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SECTION' ❖ SYSTEM Dr sCRIPTIONIPEREORMANCE 1.3 SYSTEM CAPACITY System capacity describes the message transfer capability of the data system. An accurate prediction of system capacity will reveal which component is limiting the ability of the system to transfer more messages. The limiting component will be one of the following: the inbound RF channel, the outbound RF channel, or the fixed -end network controller. Knowledge of the limiting component allows the system to be sized according to the application requirements. This ultimately determines the infrastructure required, the number of RF channels needed, the number of subscriber units that will be supported, and the potential for future expansion. While the installation of the RNC 3000 will allow for expansion of the system, and migration to RD-LAP 9.6 at a later date, Motorola does not take responsibility for system capacity. System capacity information is provided for information only. 1.4 FACTORS AFFECTING SYSTEM CAPACITY Many factors affect the capacity of a data communication system. The following factors are discussed in the paragraphs below: •3 .Inbound channel contention ❖ Over -the -air protocol ❖ Network design •:• Field device performance ❖ Customer message profile 1.4.1 Inbound Channel Contention When multiple subscriber units attempt to send inbound messages simultaneously, without determining that the channel is clear, the messages will collide. The errors caused by this type of collision will most likely cause the messages to be unreadable by the system. Each subscriber unit will delay a random amount of time and attempt to re -send the message. This form of inbound channel control is known as ALOHA. contention. A far more efficient inbound channel contention scheme, used by the RD-LAP protocol, is Slotted -Digital Sense Multiple Access (S-DSMA). This contention scheme synchronizes the inbound and outbound channels with time "slots" and allows the subscriber units to monitor the outbound channel for activity before transmission. If the outbound channel is IDLE, a message may be sent inbound. If, however, the outbound channel is BUSY, the subscriber unit must delay a random amount of time and synchronize with the outbound channel before re -sending the inbound message. This contention scheme significantly reduces the number of collisions on the inbound channel and as a result, the inbound channel capacity is significantly increased. 8MO7"ORO+Ld Confidential and Proprietary 1 :• 4