Networking: Striving for QoS
- By Peter Gray, Peter Gray
- 07/21/05
In the new converged network world, achieving true quality of service is
our goal.
THINK BACK TO when you were in third grade and class was dismissed
for recess. Would the teacher tolerate a mad scramble for the door, with kids
forming a logjam, kicking and screaming to get out? Of course not. If nothing
else than to avoid being sued, your teacher would ensure that everybody formed
a neat and orderly line and filed out in an organized fashion. Occasionally,
the teacher might let well-behaved or high-achieving students move to the head
of the line, while those who misbehaved (or forgot their homework) got stuck
at the end of the line. This is a very basic analogy of how quality of service
(QoS) works on a data network.
Why QoS?
With the growth in voice, video, and data traffic on conveged computer networks,
establishing QoS is essential. QoS is a set of rules and parameters that determines
network traffic prioritization. Think of an Ethernet switch that, like the teacher
in the example, is responsible for getting traffic out the door in the correct
order and in a timely fashion.
Still, who wants to deal with QoS? Why not just keep voice, video, and data
traffic separate? Why introduce more complexity to the network? These may be
some of the questions you are asking yourself when considering what a converged
network has to offer. Reduction of infrastructure and operation costs is a common
goal in converged network implementations, but you may find that user collaboration
and increased productivity are the ultimate benefits of a converged network.
In the past, a solution to network congestion and bottlenecks was to throw
bandwidth at them. But although a properly designed network with ample bandwidth
(and within the campus IT budget) is very important, it d'es not single-handedly
ensure that video or voice will work effectively on a converged network. Without
QoS, voice and video traffic may break up and arrive at varying intervals, causing
delay and jitter, making a conversation or video unintelligible.
QoS and VoIP. Simply put, if you choose to deploy voiceover-
IP (VoIP) on your college campus, QoS on the data network is a must. Campus
personnel and students have an inherent expectation of service and quality.
Campus IT must strive to meet or exceed that expectation, so a VoIP phone should
meet the same expectations of a regular land line. Unfortunately, that’s
not always the case. In certain situations, VoIP can act just like a cell phone
in an area of bad coverage, including problems with echo, dropped or garbled
calls, or slight delays, making a conversation intolerable. Yet, a properly
designed data network with QoS can alleviate, if not prevent, these problems.
(After all, who wants to be the person whose job it is to walk around the campus
picking up VoIP handsets and saying, “Can you hear me now?”)
QoS and video-over-IP. In addition to voice, video conferencing
and distance education have been moving into the IP world, as well. In the past,
college campuses have offered dedicated distance learning in classrooms with
closed, fixed telecommunications circuits. Now, college campuses have the opportunity
to utilize streaming video over IP and plugtheir portable video conferencing/distance
education equipment into any network data port on campus.
To obtain this flexibility, QoS is a must. In fact, QoS is more critical with
video than audio because in video, you are dealing with both video and audio.
If a person’s facial expressions or mouth movements do not match the audio,
communication can be misinterpreted. IP video also demands more bandwidth than
either data or voice-only traffic, potentially leading to new congestion on
a network that might have been fine previously.
In addition to addressing QoS when deploying converged networks, the campus
IT department should assess the overall network (bandwidth, physical infrastructure,
etc.) to guarantee that voice and video will work on the current IP network.
Often overlooked, cabling infrastructure plays an important role in any network.
The infrastructure, including all copper and fiber cables on campus, should
be reviewed to make sure it meets transmission standards. Companies such as
Viola Networks (www.violanetworks.com)
provide network assessment tools to determine whether a network can handle a
converged network deployment.
QoS Features and
Techniques
Because of the increase in multimedia applications riding on networks, many
networking equipment manufacturers have no choice but to provide QoS features
on their equipment, or be left in the technological dust. These features allow
campuses to establish end-to-end QoS on their network. 802.1p, diffserv, and
RSVP are some of the QoS techniques available for ensuring stable transmission
of voice and video traffic.
802.1p. The 802.1p standard was defined by the Institute of
Electrical and Electronics Engineers (IEEE) for assigning network traffic prioritization.
802.1p is often described as a “best effort” QoS method because
it delegates traffic prioritization on the fly, and d'es not take into consideration
the congestion of a network path.
Network traffic is prioritized into eight levels (0-7) designated by a three-bit
field at the beginning of a network frame (e.g., an Ethernet frame). The 802.1p
protocol works at layer 2 (layers 2 and 3, mentioned later, are defined by the
Open Source Interconnection model) on a network. An Ethernet switch port will
separate traffic, based on the frame’s prioritization, into different
queues: high, medium, normal, and low. The switch will ensure that all traffic
in the high queue is sent out before traffic in the middle queue, and so on.
In a perfect network, where every switch is non-blocking and there are no traffic
bottlenecks, you could get by with just 802.1p as the only means of QoS. Unfortunately,
unlimited and unhindered bandwidth d'esn’t exist on most college campus
networks, and other features may be necessary.
Diffserv. TCP/IP is capable of defining prioritization for
use with QoS. Part of the IP packet has been designated as a type of service
(ToS) field to prioritize traffic, much in the same fashion as 802.1p. Differentiated
services (diffserv) use this field to determine QoS for IP packets. Diffserv
employs various queuing methods in addition to traffic prioritization to manage
the flow of traffic across a network at layer 3. Unlike 802.1p, diffserv applies
QoS prioritization to an entire flow of IP traffic instead of individual packets.
In a high-speed network, diffserv will probably be the QoS method of choice,
but when bandwidth is constrained, resource reservation protocol (RSVP) should
be considered.
RSVP. RSVP is another form of QoS that guarantees effective
transport of traffic across a network. It works with routers along the entire
path to reserve resources at each node along the path. Basically, RSVP carves
out a guaranteed connection from end to end, for each transmission.
Bandwidth shaping and limiting. In addition to QoS, bandwidth
shaping or limiting may be necessary to ensure that heavy users (like students
downloading “non-school related materials”) do not impede the bandwidth
needed by converged network applications. Bandwidth limiting can set a threshold
on the maximum amount of bandwidth one individual can use; this prevents a user
from monopolizing the campus network bandwidth and potentially jeopardizing
needed bandwidth for other applications and users. Packeteer (www.packeteer.com)
is one vendor offering products for bandwidth control, QoS, and management.
Getting Started
If your institution is moving to a converged network, some consideration must
first go into the data network, to ensure that the traffic plays nice and follows
the rules. Initially, your campus IT department should examine the network to
see if it is up to handling the added bandwidth that voice, and especially video,
introduce. Once the network is in shape, QoS and, potentially, bandwidth shaping
and limiting can be introduced to ensure proper delivery of voice/video traffic.
In the end, the goal is to make sure your voice and video traffic will get
along, providing your campus with a state-of-the-art, multimedia-rich network.
