4. IEEE 802.11 Wireless Local Area Networks (WLAN)
This section preserves the RFC text for RAW technologies, including Wi-Fi 6/7, IEEE 802.11, TSCH, 6TiSCH, 5G NR, TSN/TSC integration, UE, gNB, RAN, UPF, PDU sessions, LDACS, PHY/MAC terms, figures, tables, and security considerations.
Original RFC Text
4. IEEE 802.11 Wireless Local Area Networks (WLAN)
In recent years, the evolution of the IEEE Std 802.11 standard has
taken a new direction, emphasizing improved reliability and reduced
latency in addition to minor improvements in speed, to enable new
fields of application such as industrial IoT and Virtual Reality
(VR).
Leveraging IEEE Std 802.11, the Wi-Fi Alliance [WFA] delivered Wi-Fi
6, 7, and now 8 with more capabilities to schedule and deliver frames
in due time at fast rates. Still, as with any radio technology, Wi-
Fi is sensitive to frame loss, which can only be combated with the
maximum use of diversity in space, time, channel, and even
technology.
In parallel, the Avnu Alliance [Avnu], which focuses on applications
of TSN for real-time data, formed a workgroup to investigate TSN
capabilities over wireless, leveraging both 3GPP and IEEE Std 802.11
standards.
To achieve the latter, the reliability must be handled at an upper
layer that can select Wi-Fi and other wired or wireless technologies
for parallel transmissions. This is where RAW comes into play.
This section surveys the IEEE 802.11 features that are most relevant
to RAW, noting that there are a great many more in the specification,
some of which may also possibly be of interest for a RAW solution.
For instance, frame fragmentation reduces the impact of a very
transient transmission loss, both on latency and energy consumption.
4.1. Provenance and Documents
The IEEE 802 LAN/MAN Standards Committee (SC) develops and maintains
networking standards and recommended practices for local,
metropolitan, and other area networks using an open and accredited
process, and it advocates them on a global basis. The most widely
used standards are for Ethernet, Bridging and Virtual Bridged LAN,
Wireless LAN, Wireless Personal Area Network (PAN), Wireless MAN,
Wireless Coexistence, Media Independent Handover Services, and
Wireless Radio Access Network (RAN). An individual working group
provides the focus for each area.
The IEEE 802.11 Wireless LAN (WLAN) standards define the underlying
Medium Access Control (MAC) and Physical (PHY) layers for the Wi-Fi
technology. While previous 802.11 generations, such as 802.11n and
802.11ac, focused mainly on improving peak throughput, more recent
generations are also considering other performance vectors, such as
efficiency enhancements for dense environments in IEEE Std 802.11ax
[IEEE802.11ax] (approved in 2021) and throughput, latency, and
reliability enhancements in IEEE Std 802.11be [IEEE802.11be]
(approved in 2024).
IEEE Std 802.11-2012 includes support for TSN time synchronization
based on IEEE 802.1AS over the 802.11 Timing Measurement protocol.
IEEE Std 802.11-2016 additionally includes an extension to the
802.1AS operation over 802.11 for Fine Timing Measurement (FTM), as
well as the Stream Reservation Protocol (IEEE 802.1Qat). 802.11 WLANs
can also be part of 802.1Q bridged networks with enhancements enabled
by the 802.11ak amendment retrofitted in IEEE Std 802.11-2020.
Traffic classification based on 802.1Q VLAN tags is also supported in
802.11. Other 802.1 TSN capabilities such as 802.1Qbv and 802.1CB,
which are media agnostic, can already operate over 802.11. The IEEE
Std 802.11ax-2021 (which has been incorporated into IEEE Std
802.11-2024) defines additional scheduling capabilities that can
enhance the timeliness performance in the 802.11 MAC and achieve
lower-bounded latency. IEEE 802.11be introduces features to enhance
the support for 802.1 TSN capabilities, especially those related to
worst-case latency, reliability, and availability.
The IEEE 802.11 Working Group has been working in collaboration with
the IEEE 802.1 Working Group for several years, extending some 802.1
features over 802.11. As with any wireless media, 802.11 imposes new
constraints and restrictions to TSN-grade QoS, and trade-offs between
latency and reliability guarantees must be considered as well as
managed deployment requirements. An overview of 802.1 TSN
capabilities and challenges for their extensions to 802.11 are
discussed in [Cavalcanti_2019].
The Wi-Fi Alliance is the worldwide network of companies that drives
global Wi-Fi adoption and evolution through thought leadership,
spectrum advocacy, and industry-wide collaboration. The WFA work
helps ensure that Wi-Fi devices and networks provide users the
interoperability, security, and reliability they have come to expect.
The Avnu Alliance is also a global industry forum developing
interoperability testing for TSN-capable devices across multiple
media including Ethernet, Wi-Fi, and 5G.
The following IEEE Std 802.11 specifications/certifications
[IEEE802.11] are relevant in the context of reliable and available
wireless services and support for TSN capabilities:
* Time synchronization: IEEE Std 802.11-2016 with IEEE Std 802.1AS;
WFA TimeSync Certification
* Congestion control: IEEE Std 802.11-2016 Admission Control; WFA
Admission Control
* Security: WFA Wi-Fi Protected Access, WPA2, and WPA3
* Interoperating with IEEE 802.1Q bridges: IEEE Std 802.11-2020
incorporating 802.11ak
* Stream Reservation Protocol (part of [IEEE802.1Qat]):
IEEE802.11-2016
* Scheduled channel access: IEEE 802.11ad enhancements for very high
throughput in the 60 GHz band [IEEE802.11ad]
* 802.11 Real-Time Applications: Topic Interest Group (TIG)
ReportDoc [IEEE_doc_11-18-2009-06]
In addition, major amendments being developed by the IEEE 802.11
Working Group include capabilities that can be used as the basis for
providing more reliable and predictable wireless connectivity and
support time-sensitive applications:
* [IEEE802.11ax]: Enhancements for High Efficiency (HE)
* [IEEE802.11be]: Extreme High Throughput (EHT)
* [IEEE802.11ay]: Enhanced throughput for operation in license-
exempt bands above 45 GHz
The main 802.11ax, 802.11be, 802.11ad, and 802.11ay capabilities and
their relevance to RAW are discussed in the remainder of this
section. As P802.11bn is still in early stages of development, its
capabilities are not included in this document.
4.2. 802.11ax High Efficiency (HE)
4.2.1. General Characteristics
The next generation Wi-Fi (Wi-Fi 6) is based on the IEEE802.11ax
amendment [IEEE802.11ax], which includes specific capabilities to
increase efficiency and control and to reduce latency. Some of these
features include higher-order 1024-QAM modulation, support for uplink
(UL) Multi-User - Multiple Input Multiple Output (MU-MIMO),
Orthogonal Frequency-Division Multiple Access (OFDMA), trigger-based
access, and Target Wake Time (TWT) for enhanced power savings. The
OFDMA mode and trigger-based access enable the Access Point (AP),
after reserving the channel using the clear channel assessment
procedure for a given duration, to schedule multi-user transmissions,
which is a key capability required to increase latency predictability
and reliability for time-sensitive flows. 802.11ax can operate in up
to 160 MHz channels, and it includes support for operation in the new
6 GHz band, which has been open to unlicensed use by the Federal
Communications Commission (FCC) and other regulatory agencies
worldwide.
4.2.1.1. Multi-User OFDMA and Trigger-Based Scheduled Access
802.11ax introduced an OFDMA mode in which multiple users can be
scheduled across the frequency domain. In this mode, the Access
Point (AP) can initiate multi-user UL transmissions in the same PHY
Protocol Data Unit (PPDU) by sending a trigger frame. This
centralized scheduling capability gives the AP much more control of
the channel in its Basic Service Set (BSS), and it can remove
contention between associated stations for UL transmissions,
therefore reducing the randomness caused by access based on Carrier
Sense Multiple Access (CSMA) between stations within the same BSS.
The AP can also transmit simultaneously to multiple users in the
downlink (DL) direction by using a DL MU OFDMA PPDU. In order to
initiate a contention-free Transmission Opportunity (TXOP) using the
OFDMA mode, the AP still follows the typical listen-before-talk
procedure to acquire the medium, which ensures interoperability and
compliance with unlicensed band access rules. However, 802.11ax also
includes a Multi-User Enhanced Distributed Channel Access (MU-EDCA)
capability, which allows the AP to get higher channel access priority
than other devices in its BSS.
4.2.1.2. Traffic Isolation via OFDMA Resource Management and Resource
Unit Allocation
802.11ax relies on the notion of an OFDMA Resource Unit (RU) to
allocate frequency chunks to different stations over time. RUs
provide a way to allow multiple stations to transmit simultaneously,
starting and ending at the same time. The way this is achieved is
via padding, where extra bits are transmitted with the same power
level. The current RU allocation algorithms provide a way to achieve
traffic isolation per station. While this does not support time-
aware scheduling per se, it is a key aspect to assist reliability, as
it provides traffic isolation in a shared medium.
4.2.1.3. Improved PHY Robustness
The 802.11ax PHY can operate with a 0.8, 1.6, or 3.2 microsecond
Guard Interval (GI). The larger GI options provide better protection
against multipath, which is expected to be a challenge in industrial
environments. The possibility of operating with smaller RUs (e.g., 2
MHz) enabled by OFDMA also helps reduce noise power and improve
Signal-to-Noise Ratio (SNR), leading to better Packet Error Rate
(PER) performance.
802.11ax supports beamforming as in 802.11ac but introduces UL MU-
MIMO, which helps improve reliability. The UL MU-MIMO capability is
also enabled by the trigger-based access operation in 802.11ax.
4.2.1.4. Support for 6 GHz Band
The 802.11ax specification [IEEE802.11ax] includes support for
operation in the 6 GHz band. Given the amount of new spectrum
available, as well as the fact that no legacy 802.11 device (prior
802.11ax) will be able to operate in this band, 802.11ax operation in
this new band can be even more efficient.
4.2.2. Applicability to Deterministic Flows
TSN capabilities, as defined by the IEEE 802.1 TSN standards, provide
the underlying mechanism for supporting deterministic flows in a
Local Area Network (LAN). The IEEE 802.11 Working Group has
incorporated support for absolute time synchronization to extend the
TSN 802.1AS protocol so that time-sensitive flows can experience
precise time synchronization when operating over 802.11 links. As
IEEE 802.11 and IEEE 802.1 TSN are both based on the IEEE 802
architecture, 802.11 devices can directly implement some TSN
capabilities without the need for a gateway/translation protocol.
Basic features required for operation in a 802.1Q LAN are already
enabled for 802.11. Some TSN capabilities, such as 802.1Qbv, can
already operate over the existing 802.11 MAC Service Access Point
(SAP) [Sudhakaran2021]. Implementation and experimental results of
TSN capabilities (802.1AS, 802.1Qbv, and 802.1CB) extended over
standard Ethernet and Wi-Fi devices have also been described in
[Fang_2021]. Nevertheless, the IEEE 802.11 MAC/PHY could be extended
to improve the operation of IEEE 802.1 TSN features and achieve
better performance metrics [Cavalcanti1287].
TSN capabilities supported over 802.11 (which also extends to
802.11ax) include:
1. 802.1AS-based time synchronization (other time synchronization
techniques may also be used)
2. Interoperating with IEEE 802.1Q bridges
3. Time-sensitive traffic stream classification
The existing 802.11 TSN capabilities listed above, and the 802.11ax
OFDMA and AP-controlled access within a BSS, provide a new set of
tools to better serve time-sensitive flows. However, it is important
to understand the trade-offs and constraints associated with such
capabilities, as well as redundancy and diversity mechanisms that can
be used to provide more predictable and reliable performance.
4.2.2.1. 802.11 Managed Network Operation and Admission Control
Time-sensitive applications and TSN standards are expected to operate
in a managed network (e.g., an industrial/enterprise network). This
enables careful management and integration of the Wi-Fi operation
with the overall TSN management framework, as defined in
[IEEE802.1Qcc].
Some of the random-access latency and interference from legacy/
unmanaged devices can be reduced under a centralized management mode
as defined in [IEEE802.1Qcc].
Existing traffic stream identification, configuration, and admission
control procedures defined in the QoS mechanism in [IEEE802.11] can
be reused. However, given the high degree of determinism required by
many time-sensitive applications, additional capabilities to manage
interference and legacy devices within tight time constraints need to
be explored.
4.2.2.2. Scheduling for Bounded Latency and Diversity
As discussed earlier, the OFDMA mode in [IEEE802.11ax] introduces the
possibility of assigning different RUs (time/frequency resources) to
users within a PPDU. Several RU sizes are defined in the
specification (26, 52, 106, 242, 484, and 996 subcarriers). In
addition, the AP can also decide on a Modulation and Coding Scheme
(MCS) and grouping of users within a given OFMDA PPDU. Such
flexibility can be leveraged to support time-sensitive applications
with bounded latency, especially:
* in a managed network where stations can be configured to operate
under the control of the AP,
* in a controlled environment (which contains only devices operating
on the unlicensed band installed by the facility owner and where
unexpected interference from other systems and/or radio access
technologies only sporadically happens), or
* in a deployment where channel and link redundancy is used to
reduce the impact of unmanaged devices and interference.
When the network is lightly loaded, it is possible to achieve
latencies under 1 ms when Wi-Fi is operated in a contention-based
mode (i.e., without OFDMA). It also has been shown that it is
possible to achieve 1 ms latencies in a controlled environment with
higher efficiency when multi-user transmissions are used (enabled by
OFDMA operation) [Cavalcanti_2019]. Obviously, there are latency,
reliability, and capacity trade-offs to be considered. For instance,
smaller RUs result in longer transmission durations, which may impact
the minimal latency that can be achieved, but the contention latency
and randomness elimination in an interference-free environment due to
multi-user transmission is a major benefit of the OFDMA mode.
The flexibility to dynamically assign RUs to each transmission also
enables the AP to provide frequency diversity, which can help
increase reliability.
4.3. 802.11be Extreme High Throughput (EHT)
4.3.1. General Characteristics
[IEEE802.11be] was the next major 802.11 amendment (after IEEE Std
802.11ax-2021) for operation in the 2.4, 5, and 6 GHz bands. 802.11be
includes new PHY and MAC features, and it is targeting extremely high
throughput (at least 30 Gbps), as well as enhancements to worst-case
latency and jitter. It is also expected to improve the integration
with 802.1 TSN to support time-sensitive applications over Ethernet
and Wireless LANs.
The main features of 802.11be that are relevant to this document
include:
1. 320 MHz bandwidth and more efficient utilization of non-
contiguous spectrum
2. Multi-Link Operation (MLO)
3. QoS enhancements to reduce latency and increase reliability
4.3.2. Applicability to Deterministic Flows
The 802.11 Real-Time Applications (RTA) Topic Interest Group (TIG)
provided detailed information on use cases, issues, and potential
solutions to improve support for time-sensitive applications in
802.11. The RTA TIG report [IEEE_doc_11-18-2009-06] was used as
input to the 802.11be project scope.
Improvements for worst-case latency, jitter, and reliability were the
main topics identified in the RTA report, which were motivated by
applications in gaming, industrial automation, robotics, etc. The
RTA report also highlighted the need to support additional TSN
capabilities, such as time-aware (802.1Qbv) shaping and packet
replication and elimination as defined in 802.1CB.
IEEE Std 802.11be builds on and enhances 802.11ax capabilities to
improve worst case latency and jitter. Some of the enhancement areas
are discussed next.
4.3.2.1. Enhanced Scheduled Operation for Bounded Latency
In addition to the throughput enhancements, 802.11be leverages the
trigger-based scheduled operation enabled by 802.11ax to provide
efficient and more predictable medium access.
802.11be introduced QoS signaling enhancements, such as an additional
QoS characteristics element, that enables stations to provide
detailed information about deterministic traffic stream to the AP.
This capability helps AP implementations to better support scheduling
for deterministic flows.
4.3.2.2. Multi-Link Operation
802.11be introduces new features to improve operation over multiple
links and channels. By leveraging multiple links and channels,
802.11be can isolate time-sensitive traffic from network congestion,
one of the main causes of large latency variations. In a managed
802.11be network, it should be possible to steer traffic to certain
links and channels to isolate time-sensitive traffic from other
traffic and help achieve bounded latency. The Multi-Link Operation
(MLO) is a major feature in the 802.11be amendment that can enhance
latency and reliability by enabling data frames to be duplicated
across links.
4.4. 802.11ad and 802.11ay (mmWave Operation)
4.4.1. General Characteristics
The IEEE 802.11ad amendment defines PHY and MAC capabilities to
enable multi-Gbps throughput in the 60 GHz millimeter wave (mmWave)
band. The standard addresses the adverse mmWave signal propagation
characteristics and provides directional communication capabilities
that take advantage of beamforming to cope with increased
attenuation. An overview of the 802.11ad standard can be found in
[Nitsche_2015].
The IEEE 802.11ay is currently developing enhancements to the
802.11ad standard to enable the next generation mmWave operation
targeting 100 Gbps throughput. Some of the main enhancements in
802.11ay include MIMO, channel bonding, improved channel access, and
beamforming training. An overview of the 802.11ay capabilities can
be found in [Ghasempour_2017].
4.4.2. Applicability to Deterministic Flows
The high-data rates achievable with 802.11ad and 802.11ay can
significantly reduce latency down to microsecond levels. Limited
interference from legacy and other unlicensed devices in 60 GHz is
also a benefit. However, the directionality and short range typical
in mmWave operation impose new challenges such as the overhead
required for beam training and blockage issues, which impact both
latency and reliability. Therefore, it is important to understand
the use case and deployment conditions in order to properly apply and
configure 802.11ad/ay networks for time-sensitive applications.
The 802.11ad standard includes a scheduled access mode in which the
central controller, after contending and reserving the channel for a
dedicated period, can allocate to stations contention-free service
periods. This scheduling capability is also available in 802.11ay,
and it is one of the mechanisms that can be used to provide bounded
latency to time-sensitive data flows in interference-free scenarios.
An analysis of the theoretical latency bounds that can be achieved
with 802.11ad service periods is provided in [Cavalcanti_2019].