Introduction to QoS (Quality of Service) by Anthony Sequeira

A lack of bandwidth is the overshadowing issue for most network quality problems. Specifically, when there is a lack of bandwidth, packets might suffer from one or more of the symptoms listed in Table 10-3

Table 10-3 Three Categories of Quality of Service

Delay Delay is the time required for a packet to travel from source to destination. You might have witnessed delay on the evening news when the news anchor is talking via satellite to a foreign news correspondent. Because of the satellite delay, the conversation begins to feel unnatural.
Jitter Jitter is the uneven arrival of packets. For example, imagine a VoIP conversation where packet 1 arrives at a destination router. Then, 20 ms later, packet 2 arrives. After another 70 ms, packet 3 arrives, and then packet 4 arrives 20 ms behind packet 3. This variation in arrival times (that is, variable delay) is not due to dropped packets, but the jitter might be interpreted by the listener as dropped packets.
Drops Packet drops occur when a link is congested and a router's interface queue overflows. Some types of traffic, such as UDP traffic carrying voice packets, are not retransmitted if packets are dropped.

Fortunately, QoS features available on many routers and switches can recognize important traffic and then treat that traffic in a special way. For example, you might want to allocate 128Kbps of bandwidth for your VoIP traffic and give that traffic priority treatment.

Because the primary challenge of QoS is a lack of bandwidth, the logical question is, "How do we increase available bandwidth?" A knee-jerk response to that question is often "Add more bandwidth." However, more bandwidth often comes at a relatively high cost.

QoS Configuration Steps

The mission statement of QoS could read something like this:

"To categorize traffic and apply a policy to those traffic categories in accordance with a QoS policy."

Understanding this underlying purpose of QoS can help you better understand the there basic steps involved in QoS configuration:

Step 1. Determine network performance requirements for various traffic types. For example, consider these design recommendations for voice, video, and data traffic:
• Voice: No more than 150 ms of one-way delay; no more than 30 ms of jitter; and no more than 1% packet loss.
• Video: No more than 150 ms of one-way delay for interactive voice applications (for example, video conferencing); no more than 30 ms of jitter; and no more than 1% packet loss.
• Data: Applications have varying delay and loss requirements. Therefore, data applications should be categorized into predefined classes of traffic, where each class is configured with specific delay and loss characteristics.
Step 2.Categorize traffic into specific categories. For example, you might have a category named Low Delay for voice and video packets in that category. You might also have a Low Priority class for traffic such as music downloads from the Internet.
Step 3. Document your QoS policy and make it available to your users. Then, for example, if users complain that their network gaming applications are running slowly, you can point them to your corporate QoS policy, which describes how applications such as network gaming have best-effort treatment, while VoIP traffic receives priority treatment.

The actual implementation of these steps varies based on the specific devices you are configuring. In some cases, you might be using the command-line interface (CLI) of a router or switch. In other cases, you might have some sort of graphical user interface (GUI) through which you configure QoS on your routers and switches.

QoS Components

QoS features are categorized into one of the three categories shown in Table 10-4.

Table 10-4 Three Categories of QoS Mechanisms

Best effort Best-effort treatment of traffic does not truly provide QoS to that traffic because there is no reordering of packets. Best-effort uses a first-in. first-out (FIFO) queuing strategy, where packets are emptied from a queue in the same order in Which they entered the queue.
Integrated Services (IntServ) IntServ is often reffered to as hard QoS because it can make strict bandwidth reservations. IntServ uses signaling among network devices to provide bandwidth reservations. Resource Reservation Protocol (RSVP) is an example of an IntServ approach to QoS. Because IntServ must be configured on every router along a packet's path, the main drawback of IntServ is its lack of scalability.
Differentiated Services (DiffServe) DiffServe, as its name suggests, differentiates between multiple traffic flows. Specifically, packets are marked, and routers and switches can then make decisions (for example, dropping or forwarding decisions) based on those markings. Because DiffServe does not make an explicit reservation, it is often called soft QoS. Most modern QoS configurations are based on the DiffServe approach.

About The Author

Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, Anthony is also a VMware Certified Professional.

CompTIA Network+ N10-008 Cert Guide contains proven study features that allow you to succeed on the exam the first time. Expert instructor Anthony Sequeira shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills, essential for successful completion of the performance-based testing items on the exam. This complete, CompTIA-approved study package includes the following:

A test-preparation routine proven to help you pass the exams
Clearly defined chapter learning objectives covering all N10-008 exam topics
Chapter-ending review questions and exam preparation exercises, which help you drill on key concepts you must know thoroughly
The powerful Pearson Test Prep practice test software, complete with hundreds of well-reviewed, exam-realistic questions, customization options, and detailed performance reports
40 performance-based exercises to help you prepare for the hands-on exam questions
A free copy of the CompTIA Network+ N10-008 Simulator Lite software, complete with meaningful lab exercises that enhance your hands-on skills
More than 60 minutes of video mentoring
A final preparation chapter that guides you through tools and resources to help you craft your review and test taking strategies
An Exam Essentials appendix that quickly recaps all major chapter topics for easy reference, both in print and interactive digital format
A key terms Glossary in both print and on the companion website, which acts as an interactive flash-card application
Study plan suggestions and templates to help you organize and optimize your study time
A 10% exam discount voucher (a $33+ value!)

Well regarded for its level of detail, study plans, assessment features, challenging review questions and exercises, video instruction, and hands-on labs, this approved study guide helps you master the concepts and techniques that ensure your exam success.

Master the topics on the CompTIA Network+ N10-008 exam, including:

Network topologies and media types
IP addressing
Network services
Data center architectures and cloud concepts
Routing, Ethernet switching, and wireless networking
Network availability and disaster recovery
Network security
Remote access
Network troubleshooting

Reader Paulo Cardoso says, "This is a great book. In addition, it comes with great additional resources."

Learn more about the CompTIA Network+ N10-008 Cert Guide at

Learn more at

More Network Troubleshooting and Support Articles:
• Campus and WAN Multilayer Network Design
• Building the Backbone Network
• Network Cabling and Installation
• Structured Network Troubleshooting Methodology Step 6 Verify Full System Functionality and, if Applicable, Implement Preventive Measures
• Why SFP Transceivers Stop Working and Their Possible Troubleshooting Guide
• Network Installation
• The Secret of Maintaining Your Fiber Optic Network
• Restrict User's Logon Hours and Access to Computers
• Network Port Monitoring
• Network Troubleshooting Basics