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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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