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QoS: Tuning Bandwidth, Delay, Jitter, and Loss 27 |
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Table 1-7 |
QoS Tools That Affect Delay (Continued) |
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Type of QoS Tool |
How It Affects Delay |
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Compression |
Compresses either payload or headers, reducing overall number of bits |
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required to transmit the data. By requiring less bandwidth, queues shrink, |
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which reduces delay. Also serialization delays shrink, because fewer bits are |
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required. Compression also adds some processing delay. |
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Traffic shaping |
Artificially increases delay to reduce drops inside a Frame Relay or ATM |
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network. |
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Jitter
Consecutive packets that experience different amounts of delay have experienced jitter. In a packet network, with variable delay components, jitter always occurs—the question is whether the jitter impacts the application enough to degrade the service. Typically, data applications expect some jitter, and do not degrade. However, some traffic, such as digitized voice, requires that the packets be transmitted in a consistent, uniform manner (for instance, every 20 ms). The packets should also arrive at the destination with the same spacing between them. (This type of traffic is called isochronous traffic.)
Jitter is defined as a variation in the arrival rate (that is, variation in delay through the network) of packets that were transmitted in a uniform manner. Figure 1-14, for example, shows three packets as part of a voice call between phones at extension 301 and 201.
Figure 1-14 Jitter Example
Server 1
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IP |
Hannah |
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FA0 |
SW1 |
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RTP |
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201 |
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20 |
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301 |
RTP |
RTP RTP |
30 20
28 Chapter 1: QoS Overview
The phone sends the packets every 20 ms. Notice that the second packet arrived 20 ms after the first, so no jitter occurred. However, the third packet arrived 30 ms after the second packet, so 10 ms of jitter occurred.
Voice and video degrade quickly when jitter occurs. Data applications tend to be much more tolerant of jitter, although large variations in jitter affect interactive applications.
QoS Tools That Affect Jitter
Several QoS features can help with jitter issues. You’ll find more detail about each of these tools in various chapters throughout this book. For now, however, knowing what each class of QoS tool accomplishes will help you sift through some of the details.
The best QoS tool for jitter issues is . . . more bandwidth—again! More bandwidth helps bandwidth-related problems, and it also helps delay-related problems. If it helps to reduce delay, and because jitter is the variation of delay, jitter will be smaller. Faster bandwidth decreases serialization delay, which will decrease jitter. For instance, if delay has been averaging between 100 ms and 200 ms, jitter would typically be up to 100 ms. If delay is reduced to between 50 ms and 100 ms by adding more bandwidth, the typical jitter can be reduced to 50 ms. Because packets exit more quickly, queuing delay decreases. If queuing delays had been between 50 and 100 ms, and now they are between 10 and 20 ms, jitter will shrink as well. In short, faster is better for bandwidth, delay, and jitter issues!
Unfortunately, more bandwidth does not solve all jitter problems, even if you could afford more bandwidth! Several classes of QoS tools improve jitter; as usual, decreasing jitter for one set of packets increases jitter for others.
The same set of tools that affect delay also affect jitter; refer to Table 1-8 for a brief list of these QoS tools.
Table 1-8 QoS Tools That Affect Jitter
Type of QoS Tool |
How It Affects Jitter |
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Queuing |
Enables you to order packets so that delay-sensitive packets leave their |
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queues more quickly than delay-insensitive packets. |
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Link fragmentation and |
Because routers do not preempt a packet that is currently being transmitted, |
interleaving |
LFI breaks larger packets into smaller fragments before sending them. |
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Smaller delay-sensitive packets can be sent after a single smaller fragment, |
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instead of having to wait for the larger original packet to be serialized. |
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Compression |
Compresses either payload or headers, reducing overall number of bits |
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required to transmit the data. By requiring less bandwidth, queues shrink, |
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which reduces delay. Also serialization delays shrink, because fewer bits |
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are required. Compression also adds some processing delay. |
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Traffic shaping |
Artificially increases delay to reduce drops inside a Frame Relay or ATM |
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network. |
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