Packet data communication system

A packet data transmission system is used to link a number of remote hand-held data-gathering units (15) such as bar code readers to a central computer (10) which maintains a database management system. Data packets are sent from the remote units by an RF link to intermediate base stations (12, 13, 14), then sent by the base stations to the central computer (10) by a serial link. Direct sequence spread spectrum modulation is used for the RF link. The remote hand-held units initiate an exchange using RF transmission to and from the base stations (12, 13, 14), receiving only during a rigid time window following a transmission from the remote unit. The base stations (12, 13, 14) cannot initiate communication to the remote units, but instead send data to the remote units only as part of the exchange.