Podcast

CTS 164: 802.11ax Target Wake Time

Objectives of TWT

Objectifs of 802.11ax

  • Increase the performance of the Wi-Fi network by a factor of 4 while improving or not impacting power requirements
  • Provide power saving mechanisms for new emerging IoT devices

All the current power saving mechanisms defined today remain usable with ax. In addition, the 802.11ax draft defines a new mechanism called Target Wake Time or TWT.

Target wake time (TWT) is used to help minimize contention between clients and reduce the amount of time a client in power save mode to be awake.

Clients will operate in non-overlapping times and/or frequencies and the frame exchanges are coordinated.

TWT was introduced in 802.11ah and is particularly useful for battery-powered devices that communicate infrequently.

TWT Modes of Operations

There are three modes of operation:

  • Individual TWT
  • Broadcast TWT
  • Opportunistic PS

TWT Power-save options in 802.11ax from Aruba 802.11ax White Paper:

TWT Power-Save options in 802.11ax

Individual TWT

Client will be assigned specific times to wake up and exchange frames. The schedule is determined and delivered by the AP. There is a different mode of TWT such as explicit TWT. A client doesn’t need to know about another client’s TWT values.

Simple process:

  • A client wants to establish a TWT agreement
  • A client communicates its waking schedule information to the AP
  • The AP devises a schedule and delivers TWT values to the client
  • The client wakes up and transmit a frame according to the schedule
  • The AP send the client the next TWT information on when to wake up again (explicit mode)
  • The client wakes up again at the next scheduled time to send a frame and receive a new TWT information
  • When TWT implicit is used, the client calculates the Next TWT by adding a fixed value to the current TWT value

A client can go off of the AP’s TWT parameters. Or a client can “demand” a TWT with indicated parameters for agreement. If agreed upon, the AP will respond with “Accept TWT”. The AP can counter the offer with an Alternate TWT.

A client wanting to utilize TWT will indicate what channel to use as a primary channel during a TWT SP.

Broadcast TWT

The AP will be in charge. The AP will send TWT parameters in the Beacon frame using the TWT Element. The TWT Element might be sent in other management frames as well such as the (Re)Association frame or the probe response frame.

The clients will use the TWT parameters from the most recently received TWT element carried in the Management frames of its associated AP. The client is also called “TWT Scheduled STA” in this case in the draft. The AP is called “TWT Scheduling AP”

The AP will provide the schedule to all the clients that supports broadcast TWT.

The AP will send a trigger frame to discover which clients are awake. The AP will then send frames to these clients that will, then, be able to doze again. This is called a trigger-based TWT SP (Service Period).

Clients can device to join or leave a broadcast TWT. This is done by an exchange of frame that carry TWT elements.

Find the information in 802.11ax Frames

Subfields to identify client support of TWT modes:

  • TWT Requester Support
  • TWT Responder Support
  • Broadcast TWT Support

TWT Element

The Control field will indicate the negotiation type: Individual, Broadcast or Wake TBTT interval.

When Individual TWT is used, the TWT parameter Information field will have this format:

When Broadcast TWT is used, the TWT Parameter Information field will have this format:

Sections Talking About TWT in the 802.11ax Draft

9.4.1.60 – TWT Information field – p.118

9.4.2.200 – TWT element – p.139

9.6.25.9 – TWT Teardown frame format – p.185

10.43 – Target wake time (TWT) – p.234

27.7 – TWT Operation – p.312

Example

Here is an example of a beacon frame coming from a 802.11ax Aerohive AP (AP630). If we look into the “HE Capabilities” information element, we can drill down into the “HE MAC Capabilities Information” and validate both the “TWT Requester Support” and “TWT Responder Support” flags. In this case, they are set to 0, which means that the feature is not yet supported by the firmware used on this AP.

Then, we can take a look at the “TWT Required” flag located in the “HE Operation Parameters”. Here again, it is set to 0.

We should be able to show you example of TWT being supported later on this year as vendors update their 802.11ax AP firmware and start enabling the feature.


This Week In Wireless

CTS 163: Cisco 802.11ax APs & Cisco Live

Cisco Live

We have an episode full of Cisco goodness. Recently, Cisco released the data sheets to two 802.11ax access points. We have the details on these two APs, their capabilities, and some of our opinions.

And speaking of Cisco, right around the corner is Cisco Live. We have our tips and the sessions we’re looking forward to.

Cisco Live Tips

The following Cisco Live tips were written on Rowell’s blog – Attendee Guide to Cisco Live 2019. It’s going to be hosted in San Diego, CA this year which should be a fun event. Check out the blog and register for Cisco Live if you haven’t done so already!

  • Bring a smile!
  • Meetups
    • Cisco Champions
    • Twitter
    • Friends
  • Exams – book early
  • Bring walking shoes.
  • Bring a jacket – sometimes the A/C is blasting inside the conference.
  • Portable battery packs – for your mobile devices.
  • Drink plenty of water – it’s best you bring your own reusable. I like to use my Hydro Flask. Empty out the contents before you pass through TSA.
  • Tylenol might be useful for those headaches. Either from learning so much by the fire hose or from partying the night before.
  • Book your sessions once they’re available. Some fill up quickly and you’ll be let on the waitlist.
  • Book your exam early! Schedule it on Saturday or Sunday before the chaos of the conference begins.
  • Download the Starbucks app. The lines for coffee get really long. Download the app and do mobile purchase. Skip the lines.
  • Don’t bring a backpack. You’ll get one at Cisco Live.
  • Leave room in your luggage. If you plan on grabbing swag, this is the conference that will fill up your bag, easily.
  • Hang around after each of your sessions and thank the presenter.
  • Check out DevNet Create and get hands on.

Sessions on Wi-Fi to Look Out For

Roadmap: Wireless and Mobility – CCP-120

Hear about the future of wireless, and how Cisco bridges 5G and WiFi 6. And, learn how our open platform delivers network and location analytics to support IT and business use cases.

Securely Designing Your Wireless LAN for Threat Mitigation, Policy and BYOD- BRKEWN-2005

The session will cover effective strategies to build a defence-in-depth security model and the role of network segmentation in implementing Software-Defined Access (VLANs, VRF and TrustSec) for wireless networks. Participants will learn the principles of secured wireless networks (encryption, 802.1X, guest access, etc.) and the latest identity services available to securely onboard different kinds of devices (laptops, tablets, smartphones, IoT etc.) and users (employees, guests, contractors, etc.). This session will guide you through designing a fabric-integrated wireless deployment enabled for security, mobility and policy.

Introduction to Next Generation Wireless Stack – BRKEWN-2010

This session will introduce the Next Generation Wireless Stack for enterprise and branch office WLAN deployments, i.e. the core technologies that drive and enable mobility services. Topics covered include protocol details like CAPWAP, deep-dives into new controller Catalyst 9800 features like High Availability, Security and integration with Cisco DNA Center and Cisco 11ax Access Points. We will also introduce Cisco DNA spaces and integration with the next generation stack.

Introduction to Catalyst 9800 Wireless Controllers – BRKEWN-2670

Your next generation Wireless controller is the Catalyst 9800, available as an appliance, cloud and embedded controller on switch form factor  with IOS XE 16.10 and later. In this introductory session, you will learn how to get your Wireless network up and running with the C9800 controller, from Day 0 provisioning, High Availability, Patching, Programmability, Telemetry to Simplified configuration and Migration. You will also learn about the capabilities of the new wireless controller appliances running the Catalyst 9800 Wireless software.

Advanced Troubleshooting of Cisco Catalyst 9800 Wireless Controller – BRKEWN-3013

The session will cover troubleshooting of new-generation Wireless LAN Controllers including real-life examples. Participants will learn how IOS-XE capabilities like packet tracing and radioactive tracing are replacing debugs. The session will cover the hardware and software architecture specifics and all the serviceability improvements pertaining to making the troubleshooting effortless in everyday scenarios. The session assumes general knowledge of Cisco Unified Wireless troubleshooting and focuses on the methods to troubleshoot the newer platform.

Design and Use Cases of a Location Enabled Wi-Fi Network Supported by Cisco DNA Spaces – BRKEWN-2012

WLAN location based services is opening a whole new ecosystem of applications applicable to numerous verticals such as retail, healthcare, hospitality, education and others. New capabilities in device silicon have improved the accuracy of the ‘blue dot’, the user experience and the networks ability to provide relevant services. This session will explain the fundamentals of WLAN location based services, architecture, design and best practices. RF triangulation and architecture will be explained in light of the AP, Controller and CMX. Several models will be covered, including: location mapping, analytics, and location based application integration.

Advanced Troubleshooting of Wireless LANs – BRKEWN-3011

This session discusses troubleshooting techniques and best practices for the Cisco Unified Wireless Network Architecture (CUWN). Major topics include RF Validation, Wireless Authentication, and Client Roaming. We will talk about common client problems by looking at the various WLC and AP information, tools, and debugs used by TAC to resolve issues. We will also discuss the best practices to ensure you keep your users online and working.

Cisco DNA Assurance for Next Generation WIreless: Isolate problems for faster troubleshooting – BRKEWN-2034

You do not have time to deal with network issues anymore. In the past you would design RF coverage, build the WLANs, hope for the best, and react to alarms and user complaints. This logic would force you to constantly react to fire drills, running every day to assess issues as they were reported to you. What if you could benefit from tools that would anticipate these issues, instead of simply reporting once they occurred? The logic of the Cisco Digital Network Architecture (Cisco DNA) is not only to provide an abstraction for automation and policies, but also provide a virtuous feedback loop, allowing you to visualize the consequences of your configurations. Intelligent analytics allows you to observe trends, work on what-if scenarios, and be warned about issues before they actually explode and collapse your network. In this session, you will get a deep dive exposure to the new Cisco DNA Center Assurance tool, to monitor trends in your network, anticipate issues, get intelligent (human readable), meaningful and synthetic messages on the state of your network, your clients and their applications, and easily troubleshoot issues once you identify an unsafe trend.

Optimize your WLANs for small and mobile devices (phones, tablets and alike) – BRKEWN-2003

Smartphones and tablets often represent 60 percent or more of your WiFi clients, and people use voice and video calls on these devices while roaming. Turn aroud, and their signal drops by 30 dBm in less than a second. Optimizing a WLAN for highly mobile devices running real-time applications requires careful design and specific configurations. Cisco has introduced new features to optimize WiFi experience for Apple iOS devices, and iOS comes with new settings to better associate, roam and optimize traffic in Cisco WiFi networks. Cisco also works with large Android and laptop system manufacturers to tailor the WLANs to these devices’ needs. In this session, you will learn how to configure both sides, and how to optimize your wifi design to offer a seamless experience to your mobile clients, especially with real time applications. At the end of this session, you should know how these different types of clients see the network, and how to deploy and configure your WLANs to offer an optimal quality of experience for these clients.

High Density Wi-Fi Design, Deployment and Optimization – BRKEWN-2013

This session will cover an array of detailed tips, tricks, and tools for configuring and optimizing High Density WiFi networks in today’s most challenging environments, including valuable real-world insights on how the Cisco Access Point and Antenna portfolio can be leveraged to deliver an unparalleled user experience. Attend this session to hear real-world hints directly from engineers who have deployed networks for some of the largest sports venues and events worldwide.

Understanding RAN Spectrum Co-ordination and Cisco RF Features – BRKEWN-3010

The RF Landscape continues to change…With 802.11ax now entering the market and 5G standard beginning to mature a paradigm shift is re-shaping the entire industry . This session will focus on how Cisco’s RRM and related tools will help you to keep pace with and manage the transition into the next generation of wireless networks. RRM, CleanAir, ClientLink and other tools, explaining how they work, and how to use them effectively to optimize your network. This use-case-driven session promises to help you put these tools into perspective, and solve and prevent issues related to changing network demographics like coverage vs capacity, 802.11ac and now 802.11ax and the Overlapping BSS (OBSS) channelization and deployment strategies. This is an advanced session, so a solid understanding of 802.11PHYbasics and a little exposure to the 802.11ax specification and a basic familiarity of RRM are advised.

Advanced Troubleshooting of Cisco Catalyst 9800 Wireless Controllers – BRKEWN-3013

The session will cover troubleshooting of new-generation Wireless LAN Controllers including real-life examples. Participants will learn how IOS-XE capabilities like packet tracing and radioactive tracing are replacing debugs. The session will cover the hardware and software architecture specifics and all the serviceability improvements pertaining to making the troubleshooting effortless in everyday scenarios. The session assumes general knowledge of Cisco Unified Wireless troubleshooting and focuses on the methods to troubleshoot the newer platform.

Advancements in Wireless Security – BRKEWN-2006

Recently, the Wi-Fi Alliance announced three new programs to advance wireless security: Wi-Fi CERTIFIED WPA3™, Wi-Fi Certified Enhanced Open™, and Wi-Fi Certified Easy Connect™. This session will explore the details around these programs, including the history of wireless security, its ties to other standards bodies, the problems they solve, and guidance on how and when to adopt them in an enterprise wireless network. Attendees will hear from and speak with Stephen Orr, the Wi-Fi Alliance’s® Security task group chairman, and Bob Sayle, a contributing member of the Wi-Fi Alliance’s® Device Provisioning Protocol task group.

Sessions on 5G

5G Technology Updates – BRKSPM-2017

The mobile industry is constantly evolving by building capabilities to enhance value to consumers. 5G technologies will not only provide higher throughput, low latency and enable connectivities to billions of devices but it will provide enable new use cases to drive industry growth. 5G will be deployed on virtualized platforms which will have big impact on infrastructures, security, automation and this will require network and operations transformation.

The speakers are doing hands-on work to build 5G use cases in Cisco laboratories and working with customers globally for 5G proof of concepts and trials. We will start with 5G industry updates and what has changed in last one year, Cisco 5G plan, use cases we are working and learning, 5G security landscape and how to manage. You will also see live demonstration of how we can auto-deploy 5G services using end-to-end automation platform.

5G Mobile Transport Design and Deployment

Mobile service providers seek a new approach to address the increasing traffic demands. They require solutions that can help them to: Lower operational expenditures Improve Time to Revenue and Average Revenue per User (ARPU) Profitably transport the massive increase in user bandwidth Provide ultra-HD video, virtual reality, and tactile internet services anywhere, anytime, to any Internet-enabled device. Cisco Mobile xhaul Solution combines best of class Mobile & IP technologies to provide a simplified programmable transport of Mobile services using High performance platforms. A converged IP/MPLS architecture greatly increases service agility and decreases Time-to-Revenue. With an Agile, open, and virtualized architecture supporting open standards, and programmatic APIs for services orchestration, the Cisco Mobile xhaul Solution is “SDN ready” today.

5G xHaul Transport – BRKSPM-2012

LTE was about connectivity, however 5G will be about immersive experience, led by new services enhanced mobile broadband access everywhere (1Gpbs per UE), massive Internet of things, tactile Internet, higher user mobility, ultra-reliable communications & enterprise use cases. These services have divergent latency and throughput requirement requiring network transformation in addition to radio evolution “5G NR”. It is critical to invest in backhaul, midhaul & fronthaul (xHaul) evolution otherwise RAN gains will not be realized This session will address transport transformation through Segment Routing, stringent synchronization through phase and frequency, CPRI evolution to address 5G fronthaul requirement, virtualization and split of packet core to bring data plane closer to the user, telemetry for network insight and proactive health monitoring and orchestration through Cisco Crosswork.

Voice Over 5G Mobile Packet Core – BRKSPM-2017

The 5G specifications developed by 3GPP have completed the specifications for voice support for both non-standalone (5G NSA) and 5G standalone (5G SA) networksThe completion of 5G NR specifications for standalone (SA) has been approved by 3GPP. Also, after the 5G NR specifications for NSA, standardization of 5G at the first stage is pretty ready. Although the main drivers for 5G evolution is data, voice and video servcies are still very important as SPs will not be able to keep the voice on their LTE netwrok for a long time. At the beginning, 5G uses the LTE voice network and to provide those services using existing IMS architecture. The 5G radio technology is NR and the voice services called VoNR in 5G. In this session, the presenters will speak about the various 5G architecture options that SPs are planning to use and the voice/video evolution over each option. Cisco packet core and 3GPP experts will provide a comprehensive view on those options including: 1-CSFB–>VoLTE–>VoNR for Option 3X, 2- EPS or RAT FB / VoLTE –>VoNR for option 4/4a, 3- EPS or RAT FB / VoLTE –>VoNR for option 2, etc. After this session, audience will walk out with a better understanding on Vo5G and the evolution path options that large service providers are taking during their journey for 5G Network

Links & Resources

Cisco Catalyst 9100 Access Points
Cisco C9115AX Data Sheet
Cisco C9117AX Data Sheet

IEEE 802.11be PAR (.docx file)
Tweet from Jim Vajda

WLPC EU in Prague
https://twitter.com/KeithRParsons/status/1108905138314334208

CTS 162: 802.11ax OFDMA Subcarriers

With OFDMA in 802.11ax, the size of the subcarriers has been divided by 4. Going from 312.5KHz wide with OFDM to 78.125KHz wide.

The symbol duration has been increased by 4 times in the meantime. Going from 3.2 microseconds with OFDM to 12.8 microseconds.

Zooming into the subcarriers of a 20 MHz channel width

View the full image here

Advantages of having more subcarriers

  1. Allow OFDMA to extend to small sub-channels. Each sub-channel requires at least one (usually two) pilot subcarriers, and with a 2 MHz minimum sub-channel size, a smaller subcarrier spacing loses a much smaller percentage of the overall bandwidth to pilots.
  2. The number of guard and null subcarriers across a channel can be reduced as a percentage of the number of usable subcarriers, again increasing the effective data rate in a given channel. The figures above show a ~10% increase in usable subcarriers compared to 802.11ac, after allowing for the 4x factor. Example: OFDM: 64 subcarriers, 12 GuardNull subcarriers = 18.75%, OFDMA: 256 subcarriers. 22 GuardNull subcarriers = 8.5%.
  3. The longer OFDM symbol allows for an increase in the cyclic prefix length without sacrificing spectral efficiency, which in turn enables increased immunity to long delay spreads, especially in outdoor conditions. The cyclic prefix can be reduced to a smaller percentage of the symbol time, increasing spectral efficiency even while more robust to multipath conditions. And it reduces the jitter-sensitivity of uplink multi-user modes.

The smallest sub-channel is composed of 26 subcarriers.

Type of subcarriers:

  • Data subcarriers
  • Pilot subcarriers
  • DC subcarriers
  • Guard subcarriers
  • Null subcarriers

A 26-tone RU consists of 24 data subcarriers and 2 pilot subcarriers.

A 52-tone RU consists of 48 data subcarriers and 4 pilot subcarriers.

A 106-tone RU consists of 102 data subcarriers and 4 pilot subcarriers.

A 242-tone RU consists of 234 data subcarriers and 8 pilot subcarriers.

A 484-tone RU consists of 468 data subcarriers and 16 pilot subcarriers.

A 996-tone RU consists of 980 data subcarriers and 16 pilot subcarriers.

DC (Direct Current) subcarriers are used for the subcarriers located in the center of the channel. Depending on the channel width and the number of tone used, the number of DC subcarriers can vary (Ex: 3 or 7 for a 20MHz wide channel). Most of the time it will be 7 for the 20MHz and 80MHz wide channels and 5 for the 40MHz wide channels.

A 20MHz wide channels has 11 guard interval: the first 6 and the last 5 of the channel.

Here are the diagrams extracted from the 802.11ax draft document detailing the structure of the subcarriers for each channel width using different RUs sizes:

Links & Resources

Meraki MR55 and MR45 802.11ax (Wi-Fi 6) access points
Meraki MR45 802.11ax (Wi-Fi 6) access point
Meraki MR55 802.11ax (Wi-Fi 6) access point

CTS 161: 802.11ax BSS Coloring

Why is there a need for BSS Coloring? To help a receiving device identify the BSS from which a receiving PPDU originates from so that there’s a reduction in BSS collision reporting a busy medium. 802.11ax allows the medium to be reused more often between OBSSs by identifying those overlapping BSSs. The primary purpose is to improve the efficiency of Wi-Fi in a dense area. BSS Coloring will tackle the issue of frequency re-use.

An AP receives a neighbor report for the purpose of including the HE Operation element of neighboring High Efficienty (HE) APs to determine BSS Color information of those neighbors.

Which frames can you find the BSS Color field? HE Operation element will contain BSS color info which can be found in the Beacon frame, Association/reassociation, Probe response. It’s in the PHY Preamble.

BSS Color within the PHY

The HE Operation Element can be found in the following frames: Beacon, Probe Response and (Re)Association frames.

HE Operation Element – Notice the BSS Color Information Field

BSS color is an identifier of a BSS to assist a receiving device in an identifying BSS from which a PPDU originates for the purposes of channel access, reduce power consumption, or update NAV.

AP selects a value from 1 to 63 which is included in the BSS Color subfield of the HE Operation element or New BSS Color subfield of the BSS Color Change Announcement element.

The device will set the BSS Color subfield of HE Operation element to value indicated in the BSS Color subfield received from the AP. AP sets the parameter for BSS_COLOR of a HE PPDU.

BSS Color field is for the active BSS color. If a device roams to another BSS the value of the active BSS color will be entered in the New BSS Color field as received in the BSS Color Change Announcement element.

Image two BSSs on the same channel, 149. One BSS would use color yellow, and the other would use color blue. The BSS coloring changes channel access methods. Devices could transmit and receive at the same time. Won’t this cause a collision? Yes, if the BSS colors are the same.

Can a collision occur between colors?

An AP can determine if there is a BSS Color collision by receiving frames from an OBSS device or AP containing the same BSS color it has selected. If this occurs, the AP sets the BSS Color Disabled subfield. The subfield is set for a duration of a BSS Color Collision Period.

It is possible to have a BSS color collision with an OBSS. And when detected, AP will set value of BSS Color Disabled subfield within HE Operation element to 1 which informs others that BSS Color is disabled.

AP selects a BSS color and may change it under certain conditions such as detecting an OBSS using the same color. There is no method defined in how selecting a new BSS color should be performed. An AP may take colors used in its surroundings into account.

When AP is changing BSS color a BSS Color Change Announcement is sent in a Beacon, Probe Response and ReAssociation Response frame or using a HE BSS Color Change Announcement frame. What could cause a color change? Another BSS using the same color.

Ultimately, you’ll have SINR.

HE BSS Color Change Announcement

The HE BSS Color Change Announcement is an Action frame. Contains a BSS Color Change Announcement. The AP can change the BSS Color. And when it does so, it sends an announcement to associated devices.

BSS Color Change Announcement element – notice the last two bits.

The BSS Color Change Announcement Element can be found in the following frames: Beacon, Probe Response and (Re)Association response.

Links & Resources

CTS 160: 802.11ax OFDMA Resource Units

802.11ax (Wi-Fi 6) brings OFDMA to wireless. It’s an enhancement over OFDM which was a single-user transmission.  When a signal is sent or received it is done with one device. In OFDMA, it allows multiple access which means simultaneous transmissions to/from multiple devices.

There is a downlink multi-user operation and an uplink multi-user operation.

In OFDMA, a channel is subdivided into smaller channels, or resource units. This is so there can be simultaneous transmissions to different devices. Most transmissions are small frames so it’s an efficient way to send data by using a smaller channel and by making it multiple access we can have more communications at the same time.

These subcarriers (tones), the smaller channels of the main channel, are called resource units. An AP can allocate varying resource units for multi-user communications.

For example, a 20 MHz channel has 242 resource units which can be further split into 2x 106 resource units, 4x 52 resource units, or 9x 26 resource units.

Resource Units in a 20 MHz channel width

OFDMA allows subcarriers to be allocated to different devices for simultaneous transmission to or from those devices.

OFDMA transmissions in DL and UL allow different stations to occupy different RUs in a PPDU. Within that RU it could be SU-MIMO or MU-MIMO.

Resource Units (RUs) are defined for DL and UL transmissions and labeled as different tones. RUs are defined as:

  • 26-tone RU
  • 52-tone RU
  • 106-tone RU
  • 242-tone RU
  • 484-tone RU
  • 996-tone RU
  • 2×996-tone RU

Number of 802.11ax (Wi-Fi 6) OFDMA Resource Units per channel bandwidth:

RU TypeCBW20CBW40CBW80CBW80+80 &
CBW160
26-tone RU9183774
52-tone RU481634
106-tone RU24816
242-tone RU1248
484-tone RUn/a124
996-tone RUn/an/a12
2×996-tone RUn/an/an/a1

Type of subcarriers:

  • Data subcarriers
  • Pilot subcarriers
  • DC subcarriers
  • Guard subcarriers
  • Null subcarriers

A 26-tone RU consists of 24 data subcarriers and 2 pilot subcarriers.
A 52-tone RU consists of 48 data subcarriers and 4 pilot subcarriers.
A 106-tone RU consists of 102 data subcarriers and 4 pilot subcarriers.
A 242-tone RU consists of 234 data subcarriers and 8 pilot subcarriers.
A 484-tone RU consists of 468 data subcarriers and 16 pilot subcarriers.
A 996-tone RU consists of 980 data subcarriers and 16 pilot subcarriers.

DC subcarriers are used for the subcarriers located in the center of the channel. Depending on the channel width and the number of tone used, the number of DC subcarriers can vary (Ex: 3 or 7 for a 20MHz wide channel). Most of the time it will be 7 for the 20MHz and 80MHz wide channels and 5 for the 40MHz wide channels.

A 20MHz wide channels has 11 guard interval: the first 6 and the last 5 of the channel.

Downlink OFDMA

An AP can transmit frames to different devices by splitting a channel into subchannels or subcarriers or resource units.

Devices tune their radios to the specific resource unit to receive their transmissions. The AP still has to contend for airtime but will allocate resource units for different devices.

Uplink OFDMA

Similar to DL OFDMA, except devices transmit at the same time on different subchannels within the same channel (RUs). The use of trigger frames by the AP must be used in order to coordinate transmissions.

AP solicits simultaneous response frames from multiple HE devices. If a client does not support TRS Control, it will not receive a solicitation for MU UL. For the AP to solicit an HE TB PPDU, it will transmit a PPDU including a trigger frame(s). Within the trigger frame is the AID12 subfield which may contain the client in which it is addressed to or for UL OFDMA-based random access.

AP must follow EDCA procedure 10.22 (HCF), contend for txop. Device in the solicitation, or trigger frame, will respond to AP’s trigger frame. Device responds with its HE TB PPDU.

The Trigger frame from AP contains duration, RU allocation, target RSSI, and MCS for the device’s HE TB PPDU.

Resource Allocations for OFDMA – Multiple Access

Resource Allocation

When an AP transmits, the AP indicates the RU allocation within the HE MU PPDU. It’s ordered from lower frequency to higher frequency.

In UL, there is a trigger frame which indicates RU allocation, duration, target RSSI, and MCS.

Triggered Response Scheduling (TRS)

Used for soliciting an HE TB PPDU which follows the HE PPDU carrying the Control subfield. RU Allocation is contained in the Control Information subfield for TRS Control

Trigger frame

Allocates resources for and solicits one or more HE TB PPDU transmissions.

The User Info field carries some interesting data in the Trigger Frame. Understanding of the AID12 subfield – contains 12 LSBs of the AID of the client that it is intended to.

The AID12 subfield, if within the range of 1 – 2007, then will indicate the RU used by the HE TB PPDU of a client identified in AID12.

The RU Allocation consists of 8 bits that indicates the size of RUs and their placement in the frequency domain. It follows the mapping presented in Table 28-24. There can be up to 9 simultaneous devices.

The RU allocation field is followed by multiple User field which specify station-specific details such as Station ID, number of spatial streams to be used and MCS to be used.

Links & Resources

Will 802.11ax Resource Units bring an impact to wireless? I think so, only if we can solely Wi-Fi 6 clients – ditching all the legacy devices. This is where 6 GHz will be big for Wi-Fi.

What do you think about Resource Units?