portant concept in GPRS and EGPRS capacity planning relates to the resource assignment priorities of the different services. Due to the relative delay insensitivity of many packet based services, it would not be unreasonable for a packet data transfer to be interrupted if a channel is required for a voice call. In this way, the capacity allocated to GPRS traffic will be time variable, allowing a degree of freedom in the overall system planning. Note, however, that it is possible to dimension additional timeslots per carrier for GPRS should a minimum level of GPRS capacity be required. Circuit Switched Traffic Dimensioning Since it is assumed that circuit switched traffic will take priority over packet traffic, and be likely to constitute the majority of the system load, it is appropriate to consider this first. Circuit Switched Traffic and the Erlang B Equation Circuit switched traffic design usually involves the application of the Erlang B formula to give a figure for the required number of channels given a required traffic load and blocking probability: (2.53) where B is the blocking probability (%), A is the traffic load (Erl) and N is the number of channels (timeslots). Fortunately, direct evaluation of Equation (2.53) is rarely required, as tables containing the results are widely available. As an example, the Erlang B calculation indicates that 22 traffic channels would support 14.9 Erlangs of traffic with <2 blocking. An area requiring, at an even distribution, an overall load of around 100 Erlangs could therefore be built up from seven cells each carrying 22 traffic channels, maintaining the <2% blocking criterion. A design based on the above would also incorporate a design C/I threshold, thus ensuring that, across the area, the C/I is sufficient for reliable transmission. This C/I is chosen to suit the transmission format in question, e.g. speech and circuit switched data. Available GPRS Resource within the Circuit Switched Design A system designed for circuit switched traffic will usually allow for a ba