BCM88335 Data Sheet
IEEE 802.11ac MAC
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.
The IFS module also contains the PTA hardware that assists the PSM in Bluetooth coexistence functions.
TSF
The timing synchronization function (TSF) module maintains the TSF timer of the MAC. It also maintains the
target beacon transmission time (TBTT). The TSF timer hardware, under the control of the PSM, is capable of
adopting timestamps received from beacon and probe response frames in order to maintain synchronization
with the network.
The TSF module also generates trigger signals for events that are specified as offsets from the TSF timer, such
as uplink and downlink transmission times used in PSMP.
NAV
The network allocation vector (NAV) timer module is responsible for maintaining the NAV information conveyed
through the duration field of MAC frames. This ensures that the MAC complies with the protection mechanisms
specified in the standard.
The hardware, under the control of the PSM, maintains the NAV timer and updates the timer appropriately based
on received frames. This timing information is provided to the IFS module, which uses it as a virtual carrier-
sense indication.
MAC-PHY Interface
The MAC-PHY interface consists of a data path interface to exchange RX/TX data from/to the PHY. In addition,
there is an programming interface, which can be controlled either by the host or the PSM to configure and control
the PHY.
Broadcom®
September 23, 2015 • 88335-DS100-R
Page 69
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