© 2006 Cisco Systems, Inc. All rights reserved. Optimizing Converged Cisco Networks (ONT) Module 3: Introduction to IP QoS

© 2006 Cisco Systems, Inc. All rights reserved. Lesson 3.1: Introducing QoS

© 2006 Cisco Systems, Inc. All rights reserved. Objectives  Explain why converged networks require QoS.  Identify the major quality issues with converged networks.  Calculate available bandwidth given multiple flows.  Describe mechanisms designed to use bandwidth more efficiently.  Describe types of delay.  Identify ways to reduce the impact of delay on quality.  Describe packet loss and ways to prevent or reduce packet loss in the network.

© 2006 Cisco Systems, Inc. All rights reserved. Traditional Nonconverged Network  Traditional data traffic characteristics: Bursty data flow FIFO access Not overly time-sensitive; delays OK Brief outages are survivable

© 2006 Cisco Systems, Inc. All rights reserved. Converged Network Realities  Converged network realities: Constant small-packet voice flow competes with bursty data flow. Critical traffic must have priority. Voice and video are time-sensitive. Brief outages are not acceptable.

© 2006 Cisco Systems, Inc. All rights reserved. Converged Network Quality Issues  Lack of bandwidth: Multiple flows compete for a limited amount of bandwidth.  End-to-end delay (fixed and variable): Packets have to traverse many network devices and links; this travel adds up to the overall delay.  Variation of delay (jitter): Sometimes there is a lot of other traffic, which results in varied and increased delay.  Packet loss: Packets may have to be dropped when a link is congested.

© 2006 Cisco Systems, Inc. All rights reserved. Measuring Available Bandwidth  The maximum available bandwidth is the bandwidth of the slowest link.  Multiple flows are competing for the same bandwidth, resulting in much less bandwidth being available to one single application.  A lack in bandwidth can have performance impacts on network applications.

© 2006 Cisco Systems, Inc. All rights reserved. Increasing Available Bandwidth  Upgrade the link (the best but also the most expensive solution).  Improve QoS with advanced queuing mechanisms to forward the important packets first.  Compress the payload of Layer 2 frames (takes time).  Compress IP packet headers.

© 2006 Cisco Systems, Inc. All rights reserved. Using Available Bandwidth Efficiently  Using advanced queuing and header compression mechanisms, the available bandwidth can be used more efficiently: Voice: LLQ and RTP header compression Interactive traffic: CBWFQ and TCP header compression Voice (Highest) Data (High) Data (Medium) Data (Low) Voice LLQ RTP header compression Data CBWFQ TCP header compression

© 2006 Cisco Systems, Inc. All rights reserved. Types of Delay  Processing delay: The time it takes for a router to take the packet from an input interface, examine the packet, and put the packet into the output queue of the output interface.  Queuing delay: The time a packet resides in the output queue of a router.  Serialization delay: The time it takes to place the “bits on the wire.”  Propagation delay: The time it takes for the packet to cross the link from one end to the other.

© 2006 Cisco Systems, Inc. All rights reserved. The Impact of Delay and Jitter on Quality  End-to-end delay: The sum of all propagation, processing, serialization, and queuing delays in the path  Jitter: The variation in the delay.  In best-effort networks, propagation and serialization delays are fixed, while processing and queuing delays are unpredictable.

© 2006 Cisco Systems, Inc. All rights reserved. Ways to Reduce Delay  Upgrade the link (the best solution but also the most expensive).  Forward the important packets first.  Enable reprioritization of important packets.  Compress the payload of Layer 2 frames (takes time).  Compress IP packet headers.

© 2006 Cisco Systems, Inc. All rights reserved. Reducing Delay in a Network  Customer routers perform: TCP/RTP header compression LLQ Prioritization  ISP routers perform: Reprioritization according to the QoS policy

© 2006 Cisco Systems, Inc. All rights reserved. The Impacts of Packet Loss  Telephone call: “I cannot understand you. Your voice is breaking up.”  Teleconferencing: “The picture is very jerky. Voice is not synchronized.”  Publishing company: “This file is corrupted.”  Call center: “Please hold while my screen refreshes.”

© 2006 Cisco Systems, Inc. All rights reserved. Types of Packet Drops  Tail drops occur when the output queue is full. Tail drops are common and happen when a link is congested.  Other types of drops, usually resulting from router congestion, include input drop, ignore, overrun, and frame errors. These errors can often be solved with hardware upgrades.

© 2006 Cisco Systems, Inc. All rights reserved. Ways to Prevent Packet Loss  Upgrade the link (the best solution but also the most expensive).  Guarantee enough bandwidth for sensitive packets.  Prevent congestion by randomly dropping less important packets before congestion occurs.

© 2006 Cisco Systems, Inc. All rights reserved. Traffic Policing and Traffic Shaping Time Traffic Traffic Rate Time Traffic Traffic Rate Time Traffic Traffic Rate Time Traffic Traffic Rate Policing Shaping

© 2006 Cisco Systems, Inc. All rights reserved. Reducing Packet Loss in a Network  Problem: Interface congestion causes TCP and voice packet drops, resulting in slowing FTP traffic and jerky speech quality.  Conclusion: Congestion avoidance and queuing can help.  Solution: Use WRED and LLQ.

© 2006 Cisco Systems, Inc. All rights reserved. Summary  Converged networks carry different types of traffic over a shared infrastructure. This creates the need to differentiate traffic and give priority to time-sensitive traffic.  Various mechanisms exist that help to maximize the use of the available bandwidth, including queuing techniques and compression mechanisms.  All networks experience delay. Delay can effect time sensitive traffic such as voice and video.  Without proper provisioning and management, networks can experience packet loss. Packet loss is especially important with voice and video, as no resending of lost packets can occur.

© 2006 Cisco Systems, Inc. All rights reserved. Q and A

© 2006 Cisco Systems, Inc. All rights reserved. Resources  Quality of Service Networking m  QoS Congestion Avoidance upport_protocol_home.html  QoS Congestion Management (queuing) upport_protocol_home.html

© 2006 Cisco Systems, Inc. All rights reserved.