Đang chuẩn bị liên kết để tải về tài liệu:
Lecture Data communications and networking: Chapter 24 - Behrouz A. Forouzan

Chapter 24 - Congestion control and quality of service. This chapter discuss two related topics: congestion control and quality of service. Although these two issues can be related to any layer, we discuss them here with some references to other layers. | Chapter 24 Congestion Control and Quality of Service Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 24. 24-1 DATA TRAFFIC The main focus of congestion control and quality of service is data traffic. In congestion control we try to avoid traffic congestion. In quality of service, we try to create an appropriate environment for the traffic. So, before talking about congestion control and quality of service, we discuss the data traffic itself. Traffic Descriptor Traffic Profiles Topics discussed in this section: 24. Figure 24.1 Traffic descriptors 24. Figure 24.2 Three traffic profiles 24. 24-2 CONGESTION Congestion in a network may occur if the load on the network—the number of packets sent to the network—is greater than the capacity of the network—the number of packets a network can handle. Congestion control refers to the mechanisms and techniques to control the congestion and keep the load below the capacity. Network Performance Topics discussed in this section: 24. Figure 24.3 Queues in a router 24. Figure Packet delay and throughput as functions of load 24. 24-3 CONGESTION CONTROL Congestion control refers to techniques and mechanisms that can either prevent congestion, before it happens, or remove congestion, after it has happened. In general, we can divide congestion control mechanisms into two broad categories: open-loop congestion control (prevention) and closed-loop congestion control (removal). Open-Loop Congestion Control Closed-Loop Congestion Control Topics discussed in this section: 24. Figure 24.5 Congestion control categories 24. Figure 24.6 Backpressure method for alleviating congestion 24. Figure 24.7 Choke packet 24. 24-4 TWO EXAMPLES To better understand the concept of congestion control, let us give two examples: one in TCP and the other in Frame Relay. Congestion Control in TCP Congestion Control in Frame Relay Topics discussed in this section: 24. . | Chapter 24 Congestion Control and Quality of Service Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 24. 24-1 DATA TRAFFIC The main focus of congestion control and quality of service is data traffic. In congestion control we try to avoid traffic congestion. In quality of service, we try to create an appropriate environment for the traffic. So, before talking about congestion control and quality of service, we discuss the data traffic itself. Traffic Descriptor Traffic Profiles Topics discussed in this section: 24. Figure 24.1 Traffic descriptors 24. Figure 24.2 Three traffic profiles 24. 24-2 CONGESTION Congestion in a network may occur if the load on the network—the number of packets sent to the network—is greater than the capacity of the network—the number of packets a network can handle. Congestion control refers to the mechanisms and techniques to control the congestion and keep the load below the capacity. Network .