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4 <br /> The existing Motorola mobile data controller is capable of transmitting and receiving <br /> data at a rate of 4,800 BPS. The land mobile radio link between a moving vehicle and a <br /> fixed radio site is a very poor transmission medium for data as there tends to be a very <br /> high bit error rate for mobile data communications. The existing data controller <br /> therefore breaks messages up into small chunks called packets and transmits each <br /> packet with some overhead bits individually. If error - checking equipment detects bit <br /> errors in the packet, the packet is retransmitted a number of times, until either an error <br /> free packet set of data is received, or else the sending device receives a message <br /> saying that the message could not be transmitted. There is overhead associated with <br /> the transmission of packets, because each packet must be identified, and so that <br /> packets with errors can be identified and instructed to retransmit so that entire message <br /> can be reconstructed in the proper order at the receiving end. That overhead uses <br /> some bits that otherwise might be data that is to be transmitted from the sender to the <br /> receiver. In addition the signaling back and forth, instructing the sender to retransmit <br /> specific packets, and the fact that data must be transmitted multiple times, all reduces <br /> the effective throughput for a data communications system. The number of packets that <br /> need to be retransmitted increases as the radio signal level decreases, and therefore <br /> units that are further away from the mobile data fixed location radio site will require <br /> more frequent transmissions of data. Overall, the effective data transmission rate of the <br /> existing mobile data communication system is in the range of 2,000 to 2,400 BPS. <br /> On the uplink (from the mobile unit to the fixed location receiver) there is the potential <br /> for contention when one person in a vehicle attempts to transmit a data message while <br /> another data message is already being transmitted to the fixed location receiver site. <br /> When that happens, neither message can be received clearly and both messages must <br /> be retransmitted. The sending unit attempts to retransmit a specific number of times <br /> before it provides a message to the sender saying that the message could not be <br /> transmitted to the receiver. As data volumes increase, the potential for contention also <br /> increases, and because of the retransmissions, contention collisions begin to occur at <br /> an exponentially increasing rate as traffic builds. From a statistical point of view, the <br /> maximum usable uplink (mobile to fixed receiver) transmission rate for a system that <br /> has an effective transmission rate of 2,000 to 2,400 BPS is no more than about 1,000 <br /> BPS. <br /> An analysis of the existing Anoka County mobile data communication system indicates <br /> that, during busier hours of busy months, the data offered on the uplink in this system <br /> probably exceeds 1,200 BPS. As a result the system will operate successfully for a <br /> period of time until more and more collisions begin to occur, and then because many of <br /> the mobile units are retransmitting messages several times, the exponential increase of <br />