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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

IssueDescription
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, StormWind.com. Anthony is also a VMware Certified Professional.

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