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:
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