ll result in blocking probability which is too high. Conversely, too many links requires too Voice Communications high an infrastructure cost, while the blocking probability will be quite low - perhaps even less than required from business or policy constraints. As noted above, a capital expenditure point of view mandates an operator to keep the number of lines to a minimum as each extra link adds to the cost of network roll-out. The question is then how many multiplexed channels are needed between two exchanges considering the likely number of calls that may be active at any one time. Assuming each call requires a dedicated cir- cuit for the duration it is active, and that all circuits are busy, then any new call attempts will not find a channel onto which they may be multiplexed. Such a call is blocked and the subscriber will be given a busy signal. On the other hand, dimensioning the cable to have extra lines increase both capital and operational expenditures and therefore it is desirable to avoid unnecessary extra lines. A field of telecommunications technology, teletraffic engineering, was developed with focus on calculating the necessary number of lines for a certain probability of call blocking. Teletraffic engineering techniques calculate the number of necessary lines for carrying voice (and later data) traffic over a finite number of telephone lines. These inform a telecommunica- tions system designer on the number of links that are necessary to connect an exchange to the rest of the network. This number is found based on the number of subscribers, and the likely amount of traffic that at any time may flow from the exchange. Through teletraffic engineering, the size of the inter-exchange links is calculated based on the expected number of calls that would be made between users in different localities, and the required probability of blocking. Call traffic per user is defined as the portion of the time a subscriber is active. For example, if a subscriber is active 3 min on average in a 1-h period, the amo