Wi-Fi standard designed for large-scale sensor, IoT applications
The IEEE’s new sub GHz Wi-Fi standard, 802.11ah, is designed for large-scale sensor and Internet of Things (IoT) applications. The standard, which is to be rolled out in early 2016, will operate in the 900 MHz ISM spectrum and will achieve a significant increase in range over higher frequency Wi-Fi networks. These networks also will operate at much slower throughputs. The standard is designed for large areas containing a vast number of client devices that typically rely on battery power and low bandwidth applications such as sensor networks.
OFDM and MIMO
The 802.11ah standard will use orthogonal frequency division multiplexing (OFDM) and multiple in, multiple out (MIMO) with a 1 MHz-wide channel. It will be possible to bond multiple channels to effect higher throughput channels. This is the same method for dynamically increasing throughput as that used in 802.11n and ac. Channel widths of 1, 2, 4, 8, and 16 MHz will be possible. There will be two modes or categories of operation depending upon the requirements of the network: 1 MHz and 2 MHz, or greater.
The 1 MHz mode is designed to provide extended range coverage; there will be 24 1-MHz channels in the 26 MHz ISM bandwidth (902 to 928 MHz), with 31.25 kHz spacing. As a result, a new modulation and coding scheme (MCS), MCS-10, was designed for long-range transmission. MCS-10 is essentially MCS-0 repeated twice to ensure transmission resilience and reliability.
The 2 MHz and higher modes of operation use bonded channels of 2, 4, 8, and 16 MHz, which allow 12, 6, 2, and 1 channel, respectively. This mode will use a symbol length of 10 times that of 802.11ac (downclocking; one-tenth the clocking rate of 802.11ac).
Many clients: 802.11ah is also designed to accommodate a large number of stations or clients. Upon association with any 802.11 access point (AP), a unique identifier, the association identifier (AID) is assigned to the client or station. For previous standards, the maximum number of AIDs is 2,007; for 802.11ah, that number goes up to 8,191. This is accomplished through the use of hierarchical AID structure that is split into four levels: page, block, sub-block, and station index within the sub-block. Each client or station belongs to a particular sub-block, which belongs to a particular block; multiple blocks make up a page.
In addition to the IoT, 802.11ah is intended for machine-to-machine (M2M) applications. The increase in the allowable number of associated devices was essential for the purposes of the standard. A typical application would be the smart grid where thousands of independent sensors and other devices would communicate with the network. An efficient network design could group sensors depending upon their location and traffic patterns; this would improve and enhance power saving, allocation of resources, and spectral efficiency.
Several improvements are being made to the MAC to increase communication efficiency. Frame, frame headers, and beacons will be shortened. Bidirectional transmission opportunity (TXOP) or TXOP sharing will also be used to make the best use of a TXOP after successful contention. This technique will essentially allow a bidirectional exchange between the station and the AP; in actuality, the TXOP will be divided and receive portions divided by a safety instrumented system (SIF). This eliminates unnecessary medium contention because only one TXOP will be negotiated.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA); email@example.com. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Dan Capano on the following topics:
Characteristics of IEEE 802.11n and 802.11ac deliver same performance for one quarter the cost of wired systems
Wireless security: Cryptology basics, fundamentals
Wireless security: Extensible authentication protocols
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.