The sink is a powerful workstation with plentiful resources whereas sensor nodes are low-power devices which have limited resources, memory space, computational capability and communication bandwidth. Desirable functions for sensor nodes include: ease of installation, self-identification, self-diagnosis, reliability, time awareness for coordination with other nodes, some software functions and digital signal processing (DSP), and standard control protocols and network interfaces. IEEE 1451 is the set of smart transducer interface standards developed by the Institute of Electrical and Electronics Engineers (IEEE) which meets these requirements.
As mentioned earlier, high degree of quality of service (QoS) or fidelity is important for sensor networks. QoS may be measured as percentage of the environment that is actually covered by active sensors. In a typical surveillance application, at least one sensor must remain active within one sub-region of the network so that the intruder may be detected with high degree of probability. Since the main function of data sinks is acquiring the description of the environment rather than receiving all raw data collected by individual nodes, the throughput is less meaningful. This means, during operation, delay may be either much more or much less important.
Digital processing. The sensed data is fed to the processing unit, which usually has a small storage unit, for processing before transmission. This is referred to as ‘in-network processing’ during which redundant, useless and spurious data are deleted and observations from different sources are combined and aggregated. In-network data processing is a must to decrease the large volume of raw observations per sensor and to reduce the number of broadcasts. The data processing and storage capacity of the sensor node is limited.
Radio transceiver. A transceiver sends collected data via radio transmitter to sink node either directly or through intermediate sensor nodes. Sensor networks have short transmission range (up to 10 metres) and low data rate (several bytes). They may not have a globally unique ID.
Radio transceiver is the dominant power consumer. Digital radio transceivers for cellular communication backhaul and ground terminal transceivers for very small apertures terminals (VSATs) already employ m-band MMICs. Most VSATs operate in Ku band (12GHz to 18GHz) but, in the future, will be moving to Ka band (26GHz to 40GHz). However, most of the excitement lies in E-band (60GHz to 90GHz) which is meant for point-to-point WLANs and broadband Internet access. Point-to-point wireless can replace fiber-optic communication. Active antenna arrays and radar transmitters at W-Band, especially 94GHz, offer superior performance through clouds, fog and smoke.
Collision and congestion
If more than two nodes attempt to send data to the same destination at the same time, the destination node my fail to receive the data. This is called congestion. Congestion in wireless networks is of two types: radio collision and buffer overflow. Congestion is very dangerous for WSNs. Collision has the following drawbacks:
1. Increases energy dissipation rates of sensor nodes
2. Causes a lot of packet loss which in turn diminishes the network throughput
3. Hinders fair event detections and reliable data transmissions