PACKET DATA COMMUNICATION SYSTEM

摘要

A packet data transmission system is used to link a number of remote hand-he ld data-gathering units such as bar code readers to a central computer which maintains a database management system. Data packets are sent from the remote units by a n RF link to intermediate base stations, then sent by the base stations to the central computer by a serial link. A frequency-hopping transmission method is used for the RF link. The remote units and base stations are all synchronized to a repeating pseud o- random sequence of frequencies in a band (902-928 MHz) which allows this type of transmis- sion. The remote hand-held units initiate an exchange using RF transmission to and from the base stations, receiving only during a rigid time window following a transmission from the remote unit. The base stations cannot initiate communication to the remote units, but instead send data to the remote units only as part of the exchange. The reply messages from the base stations to the remotes contain synchronizi ng information to maintain coherence with the pseudo-random sequence.