Traffic Characteristics of Voice Video and Data Questions

7 Describe the contents of an IP packet carrying the payload for a G.729 Voice over IP (VoIP) call.

8 Describe the amount of bandwidth required for G.711 and G.729 VoIP calls, ignoring data-link header/trailer overhead.

9 Define the meaning of the term "packetization delay" in relation to a voice call.

10 Define the term "codec delay" and discuss the two components when using a G.729 codec.

11 Describe a typical video payload flow in terms of packet sizes and packet rates.

12 Contrast the QoS characteristics needed by interactive data applications to the QoS needs of voice payload flows.

Foundation Topics

When I was a young lad in Barnesville, Georgia, I used to go to the bank with my dad. Each bank teller had his or her own line of people waiting to talk to the teller and transact their business. Invariably, we would always get behind someone who was really slow. (We called that Bubba's law—you always get behind some large, disagreeable guy named "Bubba" in line.) So, someone who came to the bank after we did would get served before we would, because he or she didn't get behind a "Bubba." But, it was the rural South, so no one was in that much of a hurry, and no one really worried about it.

Later we moved to the big city of Snellville, just outside Atlanta. At the bank in Snellville, people were in a bigger hurry. So, there was one line and many tellers. As it turns out, and as queuing theory proves, the average time in the queue is decreased with one queue served by many tellers, rather than one queue for each teller. Therefore, if one slow person (Bubba) was talking to teller 1, when teller 2 became available, my dad and I could go next, rather than the person who showed up at the bank after we did. Figure 1-1 depicts the two competing queuing methods at a typical bank or fast-food chain—multiple queues, multiple servers versus single queue, multiple servers. The single queue/multiple servers method improves average wait time, but also eliminates the possibility of your good luck in choosing a fast line—the one with no Bubbas in it.

Figure 1-1 Comparing Multiple Server/Multiple Queue to Multiple Server/Single Queue

Tellers

Tellers

Multiple Q's Multiple Servers

Single Q Multiple Servers

The bank in Snellville just chose a different queuing method, and that positively affected everyone, right? Well, the choice of using a single queue did have one negative effect—because there was only one queue, you could never show up, pick one of the many queues, and happen to get in the one with only fast people in it. In this scenario, on average everyone gets better service, but you miss out on the chance to get in and out of the bank really fast. In short, most customers' experience is improved, and some customers' experience is degraded.

In networking, QoS describes a large array of concepts and tools that can be used to affect the packet's access to some service. Most of us think of queuing features when we think of QoS— reordering the output queue so that one packet gets better service than another. But many other QoS features affect the quality—compression, drop policy, shaping, policing, and signaling, to name a few. In the end, whichever mechanism you use, you improve the behavior for one type of packet over another. Just like at the bank, implementing QoS is "managed fairness," and at the same time it is "managed unfairness"—you purposefully choose to favor one packet over another.

All of us can relate to the frustration of waiting in lines (queues) for things in our daily lives. It would be great if there were never any people in line ahead of us at the tollbooths, or waiting to get on a ride at Disneyland (or any other place). For that to be possible, however, there would need to be a lot more tollbooths, Disneyland would need to be 20 times larger, and banks would need to hire a lot more tellers. Even so, adding more capacity would not always solve the prob-lem—the tollbooth would still be crowded at rush hour, Disneyland would still be crowded when schools are not in session, and banks would still be crowded on Friday afternoons when everyone is trying to cash his or her weekly paycheck (at least where I live!). Making Disneyland 20 times larger, so that there are no queues, is financially ridiculous—likewise, the addition of 20 times more bandwidth to an existing link is probably also financially unreasonable. After all, you can afford only so much capacity, or bandwidth in the case of networking.

This chapter begins by taking a close look at the four traffic characteristics that QoS tools can affect:

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