BCM4319 Preliminary Data Sheet
802.11 a/b/g/n MAC and PHY
Section 6: 802.11 a/b/g/n MAC and PHY
Introduction to the IEEE 802.11™ Standard
The IEEE 802.11™ standard 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
(DCF) and the contention-free scheme is also called the point coordination function (PCF).
The DCF 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.
MAC Features
The BCM4319 WLAN MAC supports features specified in the 802.11 base standard plus those amended by
802.11n. The salient features are listed below:
•
•
Transmission and reception of aggregated MAC protocol data units (A-MPDUs)
Support for power management schemes, including Wi-Fi Multimedia™ (WMM®) power save, Power Save
Multipoll (PSMP) and multiphase PSMP operation
•
Support for immediate ACK and Block-ACK policies
Broadcom®
Single-Chip IEEE 802.11™ a/b/g/n MAC/Baseband/Radio
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April 2, 2014 • 4319-DS05-R
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