3.1: Introducing QoS

1. 3.1: Introducing QoS

2. 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.

3. Traditional Nonconverged Network • Traditional data traffic characteristics: Bursty data flow FIFO access Not overly time-sensitive; delays OK Brief outages are survivable

4. 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.

5. 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.

6. 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.

7. 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.

8. 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 • LLQ • RTP header compression 1 1 2 2 3 3 3 • Data • CBWFQ • TCP header compression 4 4 4 4 4 3 2 1 1 Using Available Bandwidth Efficiently

9. 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.

10. 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.

11. 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.

12. Reducing Delay in a Network • Customer routers perform: TCP/RTP header compression LLQ Prioritization • ISP routers perform: Reprioritization according to the QoS policy

13. 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.”

14. 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.

15. 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.

16. Traffic Rate Traffic Traffic Traffic Traffic Traffic Rate Policing Shaping Time Time Time Time Traffic Rate Traffic Rate Traffic Policing and Traffic Shaping

17. 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.

18. 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.