© 2006 Cisco Systems, Inc. All rights reserved. Module 4: Implement the DiffServ QoS Model Lesson 4.8: Understanding WAN Link Efficiency Mechanisms

© 2006 Cisco Systems, Inc. All rights reserved. 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.

© 2006 Cisco Systems, Inc. All rights reserved. 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)

© 2006 Cisco Systems, Inc. All rights reserved. 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

© 2006 Cisco Systems, Inc. All rights reserved. Payload and Header Compression  Payload compression reduces the size of the payload.  Header compression reduces the header overhead.  Compression increases throughput and decreases latency.

© 2006 Cisco Systems, Inc. All rights reserved. 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.

© 2006 Cisco Systems, Inc. All rights reserved. Layer 2 Payload Compression Results  Compression increases throughput and decreases delay.  Use hardware compression when possible.  Examples are Stacker, Predictor, and MPPC.

© 2006 Cisco Systems, Inc. All rights reserved. Header Compression..

© 2006 Cisco Systems, Inc. All rights reserved. Header Compression Results  Header compression increases compression delay and reduces serialization delay.

© 2006 Cisco Systems, Inc. All rights reserved. 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

© 2006 Cisco Systems, Inc. All rights reserved. Link Fragmentation and Interleaving (LFI)  LFI reduces the delay and jitter of small packets (such as VoIP).

© 2006 Cisco Systems, Inc. All rights reserved. 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.

© 2006 Cisco Systems, Inc. All rights reserved. Network Using LFI

© 2006 Cisco Systems, Inc. All rights reserved. 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.