An effective network provides more than just connectivity between endpoints.
As you add applications and leverage the convenience of networking, your network
must be intelligent enough to recognize and prioritize mission-critical and
delay-sensitive traffic. This ability to deliver data based on such policies
as importance and time is called the quality of service (QoS) capability
of the network.
The network serves a wide range of applications for your organizationmany
more applications than the mere file and printer sharing of the past. In addition
to standard IP traffic such as web (HTTP), File Transfer Protocol (FTP), Telnet,
and Simple Mail Transfer Protocol (SMTP), networks are carrying:
Real-time, mission-critical corporate datafinancial transactions,
customer orders, warehouse and shipping records, manufacturing statistics
and control data, research and development CAD/code files, and so on
Delay-sensitive datainteractive applications such as server
and mainframe logins, packetized voice, video conferences, and data
collaboration such as electronic whiteboard and client GUI sharing
Bulk data transferssystem backups, overnight data synchronization
Unknown dataUncontrolled or unknown traffic such as user-initiated
applications (networked games and shareware, for example)
Despite the broad range of users and applications, many networks give
equal access to all users and the same delivery priority to all applications.
They do not inspect or care about the contents of the data.
With an ever-increasing number of new applications on the network,
the idea of equal priority to all applications becomes a problem. The problem
is that applications vary by how they use the network,
how they behave during network congestion, and how important they are to your
Not all applications are equal. Applications that are mission-critical
to your organization deserve preferential and speedy delivery. Some applications
may be delay-sensitive and need low delay through the network in order to
function properly. Yet other applications may be low priority and should yield
network resources to high-priority applications.
After all, your network is a valuable resource that is shared and finite
A network that can vary performance based on application type is said
to have classes, or qualities of service. A QoS is a grade
of performance the network provides and differentiates from other grades of
performance in the network. A high QoS provides faster delivery and less delay
than a low QoS. By assigning applications to different QoS levels, you can
vary the performance of applications to reflect your organization's objectives.
consider many factors when defining the QoS for an application. The most common
criteria are as follows:
Mission-critical versus non-mission-critical Does this
application directly affect my organization's profits and sales? How
will my customers perceive the delays in this application, and what
is the impact? Does this application affect how quickly I can bring
my product to market?
Delay-sensitive versus delay-insensitive How easily does
the user of the application perceive delay? Even if an application
is not mission-critical, it may require minimal delay because a human
is interacting with it in real time. Login applications such as Telnet
are delay-sensitive because typing is difficult when there is a perceptible
delay between keystrokes and the display of those keystrokes. On the
other hand, non-interactive applications (such as file system backup
or FTP) may be triggered and completed with minimal or no user intervention.
Applications such as voice and video over IP are delay-sensitive. These
applications need consistent, predictable bandwidth and low delay; otherwise,
the transmission may appear garbled or choppy. This does not necessarily mean
that all users of voice and video should be given high
priority, as it might be desirable to set policies (in conjunction with QoS)
to limit these applications to certain users.
Political versus apolitical Whose data is this? What users
should get better service from the network?
offers several IOS services for delivering QoS. As we cover each service in
the following sections, keep in mind that each service has its own behavior,
purpose, and place in the network. It is not enough to know how to configure
these services, but rather, to know how they work, when they might be needed,
and where they should be placed in the network. This is covered in the following
sections as well as in Chapter 5.