PBX Sizing and Growth

When planning the purchase of a PBX, growth capability is a major consideration. Although they may be modular in design, in terms of telephones and outside lines that can be connected to the system, PBXs have a maximum capacity. In most systems, one port represents the ability to connect one telephone or one outside line. A 400 port system can accommodate a total combination of 400 outside lines and telephones.

There are no precise ratios in terms of how many outside lines are required for the number of telephones. A conservative ratio would be 25 outside lines (for incoming and outgoing calls) for every 100 telephones. If there are not a lot of calls, 10 lines may be sufficient. If it's very busy, more than 25 may be needed.

There are statistical tables and software programs available to enable you to judge the total number of outside lines needed. Using these statistics to determine the number of outside lines required is called traffic engineering. In order for this to make sense, you need to know how many calls of what duration will be handled during the busiest hour of the day. Most people do not have this information, and therefore use judgment combined with trial and error in estimating the number of outside lines needed.

In the world of telephone traffic engineering you may hear the term CCS which stands for 100 call seconds (C being the Roman numeral for 100) or, more simply put, 100 seconds worth of telephone calls. 36 CCS or 3600 call seconds are equal to one hour's worth of calls known as an Erlang. The statistical tables or software programs express call volume in this manner to determine the number of outside lines needed to handle this volume of calling. Something called grade of service is expressed in terms such as P.01 meaning that, statistically speaking, one percent of all calls will be blocked.

Another consideration in planning for the system size is that Voice Mail, paging systems and other peripheral systems also take up ports in the PBX.

Additional Considerations On PBX Sizing

Each telephone system from each different manufacturer has maximum capacities for both the number of outside lines and desktop telephones it can support. This is determined by the particular design of he manufacturer and has to do with economics as well as technology. Smaller capacity systems with fewer capabilities are sold at lower price points. As an organization grows, they may reach the point where their current system is "maxed out" and needs replacement, enabling the supplier to sell them a new system. The more basic design of these systems also enables them to be sold at a lower cost. In general, the larger systems (over 100 telephones) may be more flexible in terms of expansion, but even so, the cost of expansion can be significant and may involve the change out of both software and circuit boards. If you are considering a system from a particular manufacturer, looking at their entire product line can provide you with insight into their strategy for providing systems to organizations of varying sizes. A few of the larger manufacturers (such as Avaya, formerly Lucent, formerly AT&T and NorTel, formerly called Northern Telecom) provide systems for 5 telephones up to systems with 10,000 telephones or more. While the smaller systems are overall less profitable, there has been a need for the big companies to compete with the manufacturers of smaller systems to keep their customer base intact.

Most very large organizations have small locations as well, so for a manufacturer to provide complete coverage, it must offer the smaller systems.

Many other manufacturers target a more limited range. There are many more manufacturers of smaller telephone systems (either PBXs or more basic systems sometimes known as "Key Systems") than of large systems. Only 4-5 companies go up to the 10,000+ telephone sizes.