© 2006 Cisco Systems, Inc. All rights reserved.

Module 4: Implement the DiffServ QoS Model

Lesson 4.8: Understanding WAN Link Efficiency Mechanisms

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Objectives Describe how link efficiency mechanisms can be used

to optimize bandwidth.

Describe Layer 2 payload compression.

Describe how header compression can be used to prevent sending redundant information.

Describe Link Fragmentation and Interleaving and the issues that can be solved using this mechanism.

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Link Efficiency Mechanisms Link efficiency mechanisms are often deployed on WAN

links to increase the throughput and to decrease delay and jitter.

Cisco IOS link efficiency mechanisms include: Layer 2 payload compression

Header compression

Link Fragmentation and Interleaving (LFI)

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Compression Data compression works by the identification of

patterns in a stream of data.

Basic elements of compression:Remove redundancy as much as possible.

There is a theoretical limit, known as Shannon's limit.

Many compression algorithms exist, for different purposes:

MPEG compression for video

Huffmann compression for text and software

LZ compression, used in Stacker compression

Two methods of compression are used:Hardware compression

Software compression

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Payload and Header Compression

Payload compression reduces the size of the payload.

Header compression reduces the header overhead.

Compression increases throughput and decreases latency.

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Layer 2 Payload Compression

Layer 2 payload compression reduces the size of the frame payload.

Entire IP packet is compressed.

Software compression can add delay because of its complexity.

Hardware compression reduces the compression delay.

Serialization delay is reduced; overall latency might be reduced.

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Layer 2 Payload Compression Results

Compression increases throughput and decreases delay.

Use hardware compression when possible.

Examples are Stacker, Predictor, and MPPC.

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Header Compression

..

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Header Compression Results

Header compression increases compression delay and reduces serialization delay.

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Large Packets “Freeze Out” Voice on Slow WAN Links

Problems:Excessive delay due to slow link and MTU-sized (large) packets

Jitter (variable delay) due to variable link utilization

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Link Fragmentation and Interleaving (LFI)

LFI reduces the delay and jitter of small packets (such as VoIP).

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Applying Link Efficiency Mechanisms

Identify bottlenecks in the network.

Calculate Layer 2 and Layer 3 overhead.

Decide which type of compression to use, such as TCP header compression.

Enable compression on WAN interfaces.

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Network Using LFI

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Self Check

1. What is Shannon’s limit?

2. What is the difference between hardware compression and software compression?

3. Why is it necessary to use a technique such as LFI when transmitting delay sensitive packets such as VoIP?

© 2006 Cisco Systems, Inc. All rights reserved.

Summary WAN links can use bandwidth optimizing link efficiency

QoS mechanisms such as payload compression, header compression, and link fragmentation and interleaving (LFI). These features are applicable to low-speed WAN interfaces and are emerging for use on high-speed Ethernet interfaces.

Data compression works by identifying patterns in streams of data, and then chooses a more efficient method to represent the same information.