Content Overview As user applications
continue to drive network growth and evolution, demand for
support of different types of traffic also increases.
Applications with differing network requirements create the
need for administrative policies that control how the network
treats individual applications. The network must service
requests from business-critical and delay-sensitive
applications with priority. The deployment and enforcement of
quality of service (QoS) policies within a network plays an
essential role in enabling network administrators and
architects to meet networked application demands in converged
networks. QoS is a crucial element of any administrative policy
that decides how to handle application traffic on a network.
This module introduces the concept of QoS, explains key issues
of networked applications, lists models for providing QoS in
the network (best effort, integrated services [IntServ], and
differentiated services [DiffServ]), and describes various
methods for implementing QoS, including the Cisco Modular QoS
CLI (MQC) and Cisco Router and Security Device Manager (SDM)
QoS Wizard. Web Links Quality of Service
Networking
http://www.cisco.com/univercd/cc/td/doc/cisintwk/
ito_doc/qos.htm Network QoS using Cisco HOWTO
http://www.opalsoft.net/qos/WhyQoS.htm What Ever Happened to
Quality of Service (QoS)?
http://www.convergedigest.com/tutorials/qos1/
page1.asp
Configure QoS on your Cisco router with this
template
http://articles.techrepublic.com.com/
5100-1035-6136216.html Understanding Delay in Packet Voice
Networks
http://www.cisco.com/warp/public/788/voip/
delay-details.html QoS Congestion Avoidance
http://www.cisco.com/en/US/tech/tk543/tk760/
tsd_technology_support_protocol_home.html QoS Congestion
Management (queuing)
http://www.cisco.com/en/US/tech/tk543/tk544/
tsd_technology_support_protocol_home.html Resource
Reservation Protocol
http://www.cisco.com/univercd/cc/td/doc/cisintwk
/
ito_doc/rsvp.htm#wp1023211 Cisco AutoQoSA New
Paradigm For Automating the Delivery of Network QoS
http://www.cisco.com/en/US/tech/tk543/tk759/tk879/
tsd_technology_support_protocol_home.html
Content
3.1 Introducing QoS 3.1.1
Converged Network Quality Issues Before network
convergence was commonplace, network engineering focused on
connectivity. Figure shows how nonconverged networks met
differing traffic needs simply by connecting endpoints over
dedicated linksdata to data, voice to voice, and video to
video. Data rates delivered to the network links may result in
sporadic bursts of data. Access to the bandwidth in these
networks is on a first-come, first-served basis. The data rate
available to any one user varies depending on the number of
users accessing the network at that time. Protocols are used in
nonconverged traditional networks to handle the bursty nature
of data networks. Data networks can survive brief outages. For
example, when you retrieve e-mail, a delay of a few seconds is
generally not noticeable. A delay of several minutes is
annoying, but not serious. Traditional networks also had
requirements for applications such as data, video, and Systems
Network Architecture (SNA). Since each application has
different traffic characteristics and requirements, network
designers deployed nonintegrated networks. These nonintegrated
networks carried specific types of traffic: data network, SNA
network, voice network, and video network. Figure illustrates a
converged network in which voice, video, and data traffic use
the same network facilities. Merging these different traffic
streams with dramatically differing requirements can lead to a
number of problems. Key among these problems is the fact that
voice traffic and video traffic are very time-sensitive and
must have priority. In a converged network, constant,
small-packet voice flows compete with bursty data flows.
Although the packets carrying voice traffic on a converged
network are typically very small, the packets cannot tolerate
delay or variation in delay while they traverse the network.
When delay and delay variation occur, voices break up and words
become incomprehensible. Conversely, packets carrying file
transfer data are typically large and the nature of IP lets the
packets survive delays and drops. It is possible to retransmit
part of a dropped data file, but it is not feasible to
retransmit part of a voice conversation. Critical voice and
video traffic must have priority over data traffic. Mechanisms
must be in place to provide this priority. Another reality of
the converged network is that service providers cannot have
failures when voice and video traffic are involved. Although a
file transfer or an e-mail packet can wait until a failed
network recovers and delays are almost transparent, voice and
video packets cannot wait. Converged networks must provide
secure, predictable, measurable, and, sometimes, guaranteed
services. Even a brief network outage on a converged network
seriously disrupts business operations. Network administrators
and architects achieve required performance from the network by
managing delay, delay variation (jitter), bandwidth
provisioning, and packet loss parameters with quality of
service (QoS) techniques. Multimedia streams, such as those
used in IP telephony or videoconferencing, are very sensitive
to delivery delays and create unique QoS demands. If service
providers rely on a best-effort network model, packets may not
arrive in order, in a timely manner, or at all. The result is
unclear pictures, jerky and slow movement, and sound that is
not synchronized with images.
Content 3.1