BCM4339 Preliminary Data Sheet
IEEE 802.11ac MAC
TXE
The transmit engine (TXE) constitutes the transmit data path of the MAC. It coordinates the DMA engines to
store the transmit frames in the TXFIFO. It interfaces with WEP module to encrypt frames, and transfers the
frames across the MAC-PHY interface at the appropriate time determined by the channel access mechanisms.
The data received from the DMA engines are stored in transmit FIFOs. The MAC supports multiple logical
queues to support traffic streams that have different QoS priority requirements. The PSM uses the channel
access information from the IFS module to schedule a queue from which the next frame is transmitted. Once
the frame is scheduled, the TXE hardware transmits the frame based on a precise timing trigger received from
the IFS module.
The TXE module also contains the hardware that allows the rapid assembly of MPDUs into an A-MPDU for
transmission. The hardware module aggregates the encrypted MPDUs by adding appropriate headers and pad
delimiters as needed.
RXE
The receive engine (RXE) constitutes the receive data path of the MAC. It interfaces with the DMA engine to
drain the received frames from the RXFIFO. It transfers bytes across the MAC-PHY interface and interfaces with
the WEP module to decrypt frames. The decrypted data is stored in the RXFIFO.
The RXE module contains programmable filters that are programmed by the PSM to accept or filter frames
based on several criteria such as receiver address, BSSID, and certain frame types.
The RXE module also contains the hardware required to detect A-MPDUs, parse the headers of the containers,
and disaggregate them into component MPDUS.
IFS
The IFS module contains the timers required to determine interframe space timing including RIFS timing. It also
contains multiple backoff engines required to support prioritized access to the medium as specified by WMM.
The interframe spacing timers are triggered by the cessation of channel activity on the medium, as indicated by
the PHY. These timers provide precise timing to the TXE to begin frame transmission. The TXE uses this
information to send response frames or perform transmit frame-bursting (RIFS or SIFS separated, as within a
TXOP).
The backoff engines (for each access category) monitor channel activity, in each slot duration, to determine
whether to continue or pause the backoff counters. When the backoff counters reach 0, the TXE gets notified,
so that it may commence frame transmission. In the event of multiple backoff counters decrementing to 0 at the
same time, the hardware resolves the conflict based on policies provided by the PSM.
The IFS module also incorporates hardware that allows the MAC to enter a low-power state when operating
under the IEEE power save mode. In this mode, the MAC is in a suspended state with its clock turned off. A
sleep timer, whose count value is initialized by the PSM, runs on a slow clock and determines the duration over
which the MAC remains in this suspended state. Once the timer expires the MAC is restored to its functional
state. The PSM updates the TSF timer based on the sleep duration ensuring that the TSF is synchronized to
the network.
Broadcom®
November 17, 2014 • 4339-DS106-R
Page 88
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