Network Structures

There are basically two schemes for building distributed systems. In a multiprocessor (tightly coupled) system, the processors share memory and a clock, and communication usually takes place through the shared memory. In a distributed (loosely coupled) system, the processors do not share memory or a clock. Instead, each processor has its own local memory. The processors communicate with one another through various communication networks, such as high-speed buses or telephone lines. In this chapter, we discuss the general structure of distributed systems and the networks that interconnect them. Detailed discussions are given in Chapters 16 to 18.
Answers to Exercises
15.1 Contrast the various network topologies in terms of reliability. Answer:
Fully connected — very reliable since all the links must fail to partition the system.
Partially connected — not as reliable as a fully connected system because the failure of relatively few links (possibly 1) can partition the system.
Hierarchy — the failure of any node (except a leaf) will partition the system into several disjoint subtrees.
Star — if the central site fails, the entire network is partitioned.
Ring — in a bidirectional ring, two links, and in a unidirectional, one link must fail to partition the system.
Multi-Access Bus — The failure of one site does not affect the communications between the rest of the sites; however if the link fails, the network is completely partitioned.
Why do most WANs employ only a partially connected topology? Answer: Fully connected topologies require by de?nition a connection from each node to every other node. Such connections are expensive, especially when there are lots of nodes and they are far apart.
What are the main differences between a WAN and a LAN?
Answer: The main difference is the way in which they are geographically distributed. Computer networks are composed of a number of autonomous processors that are distributed over a large geographical area (like the U.S.). Local-area networks are composed
of processors that are distributed over small geographical areas (like a single building).
15.4 What network con?guration would best suit the following environments?
a.
A dormitory ?oor
b.
A university campus
c.
A state
d.
A nation
Answer:
a.
Multiaccess bus.
b.
Ring structure.
c.
Partially connected network.
d.
Partially connected network.
Even though the ISO model of networking speci?es seven layers of functionality, most computer systems use fewer layers to implement a network. Why do they use fewer layers? What problems could the use of fewer layers cause? Answer: No Answer
Explain why a doubling of the speed of the systems on an Ethernet segment may result in decreased network performance. What changes could be made to ameliorate the problem? Answer: Faster systems may be able to send more packets in a shorter amount of time. The network would then have more packets traveling on it, resulting in more collisions, and therefore less throughput relative to the number of packets being sent. More networks can be used, with fewer systems per network, to reduce the number of collisions.
Under what circumstances is a token-ring network more effective than an Ethernet network? Answer: A token ring is very effective under high sustained load, as no collisions can occur and each slot may be used to carry a message, providing high throughput. A token ring is less effective when the load is light (token processing takes longer than bus access, so any one packet can take longer to reach its destination), or sporadic.
Why would it be a bad idea for gateways to pass broadcast packets between networks? What would be the advantages of doing so? Answer: All broadcasts would be propagated to all networks, causing a lot of network traf?c. If broadcast traf?c were limited to important data (and very little of it), then broadcast propagation would save gateways from having to run special software to watch for this data (such as network routing information) and rebroadcast it.
Answers to Exercises 119
15.9 In what ways is using a name server better than using static host tables? What are the problems and complications associated with name servers? What methods could be used to decrease the amount of traf?c name servers generate to satisfy translation requests? Answer: Name servers require their own protocol, so they add complication to the system. Also, if a name server is down, host information may become unavailable. Backup name servers are required to avoid this problem. Caches can be used to store frequently-requested host information to cut down on network traf?c.
The original HTTP protocol used TCP/IP as the underlying network protocol. For each page, graphic, or applet, a separate TCP session was contructed, used, and torn down. Because of the overhead of building and destroying TCP/IP connections, there were performance problems with this implementation method. Would using UDP rather than TCP have been a good alternative? What other changes could be made to improve HTTP performance? Answer: No answer.
Of what use is an address resolution protocol? Why is the use of such a protocol better than making each host read each packet to determine to whom it is destined? Does a token-ring network need such a protocol? Answer: An ARP translates general-purpose addresses into hardware interface numbers so the interface can know which packets are for it. Software need not get involved. It is more ef?cient than passing each packet to the higher layers. Yes, for the same reason.