© 2006 Cisco Systems, Inc. All rights reserved. QOS Lecture 4 - Introducing QOS.

© 2006 Cisco Systems, Inc. All rights reserved.

QOS

Lecture 4 - Introducing QOS


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.


Traditional Nonconverged Network

Traditional data traffic characteristics:Bursty data flow

FIFO access

Not overly time-sensitive; delays OK

Brief outages are survivable


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.


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.


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.


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 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.


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.


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.


Reducing Delay in a Network

Customer routers perform:TCP/RTP header compression

LLQ

Prioritization

ISP routers perform:Reprioritization according to the QoS policy


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 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.


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.


Traffic Policing and Traffic Shaping

Time

Tra

ffic

Traffic Rate

Time

Tra

ffic Traffic Rate

Time

Tra

ffic

Traffic Rate

Time

Tra

ffic

Traffic Rate

Policing

Shaping


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.


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.


3.2: Implementing QoS


Objectives Describe the need for QoS as it relates to various types

of network traffic.

Identify QoS mechanisms.

Describe the steps used to implement QoS.


What Is Quality of Service? Two Perspectives

The user perspectiveUsers perceive that their applications are performing properly

Voice, video, and data

The network manager perspectiveNeed to manage bandwidth allocations to deliver the desired application performance

Control delay, jitter, andpacket loss


Different Types of Traffic Have Different Needs

Application Examples

Sensitivity to QoS Metrics

Delay JitterPacket Loss

Interactive Voice and Video Y Y Y

Streaming Video N Y Y

Transactional/ Interactive Y N N

Bulk DataEmail

File TransferN N N

Need to managebandwidth allocations

Real-time applications especially sensitive to QoS

Interactive voice

Videoconferencing

Causes of degraded performance

Congestion losses

Variable queuing delays

The QoS challenge

Manage bandwidth allocations to deliver the desired application performance

Control delay, jitter, and packet loss


Cisco IOS QoS Tools Congestion management:

PQ

CQ

WFQ

CBWFQ

Queue managementWRED

Link efficiencyLink fragmentation and interleave

RTP and CRTP

Traffic shaping and traffic policing

QoS Toolbox


Priority Queuing

PQ puts data into four levels of queues: high, medium, normal, and low.


Priority Queuing

Priority output queuing allows a network administrator to define four priorities of traffic---high, normal, medium, and low---on a given interface.

As traffic comes into the router, it is assigned to one of the four output queues.

Packets on the highest-priority queue are transmitted first.


Priority Queuing ctd..

When that queue empties, traffic on the next highest-priority queue is transmitted, and so on.

This mechanism assures that during congestion, the highest-priority data does not get delayed by lower-priority traffic.

However, if the traffic sent to a given interface exceeds the bandwidth of that interface, lower-priority traffic can experience significant delays.


Custom Queuing

CQ handles traffic by assigning a specified amount of queue space to each class of packet and then servicing up to 17 queues in a round-robin fashion.


Custom Queuing Custom queuing allows a customer to reserve a

percentage of bandwidth for specified protocols.

Customers can define up to 10 output queues for normal data and an additional queue for system messages such as LAN keepalive messages (routing packets are not assigned to the system queue).

The routers service each queue sequentially, transmitting a configurable percentage of traffic on each queue before moving on to the next one.

Custom Queuing guarantees that mission-critical data is always assigned a certain percentage of the bandwidth, but also assures predictable throughput for other traffic.


Custom Queuing … To provide this feature, routers determine how many

bytes should be transmitted from each queue, based on the interface speed and the configured percentage.

When the calculated byte count from a given queue has been transmitted, the router completes transmission of the current packet and moves on to the next queue, servicing each queue in a round-robin fashion.


Weighted Fair Queuing

•WFQ makes the transfer rates and interarrival periods of active high-volume conversations much more predictable.


Weighted Fair Queuing (Flow based)

Flow-Based WFQ: Creating Fairness Among Flows

For situations in which it is desirable to provide consistent response time to heavy and light network users alike without adding excessive bandwidth, the solution is flow-based WFQ (commonly referred to as just WFQ).

It is a flow-based queuing algorithm that creates bit-wise fairness by allowing each queue to be serviced fairly in terms of byte count.


WFQ ….

For example, if queue 1 has 100-byte packets and queue 2 has 50-byte packets, the WFQ algorithm will take two packets from queue 2 for every one packet from queue 1.

This makes service fair for each queue: 100 bytes each time the queue is serviced.

WFQ ensures that queues do not starve for bandwidth and that traffic gets predictable service.

Low-volume traffic streams that comprise the majority of traffic, receive increased service, transmitting the same number of bytes as high-volume streams.

This behavior results in what appears to be preferential treatment for low-volume traffic, when in actuality it is creating fairness.


Weighted Random Early Detection

•WRED provides a method that stochastically discards packets if congestion begins to increase.


Implementing QoS

Step 1: Identify types of traffic and their requirements.

Step 2: Divide traffic into classes.

Step 3: Define QoS policies for each class.


Step 1: Identify Types of Traffic and Their Requirements Network audit: Identify traffic on the network.

Business audit: Determine how important each type of traffic is for business.

Service levels required: Determine required response time.


Step 2: Define Traffic Classes

Scavenger Class

Less than Best Effort


Step 3: Define QoS Policy A QoS policy is a

network-wide definition of the specific levels of QoS that are assigned to different classes of network traffic.


Quality of Service OperationsHow Do QoS Tools Work?

Classification and Marking

Queuing and (Selective) Dropping

Post-Queuing Operations


Self Check

1. What types of applications are particularly sensitive to QoS issues?

2. What is WFQ? How is it different than FIFO?

3. What are the 3 basic steps involved in implementing QoS?

4. What is Scavenger Class?


Summary QoS is important to both the end user and the network

administrator. End users experience lack of QoS as poor voice quality, dropped calls or outages.

Network traffic differs in its ability to handle delay, jitter and packet loss. Traffic sensitive to these issues requires priority treatment. QoS measures can provide priority to sensitive traffic, while still providing services to more resilient traffic.

Implementing QoS involves 3 basic steps: identify the types of traffic on your network, divide the traffic into classes, and define a QoS policy for each traffic class.

© 2006 Cisco Systems, Inc. All rights reserved. QOS Lecture 4 - Introducing QOS.

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