Quality of Service: A Has Been? Or Up And
BY JOHN WOOD
Quality of Service (QoS) is a technology that has seen a slow rise to fame.
Even though it has been available to network administrators for some time,
its lack of a killer application and the challenges associated with
implementation have kept deployments to a minimum. However, with the
increasing adoption of real-time applications, such as VoIP and streaming
video, network QoS is finally becoming a reality.
QoS is based on a simple concept: it gives one class of network traffic
priority over another class of traffic to achieve acceptable application
performance. To make it work, application traffic must be tagged with a
priority identifier, and then the network must consistently handle the
traffic in accordance with the tagging.
Tagging mechanisms and the ability to enforce priorities in network devices
have been available for some time. But historically, neither tagging nor
prioritization has been easy to implement because the network administrator
has to understand all the complexity of QoS. Since application traffic is
not automatically tagged, the administrator has to design the tagging for
each application and make sure that it is applied at every network hop. This
requires purchasing and installing a special box to tag packets, or
configuring routers to do it.
Thatï¿½s only half the battle. Once the packets are tagged, all the devices in
the network must be set to prioritize the traffic consistently to apply an
overarching QoS policy. To implement this, the administrator needs to
understand different techniques for prioritizing, choose the best QoS
mechanism for each network device, and then configure it on each network
device. And the same QoS policy has to be maintained as new devices come
online or are serviced.
Understandably, anyone considering implementing QoS has to be motivated by
an urgent need. Performance for traditional business Transmission Control
Protocol (TCP) applications such as SAP and Siebel is important, but even
with slow performance the application will continue to work. If a SAP
transaction sometimes takes 10 seconds instead of the normal one second, it
may be irritating to the user, but all the data is there and they can still
do their job. If the application stopped working when response time got to
10 seconds then the need would be more urgent.
Therefore, while some network administrators adopt QoS for their TCP
applications, it hasnï¿½t gained wide acceptance. The implementation cost is
high, and it addresses a problem -- application response time -- which is
more of a nice-to-have than a must-have.
But the QoS landscape is changing. With the growing popularity of VoIP and
other real-time applications such as streaming video, QoS is becoming a
necessity to achieve expected network performance. As a result, network
vendors are working hard to lower the cost of implementation.
QoS AND VoIP
Unlike TCP business applications, which benefit from QoS, VoIP and video
rely on it. Because VoIP and video are real-time applications, they require
strict performance levels. Since users are accustomed to the high quality of
the telephone network, their demands for VoIP performance are high. Where
users of TCP applications are satisfied with response times of two or three
seconds, a VoIP call begins to see degradation after one-way delay exceeds
150 ms. When one way delay on a VoIP call reaches 500ms the quality of the
call becomes unusable. Because of the real-time nature of VoIP, if data is
lost then it is never recovered. With a TCP business application the data
will be retransmitted and the transaction is slower, but the data is not
lost. VoIP also requires a small variation in the arrival time of packets
(jitter). If the jitter on a network is too high then a packet is as good as
lost. VoIP requires networks with low delay, low data loss, and low jitter.
VoIP implementations often require network upgrades to handle the additional
demands. Additional bandwidth, conversion to an all-switched architecture,
and equipment upgrades may all be needed. While most networks require
upgrades to support VoIP, all networks require QoS to run VoIP with
acceptable quality. The network may have plenty of bandwidth, but those big
pipes feed into smaller pipes, and many pipes feed into a few. The potential
for congestion arises at each such intersection. Even in a completely
switched environment, a many-to-one case inevitably crops up. For example,
say you have 24 users on 100Mb links, all talking to a server on a 100Mb
link. If each person is using even 1/10th of the available bandwidth, the
link will experience a serious congestion problem. In congested conditions,
packets are either dropped or delayed. Those dropped or delayed packets will
severely damage VoIP call quality.
Because VoIP and video require QoS to be successful, the developers of VoIP
and video applications are very motivated to enable QoS mechanisms.
Traditionally, the manufacturers of network equipment didnï¿½t create the
applications in use on the network. Now, however, the same vendors who build
the network infrastructure are selling applications such as VoIP and video.
Recognizing the importance of QoS to the success of a VoIP deployment,
vendors have made great strides toward facilitating the deployment of VoIP
QoS. The vendors have therefore successfully addressed both QoS tagging and
implementing QoS policies in the network.
EASE OF QoS IMPLEMENTATION
The first step toward facilitating QoS is addressing the problem of tagging
an applicationï¿½s packets. VoIP phones automatically tag packets with bit
settings that conform to the 802.1p and DiffServ Code Point (DSCP) QoS
standards. 802.1p is a Layer-2 technology that places a four-byte tag on
each Ethernet frame, allowing a switch to prioritize. DSCP operates at Layer
3 by setting six bits in the IP header, providing classes for router
Since a VoIP phone sets 802.1p and DSCP automatically, a network thatï¿½s
configured correctly can prioritize VoIP above other traffic at all points
in the network. Prioritization is usually performed by means of queuing on a
given device. In a switch or router, a special queue for VoIP will be set
aside so the VoIP traffic can always flow through smoothly. Class-Based
Weighted Fair Queuing is an example of the queuing performed by many
routers. Cisco has implemented LLQ, which is essentially Class-Based
Weighted Fair Queuing, but with a specific queue set aside for VoIP.
Vendors are also working to make the implementation of QoS policy easier in
the network. Most vendors offer a QoS policy manager that will configure all
the devices in the network to prioritize VoIP. Policy managers, along with
automatic packet tagging, have greatly simplified VoIP QoS implementation.
Ciscoï¿½s has implemented AutoQoS in their devices to automatically implement
a ï¿½VoIPï¿½ policy consistently in the network. AutoQoS works in conjunction
with Ciscoworks QoS Policy Manager to automate QoS policy settings. Nortel
offers similar functionality through a product called Optivity Policy
Another reason QoS is becoming easier to deploy is that itï¿½s getting easier
to test, tune, and monitor. Verifying that QoS is operating as desired is a
vital step in any deployment. This can be done by running traffic over the
network and measuring its performance. Accurately emulating VoIP or video
transmissions is the best way to determine whether a QoS policy is working
correctly before deploying the VoIP or video application. In addition to
testing, itï¿½s also important to monitor specific devices to ensure that all
QoS settings are implemented consistently, from one end of the network to
Vendors have made great strides in recent years to make QoS easier to
deploy and manage, but there is still work to be done. One problem with the
current policy managers is that they work almost exclusively with a single
vendorï¿½s equipment. Users donï¿½t typically replace all boxes in the network
and instead tend to leverage as much of their network investment as
possible; therefore, the vendor-specific policy managers often donï¿½t handle
their situations. QoS must be implemented consistently across a network for
it to be effective.
With the increasing adoption of applications such as VoIP and video that
require QoS, and with vendorsï¿½ desire to see these applications deployed as
widely as possible, QoS is becoming much easier to implement -- not just for
VoIP and video, but for all applications. No longer an afterthought or a
painful undertaking for network administrators, QoS is becoming an integral
part of application deployment.
John Wood is development manager at NetIQ Corp. NetIQ is a leading
provider of Systems Management, Security Management and Web Analytics
solutions. Historically focused on the Windows management market, NetIQ now
delivers cross-platform solutions that enhance business performance
resulting in higher returns on infrastructure and web investments. For more
information, please visit the company online at
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