Using the BayStack 650 Wireless LAN Access Point1-4 203282-BA low-power radio operates within the 2.4000 to 2.4835 GHz band of theunlicensed Industrial, Scientific, and Medical bands (ISM). It is divided into 78,one MHz channels. The frequency hopping scheme causes the radio to broadcaston one channel for approximately 1/10 of a second, then hop to another in apredetermined pattern. Interference on one channel is avoided as soon as the radiohops to the next channel. Each BayStack 650 Access Point and the BayStack 650PC Cards attached to it follow the same frequency hopping sequence.Using Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), eachstring of data is sent with an additional signaling information designed to maintaintransmission efficiency. Every packet includes three signaling packets:• Request to send (RTS)RTS reserves the area around the sender.• Clear to send (CTS)CTS reserves the area around the receiver. In the infrastructure mode (go to“Network Overview” on page 1-5), the sender and receiver are the BayStack650 Access Point and the BayStack 650 PC Card. These RTS and CTSpackets offer protection against collisions during transmissions.• An acknowledgment (ACK)ACK confirms that the data packet was received correctly. If no ACK packetsare received, the sender retransmits the data packet. This positive ACK withretry protocol reduces the bit error rate on the cordless physical layer to alevel comparable to that of wired LAN PC Cards.The BayStack implementation of the physical layer and MAC portion of the datalink layer makes it possible for multiple BayStack 650 Wireless LAN networks toshare the same environment. Each BayStack 650 Wireless LAN network has anetwork identifier called a service set identifier (SSID). Go to “Network ServiceSet Identification” on page 1-7 for more information on SSIDs. BayStack 650Access Points with the same SSID form an extended service area (ESA). TheMAC protocol enables several laptops with BayStack 650 PC Cards to coexistwithout conflicts. The laptop computer always listens to the media beforetransmitting; if it senses another transmission, it waits. Simultaneoustransmissions (collisions) do happen; however, the data packets are resentautomatically using an algorithm designed to minimize retransmissions.
