1 - necessity of qos

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© 2006 Cisco Systems, Inc. All rights reserved. QoS v2.2—1-1

Understanding the Need for QoS

Introducing QoS




Before Converged Networks

Traditional data traffic characteristics:

• Bursty data flow

• First-come, first-served access

• Mostly not time-sensitive – delays OK

• Brief outages are survivable




After Converged Networks

Converged traffic characteristics:• Constant small-packet voice flow competes with bursty

data flow

• Critical traffic must get priority

• Voice and video are time-sensitive

• Brief outages not acceptable




Converged Networks:Quality Issues

• Telephone Call: “I cannot understand you; your voice is breaking up.”

• Teleconferencing: “The picture is very jerky.Voice not synchronized.”

• Brokerage House: “I needed that information two hours ago.Where is it?”

• Call Center: “Please hold while my screen refreshes.”




Converged Networks:Quality Issues (Cont.)

• Lack of bandwidth: Multiple flows compete for a limitedamount of bandwidth.

• End-to-end delay (fixed and variable): Packets have totraverse many network devices and links that add up to

the overall delay.• Variation of delay (jitter): Sometimes there is a lot of

other traffic, which results in more delay.

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




Lack of Bandwidth

• Maximum available bandwidth equals the bandwidth of the weakest link.

• Multiple flows are competing for the same bandwidth, resulting in much lessbandwidth being available to one single application.

Bandwidth max = min (10 Mbps, 256 kbps, 512 kbps, 100 Mbps) = 256 kbpsBandwidth avail = bandwidth max / flows

Bad Voice Due toLack of BW


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Ways to Increase Available Bandwidth

• Upgrade the link: the best solution but also the most expensive.

• Forward the important packets first.

• Compress the payload of Layer 2 frames (it takes time).

• Compress IP packet headers.



End-to-End Delay

• End-to-end delay equals a sum of all propagation, processing,and queuing delays in the path.

• In Best-Effort networks, propagation delay is fixed, processingand queuing delays are unpredictable.

Delay = P1 + Q1 + P2 + Q2 + P3 + Q3 + P4 = X ms

Bad Voice Due toDelay Variation



Types of Delay

• Processing Delay: The time it takes for a router to take the packet from an input interface,examine it, and put it 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 to transmit a packet



Processing and Queuing Delay


• Queuing Delay: The time a packets resides in the output queue of a router





Types of Delay


• Queuing Delay: The time a packet resides in the output queue of a router





Ways to Reduce Delay

• Upgrade the link; the best solution but also the most expensive.

• Forward the important packets first.

• Compress the payload of Layer 2 frames (it takes time).

• Compress IP packet headers.



Packet Loss

• Tail drops occur when the output queue is full. These are common drops, whichhappen when a link is congested.

• Many other types of drops exist, usually the result of router congestion, that areuncommon and may require a hardware upgrade (input drop, ignore, overrun,frame errors).

Bad Voice Dueto Packet Loss



Ways to Prevent Packet Loss

• Upgrade the link; the best solution but also the most expensive.

• Guarantee enough bandwidth to sensitive packets.

• Prevent congestion by randomly dropping less important packetsbefore congestion occurs.



Summary

• Traditional networks use a separate network for delay- and jitter-sensitive voice traffic. The voice network is engineeredto support the required number of calls. For data traffic,protocols like TCP have been developed to adapt to thebursty nature of a data network.

• Converged networks that support voice, video, and datacreate new requirements for managing network traffic. QoSmeets those requirements.

• Converged networks suffer from different quality issues,

including lack of adequate bandwidth, end-to-end andvariable delay, and lost packets.

• Lack of bandwidth causes networks to experience differenttypes of delay, including processing delay, queuing delay,serialization delay, and propagation delay.



Summary (Cont.)

• End-to-end delay is the sum of all propagation, processing,and queuing delays.

• Packet loss can cause applications that use TCP to slowdown. Applications that do not use TCP, such as voicetraffic, will experience poor voice quality if the packet loss istoo excessive.

• Ways to increase the available bandwidth, decrease the delayand packet loss include:

– Increasing the link capacity

– Using QoS mechanisms such as queuing, WRED, trafficpolicing and shaping, and link efficiency mechanismssuch as payload and header compression

1 - necessity of qos

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