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Orange County Emergency Management, North Carolina <br />Final VIPER Anal sis Re ort Au ust 20, 2004 <br />For Blocked Calls Delayed Trunking, an Erlang C formula is used to <br />determine the probability that a call is blocked and is a measure of the <br />GOS for a trunked LMR system that provides queuing for blocked calls. <br />The CTA traffic loading model is based on the parameters that require an <br />Erlang C statistic to be used for traffic intensity calculations. The Erlang <br />C model assumes that traffic requests obey a Poisson distribution, which <br />implies exponentially distributed call inter-arrival times. Furthermore, the <br />Erlang C model assumes a large number of channels and a large number <br />of users with similar calling patterns. <br />The loading model takes the subscriber data inputs (number of radio <br />subscriber units in the Orange County inventory) and calculates the <br />number of radio channels needed to provide sufficient capacity in a <br />trunked architecture. <br />The number of radio channels is determined by using a Delayed Call GOS <br />for the busiest hour of 1% and a maximum acceptable call delay of 1 <br />second. A Delayed Call GOS of 1% means that 1 out of 100 calls may <br />experience a delay more than 1 second. This is a generally accepted <br />standard for public safety trunked radio systems. <br />CTA's analysis of VIPER multi site traffic loading is based on Orange County's <br />subscriber unit counts identified in the TABLE 4-1. In this design approach, each <br />site operates on a different set of channels, and calls are routed to and from the <br />sites via a central controller. Working channels are dynamically assigned based <br />on units and talk groups logged in to each site. TABLE 4-2 lists the channel <br />loading for each site based on user distributions associated with their service <br />areas in the county. Unit roaming is considered and wide area call loading is <br />included in the analysis. Calculated capacity of each VIPER site listed in <br />TABLE 4-2 assumes that NC-SHP has 50 mobiles in each site based on <br />information received from NC-SHP. The initial subscriber load of 121 units <br />associated with OCEM referenced in the Phase 2A report is assumed to transfer <br />to the VIPER Durham -City site. The UNC Operations side depicts a stand alone <br />site at UNC-Chapel Hill to provide 800 MHz trunked radio service for all non <br />public safety workgroups at UNC-Chapel Hill. All sites analyzed met the target <br />1% delayed call GOS. <br />Section 4 -VIPER Coverage and Capacity <br />~£^~- Page 16 of 43 <br />