Will an IP PBX System work with DSL?

Moving to an IP PBX system can bring with it many advantages in terms of ease of use, lower cost, and advanced features. Because much of the functionality of the IP PBX is software-based, it is possible to obtain features that would be too costly, or unavailable with a standard PBX.

The purpose of course, to an IP PBX, is to deploy Voice over Internet Protocol (VoIP) in the office. VoIP is an increasingly common telephony mechanism, which has become equivalent to standard telephony in quality, and superior to it in terms of manageability, feature set and cost. But, deploying an IP PBX in an existing office with a standard PBX differs substantially from doing so in a new office. If replacing a standard PBX, the “forklift upgrade” requires other considerations, not the least of which is the cost of already-installed equipment, and the possibility of needing a bandwidth upgrade.

Since with the IP PBX, you will be transmitting voice as well as data over your Internet connection, bandwidth may have to be reconsidered, and possibly upgraded. Depending on the size of your office and the number of phone lines, a DSL may be adequate, especially for SOHO operations. An oversubscribed VoIP system over an inadequate DSL installation however, will result in reduced call quality, and a very real possibility that some people in your office may be unable to make calls during peak times. Before deploying VoIP over DSL, determine what your DSL upstream bandwidth rate is. If you have an average DSL upstream rate of 640Kbps, for example, and if an average call consumes about 90Kbps, you could have a theoretical maximum of seven calls—although that leaves nothing left for data traffic. Realistically speaking, a DSL line would not be able to adequately support more than three simultaneous calls.

The key word is “simultaneous.” If, in your capacity planning, you determine that you need six phone lines, but that at any given time, no more than half of them are in use, then you’re still good with the DSL.

However, if you have more than three lines in use at any given time, you may need to upgrade. A full T1 may not be necessary for a small business, instead, a fractional T1 may be enough, depending on your needs and the number of phone lines in your office. If you do find that your DSL line is becoming inadequate to handle your phone volume, the upgrade need not be costly, and a fractional T1 line may not be that much more expensive than the DSL. For larger installations, depending on your desired architecture, you may wish to dedicate one T1 line to VoIP and another to data.

How do you determine how many T1 lines you will need, or how much of a fractional T1 is necessary? A T1 line consists of 24 channels, which can be divided between the VoIP function and the data network as needed. A little capacity planning is again in order. Conduct a study of your phone usage, determining peak hours of usage, and the maximum number of lines that are in use at any given time. If again, you find that at peak time, 50 percent of your phones are in use, you could have a theoretical maximum of 48 phones lines on a single T1, assuming it is dedicated to VoIP exclusively. It is not necessary to dedicate your T1 to VoIP exclusively however, if you use an integrated or dynamic T1 configuration, which combines both voice and data on the same T1 link.