1. Wearable Computers and Wearable Wireless Networks
1.1 A wearable computer is a novel mobile computing system concept that originated in the 1960s. Since the 1990s, rapid advances in integrated circuits have pushed wearable computer research into a new stage. There is no universally accepted definition. In general, a wearable computer should have these characteristics: it can be used while moving; it allows hands-free operation or simultaneous manual activity; the user can exercise control; and it supports persistence and diversity, meaning devices for different applications can vary in composition and function. Compared with traditional computers, wearable computers are more tightly coupled to the user, which requires new human-machine interaction techniques to achieve harmonious integration. Human-machine interaction is a current research focus and a key challenge for wearable computing.
Designing wearable devices: a technical overview of wearable wireless network solutions
1.2 In the wearable computer systems described above, most hardware devices are connected via communication cables and the operating platforms are wired. Although suitable in some scenarios, wired systems have clear drawbacks: they lack flexibility because hardwired connections restrict the user’s range of motion, which is unacceptable in applications such as sports. They also do not consider convergence with existing personal communication terminals such as mobile phones, PDAs, and MP3 players, which can leave those terminals functionally overlapping yet physically separate from the wearable computer.
To address this, the concept of a wearable wireless network is proposed. Compared with wearable computers, wearable wireless networks have these characteristics:
- Improved flexibility.
- Full consideration of integration with existing personal communication terminals.
- Distributed control, since each communication terminal has computation and storage capability (and these capabilities will grow with advances in large-scale integration).
- Greater user-orientation: users can choose the primary communication device that controls the network according to application, convenience, and preference.
Thus, wearable wireless networks offer better applicability and broader prospects than wearable computers and can be developed on top of existing wireless LAN technologies. Because those technologies are relatively mature, development difficulty and time-to-market are reduced compared with developing standalone wearable computers.
2. System Model of Wearable Wireless Networks
2.1 Network model. Currently, technologies suitable for building wearable wireless networks include Bluetooth and ZigBee. Both belong to the IEEE 802.15 family and operate in the ISM bands. Their differences are summarized in Table 1-1. Because their network formation methods are similar, the wearable network models based on them are largely the same.
(1) Structure of a wearable wireless network built with Bluetooth or ZigBee
Bluetooth and ZigBee support three topologies: star, peer-to-peer, and hybrid. A hybrid topology is suitable for wearable networks, as shown in Figure 2.1. It forms a loose Ad Hoc network, which fits wearable use cases.
Figure 2.1 Wearable Bluetooth network model
The network is organized as a two-level hierarchy. The Level I piconet master connects the masters of multiple Level II piconets; those Level II masters act as slaves within the Level I piconet. Each piconet is managed by its master. To ease overall management, a Level I master can act as a Local Access Point (LAP) connecting to external networks; this LAP does not serve as a master or slave in Level II piconets. There can be more than one LAP, but whichever device acts as gateway should follow the network formation principles above.
(2) Wearable wireless network devices
Devices in such networks fall into two basic types by role:
Sensors or other terminal devices: these devices only need to send collected data to a master and do not handle network management. Examples include sensors, detectors, headsets, and cameras.
Network interconnection devices: these forward terminal data and manage Level II piconets while forming higher-level networks among themselves. Therefore they require storage and processing capacity. In theory any master could act as an LAP, but in practice a gateway LAP is usually a member of the Level I network responsible for Level I management only, avoiding overloading a single device with multiple roles. Examples of interconnection devices include PDAs, GPS units, head-mounted devices, MP3 players, mobile phones, and PTT (Push To Talk) devices; mobile phones can also serve as gateways to external networks.
2.2 Service principles
Regardless of the wireless technology used, wearable wireless networks should provide basic services and management functions, including internal data transport, web-based services, and network management.
(1) Internal data transport
Internal data transport is the network's primary service. Level II piconets and the Level I network may use different transport modes. In Level II piconets, terminals such as sensors typically use point-to-point transmission to their master, since most Level II terminals only need to send data upstream and direct routing between terminals is uncommon. In the Level I network, masters may use point-to-point or point-to-multipoint transmission. If slaves require peer-to-peer communication, they must leave the original piconet and form a new piconet.
Clearly, intra-Level II terminal-to-terminal data exchange must be forwarded by the master. To send data between terminals in different Level II piconets, forwarding via the Level I LAP is required. For external network access, data can be forwarded through the gateway LAP or through virtual connections.
(2) Web access
Typically only Level I devices need and can support Web access. Web access should be provided through a gateway to consider network security. After establishing a connection with a Web site, client-server models can provide services such as FTP, email, audio and video. Voice services can use VoIP technologies.
(3) Network management services
Network management maintains reliable operation. Logical addresses are typically assigned to devices for connection and management. Before wide IPv6 deployment, using real IP addresses as logical addresses is difficult; gateway devices should have real IP addresses to support Web access. Gateways could act as DHCP servers if needed, but that increases their load. Wireless technologies' native addressing schemes may also be used.
To monitor device activity, broadcast messages can be issued to the entire network or to specific Level II piconets. Responses can be used for diagnostics and to alert users. Targeted broadcasts containing a specific device address are used when a sender needs to establish contact with that device.
3. Core Technologies for Wearable Wireless Networks
Although wireless LAN technologies are mature enough to form the basis of wearable wireless networks, several key challenges must be solved to apply them effectively. These core technical issues include:
3.1 Network design
Network design must ensure system functionality and stability. Compared with wired systems, wireless wearable networks are more flexible in topology. Design should also consider human body shape and behavior. A good network structure should be easy and comfortable to wear, lightweight, portable, mechanically robust, and simple to use, particularly regarding operation and display readability.
3.2 Input and output devices
Input/output devices are critical human-machine interfaces for wearable wireless networks. Interaction flexibility and convenience affect system function and adoption. Input devices include user-operated devices such as handwriting pads, keyboards, and microphones, as well as cameras, GPS, and sensors. Inputs should support voice, data, and control functions. Output devices include headsets, displays, and haptic devices; outputs should present information and reflect user intent. Devices required vary by application and should be configured accordingly.
3.3 Multifunction integrated devices
Many devices needed by wearable networks exist on the market and some functions are already integrated, such as headsets and touchscreens. Most lack wireless interfaces and require integration and enclosure redesign for wearability. Devices performing multiple network roles require higher integration. For example, a phone used as a gateway must provide telephony, manage the wearable network, and forward data, which is challenging.
Higher integration and miniaturization better meet wearable network needs, dependent on continued progress in very-large-scale integration.
3.4 Operating platform
An operating platform should match wearable hardware and be developed per application requirements while providing management services for stable operation. Existing mobile OSes used in phones can be extended to shorten development and reuse applications, but they may be functionally limited. Alternatively, a dedicated OS based on Linux can be developed for strong targeting, though this faces greater development effort and fewer existing applications.
3.5 Network resilience
Resilience reflects the network's ability to adapt to the environment, mainly in connection reliability and anti-interference capability. Resilience largely determines QoS. Both Bluetooth and ZigBee implement measures to improve environmental adaptability: ZigBee and Bluetooth use DSSS and FHSS respectively for interference resistance; in error control, Bluetooth uses forward error correction while ZigBee uses error detection/retransmission. However, the proliferation of wireless devices operating in the ISM bands degrades the environment, so more robust measures are necessary.
3.6 Power
Devices in wearable wireless networks are typically battery powered. Average operating time per battery cycle depends on capacity, device power consumption, and power supply efficiency.
While many electronic components have become smaller due to integrated circuit advances, power supply technology has not advanced as much, so battery size and weight remain significant in mobile devices. Finding alternative energy sources is therefore important.
Until alternative energy sources are practical, improving battery life requires both hardware and software measures. Hardware efforts include increasing integration and using new components for low-power design. Software efforts include designing reasonable operating and power-saving modes and developing algorithms for adaptive transmission power control.
3.7 Security
Security is inherent to wireless networks. Mitigating risks requires user security awareness plus technical and management measures. Many wireless networks provide authentication and encryption, but these measures alone are insufficient; increasing security features increases device load and may reduce efficiency, creating a trade-off.
Wireless network security is discussed extensively in the literature and is not repeated here.
4. Applications of Wearable Wireless Networks
Wearable wireless networks can be seen as personal implementations of wireless LAN technologies. There are application needs across many fields.
4.1 Military applications
The military is a high-potential application area, especially for reconnaissance and intelligence search tasks. Systems for these applications typically include headsets, cameras, wearable GPS, and communication devices with display and storage so personnel can maintain communications, determine position, and conduct reconnaissance.
Figure 4.1 Illustration of a wearable wireless network applied to the human body
4.2 Industrial applications
In industrial production, when space or environmental constraints prevent wired detectors from operating, or when multiple detectors must work cooperatively and wiring is inconvenient, wearable wireless networks can provide convenience and enable timely data collection, processing, and transmission to improve inspection effectiveness.
4.3 Healthcare and assistive applications
Healthcare and assistive technologies are attractive application areas. Multiple sensors or medical devices can be placed on or in a patient to monitor vitals; patients requiring remote monitoring can use wearable sensors to transmit condition data. For persons with disabilities, wearable network devices can obtain information about the user or surroundings and provide assistance or alerts via audio or visual channels.
4.4 Everyday life applications
Many people carry multiple devices concurrently. Connecting these terminals with a wearable wireless network provides convenience, for example quickly downloading recommended music to a phone and transferring it to an MP3 player. Wearable networks can also assist travelers and adventurers with convenience or safety features.
