Physical Layer for Wireless Sensor Networks

The main concern of the physical layer is modulation and demodulation of digital data, i.e. transmission and reception of the data. This is done by the transceivers in the sensor nodes. The main functions of physical layer are carrier frequency selection and generation, encryption and decryption, modulation and demodulation, transmission and reception of data.

Wireless sensor network generally work in ISM bands. But many other standards like 802.11b and Bluetooth also use the same band, so all systems, in this band have to be robust against interference form other systems.
The most important parameters which are to be considered while designing Physical layer in wireless sensor networks are
1. Low Power Consumption.
2. Low Transmission and Reception range.
3. Interference from other systems, working in the same band.
4. Low complexity.
5. Low duty cycle, i.e. most of the time sensor nodes are switched off.
6. Low data rates most of the time and high data rate only for a short period of time.
The most challenging aspect in physical layer design for sensor networks is to find, low cost transceivers which consume less power, simple modulation schemes which are robust enough to provide required service.

Generally the transceivers used in sensor network are only 10% efficient. To radiate a power of 1 mw the transceivers will consume at least 10 mw of power. The power consumed for reception is some what similar to power consumed for transmission; some times one of them may be more and one less depending upon design of the transceivers. For Mica motes 21 mw is consumed in transmit mode and 15 mw is consumed in receive mode. The transmission and reception of data are the most power consuming activity in the sensor node. So transmission and reception of data should be kept as less possible. For the WIN nodes, 1500 to 2700 instructions can be executed per transmitted bit, for the MEDUSA II nodes this ratio ranges from 220:1 up to 2900:1, and for the WINS NG nodes, it is around 1400:1. The summary is computation is cheaper than communication.

The power consumed by the transceivers in idle mode will not be significantly less than the power consumed by transceivers in receive or transmit mode. So it is always preferable to put the transceiver in sleep mode rather than in idle mode when not required. But care has to be taken to see to that, the power consumed during start up and time taken to startup the transceiver does not over come the advantage of putting the transceiver in sleep mode. The most commonly used transceiver is CC2420.
The choice of modulation scheme is another important aspect to be considered here. The choice depends upon the complexity that can be supported by the node, as one cannot go for very complex processes in sensor networks. The data rate is another factor to be considered, as this tells about what amount of data can be transmitted or received when the node is on. To save more and more energy the sensor node should be in sleep as much as possible and more the data rate more amount of data can be transmitted and received in small active period. The symbol rate is yet another important factor, as it has be found that more is the symbol rate more is the power consumption. The BER has also to be considered, because to keep BER low the power radiated has to be more.

As an example we shall consider the most commonly used protocol for WSN, IEEE 802.15.4. IEEE 802.15.4 defines wireless personal area network standard for low rate devices, this specifies the standards for Physical and MAC layer. 802.15.4 Works at a data rate of less than 250kbps. It works up to a range of 75m and it supports up to 254 nodes. This protocol is designed specially for networks whose data rate is less, which have energy constraints, and which require good QoS. The network using this protocol can survive from 6 months to 2 years with two AA batteries.

802.15.4 Works in three frequency bands they are 868 MHz (20 kbps data rate), 915MHz (40 kbps data rate) and 2.4GHz (250 kbps data rate). It uses Direct Sequence Spread Spectrum for modulation and uses BPSK for first two bands and 16 bit array QPSK for the last band. The first band has 1 channel in it the second band has 10 channels in it and third band has 16 channels in it. This can be seen from the diagram below




Channels in 802.15.4 physical layer

Communication Protocols for Wireless Sensor Networks

The protocol stack used in wireless sensor networks is as shown below. It consists of physical layer, data link layer, network layer and application layer. 802.15.4 is an IEEE standard for physical and data link layers. The ZigBee protocol specifies standards above MAC and physical layer. However other than these standards, there are many other protocols and papers defined for MAC, Network, Application layers. We shall brief them in this section.



Protocol Stack of Wireless Sensor Network.

QoS in Sensor Networks

Quality of service required for different applications in sensor networks can be different. Some may require more precise and accurate information some may require less. Some applications might not tolerate any delay in notification after the detection of event and some may tolerate the delay. Some applications may monitor a large area with more number of active sensors and some applications might monitor only small area with less number of active sensors.

In sensor network the importance is on collective data delivery rather than packet form a single node, because in sensor network many sensor nodes produce similar data. So in sensor network terms such as collective latency, collective bandwidth usage, collective packet loss, collective information throughput come into picture.
Collective latency is the time between, the first packet related to an event is received by the sink and last packet related to that event is received at the sink, no matter which node produced the data. Collective bandwidth is the bandwidth required to report an event to the sink by sensor nodes al together. Collective packet loss is the number of packets lost al together during reporting of an event.
Any QoS mechanism introduced for WSN should be very simple, it should not be, computationally complex, time consuming, requiring large amount of memory. Because the sensor nodes are energy, memory constrained and sensor nodes do not use very high end processors also.

QoS support in sensor network should consider the fact that, event driven sensor networks are characterized by long periods of silence, when there are no events detected and small periods where large amount of traffic is produced, when any event is detected. And all the data produced are generally destined to sink, so traffic form nodes to sink will very large. So the QoS has to support this unbalanced traffic.

The probability of having redundant data is quite large in sensor networks, so the redundant data should be reduced as much as possible may be data aggregation. But one has to consider the delay involved and complexities involved in data aggregation and take a decision.

Sensor networks are very dynamic; there are high chances of, node failures, link failures, node mobility and node state transitions. So QoS support for sensor network should consider all the above factors.

The number of sensor nodes in sensor network is very large, probability of addition of nodes is also quite high, and so any sensor network application should be scalable.There are different types of data supported by the sensor networks QoS provided should support all different types of data.

Connection of Sensor Network with TCP/IP Network , Article -2

Continuing from the previous article ........

Unifying Micro Sensor Networks with the Internet via Overlay Networking is another paper, which proposes to use sensor network overlay over TCP/IP networks. By using such an overlay sensor network can use data centric approach, i.e. assigning unique address to each and every senor node is not essential. The sensor network overlay is formed by extending address less sensor network routing into internet as application overlay. The figure below shows the Sensor network overlay on TCP/IP networks.



Sensor Network Overlay on TCP/IP network.

The nodes in internet with Sensor network overlay are called as virtual sensor nodes, and these help in forming virtual sensor network.
TCP/IP overlay over Sensor Network paper proposes to use TCP/IP stack in the sensor nodes.
The key advantage is that it makes WSN highly compatible with internet as both them will have almost similar protocols running. However this TCP/IP stack can be loaded into sensor nodes which have higher memory and higher processing capabilities. The authors have developed something called as u-IP stack. This is similar to normal TCP/IP stack but with lesser features. The memory required to load the stack is also less compared to full TCP/IP stack. The figure below shows the TCP/IP overlay on WSN.




TCP/IP Overlay on WSN

In yet another paper called “VIP Bridge Method” authors propose to use node centric or location centric WSN. It is not applicable for data centric WSN. This paper uses a VIP Bridge it does mapping between IP address and location address or local address of the sensor node. The sensor nodes will not have IP address configured, but the IP address to each node exists in VIP Bridge. The sensors will be identified in the WSN by the local identifiers the VIP Bridge does the conversion require. The VIP Bridge does translation form sensor network to TCP/IP and vice versa. This paper explains the method to do the conversion. The advantage of this VIP Bridge method is that communication is done between TCP/IP network and WSN using IP address itself and the local sensor identifier is used for communication within the senor networks. The internet users need not know the protocols used in WSN they can use WSN as if it is a normal TCP/IP network. In other words WSN is transparent to internet users. The architecture of the VIP Bridge method is as shown below.




VIP Bridge Architecture.

Connection of Sensor Network with TCP/IP Network, Article - 1

There are many situations where sensor networks need to be connected via TCP/IP network. In other words it might be necessary to integrate many sensor networks into one virtual sensor network using TCP/IP network.
To allow users all over the world to use a WSN deployed in some remote place it is necessary to connect WSN to TCP/IP network. The users may issue a query to get information form WSN or it might be necessary to issue a command or some information to a particular node in WSN. The particular node might be one of several higher capability nodes in the WSN.

So connecting WSN with TCP/IP network is very important but due to sensor node’s limitations it is not possible to install full existing TCP/IP stack into a sensor node. So to connect WSN and TCP/IP network many new methods are proposed.

Different authors propose different methods to connect WSN and TCP/IP networks. In “Integrating Future Large-scale Wireless Sensor Networks with the Internet” application level gateway is used to connect WSN and TCP/IP network. This paper classifies WSN as homogeneous and heterogeneous WSN. It provides solutions to connect both homogeneous and heterogeneous networks to TCP/IP networks. Homogeneous sensor network is one in which all sensor nodes have similar capabilities and heterogeneous sensor network is one in which some node have more capabilities than other nodes and outside user wants to interact with the more capable nodes directly. To connect homogeneous sensor network to TCP/IP networks this paper proposes to use a gateway as shown in the figure below. Gateway Based Approach for connecting Sensor Networks with TCP/IP Networks

The gateway is viewed as front end for the distributed database. The user who wants information form WSN may issue query to the gateway. Query optimization is provided through data centric in network processing. The response is obtained from WSN by the gateway and reply is sent to the user. The gateway acts as the interface between WSN and internet.
The advantage of this approach is both WSN and internet can be completely independent; it’s the headache of gateway to do conversions required. And it is most easy approach as very less amount of changes are required to the existing architecture; a gateway need’s to be added in between internet and WSN. The drawback of this approach is that ever gateway has to be designed according to the WSN. And as we see this has single point of failure, i.e. if gateway goes down WSN becomes inaccessible. To overcome the above said problem we can use two or more gateways also.

To connect heterogeneous WSN to TCP/IP network the paper proposes to use an IP overlay over the WSN. In the heterogeneous network some nodes have more capability or some nodes may be acting as cluster heads. These nodes can be given IP address, and these nodes may be directly accessed by the user using internet. In this type of network an IP overlay network is created over the underlying WSN to connect user directly to higher capable nodes. To create an IP overlay network it is necessary to create a tunneling mechanism to allow the more capable nodes to communicate among themselves using address centric approach. To create tunnels between the more capable nodes the paper proposes to use either Directed Diffusion or ACQUIRE protocols. Directed Diffusion can be used for high IP traffic applications and ACQUIRE can be used for low IP traffic applications. The figure below makes the concept clearer.

Heterogeneous network, the red line shows the tunneling formation using Data Diffusion or ACQUIRE protocol.

The red color line in the figure above shows the tunnel between the higher capable nodes using normal sensor nodes. This tunnel makes IP overlay possible over existing WSN architecture. Once the tunnel is established it will be possible to the users to interact with the higher capable nodes directly using existing WSN architecture itself.

A paper called “A Delay-Tolerant Network Architecture for Challenged Internets” proposes to use a method to integrate the challenged networks. Challenged networks are those which have a very long end-end delay, low bandwidth, high rate of network portioning, high error rates. Sensor network can be considered as a challenged network so this paper is applicable to the sensor network also. This paper uses an application layer above transport layer in TCP/IP stack as well as in WSN stack called the bundle layer. This layer is responsible to store and forward packet. This is shown in the figure below.

Connecting WSN and Internet using DTN gateway

It looks similar to gateway approach described before, but it has several dissimilarities with the application level gateway approach. The DTN gateway is universal to connect any network, it is not application based. It takes into account many aspects of different challenged network but application level gateway fails to do so. One region itself can have two or more DTN gateways, so even within a region if partition takes place DTN gateway based approach works where as the earlier one fails.

The network is divided into different regions. One region itself has many DTN gateways. The DTN gateways communicate with each other. Initially routing is done based on the region address using the bundle layer in the DTN gateways. Once the packet reaches the destined region the address of the host is used to reach the host in the destination. The address format will be (region address, host address) host address can be of any type. So this architecture works for any network. This paper speaks about integration of all challenged networks and TCP/IP network. The figure in the next page shows DTN based architecture.

DTN based Architecture

The advantage of this architecture is that it can be used to connect any type of network, and it supports more than one DTN gateway in only one network patch. The DTN gateway takes care of interface between different networks. So every network can be designed independently. The disadvantage of this architecture is the extra bundle layer that needs to be installed.

This Article will be continued in the next post ...

Fault Tolerance in Sensor Networks

The probability of failure of sensor nodes in sensor networks can be very high in some applications. So it very necessary to design the sensor networks such that, the failure of sensor nodes do not affect the proper functioning of network. Much importance has been given to make sensor network fault tolerant and reliable.

In a paper called “Fault tolerant clustering of WSN” fault tolerance of WSN is increased using detection of failed gateways, and re associating the senor nodes attached to the failed gateway with another existing gateway. Periodic checking for failure of gateways is done. If a failure is found, all sensor nodes attached to the failed gateway are reattached to a new gateway. This method prevents re election of the gateway, and increases reliability of wireless sensor networks.

In “Prolonging Sensor Network Lifetime with Energy Provisioning and Relay Node Placement” authors propose to provide extra energy to the AFN (Aggregating and Forwarding Node) similar to gateway. As a result of providing extra energy, failure of gateways can be prevented hence sensor network become more fault tolerant. The approach also proposes to use extra relay nodes after AFN and provide extra energy to the relay nodes also. The SPINDS algorithm proposed in this paper tries to solve the joint problem of relay node placement and energy provisioning.

In another approach called “WSN with Guaranteed Capacity and Fault Tolerance” authors propose deployment of extra nodes form a robot helicopter. These extra nodes are deployed to make wireless sensor network fault tolerant. All the nodes will not be awake simultaneously. Redundant nodes will be sleeping and wake up only when an active node dies.

In a patent called “Fault Tolerance in WSN” authors try to make WSN fault tolerant by the usage of secondary and tertiary gateways along with primary gateway. The primary gateway synchronizes the secondary and tertiary gateway with itself. If the primary gateway fails, secondary gateway takes up its place and if the secondary gateway fails tertiary gateway take up its place. This increases fault tolerance of WSN.