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