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Quality of Service: An Overview
The constantly changing needs of networks have created a demand for sensitive applica-
tions (such as voice over IP (VoIP) and video conferencing over IP), and networks are being
asked to support increasingly mission-critical data traffic. Providing predictable service
levels for all of these different types of traffic has become an important task for network
administrators. Being able to provide predictable and differentiated service levels is key to
ensuring that all application traffic receives the treatment that it requires to function properly.
This chapter covers several aspects of quality of service (QoS) and discusses how to provide
QoS in Cisco networks. Specifically, this chapter covers the following topics:
• Understanding QoS
• Deploying QoS in the WAN/LAN: High-Level Overview
• Cisco AVVID (Architecture for Voice, Video, and Integrated Data)
• Overview of Integrated Services and Differentiated Services
• Differentiated Services: A Standards Approach
Understanding QoS
The following section defines QoS in terms of measurable characteristics. It is important,
however, to recognize that fully understanding QoS requires more than a definition. To truly
understand QoS, you must understand the concept of managed unfairness, the necessity for
predictability, and the goals of QoS. In addition to a definition of QoS in measurable terms,
the following section explains each of these things, to provide you with a well-rounded and
practical definition of QoS.
Definition of QoS
QoS is defined in several ways, and the combination of all of these definitions is really the
best definition of all. A technical definition is that QoS is a set of techniques to manage
bandwidth, delay, jitter, and packets loss for flows in a network. The purpose of every QoS
mechanism is to influence at least one of these four characteristics and, in some cases, all
four of these
6 Chapter 1: Quality of Service: An Overview
Bandwidth
Bandwidth itself is defined as the rated throughput capacity of a given network medium or
protocol. In the case of QoS, bandwidth more specifically means the allocation of
bandwidth, because QoS does not have the capability to influence the actual capacity of any
given link. That is to say that no QoS mechanism actually creates additional bandwidth,
rather QoS mechanisms enable the administrator to more efficiently utilize the existing
bandwidth. Bandwidth is sometimes also referred to as throughput.
Delay
Delay has several possible meanings, but when discussing QoS, processing delay is the
time between when a device receives a frame and when that frame is forwarded out of the
destination port, serialization delay is the time that it take to actually transmit a packet or
frame, and end-to-end delay is the total delay that a packet experiences from source to destination.
Jitter
Jitter is the difference between interpacket arrival and departure—that is, the variation in
delay from one packet to another.
Packet Loss
Packet loss is just losing packets along the forwarding path. Packet loss results from many
causes, such as buffer congestion, line errors, or even QoS mechanisms that intentionally
drop packets.
Table 1-1 shows examples of the varying requirements of common applications for
bandwidth, delay, jitter, and packet loss.
Managed Unfairness
Another, more pragmatic definition of QoS is managed unfairness. The best way to explain
this definition is using an analogy to airline service. Sometimes airlines are unable to sell
all of their seats in first class, but they rarely leave the gate with first class seats available,
Table 1-1 Traffic Requirements of Common Applications
Voice FTP Telnet
Bandwidth Required Low to moderate Moderate to High Low
Drop Sensitive Low Low Moderate
Delay Sensitive High Low Moderate
Jitter High Low Moderate
Understanding QoS 7
so there has to be some method by which they decide who gets those seats. The gate agent
could swing open the door to the plane and tell everyone to rush onto the plane; whoever
gets to the first class seats first gets them. Some would argue that would be the most fair
way to handle the seating, but that would be very disorderly and probably not a very
pleasant thing to watch. Anyone who flies regularly knows that frequent flier miles are
valuable because you can earn free flights and so on. If you collect enough frequent flier
miles from a specific airline in a single year, however, you will be moved into an elite
frequent flier status and get some extra benefits. One of those benefits is typically some
method by which the most frequent fliers are able to upgrade their coach seat to a first class
seat, when available. Imagine paying full price for a coach ticket to Hawaii, and having no
chance at all to upgrade, while the person beside you is able to upgrade just because he is
a frequent flier. Some would argue that this is unfair. However, it is unfair in a very
controlled way, because there is a specific policy in place that dictates who is eligible for
this upgrade and who is not. This is managed unfairness.
In QoS, managed unfairness is important because sometimes it is necessary to allocate
more bandwidth to one application than another. This doesn’t specifically indicate that
either application is more or less important than the other; rather it indicates a different level
of service that will be provided to each application. That is, the applications may well have
different bandwidth needs, and dividing available bandwidth equally between the two
applications, although fair, might not produce the best results. A good example of such a
scenario is the case of an FTP flow sharing a link with a VoIP flow. The FTP flow is charac-
terized by a large bandwidth requirement but has a high tolerance for delay, jitter, and
packet loss; the VoIP flow is characterized by a small bandwidth requirement and has a low
tolerance to delay, jitter, and packet loss. In this case, the FTP flow needs a larger share of
the bandwidth, and the voice flow needs bounded delay and jitter. It is possible to provide
each flow with what it needs without significantly impacting the service provided to the
other flow. In this case, the allocation of bandwidth is unfair, because the FTP flow will get
more bandwidth, but it is unfair in a very controlled manner. Again, this is an example of
the need for managed unfairness.
Predictability: The Goal of QoS
The successful management of bandwidth, delay, jitter, and packet loss allows for the
differentiated treatment of packets as they move through the network. Unless an implemen-
tation error occurs, all implementations of the differentiated services architecture should
provide the same treatment to each packet of the same type when those packets pass
through a given interface. In Figure 1-1, multiple packets are sent from Bob to the web
server, marked with IP precedence 2.
8 Chapter 1: Quality of Service: An Overview
Figure 1-1 Multiple Packets from Bob Are Sent to the Web Server Marked with IP Precedence 2
In this example, because all HTTP packets from Bob are marked with IP precedence 2, and
the policy on router A’s serial interface is to classify all HTTP packets with IP precedence
2 into the same class, you can assume that these packets will all receive the same treatment.
Because all packets of the same type are going to be treated the same as they egress that
interface, it’s easy to predict the treatment that the next HTTP packet from Bob will receive.
This is a simplified example of the overall goal of QoS: providing predictable service levels
to packets as they move through a network.
It is very important to be able to predict the bandwidth, delay, jitter, and packet loss that can
be expected as packets of a given flow traverse multiple hops in a network. Voice packets,
for example, must not have a one-way delay greater than 150 ms and are very intolerant of
jitter. Being able to say with confidence that voice packets will experience low latency and
jitter at each hop along a given path is critical when provisioning IP telephony solutions.
Bob
Router A
Switch A
Web
Server
All HTTP packets from
Bob are marked with IP
Precedence 2
All HTTP packets marked
IP Precedence 2 will be
treated the same when they
pass through this interface
Router B
Switch B
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