each link or the costs incurred by using each link. What is the total cost of traveling the hierarchy between Joe's computer and Sandy's? In this figure, where would you add a link of cost 15 so that the total cost between Joe's computer and Sandy's is less than it is when you travel the entire hierarchy?

Figure 1.30: Network hierarchy for Exercise 2.
In Figure 1.9, connections are added between networks in the Internet to provide a better performing path for select traffic flows. An example of this is a content delivery network (CDN). What is a CDN? Show how a CDN uses interconnectivity to provide better performance characteristics to its users.
In defining where services can be applied in a network, end-to-end is determined by where you want a service to start and stop. For example, if your WAN is supplied by a service provider (e.g., an ATM or frame relay service), you may want to define the end points and characteristics of that service. If you use IP routers at each LAN-WAN interface to that service, describe the following: (1) at which network devices would you define the end points of the service, and (2) what characteristics (service metrics) would you use to measure the service?
Service requirements flow from user to application to device to network, becoming more specific along the way. If you were given an application requirement for end-to-end delay (e.g., 100 ms) between an application server on one network and users on another network, for example, how might that translate into delay in the network and devices? What types of service metrics could you use to measure it?
For Example 1.5, the delay characteristics for the segments (including the processing at the switches) are as follows: for each GigE segment, 100 s; for the PoS OC-48 segment between routers, 1 ms; for each FE segment, 200 s; and for the security firewall, 5 ms. Draw graphs showing the end-to-end delay performance (in the direction from user PC to server) before and after the security firewall is added.
Which of the following applications require best-effort (unpredictable and unreliable), guaranteed (predictable and reliable, with accountability), or predictable service. Give reasons for your choices.
- High-quality (phone company-grade) voice calls
- Voice over IP (VoIP) calls
- File transfers via FTP
- Audio file downloads
- A commercial video-on-demand service
- User access to servers in a corporation
Show how performance boundaries and thresholds could be used in the following scenarios.
- An application has a service requirement for round-trip delay to be less than 100 ms. If delay is greater than 100 ms, notify the network administrator.
- A user requires capacity of up to 512 Kb/s but may not exceed 1.5 Mb/s. You want to keep track of how much time the user's capacity is between 512 Kb/s and 1.5 Mb/s.