Capacity Assessment Is The
Key To A Successful Network
BY PHILIP JOUNG
With the rapid growth in data networking over the past
decade, TCP/IP has emerged as the preeminent protocol running throughout
much of our global network. Myriad applications and protocols run on
TCP/IP-based networks, making its reliability and performance an important
issue. Packet-based voice solutions have recently surfaced to take advantage
of the efficiencies expected when using TCP/IP, the notable example being
VoIP. VoIP aims to make voice communications not only less expensive, but to
bring increased features to the end user, more manageability to the
enterprises that use it, and high profitability for companies that succeed
in delivering robust VoIP solutions.
Now, consider your current POTS phone line ï¿½ it has become
so reliable that barring extreme circumstances, everyone expects to get a
successful dial tone and very high voice quality during a call. Current
metrics put the uptime of POTS at 99.999 percent, making it one of the most
reliable public utilities we use. The dedicated circuit used during a call
also means a relatively high degree of security. Given this high
reliability, quality, and security, VoIP ï¿½ as a competing technology ï¿½
has much to live up to.
Of all the protocols and applications that run on TCP/IP
networks, VoIP is one of the most sensitive to network issues. Issues that
are at best annoying with many other protocols and applications can often
render VoIP unusable. E-mail and the protocols it uses, for example, can
typically tolerate latencies in transmission, and are not significantly
affected by jitter, IP fragmentation, or small amounts of packet loss.
However, introduce small amounts of jitter, latency, or packet loss, and
voice quality and usability with VoIP quickly degrades to an unacceptable
Given the complexity of most enterprise networks, adding
VoIP infrastructure and functionality is a complex undertaking. Bandwidth
must be allocated, quality of service rules devised and configured, and
security features implemented. The only way to know for sure that VoIP
service will be acceptable is through rigorous capacity assessment.
What is capacity assessment? In general, it can be defined as testing
the network under loads that replicate real-world conditions. We all
understand that live networks (especially the Internet) will have a number
of factors that complicate matters ï¿½ large numbers of users performing
large numbers of different tasks; different devices that react to and often
alter the traffic; link latencies; packet loss; jitter; and the like.
Capacity assessment incorporates these different factors during testing to
ensure that the results of testing closely resemble what is or will be
experienced in a live network. It combines the pertinent factors in user
behavior and network behavior to assess the reliability, availability,
scalability, and performance of the system under test. Capacity assessment
with realism is the best way to achieve confidence in the final results.
Unless a network is dedicated to VoIP, many other things
will be happening on the network. In most VoIP deployments, a shared network
will be a given, as one of the major advantages for VoIP centers on saving
money, and a dedicated network will typically not be cost effective. VoIP
deployments should expect a network handling traffic with many different
purposes, often with differing value and criticality to the enterprise.
Because of this diversity in network traffic, realism in the
capacity assessment of critical networks will incorporate not only the
application that is being tested (e.g., VoIP), but also all the other
traffic that is running concurrently with that application (Web browsing,
FTP, e-mail, streaming media, etc). These different protocols and
applications will cause different stresses to the network, helping to locate
possible issues and bottlenecks during testing.
Most large networks will have users that fall into a certain pattern of
usage. Some users will mostly browse the Web, others mainly download large
files, others primarily talking on the phone via VoIP, and still others
doing various other tasks. Quantifying these different user populations (and
at times, predicting how their behaviors might evolve in the future) in
order to incorporate their behaviors into testing will make capacity
assessment results much more predictable, solid, and reliable.
Because of the variability of network traffic, Iï¿½ve
included a list of user behaviors below that might be encountered in real
networks. While they do not always directly apply to VoIP, they can cause
effects that matter for other applications, indirectly affecting VoIP.
Downloads: Downloads of many small files simultaneously,
large files, or streaming media can all have detrimental effects on network
Bursty traffic: Users typically use networks in spurts, with
periods in which the traffic jumps significantly from its normal amount.
Simulating this user behavior can help ensure that the network handles these
challenges effectively. These peaks often happen during critical periods,
such as the crucial holiday buying season for an e-commerce Web site, the
busy call volumes during the morning, or the streaming broadcast of a major
Frustration thresholds/user aborts: People browsing the Web
often become frustrated when pages do not load after a few seconds and
either try to reload or hit the stop key. This can cause further load on the
system, since it continues to work on the original request even though the
user already has moved on.
SSL usage: Use of secure sockets layer to encrypt
data increases the security of network transactions, but carries a big price
in terms of processing and network bandwidth.
Use of stateful applications: This can generally be
defined as applications that keep track of the user throughout the session.
FTP logins, and SSL session IDs.
Browser versions: Different Web browsers can cause
different types of load on a system. Some Web sites tailor content to
specific browsers, which also uses processing cycles.
An important part of overall network performance will depend on what is
happening to the traffic as it flows across the network. When certain
conditions occur, they often create significant effects on system
performance. Below are some common network issues:
Packet loss: Packet loss often happens from network/
system congestion, or transmission errors. Studies have shown that FTP
performance drops by 50 percent with only three percent packet loss. Small
amounts of packet loss cause clicks and pops in VoIP, but high levels can
make the conversation unintelligible, or completely fail.
IP fragmentation: Devices often have a maximum amount
of data they can accommodate at one time. When a device receives data that
exceeds its maximum amount, it can fragment it into multiple chunks that
need to be reassembled at the final destination. This reassembly takes
processing, especially when the fragmented data arrives out of order.
Jitter: Applications that deal with real-time data,
such as VoIP and streaming media can be especially sensitive to jitter.
Jitter occurs when network data arrives at different rates. ï¿½Timing is
Latencies: High network latencies spell doom for
real-time applications like VoIP. Indeed, a VoIP conversation becomes
difficult to manage when network latency increases much above 250
milliseconds ï¿½ people often end up speaking on top of one another. Network
systems also exhibit increased loads with higher latencies because they must
deal with increased network connections and buffering.
Data corruption: Corruption is usually a rare event
in most modern networks. However, when it happens, the effects can be
similar to packet loss.
QUALITY OF SERVICE
Because of the sensitivity of VoIP to network issues, having control of
the user traffic being handled by the network is valuable. Mechanisms abound
for helping to control this traffic, with the majority falling into the QoS
(quality of service) umbrella.
QoS allows the network administrator to set priority levels
for traffic. Therefore, VoIP traffic can be given a higher priority than
say, instant messaging or Web surfing traffic. Capacity assessment will not
only help ensure that the network properly applies and follows the rules set
by the administrator, but also provide understanding into the effect of the
rules on user traffic, especially for traffic that has been given lower
priorities. Capacity assessment also provides information about how the QoS
rules and the network will degrade once it approaches its capacity. This can
be important in helping to predict failures before they occur, giving the
administrator a chance to make changes to the network in advance of any
PAY A LITTLE NOW, OR PAY A LOT LATER
Given the complexity of the networks running at most enterprises and the
business critical nature of the applications running on those networks,
real-world capacity assessment should be part of any system prototyping or
deployment. Although it involves a small amount of extra effort earlier on,
the gains in system stability, performance, availability, and ultimately
improved customer satisfaction and time-to-market more than pay for the
up-front effort. Factor in the peace of mind gained from capacity
assessment, and it becomes apparent that the justifications for not
conducting capacity assessments are poor.
Capacity assessment can take place at all stages of a
system project: Product selection: While scoping out different vendorsï¿½
products, capacity assessment can prove, using your own parameters and
network traffic mix, how the product functions and performs.
System design: Capacity assessment can help one
choose between different design ideas, demonstrate scalability, compare
performance, and can even scrutinize the tolerance of a design to certain
user and network behaviors.
System deployment: Capacity assessment can help
ensure that expected performance levels are being maintained and help adjust
Post-deployment: Realistic capacity assessment will
validate the final performance of an entire system, providing a baseline
from which future upgrades can be considered.
VoIP holds a lot of promise, and many investigations have
commenced towards determining this technologyï¿½s eventual fit into the
enterprise. Given the quality of the current POTS system in use, VoIP
deployments must be sure to come close to matching that quality, or risk
user complaints or even complete removal of the poorly functioning VoIP
system. Real-world capacity assessment offers that extra insurance that the
final VoIP deployment functions properly and scales effectively. The fact
that the rest of the network will benefit from this assessment is simply an
extra bonus that can help justify the effort.
Philip Joung is director of technical marketing at Caw
Networks. Caw Networksï¿½ products are robust, customer-ready, highly
scalable appliances that deliver unmatched performance and ease of
deployment, use, maintenance, and administration. For more information visit
the company online at www.caw.com.
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