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Orange County Emergency Management, North Carolina <br />Final VIPER Anal sis Re ort Au ust 20, 2004 <br />4.2.1 Traffic Loading Definitions <br />To help the reader understand the radio traffic analysis, some basic <br />definitions are provided: <br />A. Erlang <br />A traffic engineering term used to measure the communications <br />load for a circuit or set of circuits. One Erlang is defined as one <br />person speaking continuously. For the purpose of this analysis, 1 <br />Erlang is equivalent to 1 call lasting 3600 seconds in a 1-hour <br />period. <br />B. Erlang C Model <br />Derived from the assumption that all requested calls are held in a <br />queue that cannot be immediately assigned a radio channel. The <br />Erlang C formula is based on the following assumptions: <br />• Calls arrive randomly as characterized by a Poisson <br />distribution. <br />• Calls are served in order of arrival. <br />• Blocked calls are delayed. <br />• Blocked calls are placed in a queue with an infinite length. <br />• Top of queue is granted next available channel. <br />• Infinite number of requests. <br />• All users (including blocked) can request a channel at any <br />time. <br />• Probability of a user occupying a channel is exponentially <br />distributed (longer calls are less likely to occur). <br />Section 4 -VIPER Coverage and Capacity <br />s Page 14 of 43 <br />®.... <br />" . <br />