Multimedia Sensor Node Hardware Architecture

A multimedia sensor node device consists of a number of basic components, including sensing unit, central processing unit, communication subsystem, coordination subsystem, memory, and optional mobility/actuation unit, as depicted in Figure 1.8.

The major components of a multimedia sensor node have been described below in detail:

 Sensing Unit: As already explained in Section 1.2.2., a sensor node is capable of sensing scalar data. Going a step further, the sensing unit in a multimedia sensor node is capable of sensing not only scalar data but also video and audio streams. A sensor in a multimedia unit consists of two subunits: a sensor, which may be in the form of a camera or audio and/or scalar sensor, and an analogue-to digital converter (ADC). While the audio sensor captures sounds in the sensed event, the camera captures still or moving images. The typical resolutions for the camera are in terms of pixel/inch, and for the audio sensor in DB. The function of the ADC is to convert the analogue signals produced by the sensor, on the basis of the observed phenomenon, into digital signals.

Figure 1.8 General Hardware architecture of multimedia sensor node (Akyildiz, Melodia, and Chowdhury 2008)

 Central Processing Unit: As mentioned in section 1.2.2, the Imote2 uses Intel‟s processor, Intel PXA271, which is the central processing unit (CPU) of Imote2, working as the principal controller of this wireless sensor node. However, the main controller of a multimedia sensor node also the system software. For example, the Stargate board, designed and produced by Intel and Crossbow respectively, uses Intel‟s PXA-255 XScale RISC processor operating at 400 MHz, with 32 Mbyte of flash memory and 64 Mbyte of SDRAM. This CPU, interfaced with a memory, coordinates the sensing and communication tasks by executing the system software in charge of this function. On the whole, compared to a scalar sensor node, a multimedia sensor node is substantially more powerful, and capable of sensing and retrieving audio and video streams.

 Memory Unit: The memory unit of a multimedia sensor node usually consists of both flash memory and RAM. For example, the Stargate board has 32 Mbyte of flash memory with 64 Mbyte of SDRAM (Sharif, Potdar and Chang, 2009). The flash memory contains the program code for the multimedia node while the RAM stores any information or data required for computation. Some of the memory units also have non-volatile storage for off-line data capture, to be retrieved later.

 Power Unit: As mentioned in Section 1.2.2, the power unit of a sensor node is the most important part of the hardware as it powers the whole system. It is supported by an energy generating unit, such as a battery of solar cells. The sensing arrays, such as the CCDs, or the

multimedia sensors (Akyildiz, 2007), such as the CMOS image sensors, generally require a lot of power. The amount of power consumed in the sensing subsystem of a multimedia sensor node is considerably higher than in an ordinary, scalar sensor. For example, a scalar temperature sensor consumes 6μW in sensing the environmental temperature. However, for image capturing, Cyclops consumes 42 μW, CMU-Cam consumes 42 μW and High-end PTZ camera consumes 1W (Akyildiz, Melodia and Chowdhury, 2008).

 Communication Unit: The device is interfaced to the network through a communication subsystem which consists of a transceiver and communication software, including the communication protocol stack and the system software, for example an operating system and a middleware.

 The Coordination unit: The functioning of the different network devices is coordinated by a coordinating subsystem, carrying out operations like location management and motion control.

 Mobility Actuation unit: An optional unit in multimedia sensor node, this unit enables the node to move or manipulate the object, as per the requirements. A scalar sensor node has no actuator.

WMSNs use their various internal nodes to harvest not only scalar data, such as humidity, temperature, air pressure, light intensity, and various acoustic data, from the environment, but also multimedia information, such as audios/videos and digital images (Akyildiz, Melodia and Chowdhury, 2007). Thus, a WMSN has the imager as its main sensor, e.g. SparkFun CMUcam4, which can be seen in Figure 1.9 a. The main processor in SparkFun CMUcam4 is Parallax P8X32A (Propeller Chip) which is connected to an OmniVision 9665 CMOS camera sensor module, with a VGA resolution (640x480) RGB565/YUV655 color sensor, and an Onboard Image Processing system (QQVGA 160x120). The CMUcam open-source programmable embedded color vision sensors are low-cost, low-powered sensors, meant for mobile robots (Xiao, 2006). The Stargate 2, as shown in Figure 1.9 b, is a small form factor (3.5” × 2.5”), designed and produced by Intel and Crossbow respectively. It has Intel‟s PXA-255 Scale 400 MHz RISC processor with 32 Mbytes of flash memory, 64 Mbytes of SDRAM, and an on-board connector to connect it to

Crossbow‟s MICA2 mote, in addition to PCMCIA Bluetooth or IEEE 802.11 cards. It also has high processing ability along with more on board resources.

The visual data handled by a WMSN puts its sensor network under severe constraints. Collecting, processing and disseminating visual data is a process intensive activity and requires high bandwidth. However, WMSNs boast of many novel features as they have sensor nodes with video cameras, as well as high computation abilities.

(a) (b)

Figure 1.9 (a) SparkFun CMUcam4 (Xiao, 2006) b. Stargate (Akan, Pascal Zhang and Qian Jayant, 2008)

This section described multimedia sensor node hardware architecture. The next section will shed light on some applications of WMSNs.