Transcript CCNP-IV-ONT_Mod_3_Lesson_1

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
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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.
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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.
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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.
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Using Available Bandwidth Efficiently
Voice
1
1
• LLQ
• RTP header
compression
(Highest)
Data
2
2
3
3
3
4
4
4
(High)
Data
(Medium)
Data
Voice
4
4
3
2
1
1
Data
• CBWFQ
• TCP header
compression
(Low)
 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
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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.
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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.
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Reducing Delay in a Network
 Customer routers perform:
TCP/RTP header compression
LLQ
Prioritization
 ISP routers perform:
Reprioritization according to the QoS policy
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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.”
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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.
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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.
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Traffic Rate
Policing
Traffic
Traffic
Traffic Policing and Traffic Shaping
Time
Traffic Rate
Shaping
Time
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Traffic
Traffic
Time
Traffic Rate
Traffic Rate
Time
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.
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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.
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Q and A
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Resources
 Quality of Service Networking
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/qos.ht
m
 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_s
upport_protocol_home.html
© 2006 Cisco Systems, Inc. All rights reserved.
© 2006 Cisco Systems, Inc. All rights reserved.