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wifi: Update documentation related to CCA

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This commit is contained in:
Sebastien Deronne
2022-07-15 18:42:43 +02:00
committed by Sébastien Deronne
parent c8a81bc813
commit 71119bbb6b
1 changed files with 19 additions and 16 deletions
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35
src/wifi/doc/source/wifi-design.rst
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@@ -362,14 +362,14 @@ layer, and allows other objects to hook as *listeners* to monitor PHY
state. The main use of listeners is for the MAC layer to know when
the PHY is busy or not (for transmission and collision avoidance).
The PHY layer can be in one of seven states:
The PHY layer can be in one of these states:
#. TX: the PHY is currently transmitting a signal on behalf of its associated
MAC
#. RX: the PHY is synchronized on a signal and is waiting until it has received
its last bit to forward it to the MAC.
#. CCA_BUSY: the PHY is issuing a PHY-CCA.indication(BUSY) indication for the primary channel.
#. IDLE: the PHY is not in the TX, RX, or CCA_BUSY states.
#. CCA_BUSY: the PHY is not in TX or RX state but the measured energy is higher than the energy detection threshold.
#. SWITCHING: the PHY is switching channels.
#. SLEEP: the PHY is in a power save mode and cannot send nor receive frames.
#. OFF: the PHY is powered off and cannot send nor receive frames.
@@ -427,7 +427,8 @@ The ``PhyEntity::EndReceiveField ()`` method will check the correct reception
of the current preamble and header field and, if so, calls ``PhyEntity::StartReceiveField ()``
for the next field,
otherwise the reception is aborted and PHY is put either in IDLE state or in CCA_BUSY state,
depending on whether the measured energy is higher than the energy detection threshold.
depending on whether a PHY-CCA.indication(BUSY) is being issued on not for the primary channel
.
The next event at ``PhyEntity::StartReceiveField ()`` checks, using the interference
helper and error model, whether the header was successfully decoded, and if so,
@@ -452,17 +453,19 @@ the PHY being in a state in which it cannot receive a packet, the packet
is added to the interference helper, and the aggregate of the energy of
all such signals is compared against an energy detection threshold to
determine whether the PHY should enter a CCA_BUSY state.
The ``WifiPhy::CcaEdThreshold`` attribute
corresponds to what the standard calls the "ED threshold" for CCA Mode 1.
In section 16.4.8.5 in the 802.11-2012 standard: "CCA Mode 1: Energy above
threshold. CCA shall report a busy medium upon detection of any energy above
the ED threshold." By default, this value is set to the -62 dBm level specified
in the standard for 20 MHz channels. When using ``YansWifiPhy``, there are no
non-Wi-Fi signals, so it is unlikely that this attribute would play much of a
role in Yans wifi models if left at the default value, but if there is a strong
Wi-Fi signal that is not otherwise being received by the model, it has
the possibility to raise the CCA_BUSY while the overall energy exceeds
this threshold.
A PHY-CCA.indication(BUSY) is issued if a signal occupying the primary channel with a received
power above ``WifiPhy::CcaSensitivity`` (defaulted to -82 dBm) has been received by the PHY or if the
measured energy on the primary channel is higher than the energy detection threshold ``WifiPhy::CcaEdThreshold``
(defaulted to -62 dBm).
When channel bonding is used, CCA indication for signals not occupying the primary channel is also reported.
Since 802.11ac and above needs to sense CCA sensitivity for secondary channels larger than 20 MHz, CCA sensitivity thresholds
can be adjusted per secondary channel width using ``VhtConfiguration::SecondaryCcaSensitivityThresholds`` attribute.
For 802.11ax and above, and if the operational bandwidth is equal or larger than 40 MHz, each 20 MHz subchannel of the operational bandwidth
is being sensed and PHY-CCA.indication also reports a CCA_BUSY duration indication for each of these 20 MHz subchannel. A zero duration for
a given 20 MHz subchannel indicates the 20 MHz subchannel is IDLE.
The above describes the case in which the packet is a single MPDU. For
more recent Wi-Fi standards using MPDU aggregation, ``StartReceivePayload``
@@ -476,8 +479,8 @@ InterferenceHelper
The InterferenceHelper is an object that tracks all incoming packets and
calculates probability of error values for packets being received, and
also evaluates whether energy on the channel rises above the CCA
threshold.
also evaluates whether and for how long energy on the channel rises above
a given threshold.
The basic operation of probability of error calculations is shown in Figure
:ref:`snir`. Packets are represented as bits (not symbols) in the |ns3|