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What your manageable electronics can tell you before you implement VoIP

VoIP needs a healthy network to perform optimally. In this tip, Carrie Higbie tells you how to take your network's pulse in preparation for a VoIP implementation.

Carrie Higbie
In a recent webcast, we discussed performance management and what to look for when you examine your statistics. One of the worst statistics you can consider as a means to determine your network health is utilization. There are other statistics that are much more valuable. It is important to look at utilization, but this is only a small piece of health.

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The problem with utilization is twofold. First, it is nearly impossible to determine when the workstation is actually in use. Even if someone is sitting at his desk, he may be on the phone and not using the network. Also, many users work locally and then save their work to the network when complete. So in utilization you have to know when the network is really in use to determine how much of the bandwidth is being consumed. Look at the following two diagrams, for instance.

Figure 1. Averages over one week

Figure 2. Utilization averages over one month

In Figure 1, above, the utilization was measured on the inbound side for a week. Figure 2 shows the same circuit measured over one month. As you can see, the differences in utilization are rather large. When planning for VoIP, you should assume that the peak happens all the time. If not, when processing becomes heavy, you will degrade your voice signal because you have not planned for it.

It is also important to examine buffer space and discards on your active electronics. Switches discard packets as a function. When the buffers get too full, they will drop packets and request a retransmission from the sender. This is not a desirable "feature" for voice. While you can set up VLANs and priority, overloaded gear will not help. In particular, you want to check your discards on any uplink port, and any port that is commonly attached (for instance, where the IP switch may be).

Some errors that you will find in your SNMP data also bear investigation. The most important are bit errors. These may be expressed as InErrors and OutErrors. Not all manageable systems will allow you to drill down further into the error state. Some will allow it, and speed up the troubleshooting process. Anytime you see these errors, the first thing you should do is test your cabling channel that is connected on that port. A brief word about cable testing: Make sure the tester has the latest revision of software and firmware and has been calibrated recently. You also want to be sure that your interfaces and/or patch cords are relatively new. Each has a limited number of mating cycles, and a channel may look bad when in fact it is not.

Next, check your duplex configurations. Duplex mismatches and/or channels that have auto-negotiated to half duplex will further limit your operations. It is important to have full duplex links. A hard setting in either the switch or at the workstation, or faulty cabling, including channels that exceed the maximum distance, can cause half duplex links.

After you fix your errors, you will want to take another network pulse for 30 days. The reason that I recommend a 30-day window is to allow for such things as month-end processing and other functions that do not happen on a daily basis. A Certified Infrastructure Auditor can assist with all of these steps. For more information on specific errors, see the article Common network errors and causes.

About the author:
Carrie Higbie, global network applications market manager for The Siemon Company, has been involved in the computing and networking industries for nearly 20 years. Carrie has taught classes for Novell, Microsoft and Cisco certifications, as well as CAD/CAE, networking and programming on a collegiate level, and serves as the "Preparing your network for VoIP" site expert on

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