Based on 802.11ac architecture and PHY, 802.11ah is a variant of the wireless protocols targeted for the IoT. The design attempts to optimize for constrained sensor devices that require long battery life and can optimize range and bandwidth. 802.11ah is also referred to as HaLow, which is essentially a play on words with "ha" referring to "ah" backward and low implying low power and lower frequency. Put together, it forms a derivative of "hello".

The intent of the IEEE 802.11ah task group was to create a protocol with extended range for rural communications and offloading cell traffic. The secondary purpose was to use the protocol for low throughput wireless communications in the sub-gigahertz range. The specification was published on December 31, 2016. The architecture is most different from other forms of 802.11 standards, in the following ways, in particular:

• Operates in 900 MHz spectrum. This allows for good propagation and penetration of materials and atmospheric conditions. 
• Channel width varies and can be set to 2, 4, 8, or 16 MHz-wide channels.
• The available modulation methods are diverse and include BPSK, QPSK, 16-QAM, 64-WAM, and 256-QAM modulation techniques. 
• Modulation based on 802.11ac standard with specific changes. A total of 56 OFDM subcarriers with 52 dedicated to data and four dedicated to pilot tones. 
• Total symbol duration is 36 or 40 microseconds. 
• Supports SU-MIMO beamforming.
• Fast association for networks of thousands of STAs using two different authentication methods to limit contention.
• Provides connectivity to thousands of devices under a single access point.
• Includes the ability to relay to reduce power on STAs and allow for a crude form of mesh networking using a one-hop reach method.
• Allows for advanced power management on each 802.11ah node.
• Allows for non-star topology communication through the use of Restricted Access Windows.
• Allows for sectorization, which enables antennas to be grouped to cover different regions of a BSS (called sectors). This is accomplished using beamforming adopted from other 802.11 protocols.

The minimum throughput will be 150 kbps, based on BPSK modulation on a single MIMO stream at 1 MHz channel bandwidth. The maximum theoretical throughput will be 347 Mbps based on a 256-WAM modulation using 4 MIMO streams and 16 MHz channels. 

Channel width will vary depending on the region in which 802.11ah is deployed. Some combinations will not work due to regulations in specific regions as shown:

Every attempt in the architecture of the IEEE 802.11ah standard is aimed at optimizing overall range and efficiency. This reaches down as far as the length of the MAC headers. 

The goal to connect several thousand devices to a single AP is also accomplished using a unique association identifier (AID) assignment of 13 bits. This allows for grouping of STAs based on criteria (hallway lights, light switch, and so on). This allows an AP to connect to over 8191 STAs (802.11 could support only 2007 STAs). That many nodes, however, has the potential to induce a massive number of channel collisions. Even though the number of connected STAs increased, the goal was to reduce the amount of data in transit to address these stations. The IEEE task group accomplished this by removing a number of fields that were not particularly relevant for the IoT use cases, such as the QoS and DS fields. The following figure illustrates the 802.11ah MAC downlink and uplink frames as compared to standard 802.11.

Another improvement for power management and channel efficiency is attributed to removing acknowledgment frames. ACKs are implicit for bidirectional data. That is it both devices are sending and receiving data from each other. Normally an ACK would be used after the successful reception of a packet. In this bidi (BDT) mode, reception of the next frame implies that the previous data was successfully received and no ACK packet needs to be exchanged. 

To avoid a sea of collisions which would prevent a functional network, 802.11ah uses a Restricted Access Window (RAW). As the STAs are divided into various groups using the AID, channels will be split into time slots. Each group would be assigned a specific time slot and no others. There are exceptions, but for the general case, the grouping forms an arbitrary isolation. The additional benefit of RAW is that devices can enter a hibernation state to conserve power whenever it isn't their time slot for transmission.

Topology-wise, there are three types of stations in an 802.11ah network:

• Root access point: The principal root. Typically, serves as a gateway to other networks (WAN).
• STA: The typical 802.11 station or end point client.
• Relay node: A special node that combines an AP interface to STAs residing to a lower BSS and an STA interface to other relay nodes or a root AP on the upper BSS. 

The following figure is the IEEE802.11ah topology. This architecture differs substantially from other 802.11 protocol in the use of a single hop relay nodes that act to create identifiable BSS. The hierarchy of relays forms a larger network. Each relay acts as an AP and STA.

In addition to the basic node types, there are three power saving states an STA can reside in:

• Traffic Indication Map (TIM): Listens to AP for data transfer. Nodes will periodically receive information about data buffered for them from its access point. The message sent is called the TIM Information Element.
• Non-TIM stations: Negotiates with AP directly during association to obtain transmission time on Periodic Restricted Access Windows (PRAW).
• Unscheduled stations: Does not listen to any beacons and uses polling to access channels.

Power is critical in IoT sensor and edge devices based on coin cell batteries or energy harvesting. 802.11 protocols are notorious for high power demands. To remediate the power of this wireless protocol, 802.11ah uses a Max Idle Period value, which is part of the regular 802.11 specifications. In a general 802.11 network, the Max Idle Period is roughly 16 hours based on a time of 16-bit resolution. In 802.11ah, the first two bits of the 16-bit timer are scaling factor that allows the sleep duration to exceed five years.

Additional power is mitigated through changes to the beacon. As previously covered, beacons relay information on the availability of buffered frames. Beacons will carry a TIM bitmap, which inflates their size since 8191 STAs will cause the bitmap to grow substantially. 802.11ah uses a concept called TIM segmentation where some beacons carry portions of the overall bitmap. Each STA calculates when their respective beacon with bitmap information will arrive and allows the device to enter a power save mode until the moment it needs to wake and receive beacon information.

Another power-save feature is called the Target Wake Time (TWT) and is intended for STAs that rarely transmit or receive data. This is very common in IoT deployments such as temperature sensor data. An STA and its associated AP will negotiate to arrive at an agreed upon TWT and the STA will enter a sleep state until that timer is signaled.0;

The process of implicit ACKs is called a Speed Frame Exchange, and it is shown in the following figure: