managing interactive data traffic.
Content 3.1 Introducing QoS 3.1.5 Effects of End-to-end Delay and Jitter End-to-end delay and jitter have a severe quality impact on the network as follows: Figure shows four types of delay: Note
Many high-end routers or Layer 3 switches use advanced hardware architectures that speed up packet processing and do not require the main CPU to process the packets. Figure summarizes the impact of delay and jitter on the quality of networks. The International Telecommunication Union (ITU) considers network delay for voice applications in Recommendation G.114. This recommendation defines three bands of one-way delay as shown in Figure . These recommendations are intended for national telecom administrations. Therefore, they are more stringent than would normally be applied in private voice networks. When the location and business needs of end users are well known to the network designer, more delay can prove acceptable. For private networks, 200 ms of delay is a reasonable goal and 250 ms a limit.
Content 3.1 Introducing QoS 3.1.6 Reducing the Impact of Delay on Quality When considering solutions to the delay problem, there are two things to note: There are many ways to reduce the delay at a router. Assuming that the router has enough power to make forwarding decisions rapidly, the following factors have the most influence on queuing and serialization delays: Figure illustrates how administrators can accelerate packet dispatching for delay-sensitive flows in the following ways: By minimizing delay, network administrators can also reduce jitter (delay is more predictable than jitter and easier to reduce). Figure shows examples of ways to increase bandwidth efficiency in a network. In this scenario, an ISP providing QoS connects the offices of the customer to each other. A low-speed link (512 kbps) connects the branch office while a higher-speed link (1024 kbps) connects the main office. The customer uses both IP phones and TCP/IP-based applications to conduct daily business. Because the branch office only has a bandwidth of 512 kbps, the customer needs an appropriate QoS strategy to provide the highest possible quality for voice and data traffic. In this example, the customer needs to communicate with HTTP, FTP, e-mail, and voice services in the main office. Because the available bandwidth at the customer site is only 512 kbps, most traffic, but especially voice traffic, would suffer from end-to-end delays. In this example, the customer performs TCP and RTP header compression, LLQ, and prioritization of the various types of traffic. These mechanisms give voice traffic a higher priority than HTTP or e-mail traffic. In addition to these measures, the customer has chosen an ISP that supports QoS in the backbone. The ISP performs reprioritization on the customer's traffic, according to the QoS policy, so the traffic streams arrive on time at the customer's main office. This design guarantees that voice traffic has high priority and a guaranteed bandwidth of 128 kbps, FTP and e-mail traffic receive medium priority and a bandwidth of 256 kbps, and HTTP traffic receives low priority and a bandwidth of 64 kbps. Signaling and other management traffic uses the remaining 64 kbps.
Content 3.1 Introducing QoS 3.1.7 Packet Loss After delay, the next most serious concern for networks is packet loss. Usually, packet loss occurs when routers run out of buffer space for a particular interface (output queue). Figure gives some examples of the results of packet loss in a converged network. Figure illustrates a full interface output queue, which causes newly arriving packets to be dropped. The term that is used for such drops is “output drop” or “tail drop” (packets are dropped at the tail of the queue). Routers might also drop packets for these other less common reasons:
Content 3.1 Introducing QoS 3.1.8 Congestion Management: Ways to Prevent Packet Loss Packet loss is usually the result of congestion on an interface. Most applications that use TCP experience slowdown because TCP automatically adjusts to