Preliminary Data Sheet
6/30/09
BCM4325
Section 9: Wireless LAN Functional Description
INTRODUCTION TO IEEE STD 802.11
IEEE Std 802.11 defines two different ways to configure a wireless network: ad hoc mode and infrastructure mode. In ad
hoc mode, nodes are brought together to form a network on the fly, whereas infrastructure mode uses fixed access points
through which mobile nodes can communicate. These network access points are sometimes connected to wired networks
through bridging or routing functions.
The medium access control (MAC) layer is a contention-resolution protocol that is responsible for maintaining order in the
use of a shared wireless medium. IEEE 802.11 specifies both contention-based and contention-free channel access
mechanisms. The contention-based scheme is also called the distributed coordination function and the contention-free
scheme is also called the point coordination function.
The distributed coordination function employs a carrier sense multiple access with collision avoidance (CSMA/CA) protocol.
In this protocol, when the MAC receives a packet to be transmitted from its higher layer, the MAC first listens to ensure that
no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random backoff
factor that determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which
the channel is clear, the MAC waiting to transmit decrements its backoff counter, and when the channel is busy, it does not
decrement its backoff counter. When the backoff counter reaches zero, the MAC transmits the packet. Because the
probability that two nodes will choose the same backoff factor is low, collisions between packets are minimized. Collision
detection, as employed in Ethernet, cannot be used for the radio frequency transmissions of devices following IEEE 802.11.
The IEEE 802.11 nodes are half-duplex—when a node is transmitting, it cannot hear any other node in the system that is
transmitting because its own signal drowns out any others arriving at the node.
Optionally, when a packet is to be transmitted, the transmitting node can first send out a short request to send (RTS) packet
containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear to send
(CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as
determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back
and forth exchange is necessary to avoid the hidden node problem. Hidden node is a situation where node A can
communicate with node B, node B can communicate with node C, but node A cannot communicate with node C. For
instance, although node A can sense that the channel is clear, node C can be transmitting to node B. This protocol alerts
node A that node B is busy, and that it must wait before transmitting its packet.
IEEE 802.11A/G MAC FEATURES
The IEEE 802.11a/g MAC features include:
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Programmable independent basic service set (IBSS), or infrastructure mode
Passive scanning
Network allocation vector (NAV), inter-frame space (IFS), and timing synchronization function (TSF) functionality
Backoff
RTS/CTS procedure
Transmission of response frames (ACK/CTS)
Address filtering of RX frames as specified by IBSS rules
Broadcom Corporation
Document 4325-DS04-R
Wireless LAN Functional Description
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