IP quality of service is becoming increasingly important as customers add bandwidth-intensive services and applications to their networks. Value-added resellers (VARs) and systems integrators who can troubleshoot and improve sub-optimal performance in a converged network environment have marketable skills. Chapter 2, IP quality of service, from CCNP ONT Official Exam Certification Guide, provides an introduction to QoS, identifies and compares QoS models, and describes QoS implementation methods. This excerpt defines quality of service and the three steps to implementing it.
Defining quality of service
Following is the most recent definition that Cisco educational material provides for QoS:
QoS is the ability of the network to provide better or special service to a set of users or
applications or both to the detriment of other users or applications or both.
The earliest versions of QoS tools protected data against data. For instance, priority queuing made
sure packets that matched an access list always had the right of way on an egress interface. Another
example is WFQ, which prevents small packets from waiting too long behind large packets on an
egress interface outbound queue. When VoIP started to become a serious technology, QoS tools
were created to protect voice from data. An example of such a tool is RTP priority queue.
RTP priority queue is reserved for RTP (encapsulating voice payload). RTP priority queuing ensures
that voice packets receive right of way. If there are too many voice streams, data applications begin
experiencing too much delay and too many drops. Strict priority queue (incorporated in LLQ) was
invented to limit the bandwidth of the priority queue, which is essentially dedicated to voice
packets. This technique protects data from voice; too many voice streams do not downgrade the
quality of service for data applications. However, what if there are too many voice streams? All
the voice calls and streams must share the bandwidth dedicated to the strict priority queue that is
reserved for voice packets. If the number of voice calls exceeds the allocated resources, the quality
of those calls will drop. The solution to this problem is call admission control (CAC). CAC
prevents the number of concurrent voice calls from going beyond a specified limit and hurting the
quality of the active calls. CAC protects voice from voice. Almost all the voice requirements apply
to video applications, too; however, the video applications are more bandwidth hungry.
Enterprise networks must support a variety of applications with diverse bandwidth, drop, delay
and jitter expectations. Network engineers, by using proper devices, Cisco IOS features and
configurations, can control the behavior of the network and make it provide predictable service to
those applications. The existence of voice, video and multimedia applications in general not only
adds to the bandwidth requirements in networks but also adds to the challenges involved in having
to provide granular and strictly controlled delay, jitter and loss guarantees.
Implementing quality of service
Even though many common applications and protocols exist among enterprise networks, within
each network, the volumes and percentages of those traffic types vary. Furthermore, each enterprise
might have its own unique application types in addition to the common ones. Therefore, the
first step in implementing QoS in an enterprise is to study and discover the traffic types, and define
the requirements of each identified traffic type. If two, three or more traffic types have identical
importance and requirements, it is unnecessary to define that many traffic classes. Traffic
classification, which is the second step in implementing QoS, will define a few traffic classes,
not hundreds. The applications that end up in different traffic classes have different requirements;
therefore, the network must provide them with different service types. The definition of how each
traffic class is serviced is called the network policy. Defining and deploying the network QoS
policy for each class is step three of implementing QoS. The three steps of implementing QoS on a
network are explained next:
Step 1: Identifying traffic types and their requirements
Step 2: Classifying traffic based on the requirements identified
Step 3: Defining policies for each traffic class
Reproduced from the book CCNP ONT Official Exam Certification Guide Copyright [2007], Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all.
