802.11ah (HaLow) protocol analysis: protocol introduction-ANJIELOSMART

Regarding the 802.11ah protocol, let’s sort out the content. 802.11ah is a special protocol for IoT scenarios in the 802.11 protocol family. It has two main characteristics: a) "big connection" oriented, b) energy saving and low power consumption. 802.11ah works on the ISM channel in the Sub-1GHz frequency band. Although 802.11ah is not very successful commercially, there are many contents in its protocol design that are worthy of reference. In this article, we first introduce the 802.11ah protocol as a whole.

802.11ah and HaLow
802.11ah is the name of the protocol group, and HaLow is the name of the Wi-Fi Alliance for the 802.11ah protocol. The name HaLow is actually very good. There are several ways to say it:

1) Low represents low power consumption, and Ha can represent "Hay-" or "Halo".
2) The overall naming of HaLow is close to Hello, which is easier to remember.
3) Ha has another meaning that can be understood, that is, it is aH reversed, which means it represents 802.11ah.
As for why the Wi-Fi Alliance named it HaLow, it is roughly the same as the first three reasons.

Positioning of 802.11ah
802.11ah is a technical protocol designed by the 802.11 protocol group for large-scale connection IoT in order to enter the IoT market. This protocol works in the Sub-1G frequency band. We know that the 802.11 protocol is actually Wi-Fi (the main target is wireless LAN), but as the market continues to develop, the 802.11 protocol group will continue to go out, hoping to occupy some network services in other fields, such as IoT (i.e. 802.11ah ), short-range image transmission such as VR/AR (802.11ad/ay). These newly expanded protocols are often based on the core framework of 802.11 and are greatly improved to serve their own technical goals. So when learning 802.11ah, we completely need to treat it as a new protocol. After all, its modification volume is quite large. The features that are not consistent enough in 802.11 still exist in ah, that is, Best-effort.

Currently, in the IoT market, there are already a lot of network protocols. The following figure is roughly a summary of IoT protocols:

The relationship between 802.11ah and 802.11ax: We are mainly concerned about the IoT part of the 802.11 protocol. What we need to note here is that the IoT layout of 802.11 is actually a protocol divided into two parts:

1) Wi-Fi 6 (802.11ax), this is IoT for indoor scenarios like smart homes, while pursuing speed and power consumption. It still works in the 2.4GHz and 5GHz frequency bands and is compatible with the current 802.11 protocol. Wi-Fi itself is a protocol with a wide range of aspects. It not only includes high-speed design parts, but also adds low-power consumption functions. Specifically, equipment manufacturers can select different functions for expansion and development according to their own needs.
2) HaLow (802.11ah), 802.11ah is actually oriented to sensor network scenarios. It is actually the most classic sensor network scenario of IoT. The sensor network scenario has two characteristics: a) there are many nodes, which is a typical large connection scenario; b) sensors are usually powered by batteries, which means the requirements for energy saving are more stringent.
The relationship between 802.11ah and 802.11af: 802.11ah works in the Sub-1GH frequency band, and there is also an 802.11 protocol that also works in the Sub-1GHz frequency band, that is, 802.11af. The former is for IoT, and the latter is for TV wireless. The frequency band of the signal covers Wi-Fi type signals. The initial positioning of 802.11ah and 802.11af is completely different. One is IoT, and the other is biased towards cognitive radio. However, for many developers, the connection of two 802.11 technology-related protocols in the Sub-1GHz frequency band has actually caused internal competition.

In addition, regarding the relationship between 802.11ah and other related IoT protocols, you can refer to the following picture

Generally speaking, the description in this picture is accurate. Compared with some other protocols, 802.11ah is indeed average in terms of performance, and the quality is still good. However, the reason why it did not succeed in the end may still be the cost and stability of the technology. In my current opinion, 802.11ah has made more changes to the 802.11 protocol framework, and it retains the large framework of 802.11. But a lot of changes have actually been made to the details, so the technical cost is still somewhat high.

In summary, 802.11ah is an IoT protocol for "large connection types" and works in the Sub-1GHz frequency band. Let’s briefly talk about the specific needs of the people’s agreement below.

Design Goals of 802.11ah
The design goals of 802.11ah are mainly reflected in: 1) large connection (supporting up to 8191 nodes), 2) long distance (maximum coverage of 1Km for a single AP, supporting Relay AP), 3) high reliability, 4) low power consumption, 5 ) Good security (supports 802.11 encryption standard), this series of features. From my personal understanding, I prefer to describe the goal of 802.11ah as being based on the 802.11 protocol framework and making protocol modifications for the three factors of large connections, long distance, and low power consumption. During the specific design process, I prefer a protocol based directly on 802.11ac, which introduces the three characteristics of large connections, long distance, and low power consumption. In fact, some technical characteristics of 802.11ah and 802.11ax are similar, such as Both TWT and BSS Color are supported, and in fact ax is borrowed from ah, but in terms of detailed design, there are still big differences between the two.

Please refer to the figure below for specific parameters.

Technical characteristics of 802.11ah
802.11ah has introduced many new technologies. Generally speaking, several characteristics can be summarized, 1) bandwidth diversity of the physical layer, 2) group management of nodes and energy saving work in the form of group scheduling, 3) MAC layer Functions are decentralized to the PHY layer to improve protocol performance.

Specifically, you can refer to the picture above. The protocol features of 802.11ah include:

1) Channel with SST (Sub-channel Selective Transmission): 802.11ah supports 5 bandwidth modes, namely 1MHz, 2MHz, 4MHz, 8MHz and 16MHz, which can be selectively transmitted using SST during use.
2) Relay: 802.11ah supports Relay AP. The coverage range of a single AP is up to 1km, so if you want to extend the distance further, you need to introduce Relay AP.
3) BSS Color: BSS Color is to increase the ability of channel reuse. This technology was also introduced in Wi-Fi 6. It was first introduced in 802.11ah, and its purpose is the same as Wi-Fi 6.
4) Group sectorization: This is also a grouping method. Sector grouping is a scheduling method to improve the performance of the MAC layer.
5) 4-Level hierarchical device groups: This is also a group, mainly reflected in the TIM structure. It means that the grouping mechanism is optimized to an energy-saving working mode. Through the group polling mode, energy consumption is saved and all group nodes can be polled in an orderly manner.
6) Restricted Access Window with EDCA: Abbreviated as RAW, it mainly describes its use of RAW, an improved competition method, in competition mode to reduce the problem of high conflict probability caused by the large number of nodes in the "large connection" scenario. .
7) OFDM with 4 spatial streams: This is still for the connection rate. 802.11ah inherits from 802.11ac, allowing a maximum of 4 spatial streams.
The rest of 802.11ah also has technical features such as Bi Directional TXOP optimization, NDP control frame optimization, Traveling pilots and pilot boosting. When I sort out the details of the technical features later, I will not expand on this article.

Finally, to summarize, 802.11ah is indeed a very good and flexible application of the 802.11 protocol framework in terms of protocol design. However, the current dominance of the IoT market is still felt to be low cost, and the starting point of 802.11ah is actually 802.11ac, that is, it is designed based on 802.11ac, so the relative cost is still higher, resulting in its in IoT It has not caused a big stir in the market, but we mainly focus on the protocol design, hoping to expand our understanding of the 802.11 protocol system.