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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. 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

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  1. Module 4: Implement the DiffServ QoS Model Lesson 4.8: Understanding WAN Link Efficiency Mechanisms

  2. 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.

  3. 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)

  4. 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

  5. Payload and Header Compression • Payload compression reduces the size of the payload. • Header compression reduces the header overhead. • Compression increases throughput and decreases latency.

  6. 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.

  7. Layer 2 Payload Compression Results • Compression increases throughput and decreases delay. • Use hardware compression when possible. • Examples are Stacker, Predictor, and MPPC.

  8. Header Compression . .

  9. Header Compression Results • Header compression increases compression delay and reduces serialization delay.

  10. 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

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

  12. 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.

  13. Network Using LFI

  14. 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?

  15. 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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