Cutting edge technology facilitating Internet of Things implementation
Empowering power utilities with Internet of things technology
The MetrySense family of sensors integrates long-range radio as a standard feature (up to 10km point-to-point and over 100km in a mesh network configuration). Furthermore, each MetrySense sensor is running an IPv6 stack including a self-healing RPL routing protocol, and thus the sensors themselves embed the capability to automatically create at deployment a large scale 6LowPAN IPv6 mesh network without requiring additional routers, gateways, repeaters or transformers. This is an important advantage compared to the majority of other solutions deployed today, which require a nearby communication infrastructure that feeds from transformers and represent a costly installation and integration process. In addition, MetrySense supports cellular connectivity to any cellular network and provides full network management tools in order to manage the cellular sensor network.
Extending IP to low-power, wireless personal area networks (LoWPANs) was once considered impractical because these networks are highly constrained and must operate unattended for multi-year lifespans on a modest power supply. Many vendors embraced proprietary protocols, assuming that IP was too resource-intensive to be scaled down to operate on the micro-controllers and low-power wireless links used in LoWPAN settings. However, 6LoWPAN radically altered calculations by introducing an adaptation layer that enables efficient IPv6 communication over IEEE 802.15.4 LoWPAN links and is becoming a dominant internet protocol in the modern smart grid.
The architecture of the MetrySense solution
MetrySense sensors are arranged in clusters. The number of sensors in each cluster can be from one up to hundreds of sensors. The range between sensors is up to 10km, depending on radio regulations in the country of development. Each cluster is directly connected to the utility’s existing communication infrastructure via a gateway, or optionally via a cellular gateway as illustrated in the following figures:
(a) Using a MetrySense-3000 to interface the utility’s internal communication infrastructure
(b) Using a cellular MetrySense-3000 gateways and MetryView web server
Facts about MetrySense Radio
What are the frequency bands that the system is using?
The system is usually operating in the following unlicensed frequency bands:
- North America (FCC): 902-928MHz, up to 1W at 50 different frequencies
- Brazil: 902-907.5, 915-928 MHz
- Australia, New-Zeland: 922–928
- Europe (ETSI): 865-868MHz, 25mW, 58 different frequencies
- Israel: 315 & 325 MHz, 100mW
- Other countries usually follow the 865-868MHz or 902-928MHz range.
The system is usually used in frequency band which do not require specific radio license for the installation. However, MetrySense radio supports a wide range off frequencies, and can be adapted to specific licensed frequency bands that are used by power utilities.
How is the system maintaining stable performance in presence of radio interference?
When the system is operating in unlicensed bands other radio transmitters may exist, and data packets may be lost because of a collision with a signal transmitted by an interfering radio transmitter. Such collision may occur if the following two conditions are met:
- The interfering radio transmitter is located near a sensor, router or base unit. Radio transmitters which are located remotely are not causing collisions, because their radio signal is received below the level of the data packet from the nearby MetrySense unit.
- The interfering radio transmitter transmitted a radio signal at the same frequency and at the same time when a data packet was sent to the receiving unit.
MetrySense system uses the following mechanisms in order to keep a stable performance in the presence of other radio transmitters:
- MetrySense is using frequency hopping, and frequency is typically changed every 0.1 seconds (“time slot”)
- The system is “stubborn”: For every packet sent in a time slot, the sending unit waits for acknowledgement from the target unit. The acknowledgement is sent back in the same 0.1 second time slot. If an acknowledgement was not received, then the sender retransmits the packet several times in the next timeslots in different frequencies until the packet is delivered to the next router or base unit. The probability that the data will be lost after retransmissions is low, and may happen only in areas that are crowded with close-by interfering radios.
- Retransmission is performed again for every new radio connection (in contrary systems that perform retransmission over the entire network, or systems that do not perform retransmission at all). This quality is making the transmission much more reliable.
- Buffers are used in sensors and routers in order to allow receiving new packets (e.g. from other sensors), even if previous packets were not successfully sent yet because of an ongoing retransmission process.
- Sensors are keeping a log of data. If data was temporarily lost for any reason, then the data is automatically retrieved from the log of the sensor by the MetryView server when communication is restored. This mechanism is adding reliability on top of the retransmission mechanism.
- Adding extra routers: Routers can be easily added to the system and automatically create a new route to end-units using RPL, which is an IPv6 routing protocol. If a specific unit suffers from many close-by radio interferes or low radio signal, then a router can be very easily added in order to increase the level of the radio signal received by the remote unit. This way collisions will be avoided because the received interference level will be below the radio level of the received data from the added router.
Is the system causing interference to other systems?
Some level of interference always exists since the system is usually operating in unlicensed bands and shared with other systems. As explained above, interference of the system to other systems may happen if a unit is located closely to a unit of another system, and sends a packet in the same frequency and the same time.
However, this interference is very low, because of the following reasons:
- The duty cycle of transmissions is very low. A sensor unit typically sends an information packet of less than 0.1 seconds each tens of minutes. This means that a sensor unit typically occupies the air only for a fraction of percent of the time.
- Each transmission of is performed at a different frequency, with typically 50 frequencies used. This means that a sensor unit occupies the air at a specific frequency at a negligible time.
For these reasons, interference to other systems is rare.