Putting it all Together�Part I: VoIP Network Design

Tuesday Aug 30th 2005 by Mark A. Miller

The design of an adequately provisioned voice/data network begins with careful usage measurement. Fortunately, there are tools to help.

So you’re finally convinced—perhaps for many reasons—that a VoIP network architecture makes sense for your enterprise. But how do you glue all of these disparate piece parts together into one cohesive, converged network? If you have been following this tutorial series, you now have an understanding of the International Telecommunications Union’s (ITU-T) H.323 protocol (Understanding H.323, Part 1, 2, 3, 4), the Internet Engineering Task Force’s (IETF) Session Initiation Protocol (Understanding SIP, Part 1, 2, 3, 4, 5 6), plus issues that effect the Quality of Service (QoS) of the VoIP connection (QoS Issues, Part 1, 2, 3, 4, 5 6, 7, 8, 9). Now it’s time to put it all together, and look at the design and implementation of the VoIP network. This, and the next three tutorials will address those issues, beginning with the subject of VoIP network design.

Let’s describe the importance of the network design phase anecdotally. Suppose that you are responsible for your organization’s data network. This network has evolved over the years, and is now a distributed system, with subnetworks in several cities that are connected by wide area network (WAN) links. Your upper management goes on a cost-cutting binge, and clamps down on corporate business travel, with the intention of replacing business trips with audio/video conferences. You are now charged with the responsibility of implementing an IP-based voice-and-video conferencing network across your existing WAN. But does the existing network infrastructure have the capacity to support this type of application?

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The first place to begin would be to look at the topology of the network, and identify the number and theoretical capacity of all inter-node circuits. Next, you need to get a handle on the current utilization of these circuits, which may require the use of network analyzers and/or conversations with your carriers to determine the average and peak usage of each link. Make any measurements and/or analysis at different times of the day, week, and month, instead of just one reading, as different circumstances (such as month-end accounting) put different loads on the network. When you have finished with that analysis, take a look in the rear-view mirror, to see what type of growth has occurred. Many network managers are surprised to learn that the slow, steady growth of network traffic of a few percent per month can lead to a substantial increase when considered on an annualized basis. In other words, that T-3 circuit (operating at 45 Mbps) that you put in just last year may be getting close to its capacity during peak traffic periods. When you add voice and/or video into the mix, your capacity may well hit the red line.

To complete the growth assessment, you will need to get a handle on the amount of voice traffic that will likely be added to the converged network. Unlike data traffic, which is measured in bits per second, most telephone traffic is characterized using statistical models called the Erlang tables, which were named after A.K. Erlang, a Danish scientist who was an early pioneer in the study of telephone network design. These models are used for traffic engineering studies, and can be applied to a variety of voice-based applications, including PBXs, voicemail, and interactive voice response systems. Typical applications of Erlang analysis would be to calculate the number of lines required for a particular grade of service, the number of call agents that are needed for a given call volume, and so on.

Fortunately, there are number of both vendor- and third-party-developed tools to assist the network manager with these questions. One that has received high marks for several years is a set of tools called the Westbay Traffic Calculators from Westbay Engineers, Ltd. (Crawley, U.K.), with information available at www.erlang.com. Westbay has a varied customer base ranging from operators of local enterprise networks to the largest telecommunications carries in the world. Their flagship product is called Westplan. It allows analysis of existing networks and optimizes networks to suggest the most appropriate network facilities that should be deployed between networked locations. Other products include a suite of on-line traffic calculators, including the Lines to VoIP Bandwidth Calculator. This tool is used to estimate required bandwidth for a given number of voice paths through an IP network. Other calculators in the suite support Erlang models, call center agents and trunks, and much more. Many VoIP vendors also have network design guides specific to their architectures. Visit their websites and see what you can uncover.

Our next three tutorials will discuss various aspects of VoIP network implementation.

Copyright Acknowledgement: © 2005 DigiNet ® Corporation, All Rights Reserved

Author's Biography
Mark A. Miller, P.E. is President of DigiNet ® Corporation, a Denver-based consulting engineering firm. He is the author of many books on networking technologies, including Voice over IP Technologies, and Internet Technologies Handbook, both published by John Wiley & Sons.
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