 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 The Telecom Handbook 29 THE PBX 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 par- ticular design of he manufacturer and has to do with economics as well as technology. Smaller capacity systems with fewer ca- pabilities 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 en- ables them to be sold at a lower cost. In general, the larger systems (over 100 telephones) may be more flexible in terms of expan- s