.. include:: replace.txt
.. highlight:: cpp

++++++++++++++++++++
Design Documentation
++++++++++++++++++++


|ns3| nodes can contain a collection of NetDevice objects, much like an actual
computer contains separate interface cards for Ethernet, Wifi, Bluetooth, etc.
This chapter describes the |ns3| WifiNetDevice and related models. By adding
WifiNetDevice objects to |ns3| nodes, one can create models of 802.11-based
infrastructure and ad hoc networks.

Overview of the model
*********************

The WifiNetDevice models a wireless network interface controller based
on the IEEE 802.11 standard [ieee80211]_. We will go into more detail below but in brief,
|ns3| provides models for these aspects of 802.11:

* basic 802.11 DCF with **infrastructure** and **adhoc** modes
* **802.11a**, **802.11b**, **802.11g**, **802.11n** (both 2.4 and 5 GHz bands), **802.11ac** and **802.11ax** Draft 1.0 (both 2.4 and 5 GHz bands) physical layers
* **MSDU aggregation** and **MPDU aggregation** extensions of 802.11n, and both can be combined together (two-level aggregation)
* QoS-based EDCA and queueing extensions of **802.11e**
* the ability to use different propagation loss models and propagation delay models,
  please see the chapter on :ref:`Propagation` for more detail
* various rate control algorithms including **Aarf, Arf, Cara, Onoe, Rraa,
  ConstantRate, and Minstrel**
* 802.11s (mesh), described in another chapter
* 802.11p and WAVE (vehicular), described in another chapter

The set of 802.11 models provided in |ns3| attempts to provide an accurate
MAC-level implementation of the 802.11 specification and to provide a
packet-level abstraction of the PHY-level for different PHYs, corresponding to 
802.11a/b/e/g/n/ac/ax specifications.

In |ns3|, nodes can have multiple WifiNetDevices on separate channels, and the
WifiNetDevice can coexist with other device types.
With the use of the **SpectrumWifiPhy** framework, one can also build scenarios
involving cross-channel interference or multiple wireless technologies on
a single channel.

The source code for the WifiNetDevice and its models lives in the directory
``src/wifi``.

The implementation is modular and provides roughly three sublayers of models:

* the **PHY layer models**
* the so-called **MAC low models**: they model functions such as medium
  access (DCF and EDCA), RTS/CTS and ACK.  In |ns3|, the lower-level MAC
  is further subdivided into a **MAC low** and **MAC middle** sublayering,
  with channel access grouped into the **MAC middle**.   
* the so-called **MAC high models**: they implement non-time-critical processes
  in Wifi such as the MAC-level beacon generation, probing, and association 
  state machines, and a set of **Rate control algorithms**.  In the literature,
  this sublayer is sometimes called the **upper MAC** and consists of more 
  software-oriented implementations vs. time-critical hardware implementations. 

Next, we provide an design overview of each layer, shown in 
Figure :ref:`wifi-architecture`.

.. _wifi-architecture:

.. figure:: figures/WifiArchitecture.*
   
   WifiNetDevice architecture.

MAC high models
===============

There are presently three **MAC high models** that provide for the three
(non-mesh; the mesh equivalent, which is a sibling of these with common
parent ``ns3::RegularWifiMac``, is not discussed here) Wi-Fi topological
elements - Access Point (AP) (``ns3::ApWifiMac``), 
non-AP Station (STA) (``ns3::StaWifiMac``), and STA in an Independent
Basic Service Set (IBSS - also commonly referred to as an ad hoc
network (``ns3::AdhocWifiMac``).

The simplest of these is ``ns3::AdhocWifiMac``, which implements a
Wi-Fi MAC that does not perform any kind of beacon generation,
probing, or association. The ``ns3::StaWifiMac`` class implements
an active probing and association state machine that handles automatic
re-association whenever too many beacons are missed. Finally,
``ns3::ApWifiMac`` implements an AP that generates periodic
beacons, and that accepts every attempt to associate.

These three MAC high models share a common parent in
``ns3::RegularWifiMac``, which exposes, among other MAC
configuration, an attribute ``QosSupported`` that allows
configuration of 802.11e/WMM-style QoS support, an attribute
``HtSupported`` that allows configuration of 802.11n High Throughput
style support, an attribute ``VhtSupported`` that allows configuration
of 802.11ac Very High Throughput style support and an attribute ``HeSupported``
that allows configuration of 802.11ax High Efficiency style support.

There are also several **rate control algorithms** that can be used by the
MAC low layer.  A complete list of available rate control algorithms is 
provided in a separate section.

MAC low layer
==============

The **MAC low layer** is split into three main components:

#. ``ns3::MacLow`` which takes care of RTS/CTS/DATA/ACK transactions.
#. ``ns3::DcfManager`` and ``ns3::DcfState`` which implements the DCF and EDCAF
   functions.
#. ``ns3::DcaTxop`` and ``ns3::EdcaTxopN`` which handle the packet queue,
   packet fragmentation, and packet retransmissions if they are needed.
   The ``ns3::DcaTxop`` object is used by high MACs that are not QoS-enabled,
   and for transmission of frames (e.g., of type Management)
   that the standard says should access the medium using the DCF. 
   ``ns3::EdcaTxopN`` is is used by QoS-enabled high MACs and also
   performs 802.11n-style MSDU aggregation.

PHY layer models
================

In short, the physical layer models are mainly responsible for modeling 
the reception of packets and for tracking energy consumption.  There
are typically three main components to packet reception:

* each packet received is probabilistically evaluated for successful or
  failed reception.  The probability depends on the modulation, on
  the signal to noise (and interference) ratio for the packet, and on
  the state of the physical layer (e.g. reception is not possible while
  transmission or sleeping is taking place);
* an object exists to track (bookkeeping) all received signals so that
  the correct interference power for each packet can be computed when
  a reception decision has to be made; and
* one or more error models corresponding to the modulation and standard
  are used to look up probability of successful reception.

|ns3| offers users a choice between two physical layer models, with a
base interface defined in the ``ns3::WifiPhy`` class.  The YansWifiPhy
class has been the only physical layer model until recently; the model
implemented there is described in a paper entitled
`Yet Another Network Simulator <http://cutebugs.net/files/wns2-yans.pdf>`_
The acronym *Yans* derives from this paper title.  The SpectrumWifiPhy
class is an alternative implementation based on the Spectrum framework
used for other |ns3| wireless models.  Spectrum allows a fine-grained
frequency decomposition of the signal, and permits scenarios to
include multiple technologies coexisting on the same channel.

Scope and Limitations
*********************

The IEEE 802.11 standard [ieee80211]_ is a large specification, 
and not all aspects are covered by |ns3|; the documentation of |ns3|'s 
conformance by itself would lead to a very long document.  This section 
attempts to summarize compliance with the standard and with behavior 
found in practice.

The physical layer and channel models operate on a per-packet basis, with
no frequency-selective propagation or interference effects when using
the default YansWifiPhy model.  Directional antennas are also not
supported at this time.  For additive white Gaussian noise (AWGN)
scenarios, or wideband interference scenarios, performance is governed
by the application of analytical models (based on modulation and factors
such as channel width) to the received signal-to-noise ratio, where noise
combines the effect of thermal noise and of interference from other Wi-Fi
packets.  Moreover, interference from other technologies is not modeled.
The following details pertain to the physical layer and channel models:

* 802.11ax is still in draft phase, not all functionalities are implemented yet
* 802.11ax does not contain any of the high-density improvement
* 802.11ax MU-OFDMA is not supported
* 802.11ax can only be used with Constant rate control algorithm
* 802.11ax only supports SU PPDU format
* 802.11ac/ax MU-MIMO is not supported, and no more than 4 antennas can be configured
* 802.11n/ac/ax beamforming is not supported
* 802.11 PCF and 802.11 HCF/HCCA are not implemented
* Authentication and encryption are missing
* Processing delays are not modeled
* PLCP preamble reception is not modeled
* PHY_RXSTART is not supported
* The current implementation assumes that secondary channels are always higher than primary channels
* Cases where RTS/CTS and ACK are transmitted using HT/VHT/HE formats are not supported
* Energy consumption model does not consider MIMO

At the MAC layer, most of the main functions found in deployed Wi-Fi
equipment for 802.11a/b/e/g/n/ac/ax are implemented, but there are scattered instances
where some limitations in the models exist. Support for 802.11n and ac is evolving.

Some implementation choices that are not imposed by the standard are listed below:

* BSSBasicRateSet for 802.11b has been assumed to be 1-2 Mbit/s
* BSSBasicRateSet for 802.11a/g has been assumed to be 6-12-24 Mbit/s
* The wifi manager always selects the lowest basic rate for management frames.

Design Details
**************

The remainder of this section is devoted to more in-depth design descriptions
of some of the Wi-Fi models.  Users interested in skipping to the section
on usage of the wifi module (User Documentation) may do so at this point.
We organize these more detailed sections from the bottom-up, in terms of
layering, by describing the channel and PHY models first, followed by
the MAC models.

We focus first on the choice between physical layer frameworks.  |ns3| 
contains support for a Wi-Fi-only physical layer model called YansWifiPhy
that offers no frequency-level decomposition of the signal.  For simulations
that involve only Wi-Fi signals on the Wi-Fi channel, and that do not
involve frequency-dependent propagation loss or fading models, the default
YansWifiPhy framework is a suitable choice.  For simulations involving
mixed technologies on the same channel, or frequency dependent effects,
the SpectrumWifiPhy is more appropriate.  The two frameworks are very
similarly configured.

The SpectrumWifiPhy framework uses the ``Spectrum`` channel
framework, which is not documented herein but in the Spectrum module
documentation.

The YansWifiChannel is the only concrete channel model class in 
the |ns3| wifi module.  The 
``ns3::YansWifiChannel`` implementation uses the propagation loss and 
delay models provided within the |ns3| :ref:`Propagation` module.
In particular, a number of propagation models can be added (chained together,
if multiple loss models are added) to the channel object, and a propagation 
delay model also added. Packets sent from a ``ns3::YansWifiPhy`` object
onto the channel with a particular signal power, are copied to all of the
other ``ns3::YansWifiPhy`` objects after the signal power is reduced due
to the propagation loss model(s), and after a delay corresponding to
transmission (serialization) delay and propagation delay due 
any channel propagation delay model (typically due to speed-of-light
delay between the positions of the devices).

Only objects of ``ns3::YansWifiPhy`` may be attached to a 
``ns3::YansWifiChannel``; therefore, objects modeling other 
(interfering) technologies such as LTE are not allowed.    Furthermore,
packets from different channels do not interact; if a channel is logically
configured for e.g. channels 5 and 6, the packets do not cause 
adjacent channel interference (even if their channel numbers overlap).

WifiPhy and related models
==========================

The ``ns3::WifiPhy`` is an abstract base class representing the 802.11
physical layer functions.  Packets passed to this object (via a
``SendPacket()`` method are sent over a channel object, and
upon reception, the receiving PHY object decides (based on signal power
and interference) whether the packet was successful or not.  This class
also provides a number of callbacks for notifications of physical layer
events, exposes a notion of a state machine that can be monitored for
MAC-level processes such as carrier sense, and handles sleep/wake models
and energy consumption.  The ``ns3::WifiPhy`` hooks to the ``ns3::MacLow``
object in the WifiNetDevice.

There are currently two implementations of the ``WifiPhy``: the
``ns3::YansWifiPhy`` and the ``ns3::SpectrumWifiPhy``.  They each work in 
conjunction with three other objects:

* **WifiPhyStateHelper**:  Maintains the PHY state machine
* **InterferenceHelper**:  Tracks all packets observed on the channel
* **ErrorModel**:  Computes a probability of error for a given SNR

YansWifiPhy and WifiPhyStateHelper
##################################

Class ``ns3::YansWifiPhy`` is responsible for taking packets passed to
it from the MAC (the ``ns3::MacLow`` object) and sending them onto the
``ns3::YansWifiChannel`` to which it is attached.  It is also responsible
to receive packets from that channel, and, if reception is deemed to have
been successful, to pass them up to the MAC. 

Class ``ns3::WifiPhyStateHelper`` manages the state machine of the PHY 
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 six 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.
#. 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.

Packet reception works as follows.  The ``YansWifiPhy`` attribute 
CcaMode1Threshold 
corresponds to what the standard calls the "ED threshold" for CCA Mode 1.  
In section 16.4.8.5:  "CCA Mode 1: Energy above threshold. CCA shall report 
a busy medium upon detection of any energy above the ED threshold."  

There is a "noise ED threshold" in the standard for non-Wi-Fi signals, and 
this is usually set to 20 dB greater than the "carrier sense ED threshold".  
However, the model doesn't support this, because there are no 'foreign' 
signals in the YansWifi model-- everything is a Wi-Fi signal.

In the standard, there is also what is called the "minimum modulation
and coding rate sensitivity" in section 18.3.10.6 CCA requirements. This is 
the -82 dBm requirement for 20 MHz channels.  This is analogous to the 
EnergyDetectionThreshold attribute in ``YansWifiPhy``.  CCA busy state is 
not raised in this model when this threshold is exceeded but instead RX 
state is immediately reached, since it is assumed that PLCP sync always 
succeeds in this model.  Even if the PLCP header reception fails, the 
channel state is still held in RX until YansWifiPhy::EndReceive().

In ns-3, the values of these attributes are set to small default values 
(-96 dBm for EnergyDetectionThreshold and -99 dBm for CcaMode1Threshold).  
So, if a signal comes in at > -96 dBm and the state is IDLE or CCA BUSY, 
this model will lock onto it for the signal duration and raise RX state.  
If it comes in at <= -96 dBm but >= -99 dBm, it will definitely raise 
CCA BUSY but not RX state.  If it comes in < -99 dBm, it gets added to 
the interference tracker and, by itself, it will not raise CCA BUSY, but 
maybe a later signal will contribute more power so that the threshold 
of -99 dBm is reached at a later time.

The energy of the signal intended to be received is 
calculated from the transmission power and adjusted based on the Tx gain
of the transmitter, Rx gain of the receiver, and any path loss propagation
model in effect.

The packet reception occurs in two stages.   First, an event is scheduled
for when the PLCP header has been received. PLCP header is often transmitted
at a lower modulation rate than is the payload.  The portion of the packet
corresponding to the PLCP header is evaluated for probability of error 
based on the observed SNR.  The InterferenceHelper object returns a value
for "probability of error (PER)" for this header based on the SNR that has
been tracked by the InterferenceHelper.  The ``YansWifiPhy`` then draws
a random number from a uniform distribution and compares it against the 
PER and decides success or failure.  The process is again repeated after 
the payload has been received (possibly with a different error model 
applied for the different modulation).  If both the header and payload 
are successfully received, the packet is passed up to the ``MacLow`` object.  

Even if packet objects received by the PHY are not part of the reception
process, they are remembered by the InterferenceHelper object for purposes
of SINR computation and making clear channel assessment decisions.

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.

The basic operation of probability of error calculations is shown in Figure
:ref:`snir`.  Packets are represented as bits (not symbols) in the |ns3|
model, and the InterferenceHelper breaks the packet into one or more
"chunks" each with a different signal to noise (and interference) ratio
(SNIR).  Each chunk is separately evaluated by asking for the probability
of error for a given number of bits from the error model in use.  The
InterferenceHelper builds an aggregate "probability of error" value
based on these chunks and their duration, and returns this back to
the ``YansWifiPhy`` for a reception decision.

.. _snir:

.. figure:: figures/snir.*
   
   *SNIR function over time.*

From the SNIR function we can derive the Bit Error Rate (BER) and Packet 
Error Rate (PER) for
the modulation and coding scheme being used for the transmission.  

ErrorModel
##########

The error models are described in more detail in outside references.  Please refer to [pei80211ofdm]_, [pei80211b]_, [lacage2006yans]_, [Haccoun]_ and [Frenger]_ for a detailed description of the available BER/PER models.

The current |ns3| error rate models are for additive white gaussian
noise channels (AWGN) only; any potential fast fading effects are not modeled.

The original error rate model was called the ``ns3::YansErrorRateModel`` and
was based on analytical results.  For 802.11b modulations, the 1 Mbps mode 
is based on DBPSK. BER is from equation 5.2-69 from [proakis2001]_.
The 2 Mbps model is based on DQPSK. Equation 8 of [ferrari2004]_.  
More details are provided in [lacage2006yans]_.

The ``ns3::NistErrorRateModel`` was later added and became the |ns3| default.
The model was largely aligned with the previous ``ns3::YansErrorRateModel``
for DSSS modulations 1 Mbps and 2 Mbps, but the 5.5 Mbps and 11 Mbps models
were re-based on equations (17) and (18) from [pursley2009]_.
For OFDM modulations, newer results were
obtained based on work previously done at NIST [miller2003]_.  The results
were also compared against the CMU wireless network emulator, and details
of the validation are provided in [pei80211ofdm]_.  Since OFDM modes use
hard-decision of punctured codes, the coded BER is calculated using
Chernoff bounds.

The 802.11b model was split from the OFDM model when the NIST error rate
model was added, into a new model called DsssErrorRateModel.  The current
behavior is that users may 

Furthermore, the 5.5 Mbps and 11 Mbps models for 802.11b rely on library
methods implemented in the GNU Scientific Library (GSL).  The Waf build
system tries to detect whether the host platform has GSL installed; if so,
it compiles in the newer models from [pursley2009]_ for 5.5 Mbps and 11 Mbps;
if not, it uses a backup model derived from Matlab simulations.

As a result, there are three error models:

#. ``ns3::DsssErrorRateModel``:  contains models for 802.11b modes.  The
   802.11b 1 Mbps and 2 Mbps error models are based on classical modulation
   analysis.  If GNU GSL is installed, the 5.5 Mbps and 11 Mbps from 
   [pursley2009]_ are used; otherwise, a backup Matlab model is used.
#. ``ns3::NistErrorRateModel``: is the default for OFDM modes and reuses
   ``ns3::DsssErrorRateModel`` for 802.11b modes. 
#. ``ns3::YansErrorRateModel``: is the legacy for OFDM modes and reuses
   ``ns3::DsssErrorRateModel`` for 802.11b modes. 

Users should select either Nist or Yans models for OFDM (Nist is default), 
and Dsss will be used in either case for 802.11b.

SpectrumWifiPhy
###############

This section describes the implementation of the ``SpectrumWifiPhy``
class that can be found in ``src/wifi/model/spectrum-wifi-phy.{cc,h}``.

The implementation also makes use of additional classes found in the
same directory:

* ``wifi-spectrum-phy-interface.{cc,h}``
* ``wifi-spectrum-signal-parameters.{cc,h}``

and classes found in the spectrum module:

* ``wifi-spectrum-value-helper.{cc,h}``

The current ``SpectrumWifiPhy`` class 
reuses the existing interference manager and error rate models originally
built for YansWifiPhy, but allows, as a first step, foreign (non Wi-Fi)
signals to be treated as additive noise.

Two main changes were needed to adapt the Spectrum framework to Wi-Fi.
First, the physical layer must send signals compatible with the
Spectrum channel framework, and in particular, the
``MultiModelSpectrumChannel`` that allows signals from different
technologies to coexist.  Second, the InterferenceHelper must be
extended to support the insertion of non-Wi-Fi signals and to
add their received power to the noise, in the same way that
unintended Wi-Fi signals (perhaps from a different SSID or arriving
late from a hidden node) are added to the noise.

Third, the default value for CcaMode1Threshold attribute is -62 dBm
rather than the value of -99 dBm used for YansWifiPhy.  This is because,
unlike YansWifiPhy, where there are no foreign signals, CCA BUSY state
will be raised for foreign signals that are higher than this 'energy
detection' threshold (see section 16.4.8.5 in the 802.11-2012 standard
for definition of CCA Mode 1).  

To support the Spectrum channel, the ``YansWifiPhy`` transmit and receive methods
were adapted to use the Spectrum channel API.  This required developing
a few ``SpectrumModel``-related classes.  The class
``WifiSpectrumValueHelper`` is used to create Wi-Fi signals with the
spectrum framework and spread their energy across the bands.  The 
spectrum is sub-divided into 312.5 kHz sub-bands (the width of an OFDM
subcarrier).  The power allocated to a particular channel
is spread across the sub-bands roughly according to how power would 
be allocated to sub-carriers using an even distribution of power and
assuming perfect transmit filters.  This could be extended in the 
future to place power outside of the
channel according to the real spectral mask.  This should be
done for future adjacent channel models but is not presently implemented.
Similarly, on the receive side, a receiver filter mask can be defined; 
for this initial implementation, we implemented a perfect brick wall 
filter that is centered on the channel center frequency.

To support an easier user configuration experience, the existing
YansWifi helper classes (in ``src/wifi/helper``) were copied and
adapted to provide equivalent SpectrumWifi helper classes.

Finally, for reasons related to avoiding C++ multiple inheritance
issues, a small forwarding class called ``WifiSpectrumPhyInterface``
was inserted as a shim between the ``SpectrumWifiPhy`` and the
Spectrum channel.

The MAC model
=============

The 802.11 Distributed Coordination Function is used to calculate when to grant
access to the transmission medium. While implementing the DCF would have been
particularly easy if we had used a recurring timer that expired every slot, we
chose to use the method described in [ji2004sslswn]_
where the backoff timer duration is lazily calculated whenever needed since it
is claimed to have much better performance than the simpler recurring timer
solution.

The backoff procedure of DCF is described in section 9.2.5.2 of [ieee80211]_.

*  “The backoff procedure shall be invoked for a STA to transfer a frame 
   when finding the medium busy as indicated by either the physical or 
   virtual CS mechanism.”
*  “A backoff procedure shall be performed immediately after the end of 
   every transmission with the More Fragments bit set to 0 of an MPDU of 
   type Data, Management, or Control with subtype PS-Poll, even if no 
   additional transmissions are currently queued.”

Thus, if the queue is empty, a newly arrived packet should be transmitted 
immediately after channel is sensed idle for DIFS.  If queue is not empty 
and after a successful MPDU that has no more fragments, a node should 
also start the backoff timer.

Some users have observed that the 802.11 MAC with an empty queue on an 
idle channel will transmit the first frame arriving to the model 
immediately without waiting for DIFS or backoff, and wonder whether this 
is compliant.  According to the standard, “The backoff procedure shall 
be invoked for a STA to transfer a frame when finding the medium busy 
as indicated by either the physical or virtual CS mechanism.”  So in 
this case, the medium is not found to be busy in recent past and the 
station can transmit immediately. 

The higher-level MAC functions are implemented in a set of other C++ classes and
deal with:

* packet fragmentation and defragmentation,
* use of the RTS/CTS protocol,
* rate control algorithm,
* connection and disconnection to and from an Access Point,
* the MAC transmission queue,
* beacon generation,
* MSDU aggregation,
* etc.

Rate control algorithms
#######################

Multiple rate control algorithms are available in |ns3|.
Some rate control algorithms are modeled after real algorithms used in real devices;
others are found in literature.
The following rate control algorithms can be used by the MAC low layer:

Algorithms found in real devices:

* ``ArfWifiManager`` (default for ``WifiHelper``)
* ``OnoeWifiManager``
* ``ConstantRateWifiManager``
* ``MinstrelWifiManager``

Algorithms in literature:

* ``IdealWifiManager``
* ``AarfWifiManager`` [lacage2004aarfamrr]_
* ``AmrrWifiManager`` [lacage2004aarfamrr]_
* ``CaraWifiManager`` [kim2006cara]_
* ``RraaWifiManager`` [wong2006rraa]_
* ``AarfcdWifiManager`` [maguolo2008aarfcd]_
* ``ParfWifiManager`` [akella2007parf]_
* ``AparfWifiManager`` [chevillat2005aparf]_

ConstantRateWifiManager
#######################

The constant rate control algorithm always uses the same
transmission mode for every packet. Users can set a desired
'DataMode' for all 'unicast' packets and 'ControlMode' for all
'request' control packets (e.g. RTS).

To specify different data mode for non-unicast packets, users
must set the 'NonUnicastMode' attribute of the
WifiRemoteStationManager.  Otherwise, WifiRemoteStationManager
will use a mode with the lowest rate for non-unicast packets.

The 802.11 standard is quite clear on the rules for selection
of transmission parameters for control response frames (e.g.
CTS and ACK).  |ns3| follows the standard and selects the rate
of control response frames from the set of basic rates or
mandatory rates. This means that control response frames may
be sent using different rate even though the ConstantRateWifiManager
is used.  The ControlMode attribute of the ConstantRateWifiManager
is used for RTS frames only.  The rate of CTS and ACK frames are
selected according to the 802.11 standard.  However, users can still
manually add WifiMode to the basic rate set that will allow control
response frames to be sent at other rates.  Please consult the
`project wiki <http://www.nsnam.org/wiki>`_ on how to do this.

Available attributes:

* DataMode (default WifiMode::OfdmRate6Mbps): specify a mode for
  all non-unicast packets
* ControlMode (default WifiMode::OfdmRate6Mbps): specify a mode for
  all 'request' control packets

IdealWifiManager
################

The ideal rate control algorithm selects the best
mode according to the SNR of the previous packet sent.
Consider node *A* sending a unicast packet to node *B*.
When *B* successfully receives the packet sent from *A*,
*B* records the SNR of the received packet into a ``ns3::SnrTag``
and adds the tag to an ACK back to *A*.
By doing this, *A* is able to learn the SNR of the packet sent to *B*
using an out-of-band mechanism (thus the name 'ideal').
*A* then uses the SNR to select a transmission mode based
on a set of SNR thresholds, which was built from a target BER and
mode-specific SNR/BER curves.

Available attribute:

* BerThreshold (default 10e-6): The maximum Bit Error Rate
  that is used to calculate the SNR threshold for each mode.

MinstrelWifiManager
###################

The minstrel rate control algorithm is a rate control algorithm originated from
madwifi project.  It is currently the default rate control algorithm of the Linux kernel.

Minstrel keeps track of the probability of successfully sending a frame of each available rate.
Minstrel then calculates the expected throughput by multiplying the probability with the rate.
This approach is chosen to make sure that lower rates are not selected in favor of the higher
rates (since lower rates are more likely to have higher probability).

In minstrel, roughly 10 percent of transmissions are sent at the so-called lookaround rate.
The goal of the lookaround rate is to force minstrel to try higher rate than the currently used rate.

For a more detailed information about minstrel, see [linuxminstrel]_.

Modifying Wifi model
####################

Modifying the default wifi model is one of the common tasks when performing research.
We provide an overview of how to make changes to the default wifi model in this section.
Depending on your goal, the common tasks are (in no particular order):

* Creating or modifying the default Wi-Fi frames/headers by making changes to ``wifi-mac-header.*``.
* MAC low modification. For example, handling new/modified control frames (think RTS/CTS/ACK/Block ACK),
  making changes to two-way transaction/four-way transaction.  Users usually make changes to 
  ``mac-low.*`` to accomplish this.  Handling of control frames is performed in
  ``MacLow::ReceiveOk``.
* MAC high modification. For example, handling new management frames (think beacon/probe), 
  beacon/probe generation.  Users usually make changes to ``regular-wifi-mac.*``, 
  ``sta-wifi-mac.*``, ``ap-wifi-mac.*``, or ``adhoc-wifi-mac.*`` to accomplish this.
* Wi-Fi queue management.  The files ``dca-txop.*`` and ``edca-txop-n.*`` are of interested for this task.
* Channel access management.  Users should modify the files ``dcf-manager.*``, which grant access to
  ``DcaTxop`` and ``EdcaTxopN``.
* Fragmentation and RTS threholds are handled by Wi-Fi remote station manager.  Note that Wi-Fi remote
  station manager simply indicates if fragmentation and RTS are needed.  Fragmentation is handled by
  ``DcaTxop`` or ``EdcaTxopN`` while RTS/CTS transaction is hanled by ``MacLow``.
* Modifying or creating new rate control algorithms can be done by creating a new child class of Wi-Fi remote
  station manager or modifying the existing ones.

