New Wi-Fi for the Internet of Things (Part 1)
At the end of the last century, computers drove people out of many fields of activity related to routine operations. United by the Internet, computers have destroyed the boundaries of information dissemination.
In the early 2000s, social networks and mobile gadgets brought people together from all over the world: replacing personal interaction, they presented us with a network that it would be better to call the “Internet of people”.
Both phenomena, computers and the Internet, unpredictably changed even people's habits, prompting us to check our mail in the morning before brushing our teeth. Without a doubt, the further desire to automate everything that is possible, radically change the economy, politics and personal life. Analysts believe that the next ICT revolution will be linked to the Internet of Things - an ecosystem of billions of autonomous devices: sensors, controllers, machines, devices, etc. The scale of the revolution is indicated by the fact that, according to forecasts, the total number of devices in such a network will reach 50 billion by 2020. Obviously, the best way to connect such a huge number of devices is a wireless network.
')
While futurologists predict our life in 2020, engineers and researchers ask if modern wireless technologies can handle such a huge number of devices, most of which are powered by rechargeable batteries.
Although today there are already a number of technologies for personal wireless networks based on RFID, ZigBee, Bluetooth, supporting the operation of low-power devices, their capabilities are limited by the number of devices in one network, bandwidth, range and other parameters. On the other hand, the technology of urban and regional networks, such as today's WiMAX and LTE, is also not suitable for the Internet of things because of the high energy consumption and relatively high cost of use. That is why 3GPP, IEEE and other international organizations today are busy trying to adapt their technologies for the needs of the Internet of Things.
Consider, for example, IEEE and IEEE 802.11 networks, known as Wi-Fi. This wireless technology was originally created for high-speed connection of a limited number of stations located indoors at a short distance between each other. Therefore, it is not effective for the transmission of short messages by a large number of devices of the Internet of things remote from each other. To meet the requirements of the Internet of Things, the IEEE 802 LAN / MAN Standard Committee (LMSC) created the IEEE 802.11ah group (Task Group ah, TGah), which aims to expand the scope of IEEE 802.11 networks by developing an energy efficient protocol that allows thousands of stations located as indoors and outside, work in the same frequency-spatial domain.
In this series of articles, we will look at the activities of TGah and its tasks, as well as the basic mechanisms that will be included in addition to IEEE 802.11ah (.11ah). Work on the .11ah should be completed in early 2016, which means that in about a year the first devices using the new technology will appear on the market.
In 2010, after examining the properties of the range <1 GHz (S1G), the Committee identifies this range as promising for Wi-Fi devices operating outdoors. However, due to the lack of available spectrum, the S1G does not allow the use of wide bands, especially> 20 MHz, introduced in .11n and .11ac. However, signal code constructions (CCMs), developed in .11ac and adapted to narrow channels .11ah, are capable of providing hundreds of MB / s, if the channel conditions are good enough.
At the same time, S1G has definitely better radio wave propagation characteristics in street scenarios compared to the traditional 2.4 and 5 GHz Wi-Fi band, which increases the radius of the coverage area to 1 km with a standard transmit power of 200 mW. Effective CCMs and favorable propagation characteristics of the waves allow using S1G to build sensor networks that are superior to ZigBee, Bluetooth, NFC, etc. in bandwidth and coverage, while remaining very energy efficient.
Thus, the main group of .11ah usage scenarios is as follows:
In all of these applications, an access point (AP) covers hundreds or even thousands of devices — sensors or controllers — that transmit short packets from time to time. The huge number of stations competing for channel access leads to collisions, and the use of standard packets for transmitting short messages increases the overhead caused by relatively long packet headers. Despite the fact that the required aggregate bandwidth in the considered scenarios does not exceed 1 Mbit / s, all these features reduce the efficiency of using channel resources. An equally important problem is the need to reduce energy consumption, since the sensors are mainly powered by batteries.
IEEE 802.15.4g wireless devices that are widely used in industry can run on battery power for a long time, but the range and available data rates are very low. Therefore, the second group of scenarios is the construction of a transport network of communication between IEEE 802.15.4g sensors and remote servers. IEEE 802.15.4g routers collect data from devices and relay them to servers over the .11ah network. In other words, .11ah extends the network coverage area of .15.4g. In addition, since the IEEE 802.15.4g speeds are not sufficient for transmitting video streams, IEEE 802.11ah can also be used in these scenarios for transmitting images from surveillance cameras.
High bandwidth and a large coverage area make S1G attractive for increasing the coverage area of a Wi-Fi access point and for reducing mobile network traffic offloading, which is a successful solution to the problem of telecom operators arising from the ever-increasing volume of mobile traffic. Although in data networks .11n and .11ac data rates are comparable to data rates in an LTE network (or even higher), these technologies can hardly be used to offload mobile networks outdoors because of the small visibility radius. In contrast, IEEE 802.11ah will be very useful, especially in countries with a wide affordable S1G channel, for example, the United States.
The table below contains the basic requirements of the use cases described above:
Having studied the regulation in the S1G range in various countries, TGah faced two problems.
The first problem is that the bands available in S1G for industrial, scientific and medical communications vary from country to country. The current version of the draft standard defines which channels should be used in the United States, Europe, Japan, China, South Korea and Singapore.
The second problem is the lack of free frequencies. Therefore, the channels used in .11ah are 10 times narrower than in .11ac (one of the latest additions to the Wi-Fi standard): 1, 2, 4, 8, and 16 MHz (Only 1 and 2 MHz are mandatory). Moreover, the physical layer of the IEEE 802.11ah standard is inherited from .11ac and adapted to the available S1G bands.
For channels> = 2 MHz, the CCM are 10 times slowed down by the .11ac standard CCM, that is, the length of OFDM symbols in .11ah is 10 times longer than .11ac, while the number of subcarriers in .11ah channels is the same as in the corresponding channels .11ac. For example, both channels - 2 MHz at .11ah and 20 MHz at .11ac - contain 64 subcarriers, of which only 52 transmit data. For the 1 MHz channel, the total number of subcarriers is two times lower, but only 24 of them (which is less than 52/2) transmit data.
The .11ah inherits from the standard .11ac 10 CCM (called MCS0, ..., MCS9) with different speeds and reliability. To extend the range of the channel 1 MHz, the standard defines a new MCS MCK10, which is nothing but a modification of the MCS0 with double repetition, increasing the reliability of transmission. Preliminary studies show that, thanks to MCS0, outdoors .11ah will allow data to be transmitted in a 1 MHz channel at a power of 200mW over a distance substantially greater than 1 km.
Available data rates for various channels and CCMs are listed in the table below:
The reduced rates can be improved by reducing the length of the OFDM symbol and using multiple spatial streams (MIMO).
So, the duration of a regular OFDM symbol in .11ah networks is 40 µs, 20% of which is a guard interval containing redundant data and preventing intersymbol interference. IEEE 802.11ah allows you to shorten the guard interval by half, which increases the data transfer speed by 11%.
IEEE 802.11ah stations can use up to 4 spatial streams. As is known, N spatial streams increase data transfer speed N times. Thus, the maximum data transfer rate in .11ah networks reaches almost 350 Mbit / s. However, 350 Mbps is the maximum data transfer rate measured at the physical layer. In fact, if the data link layer protocols are not changed, then the data transfer rate, say, at the network level will be significantly lower.
Why this happens and how TGah changed the data link layer protocols to reduce protocol losses will be discussed in the next article.
In the early 2000s, social networks and mobile gadgets brought people together from all over the world: replacing personal interaction, they presented us with a network that it would be better to call the “Internet of people”.
Both phenomena, computers and the Internet, unpredictably changed even people's habits, prompting us to check our mail in the morning before brushing our teeth. Without a doubt, the further desire to automate everything that is possible, radically change the economy, politics and personal life. Analysts believe that the next ICT revolution will be linked to the Internet of Things - an ecosystem of billions of autonomous devices: sensors, controllers, machines, devices, etc. The scale of the revolution is indicated by the fact that, according to forecasts, the total number of devices in such a network will reach 50 billion by 2020. Obviously, the best way to connect such a huge number of devices is a wireless network.
')
While futurologists predict our life in 2020, engineers and researchers ask if modern wireless technologies can handle such a huge number of devices, most of which are powered by rechargeable batteries.
Although today there are already a number of technologies for personal wireless networks based on RFID, ZigBee, Bluetooth, supporting the operation of low-power devices, their capabilities are limited by the number of devices in one network, bandwidth, range and other parameters. On the other hand, the technology of urban and regional networks, such as today's WiMAX and LTE, is also not suitable for the Internet of things because of the high energy consumption and relatively high cost of use. That is why 3GPP, IEEE and other international organizations today are busy trying to adapt their technologies for the needs of the Internet of Things.
Consider, for example, IEEE and IEEE 802.11 networks, known as Wi-Fi. This wireless technology was originally created for high-speed connection of a limited number of stations located indoors at a short distance between each other. Therefore, it is not effective for the transmission of short messages by a large number of devices of the Internet of things remote from each other. To meet the requirements of the Internet of Things, the IEEE 802 LAN / MAN Standard Committee (LMSC) created the IEEE 802.11ah group (Task Group ah, TGah), which aims to expand the scope of IEEE 802.11 networks by developing an energy efficient protocol that allows thousands of stations located as indoors and outside, work in the same frequency-spatial domain.
In this series of articles, we will look at the activities of TGah and its tasks, as well as the basic mechanisms that will be included in addition to IEEE 802.11ah (.11ah). Work on the .11ah should be completed in early 2016, which means that in about a year the first devices using the new technology will appear on the market.
Usage scenarios
In 2010, after examining the properties of the range <1 GHz (S1G), the Committee identifies this range as promising for Wi-Fi devices operating outdoors. However, due to the lack of available spectrum, the S1G does not allow the use of wide bands, especially> 20 MHz, introduced in .11n and .11ac. However, signal code constructions (CCMs), developed in .11ac and adapted to narrow channels .11ah, are capable of providing hundreds of MB / s, if the channel conditions are good enough.
At the same time, S1G has definitely better radio wave propagation characteristics in street scenarios compared to the traditional 2.4 and 5 GHz Wi-Fi band, which increases the radius of the coverage area to 1 km with a standard transmit power of 200 mW. Effective CCMs and favorable propagation characteristics of the waves allow using S1G to build sensor networks that are superior to ZigBee, Bluetooth, NFC, etc. in bandwidth and coverage, while remaining very energy efficient.
Thus, the main group of .11ah usage scenarios is as follows:
- smart meters (gas, water and power consumption);
- intelligent energy systems (Smart-grid, for Russia with its subsoil, this is of little relevance, but in the West ...);
- monitoring of the environment and agricultural land (temperature, humidity, wind, water level, environmental pollution, the state of animals, detection of forest fires, etc.);
- automation of production processes (extraction and processing of oil, ore; chemical and pharmaceutical industry, etc.);
- health systems / fitness system (remote measurement of blood pressure, heart rate, weight);
- care system for the elderly and newborns;
- smart House.
In all of these applications, an access point (AP) covers hundreds or even thousands of devices — sensors or controllers — that transmit short packets from time to time. The huge number of stations competing for channel access leads to collisions, and the use of standard packets for transmitting short messages increases the overhead caused by relatively long packet headers. Despite the fact that the required aggregate bandwidth in the considered scenarios does not exceed 1 Mbit / s, all these features reduce the efficiency of using channel resources. An equally important problem is the need to reduce energy consumption, since the sensors are mainly powered by batteries.
IEEE 802.15.4g wireless devices that are widely used in industry can run on battery power for a long time, but the range and available data rates are very low. Therefore, the second group of scenarios is the construction of a transport network of communication between IEEE 802.15.4g sensors and remote servers. IEEE 802.15.4g routers collect data from devices and relay them to servers over the .11ah network. In other words, .11ah extends the network coverage area of .15.4g. In addition, since the IEEE 802.15.4g speeds are not sufficient for transmitting video streams, IEEE 802.11ah can also be used in these scenarios for transmitting images from surveillance cameras.
High bandwidth and a large coverage area make S1G attractive for increasing the coverage area of a Wi-Fi access point and for reducing mobile network traffic offloading, which is a successful solution to the problem of telecom operators arising from the ever-increasing volume of mobile traffic. Although in data networks .11n and .11ac data rates are comparable to data rates in an LTE network (or even higher), these technologies can hardly be used to offload mobile networks outdoors because of the small visibility radius. In contrast, IEEE 802.11ah will be very useful, especially in countries with a wide affordable S1G channel, for example, the United States.
The table below contains the basic requirements of the use cases described above:
Physical level
Having studied the regulation in the S1G range in various countries, TGah faced two problems.
The first problem is that the bands available in S1G for industrial, scientific and medical communications vary from country to country. The current version of the draft standard defines which channels should be used in the United States, Europe, Japan, China, South Korea and Singapore.
The second problem is the lack of free frequencies. Therefore, the channels used in .11ah are 10 times narrower than in .11ac (one of the latest additions to the Wi-Fi standard): 1, 2, 4, 8, and 16 MHz (Only 1 and 2 MHz are mandatory). Moreover, the physical layer of the IEEE 802.11ah standard is inherited from .11ac and adapted to the available S1G bands.
For channels> = 2 MHz, the CCM are 10 times slowed down by the .11ac standard CCM, that is, the length of OFDM symbols in .11ah is 10 times longer than .11ac, while the number of subcarriers in .11ah channels is the same as in the corresponding channels .11ac. For example, both channels - 2 MHz at .11ah and 20 MHz at .11ac - contain 64 subcarriers, of which only 52 transmit data. For the 1 MHz channel, the total number of subcarriers is two times lower, but only 24 of them (which is less than 52/2) transmit data.
The .11ah inherits from the standard .11ac 10 CCM (called MCS0, ..., MCS9) with different speeds and reliability. To extend the range of the channel 1 MHz, the standard defines a new MCS MCK10, which is nothing but a modification of the MCS0 with double repetition, increasing the reliability of transmission. Preliminary studies show that, thanks to MCS0, outdoors .11ah will allow data to be transmitted in a 1 MHz channel at a power of 200mW over a distance substantially greater than 1 km.
Available data rates for various channels and CCMs are listed in the table below:
The reduced rates can be improved by reducing the length of the OFDM symbol and using multiple spatial streams (MIMO).
So, the duration of a regular OFDM symbol in .11ah networks is 40 µs, 20% of which is a guard interval containing redundant data and preventing intersymbol interference. IEEE 802.11ah allows you to shorten the guard interval by half, which increases the data transfer speed by 11%.
IEEE 802.11ah stations can use up to 4 spatial streams. As is known, N spatial streams increase data transfer speed N times. Thus, the maximum data transfer rate in .11ah networks reaches almost 350 Mbit / s. However, 350 Mbps is the maximum data transfer rate measured at the physical layer. In fact, if the data link layer protocols are not changed, then the data transfer rate, say, at the network level will be significantly lower.
Why this happens and how TGah changed the data link layer protocols to reduce protocol losses will be discussed in the next article.
Source: https://habr.com/ru/post/240201/
All Articles