Answers to your VoIP questions

By Phil Hochmuth and Tim Greene, Network World
October 26, 2002 08:11 PM ET

. What's this?

If you're seriously considering making the voice-over-IP plunge, you no doubt have loads of questions ranging from the technical to the financial to the political.

Can VoIP traffic traverse firewalls? Can it really save us money? What is the best way to get IT and telecom staffs to work together?

In wrapping up our series on convergence, we've taken a crack at answering a few of those questions.

Can my LAN handle VoIP?

IP telephony works best on LANs running switched 10/100M bit/sec Ethernet to the desktop and switched Gigabit Ethernet in the backbone. If you're still connecting desktops with hubs, and you want IP telephony, an upgrade is probably in order.

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Having LAN switches that support quality-of-service (QoS) technologies, such as 802.1p traffic prioritization, virtual LAN tagging or Differentiated Services, also is important. Many IP telephony vendors build QoS into their equipment, so a network lacking QoS-enabled switches also is a candidate for an upgrade if IP telephony is your goal - though there are users who have QoS-capable switches and get by without flicking the QoS switch, preferring to just tolerate the occasional snaps and pops on the line.

Do I have to throw out my PBX?

Companies such as Cisco and 3Com, which have no traditional PBX installed base, have pushed their customers to make a wholesale swap from circuit-switched telephony to IP. Avaya, Siemens, Nortel, NEC and other sellers of traditional PBXs offer IP cards for their systems as a way to "IP-enable" them. IP enabling a PBX lets customers extend their PBXs to branch offices via IP WAN connections, or even to IP or digital phones inside a corporate headquarters.

What's the voice quality like?

It can be as good or better than standard voice quality. But quality is in the ear of the beholder, so the answer depends on how discriminating your company's end users are. Some companies don't worry much about the quality of VoIP for certain internal calls. But if the calls are involved directly with revenue generation, companies typically have a higher standard.

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If you want to measure the quality of a VoIP call, there are methods, including the MedianOpinion Score (MOS) test, endorsed by the International Telecommunications Union (ITU). MOS involves gathering people into a room to listen to calls, after which group members rate quality on a scale from 1 to 5. Voice-quality testing tools based on computer algorithms also are available from vendors such as Agilent and Empirix.

Is VoIP really less expensive?

You can make a good case for it, but it's hard to give a blanket answer. Theoretically you can get rid of some phone trunks if you use a single network for all traffic. You can avoid expensive toll fees, particularly for international calls. You can cut the administrative cost of moving phones when someone changes offices or someone is hired or fired. You might get by with fewer employees if you merge data and telecom staffs. But you have to factor in the cost of new equipment, increased traffic on your data network that can require bigger, more expensive links to service providers and higher-priced services based on stringent service-level agreements that voice requires. Some users worry that because VoIP is relatively new, software updates will be more frequent than with traditional PBXs, making the maintenance of IP PBXs more expensive. You have to crunch your own numbers and determine whether it makes sense for you.