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Module 4: Implement the DiffServ QoS Model. Lesson 4.8: Understanding WAN Link Efficiency Mechanisms. Objectives. Describe how link efficiency mechanisms can be used to optimize bandwidth. Describe Layer 2 payload compression.

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Module 4 implement the diffserv qos model

Module 4: Implement the DiffServ QoS Model

Lesson 4.8: Understanding WAN Link Efficiency Mechanisms


Objectives
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.


Link efficiency mechanisms
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)


Compression
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


Payload and header compression
Payload and Header Compression

  • Payload compression reduces the size of the payload.

  • Header compression reduces the header overhead.

  • Compression increases throughput and decreases latency.


Layer 2 payload compression
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.


Layer 2 payload compression results
Layer 2 Payload Compression Results

  • Compression increases throughput and decreases delay.

  • Use hardware compression when possible.

  • Examples are Stacker, Predictor, and MPPC.



Header compression results
Header Compression Results

  • Header compression increases compression delay and reduces serialization delay.


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

Large Packets “Freeze Out” Voice on Slow WAN Links


Link fragmentation and interleaving lfi
Link Fragmentation and Interleaving (LFI)

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


Applying link efficiency mechanisms
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.



Self check
Self Check

  • What is Shannon’s limit?

  • What is the difference between hardware compression and software compression?

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


Summary
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.


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