A queing system approach for the design of coast guard vessel traffic services communications

In order to provide for the safe and expeditious passage of maritime traffic in congested waters, the U.S. Coast Guard is authorized by the Ports and Waterways Safety Act of 1972 to establish, operate, and maintain Vessel Traffic Services (VTS) where needed. In larger areas, a VTS will generally require a communications system to enable the vessel traffic center and the participating vessels to exchange information. In designing such a system, it is necessary to assess the expected communications loading in order to determine frequency requirements and evaluate alternative configurations for the system. Here, VTS communications are viewed as a queuing system. The customers (messages) arrive at the service facility (communications channel) according to some probabilistic process, and are then serviced (transmitted) according to some other probabilistic process. Queues or waiting lines form as arriving messages wait to be transmitted, because the communications channels are busy. Three classes of messages are considered in the arrival process: check in/check out (basic VTS) messages; Vessel Movement Reporting System (VMRS) messages; and bridge-to-bridge messages. Each class is characterized by an independent Poisson distribution, and the resultant arrival process is a well-defined nonhomogeneous Poisson process. The service time is characterized by a general distribution with a known mean and variance. The queuing results, which are developed, include the utilization factor, the expected waiting time, and the expected number of messages waiting to be transmitted. The arrival process and the queuing results vary according to the time of the day, reflecting the varying traffic load throughout the day. A detailed example is given for a preliminary analysis of New York Harbor VTS communications.