The use of imaging technology is exploding with the advent of low-cost mega pixel digital cameras and cameras inside cell phones. Cities are routinely rolling out cameras in high-risk areas to help deter crime or provide background on events. Webcams, or cameras attached to a personal computer, continue to grow in popularity and free services are expanding to support their use, such as Yahoo! Messenger. Wireless home monitoring and control products are hitting the market with wireless 802.11 cameras which require tethering to a power source but can send their images and video to a personal computer located somewhere nearby. Society is becoming more aware of digital imaging technologies and their advantages.
CMOS (Complementary Metal Oxide Semiconductor) is the technology popularly used to make computer processors such as the Pentium. As a substitute for CCD (charge coupled device) chips, CMOS imagers allow a camera with lower power consumption, lower signal-to-noise ratio, and smaller overall design. CMOS imagers have been on the market since the late 1990's, but have seen a spike in popularity as they have been adopted into cell phones since about 2002. With the huge volumes of cellular phones, the price and performance of CMOS images have been rapidly improving, and they are challenging CCD's for image quality.
A brief history of the IEEE 802.15.4 protocol development begins as follows: whereas IEEE 802.11 (WiFi) was concerned with features such as ethernet matching speed, long range (100 m), complexity to handle seamless roaming, message forwarding, and data throughput of 2-11 Mbps; WPANs (Wireless Personal Area Networks) are focused on a space around a person or object that typically extends up to 10 m in all directions. The focus of WPANs is low-cost, low power, short range, and very small size. The IEEE 802.15 working group currently defined three classes of WPANs that are differentiated by data rate, battery drain, and quality of service (QoS). The present invention concerns the last class. The first class, a high data rate WPAN (IEEE 802.15.3) is suitable for multi-media applications that require very high QoS. Medium rate WPANs (IEEE 802.15.1/Bluetooth) will handle a variety of tasks ranging from cell phones to PDA communications and have QoS suitable for voice communications. The last class, a low rate WPANs (IEEE 802.15.4/LR-WPAN) is intended to serve a set of industrial, residential, and medical applications. These applications have very low power consumption, a cost requirement not considered by the above WPANs, and relaxed needs for data rate and QoS. The low data rate enables the LR-WPAN to consume very little power.
The IEEE 802.15.4 wireless protocol is still in its infancy and is being rolled out primarily in applications such as sensors, interactive toys, smart badges, remote controls, remote metering, and home and industrial automation. The 802.15.4 protocol supports data rates of 250 kbps at 2.405-2.480 Ghz with 16 channels (world-wide), 40 kbps at 902-928 Mhz with 10 channels (Americas), and 20 kbps at 868.3 Mhz with 1 channel (Europe). The protocol supports automatic network establishment by the coordinator; a fully hand-shaked protocol for transfer reliability; and power management to ensure low power consumption. The wireless IEEE 802.15.4-2003 standard was approved in May of 2003 and was published in October of the same year. The standard is still under further development with 2 additional task groups, 802.15.4a and 802.15.4b continuing the development. Current areas of development (as of September 2005) include resolving ambiguities, reducing unnecessary complexity, increasing flexibility in security key usage, and considerations for newly available frequency allocations among others.
General requirements of sensor/control networks include that they can be quite large, employing 255 clusters of 254 nodes each (64,770 nodes); are suitable for latency-tolerant applications; can operate very reliably for years without any operator intervention; have very long battery life (up to several years from an AA cell); very low infrastructure cost (low device and setup costs); very low complexity and small size; and device data rates and QoS (Quality of Service, i.e., delay, jitter, throughput, and reliability) needs are low.
The IEEE 802.15.4 standard was developed to address the low power, low-bandwidth market; primarily focused on controls signals. In general terms, 802.15.4 is seen as one of the lowest-bandwidth wireless technologies available on the market today, and provides the corresponding benefit of long battery life. Presentations typically show the following:
TABLE 1TechnologyRangeData Rate802.15.4WPAN to WLAN<0.25 Mbps802.15.1 (Bluetooth)WPAN>0.1 Mbps; <1 Mbps802.11 (WiFi)WLAN>1 Mbps; <100 Mbps
Zigbee is a protocol layer that sits “on top” of 802.15.4, and seeks to establish an interoperability standard for many companies to adopt, and to enable a smarter network with intelligence. Zigbee, or 802.15.4, sits below Bluetooth in terms of data rate. The operational range of ZigBee is typically stated as 10-75 m compared to 10 m for Bluetooth. Zigbee uses a basic master-slave configuration suited to static star networks of many infrequently use devices that talk via small data packets. Bluetooth's protocol is more complex since it is geared towards handling voice, images, and file transfers in ad-hoc networks. Bluetooth devices can support scatternets of multiple smaller non-synchronized networks (piconets). It only allows up to 8 slave nodes in a basic master-slave piconet set-up. Zigbee nodes spend much of their time sleeping, but the protocol is optimized for quick wake up and response. When a Zigbee node is powered down, it can wake up and get a packet in around 15 msec whereas a Bluetooth device would take around 3 sec to wake up and respond.
Another way of looking at the various technologies and where 802.15.4 fits:
TABLE 2BluetoothNameGPRS/GSMWi-Fi 802.11b802.15.1Zigbee 802.15.4ApplicationWide Area VoiceWeb, E-mail,CableMonitoring andFocus& DataVideoReplacementControlSystem16 MB+1 MB+250 KB+4 KB-32 KBResourcesBattery Life1-70.5-5  1-7 100-1000+(days)Network Size  1  32 7255/65,000Bandwidth 64-128+11,000+72020-250(KB/s)Transmission1,000+ 1-100 1-10+ 1-100+Range (meters)Success MetricsReach, QualitySpeed,Cost,Reliability,FlexibilityConveniencePower, Cost
The wireless cameras available today use the high data rate 802.11b wireless technology and due to their high power consumption, typically require 110 or 220-volt “wall” power. These solutions suffer from several drawbacks. Some of these units are battery powered but require many batteries, such as 6 AA cells, and only work for a short time period, such as 2 to 4 hours, before exhausting the batteries. As a result of the many battery cells, these units are large and heavy. As a result of the power cords, placement and view are very limited, or power cords must be run making installation and movement inflexible. Current 802.11b wireless imaging transfer solutions require complex setup and configuration, as they are typically “IP Addressable” and connect to the internet via an 802.11b wireless ethernet connection. They must be managed and configured as if they are other computers on the internet. While current 802.11b wireless imaging transfer solutions may allow from 1 to N cameras within a local area network, these are not automatically configured for 1 to 254 nodes as with the 802.15.4 protocol. The 802.11b solutions are relatively expensive and run from $200 to $300 per camera. Finally, current 80211b wireless imaging transfer solutions do not allow the passing of messages from node to node, so are limited by their direct range from end node to hub.
Digital images typically require anywhere from 10 Kilo Bytes up to 2,000 Kilo Bytes of storage. VGA images are 640×480 pixels, or 307,200 pixels. Each Pixel typically has 2 bytes of data associated with it. Therefore, a regular VGA image will contain 614,000 Bytes of data. JPEG compression routines can compress this image 10:1; 20:1; 30:1; and more, down to under 20K Bytes. At a 30:1 compression, a 20K Byte image can be transported at 15K Bytes/Second in 1.25 seconds. Additional JPEG compression and information reduction routines can bring this image down to, say, 9K. However, each reduction in size will correspond to a reduction in the information content of the picture, and a reduction in the clarity of the resulting picture. Lesser quality picture standards, such as QCIF (176×144) or QQVGA (160×120) are available to reduce the initial image size, however, these will display on correspondingly smaller screens or views such as a cell phone screen. A VGA image displays as roughly a 6″×8″ image on a standard computer screen.
Processing techniques can “shrink” the image size at the expense of image quality. For a given image size, say 30 Kilo Bytes, a high bandwidth system can transfer images quickly; while a low bandwidth system requires more time. The IEEE 802.15.4 wireless protocol operates at several speeds: 240, 40, and 20 kilo bits per second. At 240 kilo bits per second, a 30 kilo byte image would take 1 second to transfer, and with overhead would take up to 100% longer, or 2 seconds. At 40 kilo bits per second, a 30 kilo byte image would take 6 seconds to transfer, and with overhead would take up to 100% longer, or 12 seconds. This is seen as very long latency. Since most image transfer applications require low latency (i.e., you hear a baby cry and want to immediately see the image) and since the IEEE 802.15.4 wireless protocol is considered low-bandwidth, in the past it has not been considered suitable for image transfer applications. Previously, any file size greater than an order of magnitude greater than the 802.15.4 packet size has not been feasible.
A Zigbee system includes several components. The most basic is the device. A device can be a full-function device (FFD) or a reduced-function device (RFD). A network should include at least one FFD, operating as the PAN coordinator.
A solution is needed that enables many, low-cost, low-maintenance, small, battery-powered cameras to be flexibly placed in a network configuration. These cameras should be light enough to be placed with adhesive, thus allowing the user to “peel n stick” the cameras in out-of-the-way places. These cameras should cost less than $50 a piece, compared to the $200-$300 802.11b solutions available on the market today. They should be self-configuring and announce themselves to the WPAN coordinator (FFD) that they exist. The software that runs on the desktop should capture the presence of each camera end node and show it's health with signal strength and battery life, so that setup and maintenance of the 802.15.4 network is absolutely simple. Wireless ethernet should not be required to arrange a network, but rather, the devices themselves should create their own network and the coordinator can be powered directly from the USB connection of a mobile lap-top computer, thus, a network can be established anywhere the camera RFD's can communicate with their FFD coordinator. This enables extreme mobility of the network. Finally, the cameras should provide for some kind of alert, either a beep or light flash, to indicate that they are capturing images to address general public concerns about being “watched”.
Thus a need exists to bring together CMOS imaging technology, the IEEE 802.15.4 wireless protocol, and control software to create a flexible, low-cost solution to information delivery using images.
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