34.1. 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.

34.1.1. 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, 802.11ax (2.4, 5 and 6 GHz bands) and 802.11be 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 (including support for the MU EDCA Parameter Set)
  • 802.11be Multi-link discovery and setup
  • 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/be 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 a design overview of each layer, shown in Figure WifiNetDevice architecture. For 802.11be Multi-Link Devices (MLDs), there as many instances of WifiPhy, FrameExchangeManager and ChannelAccessManager as the number of links.


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::WifiMac, 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::WifiMac, 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.

34.1.2. 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:

  • 802.11n/ac/ax/be beamforming is not supported
  • 802.11n RIFS is not supported
  • 802.11 PCF/HCF/HCCA are not implemented
  • Channel Switch Announcement is not supported
  • Authentication and encryption are missing
  • Processing delays are not modeled
  • Cases where RTS/CTS and ACK are transmitted using HT/VHT/HE/EHT formats are not supported
  • Energy consumption model does not consider MIMO
  • 802.11ax preamble puncturing is supported by the PHY but is currently not exploited by the MAC
  • Only minimal MU-MIMO is supported (ideal PHY assumed, no MAC layer yet)

At the MAC layer, most of the main functions found in deployed Wi-Fi equipment for 802.11a/b/e/g/n/ac/ax/be are implemented, but there are scattered instances where some limitations in the models exist. Support for 802.11n, ac, ax and be 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.
  • If a STA (AP or non-AP) supports VHT, a Block Ack agreement is always setup once a first packet is enqueued regardless of whether it will be transmitted in an A-MPDU.
  • Once an A-MSDU is created, it is no longer modified, even before it is actually transmitted for the first time. This means that this A-MSDU cannot be aggregated to other MSDUs using A-MSDU aggregation.

34.1.3. 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). The MAC model Infrastructure association

Association in infrastructure mode is a high-level MAC function performed by the Association Manager, which is implemented through a base class (WifiAssocManager) and a default subclass (WifiDefaultAssocManager). The interaction between the station MAC, the Association Manager base class and subclass is illustrated in Figure Scanning procedure.


Scanning procedure

The STA wifi MAC requests the Association Manager to start a scanning procedure with specified parameters, including the type of scanning (active or passive), the desired SSID, the list of channels to scan, etc. The STA wifi MAC then expects to be notified of the best AP to associate with at the end of the scanning procedure. Every Beacon or Probe Response frame received during scanning is forwarded to the Association Manager, which keeps a list of candidate APs that match the scanning parameters. The sorting criterium for such a list is defined by the Association Manager subclass. The default Association Manager sorts APs in decreasing order of the SNR of the received Beacon/Probe Response frame.

When notified of the start of a scanning procedure, the default Association Manager schedules a call to a method that processes the information included in the frames received up to the time such a method is called. When both the AP and the STA have multiple links (i.e., they are 802.11be MLDs), the default Association Manager attempts to setup as many links as possible. This involves switching operating channel on some of the STA’s links to match those on which the APs affiliated with the AP MLD are operating.

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). In case of non-AP MLDs, in order for losing association, it is necessary that no beacon is received on any link for an interval of duration equal to the maximum number of missed beacons times the interval between two consecutive Beacon frames. 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 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 ( 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 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 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 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 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 choosing not to transmit (to be included in the version of the standard following IEEE 802.11-2020)
  • 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.

When the Channel Access Manager determines that channel access can be granted, it determines the largest primary channel that is considered idle based on the CCA-BUSY indication provided by the PHY. Such an information is passed to the Frame Exchange Manager, which in turn informs the Multi-User Scheduler (if any) and the Wifi Remote Station Manager. As a result, PPDUs are transmitted on the largest idle primary channel. For example, if a STA is operating on a 40 MHz channel and the secondary20 channel is indicated to be busy, transmissions will occur on the primary20 channel.

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. MAC queues

Each EDCA function (on QoS stations) and the DCF (on non-QoS stations) have their own MAC queue (an instance of the WifiMacQueue class) to store packets received from the upper layer and waiting for transmission. On QoS stations, each received packet is assigned a User Priority based on the socket priority (see, e.g., the wifi-multi-tos or the wifi-mac-ofdma examples), which determines the Access Category that handles the packet. By default, wifi MAC queues support flow control, hence upper layers do not forward a packet down if there is no room for it in the corresponding MAC queue. Wifi MAC queues do not support dynamic queue limits (byte queue limits); therefore, there is no backpressure into the traffic control layer until the WifiMacQueue for an access category is completely full (i.e., when the queue depth reaches the value of the MaxSize attribute, which defaults to 500 packets). TCP small queues (TSQ) [corbet2012] is a Linux feature that provides feedback from the Wi-Fi device to the socket layer, to control how much data is queued at the Wi-Fi level. ns-3 TCP does not implement TSQ, nor does the WifiNetDevice provide that specific feedback (although some use of the existing trace sources may be enough to support it). Regardless, experimental tests have demonstrated that TSQ interferes with Wi-Fi aggregation on uplink transfers [grazia2022]. Packets stay in the wifi MAC queue until they are acknowledged or discarded. A packet may be discarded because, e.g., its lifetime expired (i.e., it stayed in the queue for too long) or the maximum number of retries was reached. The maximum lifetime for a packet can be configured via the MaxDelay attribute of WifiMacQueue. There are a number of traces that can be used to track the outcome of a packet transmission (see the corresponding doxygen documentation):

  • WifiMac trace sources: AckedMpdu, NAckedMpdu, DroppedMpdu, MpduResponseTimeout, PsduResponseTimeout, PsduMapResponseTimeout
  • WifiMacQueue trace source: Expired

Internally, a wifi MAC queue is made of multiple sub-queues, each storing frames of a given type (i.e., data or management) and having a given receiver address and TID. For single-user transmissions, the next station to serve is determined by a wifi MAC queue scheduler (held by the WifiMac instance). A wifi MAC queue scheduler is implemented through a base class (WifiMacQueueScheduler) and subclasses defining specific scheduling policies. The default scheduler (FcfsWifiQueueScheduler) gives management frames higher priority than data frames and serves data frames in a first come first serve fashion. For multi-user transmissions (see below), scheduling is performed by a Multi-User scheduler, which may or may not consult the wifi MAC queue scheduler to identify the stations to serve with a Multi-User DL or UL transmission. 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 Frame exchange sequences using UL OFDMA. 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. Note that, in order for the two frame exchange sequences to be separated by a SIFS (as shown in Figure Frame exchange sequences using UL OFDMA), it is necessary that the transmitting Access Category has a non-zero TXOP Limit, there is enough remaining time in the TXOP to perform the frame exchange sequence initiated by the Basic Trigger Frame and the Multi-User scheduler (described next) chooses to send a Basic Trigger Frame after a BSRP Trigger Frame.


Frame exchange sequences using UL OFDMA 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). A TXOP is gained (some time) after requesting channel access, which is normally done by DCF/EDCA (Txop/QosTxop) if the device has frames to transmit. In order for an AP to coordinate UL MU transmissions even without DL traffic, the duration of the access request interval can be set to a non-zero value through the AccessReqInterval attribute. The access request interval is the interval between two consecutive requests for channel access made by the MultiUserScheduler; such requests are made independently of the presence of frames in the queues of the AP. It is also possible to set the Access Category for which the MultiUserScheduler makes requests for channel access (via the AccessReqAc attribute) and to choose whether the access request interval is measured starting from the last time the MultiUserScheduler made a request for channel access or from the last time channel access was obtained by DCF/EDCA (via the DelayAccessReqUponAccess attribute).

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 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
  • OnoeWifiManager
  • ConstantRateWifiManager
  • MinstrelWifiManager
  • MinstrelHtWifiManager

Algorithms in literature: 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 \alpha_i and the number of unsuccessful transmissions \beta_i for each MCS i, both of which are initially set to zero.

To select MCS for a data frame, the algorithm draws a sample frame success rate q_i from the beta distribution with shape parameters (1 + \alpha_i, 1 + \beta_i) 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 \alpha_i and \beta_i. 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 occurred 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 frame-exchange-manager.* or its subclasses to accomplish this. Handling of control frames is performed in FrameExchangeManager::ReceiveMpdu.
  • MAC high modification. For example, handling new management frames (think beacon/probe), beacon/probe generation. Users usually make changes to 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 thresholds 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 FrameExchangeManager.
  • 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.