Module 4 Implement the Diff Serv Qo S

Module 4: Implement the Diff. Serv Qo. S 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 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 Vo. IP). © 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 Vo. IP? © 2006 Cisco Systems, Inc. All rights reserved.

Summary § WAN links can use bandwidth optimizing link efficiency Qo. S mechanisms such as payload compression, header compression, and link fragmentation and interleaving (LFI). These features are applicable to lowspeed 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. © 2006 Cisco Systems, Inc. All rights reserved.