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200 C
HAPTER
10: C
LASS OF
S
ERVICE
(C
O
S)
Overview
Many network technologies, such as Ethernet and Fiber Distributed Data
Interface (FDDI), have no inherent ability to distinguish between different
types of traffic such as data, voice, and video, or even perhaps between
different data applications. Thus, all traffic competes for the same
bandwidth and is processed in a single queue by network devices. This
approach to network service is described as “best effort” because there is
no way to prioritize certain traffic ahead of other traffic.
If the network load is high and network devices become congested,
certain bandwidth-intensive applications may receive a poor
quality of
service (QoS) . A jittery video conference display that does not reflect
real-time movement or a crisp picture is an example of poor quality of
service.
To overcome this limitation in Ethernet and FDDI, switch and router
vendors developed a variety of QoS-oriented features that work at higher
levels in the Open Systems Interconnection (OSI) model. Users can
configure these features to better control how different types of traffic
are processed and forwarded through the switch and ultimately the
network as whole. QoS techniques are designed to address the different
latency and throughput needs of time-sensitive applications, as well as to
address the desire to prioritize business-critical information over
non-critical information.
While QoS features clearly benefit a network with bandwidth constraints,
they can add complexity and cost into network equipment and
administration activities. Thus, the practical aim of the IEEE 802.1p
standard is to outline a simplified version or subset of QoS techniques
that preserves the high speed, low cost nature of traditional LAN
bridging. Because the IEEE 802.1p standard addresses queuing and
prioritization based on a numeric traffic class but it does not address
bandwidth reservation or other approaches to QoS, the approach is often
distinguished with the term
Class of Service (CoS) .