Nortel BayStack 650 User Manual page 24

Using the BayStack 650 Wireless LAN Access Point
A low-power radio operates within the 2.4000 to 2.4835 GHz band of the
unlicensed Industrial, Scientific, and Medical bands (ISM). It is divided into 78,
one MHz channels. The frequency hopping scheme causes the radio to broadcast
on one channel for approximately 1/10 of a second, then hop to another in a
predetermined pattern. Interference on one channel is avoided as soon as the radio
hops to the next channel. Each BayStack 650 Access Point and the BayStack 650
PC Cards attached to it follow the same frequency hopping sequence.
Using Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), each
string of data is sent with an additional signaling information designed to maintain
transmission 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
650 Access Point and the BayStack 650 PC Card. These RTS and CTS
packets offer protection against collisions during transmissions.
• An acknowledgment (ACK)
ACK confirms that the data packet was received correctly. If no ACK packets
are received, the sender retransmits the data packet. This positive ACK with
retry protocol reduces the bit error rate on the cordless physical layer to a
level comparable to that of wired LAN PC Cards.
The BayStack implementation of the physical layer and MAC portion of the data
link layer makes it possible for multiple BayStack 650 Wireless LAN networks to
share the same environment. Each BayStack 650 Wireless LAN network has a
network identifier called a service set identifier (SSID). Go to
Set Identification" on
Access Points with the same SSID form an extended service area (ESA). The
MAC protocol enables several laptops with BayStack 650 PC Cards to coexist
without conflicts. The laptop computer always listens to the media before
transmitting; if it senses another transmission, it waits. Simultaneous
transmissions (collisions) do happen; however, the data packets are resent
automatically using an algorithm designed to minimize retransmissions.
1-4
Overview" on page 1-5), the sender and receiver are the BayStack
page 1-7 for more information on SSIDs. BayStack 650
"Network Service
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