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Testing the Experts
Question:
"Customers are asking for VoIP telephones as part of the cabling package. Is there anything special I need to know about cabling for VoIP?"
Answer:
As with so many topics in our business, there is both a short answer and a long answer. The short answer is "No; if you install and certify to standards, your VoIP installation should perform just fine." The long answer is "it depends how the installation was designed, and that may not be in your hands."
One of the advantages of VoIP is that it can travel on the same network that handles data traffic. So in general, if you have installed all the structured cabling with normal care, and certified each link to at least Cat 5e standards, the cabling should be able to transmit VoIP successfully.
Now comes the tricky part. VoIP's big advantage, using the same network that handles data, is also the big reason that VoIP call quality may not meet expectations. Phone calls happen in real-time. A person listening to a call expects a steady stream of understandable sound coming from the phone. But since VoIP sends digitized voice packets over the data network, there may be conflicts on which packets reach their destination first.
The time interval it takes for a packet to arrive at the receiving phone is called latency. Excessive latency is noticeable to the listener. Even more noticeable is jitter. Jitter is packets arriving with different amounts of latency, and can make the call sound like it is speeding up, slowing down or not fitting together correctly. Finally, dropped packets can lead to gaps in the dialog and poor listening quality.
The solution to these problems is not really in the hands of the cable installer. The network must be designed and configured to prioritize VoIP traffic. This is the responsibility of the network designer and more importantly, the network manager. But there are some things the installer can do that go beyond certification of the cabling. If the installer has access to the network when it is activated, tests can be performed to verify Power over Ethernet (PoE) levels that are used to power the VoIP phones. Often times this can be done with a tool that also checks network connectivity and communication ability. At the same time, the installer can also conduct physical layer tests that include wiring, patch cables and switch port configuration.
In addition, the installer can check for communication between the VoIP phone and the nearest switch. Call setup, configuration and tear down signaling should all be visible using the latest network connectivity testers that include VoIP monitoring capability.
If your customers plan on using VoIP phones, most installers should be able to supply the cabling needed. Use your normal installation practices, certify, and if you can, test for power and connectivity between the phone and the switch. That part of the installation is in your control.
Answer provided by Robert Finlay, Fluke Networks Product Manager
Reprinted with permission of Cabling Business Magazine www.cablingbusiness.com
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