The vast majority of conversations about the convergence of voice and data could benefit greatly from a generally accepted vocabulary. The primary culprits in this smorgasbord of confusion are the terms "VoIP" and "IP telephony," which are used interchangeably by some but carry subtle distinctions or connotations for others. For instance, Voice over IP (VoIP), to some, refers more specifically to network protocols and traffic, whereas IP telephony includes VoIP, gateways, softswitches, and Ethernet-based phones.
This is mildly annoying to technicians, but it can cause real problems for others who don't realize that there are many distinct voice services in the data network and that -- most important -- you don't necessarily have to implement all of them to take advantage of one. So, in this tip, I categorize a few of these services and explain how you can use them together or individually.
Somewhere, something in your network needs to know where to send calls (based on the dialed digits) and coordinate communication between endpoints (i.e., phones and gateways). This can be a traditional PBX or an IP-based softswitch, or it can be provided by a centrex service. While your call control and phones need to match (i.e., both IP or both TDM), use of one doesn't preclude the use of the other elsewhere in the network, as we'll discuss below.
Often, when people use the term "VoIP," they mean "toll-bypass," which in turn means transporting voice media over data circuits to avoid the per-minute charges associated with metered usage of traditional long-distance service. Of course, if you're currently using a tie-line or local service that isn't metered, then a proposed VoIP solution would technically bypass any tolls, even though it would be cheaper. Either way, it is possible, and very common, to use VoIP trunks to avoid long-distance charges without replacing your legacy PBX or phones.
Connectivity to PSTN
Although less common, it is possible to use VoIP trunks over the Internet to a service provider for local phone service (long distance is typically included). This would be the equivalent of your business signing up for Vonage. Again, it is relatively easy to accomplish this without changing any of your phones or PBX.
I've split voice services into just three categories, which have some overlap. You should realize that you could continue breaking these services down into much more specific or even atomic distinctions. I don't have space to get into Automatic Call Distributor (ACD), Voice Response Unit (VRU), computer-telephony integration (CTI), or solutions that address other aspects, such as mobility.
Again, though, the point is that you should understand you can use almost any of these services by itself or together with other services. This becomes important when you start to justify the return on investment (ROI) of VoIP, because you need to know which specific component or service you're analyzing. Clearly, the ROI of toll-bypass is different from the ROI of replacing your PBX and phones with new IP phones. Don't fall into the trap of comparing the price of a brand-new phone system with the monthly long-distance savings, when you don't need the phone system to get the savings.
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years experience in the networking industry, and co-author of several books on networking, most recently, CCSP: Secure PIX and Secure VPN Study Guide published by Sybex.