Lesson 1 - Introducing QoS 3 1

8/12/2019 Lesson 1 - Introducing QoS 3 1

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© 2006 Cisco Systems, Inc. All rights reserved.

Optimizing Converged

Cisco Networks (ONT)

Module 3: Introduction to IP QoS




Lesson 3.1:

Introducing QoS






Traditional Nonconverged Network

Traditional data traffic characteristics:Bursty data flow

FIFO access

Not overly time-sensitive; delays OK

Brief outages are survivable

http://www.cisco.com/

http://www.cisco.com/




Converged Network Realities

Converged network realities:

Constant small-packet voice flow competeswith bursty data flow.

Critical traffic must have priority.

Voice and video are time-sensitive.

Brief outages are not acceptable.




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 totraverse many network devices and links; this traveladds up to the overall delay.

Variation of delay (jitter): Sometimes there is a lot ofother traffic, which results in varied and increaseddelay.

Packet loss: Packets may have to be dropped when alink is congested.


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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 lessbandwidth being available to one single application.

A lack in bandwidth can have performance impacts on network applications.



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.



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)

1 1

2 2

3 3 3

4 4 4 4

4 3 2 1 1

Voice• LLQ

• RTP header

compression

Data• CBWFQ

• TCP header

compression



Types of Delay

Processing delay: The time it takes for a router to take the packet from an inputinterface, examine the packet, and put the packet into the output queue of theoutput 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 tothe other.



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.







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.”



Types of Packet Drops

Tail drops occur when the output queue is full. Tail drops are commonand happen when a link is congested.

Other types of drops, usually resulting from router congestion, includeinput drop, ignore, overrun, and frame errors. These errors can oftenbe solved with hardware upgrades.









Summary

Converged networks carry different types of traffic over

a shared infrastructure. This creates the need todifferentiate traffic and give priority to time-sensitivetraffic.

Various mechanisms exist that help to maximize the

use of the available bandwidth, including queuingtechniques and compression mechanisms.

All networks experience delay. Delay can effect timesensitive traffic such as voice and video.

Without proper provisioning and management,networks can experience packet loss. Packet loss isespecially important with voice and video, as noresending of lost packets can occur.





Resources

Quality of Service Networking

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.htm

QoS Congestion Avoidance

http://www.cisco.com/en/US/tech/tk543/tk760/tsd_technology_s

upport_protocol_home.html

QoS Congestion Management (queuing)

http://www.cisco.com/en/US/tech/tk543/tk544/tsd_technology_support_protocol_home.html















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Lesson 1 - Introducing QoS 3 1