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Redwood City Police Department System Performance � the same message caused by multi-path radio transmissions or � receipt by more than one data radio infiastructure site. Whenever a Mobile Data Terminal or application server receives a completely correct message,it automatically retiuns an aclrnowledgment If the message has been corrupted by the radio environment or other sources, an instruction is issued for the message initiating device to retransmit the data. If a full acknowledgment is not received by the initiating device after a preset time interval,it will automatically retransmit the message. Throughout this process,the message transaction number and device ID ensure that users are never presented duplicate copies of the same message. In an environment of multiple Mobile Data Terminals, it is necessary to regulate the RF channel to establish an efficient manner by which all terminals may access the single communications path. Thus, systems must use a contention control scheme to prevent Mobile Data Terminals from initiating a transmission while an inbound message is in progress. Additionally,when two terminals initiate messages at the exact � same time, collisions occur and,unless some means of t retransmission control is employed,the efficiency and capacity of the channel is diminished. Motorola's RD-LAP uses Slotted Digital Sense Multiple Access (SDSMA) contention c�ntrol to separate inbound(Mobile Data Terminal to radio infrastructure)messages on this full duplex Mobile Data System. With SDSMA,the Data Station transmitter is keyed to give tenninals an outbound stream. When an inbound message transmission is in pr+ogress, "busy" flags are set in this - outbound stream to alert aIl other terminals to hold any transmissions until the channel is free. Mobile Data Terminals always check this streazn for the presence of"busy" flags before . transmitting. This-orderly control of the inbound frequency maacimize5 system throughput by reducing interference between the tenninals which would otherw�ise result in repeated transmissions of the same message The RNC has been equipped to recognize the unusual condition of two Mobile Data terminals transnutting at the eacact same time(or at least in the time before the "busy" can be set). In this case,the controller issues a"random time n:transmit" command. If a � -: tenninal receives this command, it will wait a short but random QMOrofro�..4 • 3 - 4 _ _ _ __ __. __ __ __