Design Documentation

ns-3 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 ns-3 WifiNetDevice and related models. By adding WifiNetDevice objects to ns-3 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, ns-3 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 (2.4, 5 and 6 GHz bands) physical layers
• MSDU aggregation and MPDU aggregation extensions of 802.11n, and both can be combined together (two-level aggregation)
• 802.11ax DL OFDMA and UL OFDMA
• 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 Propagation for more detail
• packet error models and frame detection models that have been validated against link simulations and other references
• various rate control algorithms including Aarf, Arf, Cara, Onoe, Rraa, ConstantRate, Minstrel and Minstrel-HT
• 802.11s (mesh), described in another chapter
• 802.11p and WAVE (vehicular), described in another chapter

The set of 802.11 models provided in ns-3 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 ns-3, 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: they model amendment-specific and common PHY layer operations and functions.
• the so-called MAC low models: they model functions such as medium access (DCF and EDCA), frame protection (RTS/CTS) and acknowledgment (ACK/BlockAck). In ns-3, the lower-level MAC is comprised of a Frame Exchange Manager hierarchy, a Channel Access Manager and a MAC middle entity.
• 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 WifiNetDevice architecture.

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.

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:

1. ns3::FrameExchangeManager a class hierarchy which implement the frame exchange sequences introduced by the supported IEEE 802.11 amendments. It also handles frame aggregation, frame retransmissions, protection and acknowledgment.
2. ns3::ChannelAccessManager which implements the DCF and EDCAF functions.
3. ns3::Txop and ns3::QosTxop which handle the packet queue. The ns3::Txop 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::QosTxop is used by QoS-enabled high MACs.

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.

ns-3 offers users a choice between two physical layer models, with a base interface defined in the ns3::WifiPhy class. The YansWifiPhy class implements a simple physical layer model, which is described in a paper entitled Yet Another Network Simulator The acronym Yans derives from this paper title. The SpectrumWifiPhy class is a more advanced implementation based on the Spectrum framework used for other ns-3 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 ns-3; the documentation of ns-3’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 nor 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. Interference from other wireless technologies is only modeled when the SpectrumWifiPhy is used. The following details pertain to the physical layer and channel models:

• 802.11ax MU-RTS/CTS and the MU EDCA Parameter set are not yet supported
• 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
• 802.11n RIFS is not supported
• 802.11 PCF/HCF/HCCA are not implemented
• Authentication and encryption are missing
• Processing delays are not modeled
• Channel bonding implementation only supports the use of the configured channel width and does not perform CCA on secondary 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, ac and ax 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
• OperationalRateSet is assumed to contain all mandatory rates (see issue 183)
• 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. ns-3 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 Module channel framework.

The YansWifiChannel is the only concrete channel model class in the ns-3 wifi module. The ns3::YansWifiChannel implementation uses the propagation loss and delay models provided within the ns-3 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 to 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 Send() 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/off 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 five other objects:

• PhyEntity: Contains the amendment-specific part of the PHY processing
• WifiPpdu: Models the amendment-specific PHY protocol data unit (PPDU)
• WifiPhyStateHelper: Maintains the PHY state machine
• InterferenceHelper: Tracks all packets observed on the channel
• ErrorModel: Computes a probability of error for a given SNR

PhyEntity

A bit of background

Some restructuring of ns3::WifiPhy and ns3::WifiMode (among others) was necessary considering the size and complexity of the corresponding files. In addition, adding and maintaining new PHY amendments had become a complex task (especially those implemented inside other modules, e.g. DMG). The adopted solution was to have PhyEntity classes that contain the “clause” specific (i.e. HT/VHT/HE etc) parts of the PHY process.

The notion of “PHY entity” is in the standard at the beginning of each PHY layer description clause, e.g. section 21.1.1 of IEEE 802.11-2016:

:: Clause 21 specifies the PHY entity for a very high throughput (VHT) orthogonal frequency division multiplexing (OFDM) system.

Note that there is already such a name inside the wave module (e.g. ``WaveNetDevice::AddPhy``) to designate the WifiPhys on each 11p channel, but the wording is only used within the classes and there is no file using that name, so no ambiguity in using the name for 802.11 amendments.

Architecture

The abstract base class ns3::PhyEntity enables to have a unique set of APIs to be used by each PHY entity, corresponding to the different amendments of the IEEE 802.11 standard. The currently implemented PHY entities are:

• ns3::DsssPhy: PHY entity for DSSS and HR/DSSS (11b)
• ns3::OfdmPhy: PHY entity for OFDM (11a and 11p)
• ns3::ErpOfdmPhy: PHY entity for ERP-OFDM (11g)
• ns3::HtPhy: PHY entity for HT (11n)
• ns3::VhtPhy: PHY entity for VHT (11ac)
• ns3::HePhy: PHY entity for HE (11ax)

Their inheritance diagram is given in Figure PhyEntity hierarchy and closely follows the standard’s logic, e.g. section 21.1.1 of IEEE 802.11-2016:

:: The VHT PHY is based on the HT PHY defined in Clause 19, which in turn is based on the OFDM PHY defined in Clause 17.

PhyEntity hierarchy

Such an architecture enables to handle the following operations in an amendment- specific manner:

• WifiMode handling and data/PHY rate computation,
• PPDU field size and duration computation, and
• Transmit and receive paths.

WifiPpdu

In the same vein as PhyEntity, the ns3::WifiPpdu base class has been specialized into the following amendment-specific PPDUs:

• ns3::DsssPpdu: PPDU for DSSS and HR/DSSS (11b)
• ns3::OfdmPpdu: PPDU for OFDM (11a and 11p)
• ns3::ErpOfdmPpdu: PPDU for ERP-OFDM (11g)
• ns3::HtPpdu: PPDU for HT (11n)
• ns3::VhtPpdu: PPDU for VHT (11ac)
• ns3::HePpdu: PPDU for HE (11ax)

Their inheritance diagram is given in Figure WifiPpdu hierarchy and closely follows the standard’s logic, e.g. section 21.3.8.1 of IEEE 802.11-2016:

:: To maintain compatibility with non-VHT STAs, specific non-VHT fields are defined that can be received by non-VHT STAs compliant with Clause 17 [OFDM] or Clause 19 [HT].

WifiPpdu hierarchy

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.

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.

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 seven states:

1. TX: the PHY is currently transmitting a signal on behalf of its associated MAC
2. RX: the PHY is synchronized on a signal and is waiting until it has received its last bit to forward it to the MAC.
3. IDLE: the PHY is not in the TX, RX, or CCA_BUSY states.
4. CCA_BUSY: the PHY is not in TX or RX state but the measured energy is higher than the energy detection threshold.
5. SWITCHING: the PHY is switching channels.
6. SLEEP: the PHY is in a power save mode and cannot send nor receive frames.
7. OFF: the PHY is powered off and cannot send nor receive frames.

Packet reception works as follows. For YansWifiPhy, most of the logic is implemented in the WifiPhy base class. The YansWifiChannel calls WifiPhy::StartReceivePreamble (). The latter calls PhyEntity::StartReceivePreamble () of the appropriate PHY entity to start packet reception, but first there is a check of the packet’s notional signal power level against a threshold value stored in the attribute WifiPhy::RxSensitivity. Any packet with a power lower than RxSensitivity will be dropped with no further processing. The default value is -101 dBm, which is the thermal noise floor for 20 MHz signal at room temperature. The purpose of this attribute is two-fold: 1) very weak signals that will not affect the outcome will otherwise consume simulation memory and event processing, so they are discarded, and 2) this value can be adjusted upwards to function as a basic carrier sense threshold limitation for experiments involving spatial reuse considerations. Users are cautioned about the behavior of raising this threshold; namely, that all packets with power below this threshold will be discarded upon reception.

In StartReceivePreamble (), the packet is immediately added to the interference helper for signal-to-noise tracking, and then further reception steps are decided upon the state of the PHY. In the case that the PHY is transmitting, for instance, the packet will be dropped. If the PHY is IDLE, or if the PHY is receiving and an optional FrameCaptureModel is being used (and the packet is within the capture window), then PhyEntity::StartPreambleDetectionPeriod () is called next.

The PhyEntity::StartPreambleDetectionPeriod () will typically schedule an event, PhyEntity::EndPreambleDetectionPeriod (), to occur at the notional end of the first OFDM symbol, to check whether the preamble has been detected. As of revisions to the model in ns-3.30, any state machine transitions from IDLE state are suppressed until after the preamble detection event.

The PhyEntity::EndPreambleDetectionPeriod () method will check, with a preamble detection model, whether the signal is strong enough to be received, and if so, an event PhyEntity::EndReceiveField () is scheduled for the end of the preamble and the PHY is put into the CCA_BUSY state. Currently, there is only a simple threshold-based preamble detection model in ns-3, called ThresholdPreambleDetectionModel. If there is no preamble detection model, the preamble is assumed to have been detected. It is important to note that, starting with the ns-3.30 release, the default in the WifiPhyHelper is to add the ThresholdPreambleDetectionModel with a threshold RSSI of -82 dBm, and a threshold SNR of 4 dB. Both the RSSI and SNR must be above these respective values for the preamble to be successfully detected. The default sensitivity has been reduced in ns-3.30 compared with that of previous releases, so some packet receptions that were previously successful will now fail on this check. More details on the modeling behind this change are provided in [lanante2019].

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.

The next event at PhyEntity::StartReceiveField () checks, using the interference helper and error model, whether the header was successfully decoded, and if so, a PhyRxPayloadBegin callback (equivalent to the PHY-RXSTART primitive) is triggered. The PHY header is often transmitted at a lower modulation rate than is the payload. The portion of the packet corresponding to the PHY 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 PhyEntity then draws a random number from a uniform distribution and compares it against the PER and decides success or failure.

This is iteratively performed up to the beginning of the data field upon which PhyEntity::StartReceivePayload () is called.

Even if packet objects received by the PHY are not part of the reception process, they are tracked by the InterferenceHelper object for purposes of SINR computation and making clear channel assessment decisions. If, in the course of reception, a packet is errored or dropped due to 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.

The above describes the case in which the packet is a single MPDU. For more recent Wi-Fi standards using MPDU aggregation, StartReceivePayload schedules an event for reception of each individual MPDU (ScheduleEndOfMpdus), which then forwards each MPDU as they arrive up to MacLow, if the reception of the MPDU has been successful. Once the A-MPDU reception is finished, MacLow is also notified about the amount of successfully received MPDUs.

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 SNIR function over time. Packets are represented as bits (not symbols) in the ns-3 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 WifiPhy for a reception decision.

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.

If MIMO is used and the number of spatial streams is lower than the number of active antennas at the receiver, then a gain is applied to the calculated SNIR as follows (since STBC is not used):

Having more TX antennas can be safely ignored for AWGN. The resulting gain is:

antennas   NSS    gain
2 x 1       1     0 dB
1 x 2       1     3 dB
2 x 2       1     3 dB
3 x 3       1   4.8 dB
3 x 3       2   1.8 dB
3 x 3       3     0 dB
4 x 4       1     6 dB
4 x 4       2     3 dB
4 x 4       3   1.2 dB
4 x 4       4     0 dB
...

ErrorRateModel

ns-3 makes a packet error or success decision based on the input received SNR of a frame and based on any possible interfering frames that may overlap in time; i.e. based on the signal-to-noise (plus interference) ratio, or SINR. The relationship between packet error ratio (PER) and SINR in ns-3 is defined by the ns3::ErrorRateModel, of which there are several. The PER is a function of the frame’s modulation and coding (MCS), its SINR, and the specific ErrorRateModel configured for the MCS.

ns-3 has updated its default ErrorRateModel over time. The current (as of ns-3.33 release) model for recent OFDM-based standards (i.e., 802.11n/ac/ax), is the ns3::TableBasedErrorRateModel. The default for 802.11a/g is the ns3::YansErrorRateModel, and the default for 802.11b is the ns3::DsssErrorRateModel. The error rate model for recent standards was updated during the ns-3.33 release cycle (previously, it was the ns3::NistErrorRateModel).

The error models are described in more detail in outside references. The current OFDM model is based on work published in [patidar2017], using link simulations results from the MATLAB WLAN Toolbox, and validated against IEEE TGn results [erceg2004]. For publications related to other error models, please refer to [pei80211ofdm], [pei80211b], [lacage2006yans], [Haccoun], [hepner2015] and [Frenger] for a detailed description of the legacy PER models.

The current ns-3 error rate models are for additive white gaussian noise channels (AWGN) only; any potential frequency-selective fading effects are not modeled.

In summary, there are four error models:

1. ns3::TableBasedErrorRateModel: for OFDM modes and reuses ns3::DsssErrorRateModel for 802.11b modes. This is the default for 802.11n/ac/ax.
2. ns3::YansErrorRateModel: for OFDM modes and reuses ns3::DsssErrorRateModel for 802.11b modes. This is the default for 802.11a/g.
3. 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 Scientific Library (GSL) is installed, the 5.5 Mbps and 11 Mbps from [pursley2009] are used for CCK modulation; otherwise, results from a backup MATLAB-based CCK model are used.
4. ns3::NistErrorRateModel: for OFDM modes and reuses ns3::DsssErrorRateModel for 802.11b modes.

Users may select either NIST, YANS or Table-based models for OFDM, and DSSS will be used in either case for 802.11b. The NIST model was a long-standing default in ns-3 (through release 3.32).

TableBasedErrorRateModel

The ns3::TableBasedErrorRateModel has been recently added and is now the ns-3 default for 802.11n/ac/ax, while ns3::YansErrorRateModel is the ns-3 default for 802.11a/g.

Unlike analytical error models based on error bounds, ns3::TableBasedErrorRateModel contains end-to-end link simulation tables (PER vs SNR) for AWGN channels. Since it is infeasible to generate such look-up tables for all desired packet sizes and input SNRs, we adopt the recommendation of IEEE P802.11 TGax [porat2016] that proposed estimating PER for any desired packet length using BCC FEC encoding by extrapolating the results from two reference lengths: 32 (all lengths less than 400) bytes and 1458 (all lengths greater or equal to 400) bytes respectively. In case of LDPC FEC encoding, IEEE P802.11 TGax recommends the use of a single reference length. Hence, we provide two tables for BCC and one table for LDPC that are generated using a reliable and publicly available commercial link simulator (MATLAB WLAN Toolbox) for each modulation and coding scheme. Note that BCC tables are limited to MCS 9. For higher MCSs, the models fall back to the use of the YANS analytical model.

The validation scenario is set as follows:

1. Ideal channel and perfect channel estimation.
2. Perfect packet synchronization and detection.
3. Phase tracking, phase correction, phase noise, carrier frequency offset, power amplifier non-linearities etc. are not considered.

Several packets are simulated across the link to obtain PER, the number of packets needed to reliably estimate a PER value is computed using the consideration that the ratio of the estimation error to the true value should be within 10 % with probability 0.95. For each SNR value, simulations were run until a total of 40000 packets were simulated.

The obtained results are very close to TGax curves as shown in Figure Comparison of table-based OFDM Error Model with TGax results.

Comparison of table-based OFDM Error Model with TGax results.

Legacy ErrorRateModels

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. 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 [hepner2015].

The 802.11b model was split from the OFDM model when the NIST error rate model was added, into a new model called DsssErrorRateModel.

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.

The error curves for analytical models are shown to diverge from link simulation results for higher MCS in Figure YANS and NIST error model comparison with TGn results. This prompted the move to a new error model based on link simulations (the default TableBasedErrorRateModel, which provides curves close to those depicted by the TGn dashed line).

YANS and NIST error model comparison with TGn results

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.

Unlike YansWifiPhy, where there are no foreign signals, CCA_BUSY state will be raised for foreign signals that are higher than CcaEdThreshold (see section 16.4.8.5 in the 802.11-2012 standard for definition of CCA Mode 1). The attribute WifiPhy::CcaEdThreshold therefore potentially plays a larger role in this model than in the YansWifiPhy model.

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 sub-bands (the width of an OFDM subcarrier, which depends on the technology). The power allocated to a particular channel is spread across the sub-bands roughly according to how power would be allocated to sub-carriers. Adjacent channels are models by the use of OFDM transmit spectrum masks as defined in the standards.

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 WifiSpectrumPhyInterface calls a different SpectrumWifiPhy::StartRx () method to start the reception process. This method performs the check of the signal power against the WifiPhy::RxSensitivity attribute and discards weak signals, and also checks if the signal is a Wi-Fi signal; non-Wi-Fi signals are added to the InterferenceHelper and can raise CCA_BUSY but are not further processed in the reception chain. After this point, valid Wi-Fi signals cause WifiPhy::StartReceivePreamble to be called, and the processing continues as described above.

The MAC model

Infrastructure association

Association in infrastructure mode is a high-level MAC function. Either active probing or passive scanning is used (default is passive scan). At the start of the simulation, Wi-Fi network devices configured as STA will attempt to scan the channel. Depends on whether passive or active scanning is selected, STA will attempt to gather beacons, or send a probe request and gather probe responses until the respective timeout occurs. The end result will be a list of candidate AP to associate to. STA will then try to associate to the best AP (i.e., best SNR).

If association is rejected by the AP for some reason, the STA will try to associate to the next best AP until the candidate list is exhausted which then sends STA to ‘REFUSED’ state. If this occurs, the simulation user will need to force reassociation retry in some way, perhaps by changing configuration (i.e. the STA will not persistently try to associate upon a refusal).

When associated, if the configuration is changed by the simulation user, the STA will try to reassociate with the existing AP.

If the number of missed beacons exceeds the threshold, the STA will notify the rest of the device that the link is down (association is lost) and restart the scanning process. Note that this can also happen when an association request fails without explicit refusal (i.e., the AP fails to respond to association request).

Roaming

Roaming at layer-2 (i.e. a STA migrates its association from one AP to another) is not presently supported. Because of that, the Min/Max channel dwelling time implementation as described by the IEEE 802.11 standard [ieee80211] is also omitted, since it is only meaningful on the context of channel roaming.

Channel access

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 DCF basic access is described in section 10.3.4.2 of [ieee80211-2016].

• “A STA may transmit an MPDU when it is operating under the DCF access method [..] when the STA determines that the medium is idle when a frame is queued for transmission, and remains idle for a period of a DIFS, or an EIFS (10.3.2.3.7) from the end of the immediately preceding medium-busy event, whichever is the greater, and the backoff timer is zero. Otherwise the random backoff procedure described in 10.3.4.3 shall be followed.”

Thus, a station is allowed not to invoke the backoff procedure if all of the following conditions are met:

• the medium is idle when a frame is queued for transmission
• the medium remains idle until the most recent of these two events: a DIFS from the time when the frame is queued for transmission; an EIFS from the end of the immediately preceding medium-busy event (associated with the reception of an erroneous frame)
• the backoff timer is zero

The backoff procedure of DCF is described in section 10.3.4.3 of [ieee80211-2016].

• “A STA shall invoke the backoff procedure 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.”

The EDCA backoff procedure is slightly different than the DCF backoff procedure and is described in section 10.22.2.2 of [ieee80211-2016]. The backoff procedure shall be invoked by an EDCAF when any of the following events occur:

• a frame is “queued for transmission such that one of the transmit queues associated with that AC has now become non-empty and any other transmit queues associated with that AC are empty; the medium is busy on the primary channel”
• “The transmission of the MPDU in the final PPDU transmitted by the TXOP holder during the TXOP for that AC has completed and the TXNAV timer has expired, and the AC was a primary AC”
• “The transmission of an MPDU in the initial PPDU of a TXOP fails [..] and the AC was a primary AC”
• “The transmission attempt collides internally with another EDCAF of an AC that has higher priority”
• (optionally) “The transmission by the TXOP holder of an MPDU in a non-initial PPDU of a TXOP fails”

Additionally, section 10.22.2.4 of [ieee80211-2016] introduces the notion of slot boundary, which basically occurs following SIFS + AIFSN * slotTime of idle medium after the last busy medium that was the result of a reception of a frame with a correct FCS or following EIFS - DIFS + AIFSN * slotTime + SIFS of idle medium after the last indicated busy medium that was the result of a frame reception that has resulted in FCS error, or following a slotTime of idle medium occurring immediately after any of these conditions.

On these specific slot boundaries, each EDCAF shall make a determination to perform one and only one of the following functions:

• Decrement the backoff timer.
• Initiate the transmission of a frame exchange sequence.
• Invoke the backoff procedure due to an internal collision.
• Do nothing.

Thus, if an EDCAF decrements its backoff timer on a given slot boundary and, as a result, the backoff timer has a zero value, the EDCAF cannot immediately transmit, but it has to wait for another slotTime of idle medium before transmission can start.

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.

Frame Exchange Managers

As the IEEE 802.11 standard evolves, more and more features are added and it is more and more difficult to have a single component handling all of the allowed frame exchange sequences. A hierarchy of FrameExchangeManager classes has been introduced to make the code clean and scalable, while avoiding code duplication. Each FrameExchangeManager class handles the frame exchange sequences introduced by a given amendment. The FrameExchangeManager hierarchy is depicted in Figure FrameExchangeManager hierarchy.

FrameExchangeManager hierarchy

The features supported by every FrameExchangeManager class are as follows:

• FrameExchangeManager is the base class. It handles the basic sequences for non-QoS stations: MPDU followed by Normal Ack, RTS/CTS and CTS-to-self, NAV setting and resetting, MPDU fragmentation
• QosFrameExchangeManager adds TXOP support: multiple protection setting, TXOP truncation via CF-End, TXOP recovery, ignore NAV when responding to an RTS sent by the TXOP holder
• HtFrameExchangeManager adds support for Block Ack (compressed variant), A-MSDU and A-MPDU aggregation, Implicit Block Ack Request policy
• VhtFrameExchangeManager adds support for S-MPDUs
• HeFrameExchangeManager adds support for the transmission and reception of multi-user frames via DL OFDMA and UL OFDMA, as detailed below.

Multi-user transmissions

Since the introduction of the IEEE 802.11ax amendment, multi-user (MU) transmissions are possible, both in downlink (DL) and uplink (UL), by using OFDMA and/or MU-MIMO. Currently, ns-3 only supports multi-user transmissions via OFDMA. Three acknowledgment sequences are implemented for DL OFDMA.

The first acknowledgment sequence is made of multiple BlockAckRequest/BlockAck frames sent as single-user frames, as shown in Figure Acknowledgment of DL MU frames in single-user format.

Acknowledgment of DL MU frames in single-user format

For the second acknowledgment sequence, an MU-BAR Trigger Frame is sent (as a single-user frame) to solicit BlockAck responses sent in TB PPDUs, as shown in Figure Acknowledgment of DL MU frames via MU-BAR Trigger Frame sent as single-user frame.

Acknowledgment of DL MU frames via MU-BAR Trigger Frame sent as single-user frame

For the third acknowledgment sequence, an MU-BAR Trigger Frame is aggregated to every PSDU included in the DL MU PPDU and the BlockAck responses are sent in TB PPDUs, as shown in Figure Acknowledgment of DL MU frames via aggregated MU-BAR Trigger Frames.

Acknowledgment of DL MU frames via aggregated MU-BAR Trigger Frames

For UL OFDMA, both BSRP Trigger Frames and Basic Trigger Frames are supported, as shown in Figure Acknowledgment of DL MU frames via aggregated MU-BAR Trigger Frames. A BSRP Trigger Frame is sent by an AP to solicit stations to send QoS Null frames containing Buffer Status Reports. A Basic Trigger Frame is sent by an AP to solicit stations to send data frames in TB PPDUs, which are acknowledged by the AP via a Multi-STA BlockAck frame.

Acknowledgment of DL MU frames via aggregated MU-BAR Trigger Frames

Multi-User Scheduler

A new component, named MultiUserScheduler, is in charge of determining what frame exchange sequence the aggregated AP has to perform when gaining a TXOP (DL OFDMA, UL OFDMA or BSRP Trigger Frame), along with the information needed to perform the selected frame exchange sequence (e.g., the set of PSDUs to send in case of DL OFDMA). MultiUserScheduler is an abstract base class. Currently, the only available subclass is RrMultiUserScheduler. By default, no multi-user scheduler is aggregated to an AP (hence, OFDMA is not enabled).

Round-robin Multi-User Scheduler

The Round-robin Multi-User Scheduler dynamically assigns a priority to each station to ensure airtime fairness in the selection of stations for DL multi-user transmissions. The NStations attribute enables to set the maximum number of stations that can be the recipients of a DL multi-user frame. Therefore, every time an HE AP accesses the channel to transmit a DL multi-user frame, the scheduler determines the number of stations the AP has frames to send to (capped at the value specified through the mentioned attribute) and attempts to allocate equal sized RUs to as many such stations as possible without leaving RUs of the same size unused. For instance, if the channel bandwidth is 40 MHz and the determined number of stations is 5, the first 4 stations (in order of priority) are allocated a 106-tone RU each (if 52-tone RUs were allocated, we would have three 52-tone RUs unused). If central 26-tone RUs can be allocated (as determined by the UseCentral26TonesRus attribute), possible stations that have not been allocated an RU are assigned one of such 26-tone RU. In the previous example, the fifth station would have been allocated one of the two available central 26-tone RUs.

When UL OFDMA is enabled (via the EnableUlOfdma attribute), every DL OFDMA frame exchange is followed by an UL OFDMA frame exchange involving the same set of stations and the same RU allocation as the preceding DL multi-user frame. The transmission of a BSRP Trigger Frame can optionally (depending on the value of the EnableBsrp attribute) precede the transmission of a Basic Trigger Frame in order for the AP to collect information about the buffer status of the stations.

Ack manager

Since the introduction of the IEEE 802.11e amendment, multiple acknowledgment policies are available, which are coded in the Ack Policy subfield in the QoS Control field of QoS Data frames (see Section 9.2.4.5.4 of the IEEE 802.11-2016 standard). For instance, an A-MPDU can be sent with the Normal Ack or Implicit Block Ack Request policy, in which case the receiver replies with a Normal Ack or a Block Ack depending on whether the A-MPDU contains a single MPDU or multiple MPDUs, or with the Block Ack policy, in which case the receiver waits to receive a Block Ack Request in the future to which it replies with a Block Ack.

WifiAckManager is the abstract base class introduced to provide an interface for multiple ack managers. Currently, the default ack manager is the WifiDefaultAckManager.

WifiDefaultAckManager

The WifiDefaultAckManager allows to determine which acknowledgment policy to use depending on the value of its attributes:

• UseExplicitBar: used to determine the ack policy to use when a response is needed from the recipient and the current transmission includes multiple frames (A-MPDU) or there are frames transmitted previously for which an acknowledgment is needed. If this attribute is true, the Block Ack policy is used. Otherwise, the Implicit Block Ack Request policy is used.
• BaThreshold: used to determine when the originator of a Block Ack agreement needs to request a response from the recipient. A value of zero means that a response is requested at every frame transmission. Otherwise, a non-zero value (less than or equal to 1) means that a response is requested upon transmission of a frame whose sequence number is distant at least BaThreshold multiplied by the transmit window size from the starting sequence number of the transmit window.
• DlMuAckSequenceType: used to select the acknowledgment sequence for DL MU frames (acknowledgment in single-user format, acknowledgment via MU-BAR Trigger Frame sent as single-user frame, or acknowledgment via MU-BAR Trigger Frames aggregated to the data frames).

Protection manager

The protection manager is in charge of determining the protection mechanism to use, if any, when sending a frame.

WifiProtectionManager is the abstract base class introduced to provide an interface for multiple protection managers. Currently, the default protection manager is the WifiDefaultProtectionManager.

WifiDefaultProtectionManager

The WifiDefaultProtectionManager selects a protection mechanism based on the information provided by the remote station manager.

Rate control algorithms

Multiple rate control algorithms are available in ns-3. 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
• MinstrelHtWifiManager

Algorithms in literature:

• IdealWifiManager
• AarfWifiManager [lacage2004aarfamrr]
• AmrrWifiManager [lacage2004aarfamrr]
• CaraWifiManager [kim2006cara]
• RraaWifiManager [wong2006rraa]
• AarfcdWifiManager [maguolo2008aarfcd]
• ParfWifiManager [akella2007parf]
• AparfWifiManager [chevillat2005aparf]
• ThompsonSamplingWifiManager [krotov2020rate]

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). ns-3 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 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 1e-6): The maximum Bit Error Rate that is used to calculate the SNR threshold for each mode.

Note that the BerThreshold has to be low enough to select a robust enough MCS (or mode) for a given SNR value, without being too restrictive on the target BER. Indeed we had noticed that the previous default value (i.e. 1e-5) led to the selection of HE MCS-11 which resulted in high PER. With this new default value (i.e. 1e-6), a HE STA moving away from a HE AP has smooth throughput decrease (whereas with 1e-5, better performance was seen further away, which is not “ideal”).

ThompsonSamplingWifiManager

Thompson Sampling (TS) is a classical solution to the Multi-Armed Bandit problem. ThompsonSamplingWifiManager implements a rate control algorithm based on TS with the goal of providing a simple statistics-based algorithm with a low number of parameters.

The algorithm maintains the number of successful transmissions and the number of unsuccessful transmissions for each MCS , both of which are initially set to zero.

To select MCS for a data frame, the algorithm draws a sample frame success rate from the beta distribution with shape parameters for each MCS and then selects MCS with the highest expected throughput calculated as the sample frame success rate multiplied by MCS rate.

To account for changing channel conditions, exponential decay is applied to and . The rate of exponential decay is controlled with the Decay attribute which is the inverse of the time constant. Default value of 1 Hz results in using exponential window with the time constant of 1 second. Setting this value to zero effectively disables exponential decay and can be used in static scenarios.

Control frames are always transmitted using the most robust MCS, except when the standard specifies otherwise, such as for ACK frames.

As the main goal of this algorithm is to provide a stable baseline, it does not take into account backoff overhead, inter-frame spaces and aggregation for MCS rate calculation. For an example of a more complex statistics-based rate control algorithm used in real devices, consider Minstrel-HT described below.

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].

MinstrelHtWifiManager

This is the extension of minstrel for 802.11n/ac/ax.

802.11ax OBSS PD spatial reuse

802.11ax mode supports OBSS PD spatial reuse feature. OBSS PD stands for Overlapping Basic Service Set Preamble-Detection. OBSS PD is an 802.11ax specific feature that allows a STA, under specific conditions, to ignore an inter-BSS PPDU.

OBSS PD Algorithm

ObssPdAlgorithm is the base class of OBSS PD algorithms. It implements the common functionalities. First, it makes sure the necessary callbacks are setup. Second, when a PHY reset is requested by the algorithm, it performs the computation to determine the TX power restrictions and informs the PHY object.

The PHY keeps tracks of incoming requests from the MAC to get access to the channel. If a request is received and if PHY reset(s) indicating TX power limitations occured before a packet was transmitted, the next packet to be transmitted will be sent with a reduced power. Otherwise, no TX power restrictions will be applied.

Constant OBSS PD Algorithm

Constant OBSS PD algorithm is a simple OBSS PD algorithm implemented in the ConstantObssPdAlgorithm class.

Once a HE preamble and its header have been received by the PHY, ConstantObssPdAlgorithm:: ReceiveHeSig is triggered. The algorithm then checks whether this is an OBSS frame by comparing its own BSS color with the BSS color of the received preamble. If this is an OBSS frame, it compares the received RSSI with its configured OBSS PD level value. The PHY then gets reset to IDLE state in case the received RSSI is lower than that constant OBSS PD level value, and is informed about a TX power restrictions.

Note: since our model is based on a single threshold, the PHY only supports one restricted power level.

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 txop.* and qos-txop.* are of interest for this task.
• Channel access management. Users should modify the files channel-access-manager.*, which grant access to Txop and QosTxop.
• 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 Txop or QosTxop while RTS/CTS transaction is handled 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.