Building a Practical Wireless Sensor Network
by Brian Macdonald
Director, ANT Networks
Nikkei Electronics Asia -- December 2007
Wireless sensor networks (WSN) will open the floodgates to the wireless revolution. But building a practical wireless network can be a daunting challenge unless the concepts are kept simple. In time, the new wireless technologies will likewise reshape society in unpredictable ways.
Nobody can question that the wireless revolution has already begun: CDMA or GSM for long-range voice and data, Wi-Fi for wireless local area networks (WLAN) and Bluetooth for consumer-oriented personal area networks (PAN) are all flourishing. Although each is a very successful commercial technology, they are restricted to particular applications areas by virtue of range, bandwidth and power requirements. For a wireless revolution to occur, a truly pervasive networking technology that can build networks consisting of hundreds of nodes is required. These nodes will need to be capable of communicating with each other at any time without being compromised by interference from other RF sources. The wireless networks these nodes build up will be characterised by inexpensive, ultra-low power radios, with modest bandwidth requirements - able to transmit small amounts of sensor data perhaps a few times a second - and typically operating in the globally accepted, licence-free 2.4GHz industrial, scientific, medical (ISM) band. There are however some major design constraints: if a network is going to comprise many nodes then each has to be inexpensive - of the order of less than US$5 today and even lower in the long term - and virtually maintenance free. There are also likely to be nodes sited in inaccessible places, so battery life of months or years from inexpensive cells is vital.ZigBee, the IEEE802.15.4-based solution is one option, and there are a slew of proven proprietary alternatives. Yet at present no single technology dominates or has even been installed in high volumes, because designing a WSN can be so difficult that designers struggle to come up with commercial solutions.It's not just a case of switching on one radio and expecting it to talk to another. Technical challenges that have to be resolved include: how to avoid interference between nodes and other RF sources; whether the network is scalable; how many nodes can be supported; whether nodes can be added in an ad hoc manner without reconfiguring the rest of the network; what bandwidth is required; how the power consumption can be minimized; and what microcontroller resources will be needed.
Mesh Networks Overly Complicated
Creating a mesh network is one of the ways to build a WSN. Mesh networks are touted as the best way to maximize the potential of ultra-low power wireless sensors where every node can communicate with many (or even all) of its neighbors in a self-managing and healing topology.Unfortunately, while mesh networks make for compelling academic debate, in commercial implementations with even a modest number of nodes they invariably prove difficult to set up and introduce a level of complexity that can't be justified for almost all contemporary practical applications. Engineers quickly conclude, often after a lengthy development program, that mesh networks are overly complicated, demanding lots of computing resource and electrical power, and are expensive. Fortunately, 99.5% of all envisaged WSN applications can be designed without a mesh, eliminating the need to waste time grappling with the challenge.Virtually all practical networking problems can be resolved using a simple pre-determined structure comprising two to several dozen nodes at most. The simplest of these is the peer-to-peer network where only one node communicates with another. The simplest application example of this peer-to-peer networking is a humble switch controlling a light. A more complicated wireless network will comprise several peripheral nodes talking to a single receiving node.Consider, for example, a cyclist wearing a sports watch (node 1) where node 2 is a GPS tracker, node 3 is a speed indicator and node 4 is a heart rate monitor all communicating simultaneously with the sports watch node via their own dedicated channels, A, B, and C (Fig 1). This type of network is often referred to as star network because it features a central hub that can be schematically shown communicating in a star-like fashion with peripheral nodes.
Importance of Protocol
Practical wireless networks should be low-cost, immune to interference from other radio sources (including neighboring nodes), reliable, and perhaps most importantly, consume little power. The last thing a user wants is a reliable network compromised by the need to change batteries every few days.Each node requires a silicon radio allied to a microcontroller, often referred to as the physical layer (PHY), forming the hardware that drives the node. Some 2.4GHz radios integrate the radio and microcontroller into a single chip. The PHY supports a protocol stack and an application layer that forms the specific instruction set for the application supported by the network.The protocol is perhaps the most vital element in ensuring the practical wireless network performs to expectations. It determines how the node communicates across a wireless link with other nodes by establishing standard rules for co-existence, data representation, signalling, authentication and error detection. One way to compare the various offerings from wireless communications companies is to consider the protocol's efficiency by comparing a packet's ratio of overhead (information required to set-up the communication with a specific node and to determine how the information will be reliably sent) to payload (the actual useful data). A high ratio of data to overhead means the time that the radio transmits (when the power consumption is highest) is shortened and hence the radio can go back into ultra-low power sleep mode faster.There is a bit more to it than this though; the key is the bandwidth and hardware efficiency of the radio itself allied with how this is managed in terms of the physical layer efficiency when communicating. The bandwidth of the radio broadly correlates to how much time the radio will need to spend transmitting in a relatively high power "on" mode for a given amount of data. Theoretically, the wider the bandwidth, the faster the transmission and the less time the radio will need to spend out of sleep mode. In the real world, bandwidth costs power and the optimal trade-off point is generally considered to be 1Mbps before the added power losses begin to outweigh the gains.But all these radio hardware efficiency savings can be swept away in an instant by a flabby physical layer efficiency. Power consumed by the radio when on will have the biggest effect on overall power consumption because this will usually be an order of magnitude higher than the power consumed by the radio when off (although this figure is important too due to the amount of time a radio will spend in this state). The problem is that the radio will diligently transmit what it's told to transmit by the protocol. Unless the data is packaged in a way that optimizes off time per bit of data sent, the proportion of the time the radio spends "on" will rise significantly. The real challenge, therefore, is to maximize the amount of time the radio spends "off" in minimum power sleep mode.WSNs are characterized by small amounts of data sent occasionally. Usually, if data is sent but occasionally not received this is not a problem because updated data follows soon after. This technique is suited to sensor applications and is field proven as the most economical method of operation. However, if it's essential that every piece of data is received, the protocol should include instructions for the receiving node to return an acknowledgement that the message was received successfully. If the acknowledgement is not received, the message is resent.
Building Practical Infrastructure
Identifying a good silicon radio and efficient protocol is only part of designing a practical wireless network. Whether the network is going to comprise two, ten or a hundred nodes, the biggest challenge is linking those nodes into a reliable, scalable network.The key to this is to choose a technology where at the physical connection layer all nodes have equal functionality, so are capable of acting as clients or masters within a practical wireless network and can swap roles at any time. In other words, the nodes should be able to perform as transmitters, receivers or transceivers in order to route traffic to other nodes.
In addition, every node should be capable of determining the best time to transmit based on the activity of its neighbors, eliminating the need for a network restricting coordinator or supervisory node. This combination of features means it's easy to add another node to a network, of any topology, in an ad hoc fashion. There is no need to plan in order to consider what type of node will be required to extend the network, or to make provision for a coordinator node to tie the network together when it reaches a certain size. Setting up a network to perform a practical function requires more than establishing a network of nodes that can communicate with each other. The nodes need to be configured to perform a function such as measuring temperature, humidity or heart rate. The configuration for each node to perform this function can be made considerably easier by selecting the appropriate technology. In operation, the sensor is configured at start-up with the Flash memory storing the sensor profile and the relevant sensor communication protocol. An application host MCU isn't required, further cutting system cost, power consumption and size.Ultra-low power is essential for a practical wireless network because the coin cell batteries powering the nodes need to last for months or preferably years to minimize maintenance. Let's look at some typical numbers for a proprietary technology.For an application sending 8 bytes of data once a second for an hour a day, battery lives of the transmitter and receiver are 6.4 and 5.6 years respectively. Note that battery life is heavily dependent on the application and this example is a low usage case. Fig 2 shows how battery life varies with messaging frequency for a particular use case. For instance, in an industrial setting the sensor may be required to be in use 24 hours a day with a message period of, typically 0.5Hz. The transmitter battery life would then be 7. 2 months and the receiver life would be 6.3 months. This compares very favorably with a commercially available ZigBee solution. Battery lives for ZigBee transmitters and receivers in the industrial application described above would be 8 to 10 weeks.
Crowded Environment
Wireless sensor networks, in keeping with many contemporary 2.4GHz technologies, operate in an increasingly crowded part of the radio spectrum. Network nodes will have to compete with Wi-Fi, Bluetooth, cordless phones and each other when trying to get their message through. However, network nodes do have one big advantage: they don't have to transmit very often, and when they do, it's for a very short time. Nonetheless, an interference avoidance strategy is vital. Some common techniques for minimizing the impact of interference for devices operating in the 2.4GHz band include time slot allocation schemes such as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS).Both DSSS and FHSS work well, but require the transmitter and receiver to be synchronized. In the case of FHSS this is to ensure the devices are tuned to the same narrow band simultaneously, and for DSSS so that the de-spreading by the same pseudo-random sequence used to spread the signal in the first place works properly. Synchronization adds complexity to the network and increases power consumption. Although synchronization can be switched off to save power when communication isn't needed, re-acquisition can take several seconds and uses even more power.
Ubiquitous Wireless Connectivity
WSNs have the potential to make wireless connectivity ubiquitous and truly unleash the full power of the wireless revolution in myriad applications, where most of which have yet to be conceived. However, this revolution won't start unless networks become a lot simpler to set-up, maintain and scale.
by Brian Macdonald
Director, ANT Networks
Nikkei Electronics Asia -- December 2007
Wireless sensor networks (WSN) will open the floodgates to the wireless revolution. But building a practical wireless network can be a daunting challenge unless the concepts are kept simple. In time, the new wireless technologies will likewise reshape society in unpredictable ways.
Nobody can question that the wireless revolution has already begun: CDMA or GSM for long-range voice and data, Wi-Fi for wireless local area networks (WLAN) and Bluetooth for consumer-oriented personal area networks (PAN) are all flourishing. Although each is a very successful commercial technology, they are restricted to particular applications areas by virtue of range, bandwidth and power requirements. For a wireless revolution to occur, a truly pervasive networking technology that can build networks consisting of hundreds of nodes is required. These nodes will need to be capable of communicating with each other at any time without being compromised by interference from other RF sources. The wireless networks these nodes build up will be characterised by inexpensive, ultra-low power radios, with modest bandwidth requirements - able to transmit small amounts of sensor data perhaps a few times a second - and typically operating in the globally accepted, licence-free 2.4GHz industrial, scientific, medical (ISM) band. There are however some major design constraints: if a network is going to comprise many nodes then each has to be inexpensive - of the order of less than US$5 today and even lower in the long term - and virtually maintenance free. There are also likely to be nodes sited in inaccessible places, so battery life of months or years from inexpensive cells is vital.ZigBee, the IEEE802.15.4-based solution is one option, and there are a slew of proven proprietary alternatives. Yet at present no single technology dominates or has even been installed in high volumes, because designing a WSN can be so difficult that designers struggle to come up with commercial solutions.It's not just a case of switching on one radio and expecting it to talk to another. Technical challenges that have to be resolved include: how to avoid interference between nodes and other RF sources; whether the network is scalable; how many nodes can be supported; whether nodes can be added in an ad hoc manner without reconfiguring the rest of the network; what bandwidth is required; how the power consumption can be minimized; and what microcontroller resources will be needed.Mesh Networks Overly Complicated
Creating a mesh network is one of the ways to build a WSN. Mesh networks are touted as the best way to maximize the potential of ultra-low power wireless sensors where every node can communicate with many (or even all) of its neighbors in a self-managing and healing topology.Unfortunately, while mesh networks make for compelling academic debate, in commercial implementations with even a modest number of nodes they invariably prove difficult to set up and introduce a level of complexity that can't be justified for almost all contemporary practical applications. Engineers quickly conclude, often after a lengthy development program, that mesh networks are overly complicated, demanding lots of computing resource and electrical power, and are expensive. Fortunately, 99.5% of all envisaged WSN applications can be designed without a mesh, eliminating the need to waste time grappling with the challenge.Virtually all practical networking problems can be resolved using a simple pre-determined structure comprising two to several dozen nodes at most. The simplest of these is the peer-to-peer network where only one node communicates with another. The simplest application example of this peer-to-peer networking is a humble switch controlling a light. A more complicated wireless network will comprise several peripheral nodes talking to a single receiving node.Consider, for example, a cyclist wearing a sports watch (node 1) where node 2 is a GPS tracker, node 3 is a speed indicator and node 4 is a heart rate monitor all communicating simultaneously with the sports watch node via their own dedicated channels, A, B, and C (Fig 1). This type of network is often referred to as star network because it features a central hub that can be schematically shown communicating in a star-like fashion with peripheral nodes.
Importance of Protocol
Practical wireless networks should be low-cost, immune to interference from other radio sources (including neighboring nodes), reliable, and perhaps most importantly, consume little power. The last thing a user wants is a reliable network compromised by the need to change batteries every few days.Each node requires a silicon radio allied to a microcontroller, often referred to as the physical layer (PHY), forming the hardware that drives the node. Some 2.4GHz radios integrate the radio and microcontroller into a single chip. The PHY supports a protocol stack and an application layer that forms the specific instruction set for the application supported by the network.The protocol is perhaps the most vital element in ensuring the practical wireless network performs to expectations. It determines how the node communicates across a wireless link with other nodes by establishing standard rules for co-existence, data representation, signalling, authentication and error detection. One way to compare the various offerings from wireless communications companies is to consider the protocol's efficiency by comparing a packet's ratio of overhead (information required to set-up the communication with a specific node and to determine how the information will be reliably sent) to payload (the actual useful data). A high ratio of data to overhead means the time that the radio transmits (when the power consumption is highest) is shortened and hence the radio can go back into ultra-low power sleep mode faster.There is a bit more to it than this though; the key is the bandwidth and hardware efficiency of the radio itself allied with how this is managed in terms of the physical layer efficiency when communicating. The bandwidth of the radio broadly correlates to how much time the radio will need to spend transmitting in a relatively high power "on" mode for a given amount of data. Theoretically, the wider the bandwidth, the faster the transmission and the less time the radio will need to spend out of sleep mode. In the real world, bandwidth costs power and the optimal trade-off point is generally considered to be 1Mbps before the added power losses begin to outweigh the gains.But all these radio hardware efficiency savings can be swept away in an instant by a flabby physical layer efficiency. Power consumed by the radio when on will have the biggest effect on overall power consumption because this will usually be an order of magnitude higher than the power consumed by the radio when off (although this figure is important too due to the amount of time a radio will spend in this state). The problem is that the radio will diligently transmit what it's told to transmit by the protocol. Unless the data is packaged in a way that optimizes off time per bit of data sent, the proportion of the time the radio spends "on" will rise significantly. The real challenge, therefore, is to maximize the amount of time the radio spends "off" in minimum power sleep mode.WSNs are characterized by small amounts of data sent occasionally. Usually, if data is sent but occasionally not received this is not a problem because updated data follows soon after. This technique is suited to sensor applications and is field proven as the most economical method of operation. However, if it's essential that every piece of data is received, the protocol should include instructions for the receiving node to return an acknowledgement that the message was received successfully. If the acknowledgement is not received, the message is resent.
Building Practical Infrastructure
Identifying a good silicon radio and efficient protocol is only part of designing a practical wireless network. Whether the network is going to comprise two, ten or a hundred nodes, the biggest challenge is linking those nodes into a reliable, scalable network.The key to this is to choose a technology where at the physical connection layer all nodes have equal functionality, so are capable of acting as clients or masters within a practical wireless network and can swap roles at any time. In other words, the nodes should be able to perform as transmitters, receivers or transceivers in order to route traffic to other nodes.
In addition, every node should be capable of determining the best time to transmit based on the activity of its neighbors, eliminating the need for a network restricting coordinator or supervisory node. This combination of features means it's easy to add another node to a network, of any topology, in an ad hoc fashion. There is no need to plan in order to consider what type of node will be required to extend the network, or to make provision for a coordinator node to tie the network together when it reaches a certain size. Setting up a network to perform a practical function requires more than establishing a network of nodes that can communicate with each other. The nodes need to be configured to perform a function such as measuring temperature, humidity or heart rate. The configuration for each node to perform this function can be made considerably easier by selecting the appropriate technology. In operation, the sensor is configured at start-up with the Flash memory storing the sensor profile and the relevant sensor communication protocol. An application host MCU isn't required, further cutting system cost, power consumption and size.Ultra-low power is essential for a practical wireless network because the coin cell batteries powering the nodes need to last for months or preferably years to minimize maintenance. Let's look at some typical numbers for a proprietary technology.For an application sending 8 bytes of data once a second for an hour a day, battery lives of the transmitter and receiver are 6.4 and 5.6 years respectively. Note that battery life is heavily dependent on the application and this example is a low usage case. Fig 2 shows how battery life varies with messaging frequency for a particular use case. For instance, in an industrial setting the sensor may be required to be in use 24 hours a day with a message period of, typically 0.5Hz. The transmitter battery life would then be 7. 2 months and the receiver life would be 6.3 months. This compares very favorably with a commercially available ZigBee solution. Battery lives for ZigBee transmitters and receivers in the industrial application described above would be 8 to 10 weeks.Crowded Environment
Wireless sensor networks, in keeping with many contemporary 2.4GHz technologies, operate in an increasingly crowded part of the radio spectrum. Network nodes will have to compete with Wi-Fi, Bluetooth, cordless phones and each other when trying to get their message through. However, network nodes do have one big advantage: they don't have to transmit very often, and when they do, it's for a very short time. Nonetheless, an interference avoidance strategy is vital. Some common techniques for minimizing the impact of interference for devices operating in the 2.4GHz band include time slot allocation schemes such as direct sequence spread spectrum (DSSS) or frequency hopping spread spectrum (FHSS).Both DSSS and FHSS work well, but require the transmitter and receiver to be synchronized. In the case of FHSS this is to ensure the devices are tuned to the same narrow band simultaneously, and for DSSS so that the de-spreading by the same pseudo-random sequence used to spread the signal in the first place works properly. Synchronization adds complexity to the network and increases power consumption. Although synchronization can be switched off to save power when communication isn't needed, re-acquisition can take several seconds and uses even more power.
Ubiquitous Wireless Connectivity
WSNs have the potential to make wireless connectivity ubiquitous and truly unleash the full power of the wireless revolution in myriad applications, where most of which have yet to be conceived. However, this revolution won't start unless networks become a lot simpler to set-up, maintain and scale.
Copyright © 1995-2007 Nikkei Business Publications, Inc.
No comments:
Post a Comment