Introduction

Computing in general, and networking in particular, must deal with the issues relating to constrained resources. For computers, operating systems must find a way to equitably distribute the CPU time and memory among the various programs running on the computer. When the need for memory exceeds the available memory, the CPU spends more time performing memory management, moving data from memory to permanent storage, typically on a hard disk. Of course, the computer might be low on CPU resources at the same time, meaning the CPU has less available time to devote to overhead tasks like memory management. With only a small load on the computer, all is well. When the load exceeds the capacity of the CPU, memory, and other resources, a lower volume of useful work is accomplished, and the users get worse response time from the computer.

The competition for bandwidth is the classic battle for resources in networking. If the offered load sent into the network exceeds the available bandwidth, the network must react by either discarding packets, or queuing them in memory waiting for the bandwidth to become available. The packets that are queued experience more delay in the network than do packets that happen to be sent when the network is not congested. When consecutive packets experience different amounts of delay, variable delay, or jitter, has occurred. So, although bandwidth might be the constrained resource for which many network attached devices compete, other side effectsdelay, jitter, and lossoccur as a result.

Cisco calls the general topic of how to manipulate bandwidth, delay, jitter, and loss characteristics in a network quality of service, or QoS. The Cisco QOS exam 642-642 tests your knowldege of QoS features and configurations covered in the course "Implementing Cisco Quality of Service (QOS)." This book covers the topics on the QOS exam, with some additional detailed explanations beyond what you find in the QOS course. By going deeper, you can approach the exam with more confidence, while learning valuable information that will help you deploy QoS in real networks. This book also attempts to cover the same breadth of topics found in the QOS course and exam, so it will keep you focused on what's on the exam.

In years past, Cisco actually had two QoS courses, and exams based on each course. With the availability of the QOS 642-642 exam, and the course of the same name, Cisco converged the two courses into a single course.

This introduction discusses the QOS exam, including the exam topics covered, and some reasons why you might be interested in the exam.