Simple things you can do with VoIP

You can avoid disaster by starting small

All too many voice-over-IP (VoIP) vendors want to sell you "complete solutions," from the telephone on your desk and voice mail to the wide-area connectivity between your sites and to the public switched telephone network (PSTN).

But I've seen that unless the customer and vendor are thoroughly experienced and check every detail, complete VoIP systems can be recipes for failure.

Enterprises that have had problems with VoIP may have done so because they jumped into a covered wagon and leaped into the technology frontier, rather than first learning to ride a horse.

But it's easy to move incrementally into VoIP technology, especially for larger businesses. Small and home businesses, with a limited range of conventional plain old telephone services (POTS), may have to make more of a leap of faith -- but I always counsel having a backup plan for advanced telephone services.

If, for example, you want to move your POTS service to the much cheaper -- and often quite reliable -- voice-over-cable system, it's wise to ensure that you have a cell phone or keep one POTS line. Even with large organizations, one of my basic questions to be sure they understand telephony requirements: If your VoIP switchboard catches on fire, how do you call the fire department?

Basic Connectivity

Most enterprises use an on-site private branch exchange (PBX) or the similar Centrex located in telephone company facilities. Whichever is used, its functionality allows things such as dialing internal extensions rather than full telephone numbers, as well as more powerful features such as call transfer and forwarding. Some PBX -- and all Centrex -- systems allow dialing off-premises extensions as if they were in the local building. Enterprises with multiple locations often have direct connections among the offices' PBX, as well as PBX connections to the PSTN.

To begin reducing the cost of these links, you will need to know what you now have. To do this, you will need to know a bit of telephone terminology. While I would normally spell out three-letter acronyms (TLA), the reality is that the words that make up some telephony TLAs are lost to antiquity. Remember that it's been over a century since Alexander Graham Bell wanted to order a pizza and realized he had to invent the telephone first.

The first place to start your analysis is your detailed local telephone bill. From a detailed bill, find out how many of each of the interface types in Table 1 for which you are being charged. You may have to get clarification from an account manager, since telecommunications providers often use unique marketing terms for lines and interfaces, not standard terminology.

Table 1: What links do I have, and how much do they cost?

FXS The POTS interface between a telephone and a PBX, or from a telephone to the local telephone office.
FXO One possible POTS interface of a PBX or PBX-like device to the PSTN. It is a two-wire/one-pair physical interface.
E&M Another PBX interface, using a four-wire, two-pair analog interface.

Multiplexed Connectivity

When there is need for more than a very few voice or data lines at a small to midsize site, the telecommunications provider sensibly combines them into a multiplexed links, with the combined traffic in a DS1 (also called T1, at 1.544Mbit/sec.) signal format. T1 is actually a specific cabling system, and the service provider might provide the DS1 signal through other means. At each site, either you or the telecommunications firm may have a "splitter" known as an integrated access device (such as Adtran Atlas or Total Access) or Cisco's term, integrated service Router. These break out some number of DS1 interfaces, and use the rest for data applications. IADs are usually limited in the speeds and types of interfaces they support.

It's a very low-risk approach to replace the IAD, or individual DS1 links, with routers that provide both voice and data interfaces. If you own the IAD, however, before replacing it, be sure it can't be upgraded with new software to full router functionality. The only "VoIP" present here are digitized voice endpoints between the PBX external connections.

A good first start, for an enterprise with more than one site, is to begin replacing the direct telephone lines among your sites with VoIP. At this stage, you are not replacing your PBX itself. You can set up a secure virtual private networks (VPN) to telephones on small office/home office (SOHO) routers that have FXS interfaces for telephones.

If you use Centrex service, there still may be advantages to using incremental VoIP, the details of which will be discussed in a future article.

Table 2: Checklist for Tie Line and Related POTS Replacement

Does my organization have more than one site? Find out how your tie lines are provided, and how they can be replaced
 Do you have remote telephone users? Consider VPN
If not, does it have a PBX at its single site or intelligent telephones that can transfer calls and the like? Consider virtual PBX


There is a wide range of business applications for phones that don't provide dialing, but call a predetermined number as soon as their handset is lifted from the cradle. These are used in applications as critical as calling for "code blue" teams in a hospital and as mundane as calling a cab to take you away from the car repair shop. In telephony jargon, this function is called private-line automatic ringdown (PLAR), and it often lends itself to VoIP.

To implement these services, you program what is effectively automatic dialing into the voice-capable router at both ends.

A caution: proceed slowly and carefully if this is a life-critical function. VoIP actually can add value if used properly in such situations. For example, many hospitals will have at critical locations one line that goes through the switchboard and one that does not, to protect against PBX failure. Having one VoIP phone that goes through a data connection (e.g., cable) while another phone goes to a POTS or VoIP PBX protects you further from a single point of failure.


In this article, we've shown one incremental approach to introducing VoIP. In future articles, we will deal with other incremental capabilities such as using facsimile over IP (FoIP) and reducing what is sometimes a huge and unexpected cost for fax services. We will explore replacing the PBX or Centrex service, emerging services such as virtual PBX, and also when it is wise to retain some of the PBX-associated capabilities, such as voice mail, in the early stages of VoIP introduction.

Copyright © 2006 IDG Communications, Inc.

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