Originally Published MDDI
Originally Published MDDI June 2004
Bluetooth and Beyond: Wireless Options for Medical Devices
As wireless technologies such as Bluetooth reach maturity, medical device manufacturers now have the option of designing without cables and wires.
William E. Saltzstein
Over the last several years, wireless technologies have made significant progress, and they are now being integrated into many mainstream applications. In particular, Bluetooth is now seeing increased use in a variety of medical applications ranging from home-healthcare devices to operating room equipment.
The choice of wireless technology needs to be made with an understanding of the major factors motivating a given product’s wireless needs. In addition, consideration to the new attributes that are inherent in wireless must be considered: usability, power, distance, data rates, and coexistence, for example (see Figure 1).
This article presents an overview of Bluetooth technology and a brief look at other major wireless technologies.
Wireless Technology Options
|Figure 1. Industry standard wireless technologies: applications and range (click to enlarge).|
Designers have alternative technologies available for wireless connectivity, and their choice is often based on the intended use of the device (see the sidebar “Wireless Technologies” on page 85). It is helpful to compare a few major characteristics to understand how to determine when to use which technology. Bluetooth, for example, offers the lowest power consumption of all of the networked technologies. It typically requires 1¼10 to 1¼5 the power of IEEE 802.11b solutions. Typical PDA implementations yield 6–10 hours of usage compared with 2–4 hours for 802.11 solutions using the same batteries. With Bluetooth, no IP addressing is involved, so it is relatively quick and easy to set up small networks of devices. Unlike IrDA, Bluetooth can be used in a small network.
When appropriately used, Bluetooth provides security that meets the needs of HIPAA for patient data. Security requirements in the approved IEEE 802.11 standards raise issues that have yet to be addressed in a standardized way. Unfortunately, many designs produce incompatibilities when connections are implemented between vendors.
Bluetooth has yielded to attacks only in products that have not implemented security appropriately or that leave security turned off by default. With the exception of IrDA, Bluetooth is the most cost-effective wireless technology currently available.
Like commercial applications, medical devices also face many development issues that can be addressed with wireless technologies. Bluetooth is particularly well suited for cable replacement, allowing for mobile connectivity. It also provides excellent security and reliability and coexists well with other wireless technologies. And, it is a relatively low-cost technology to implement. Several medical systems have been implemented successfully (see the sidebar “Bluetooth Medical Applications” on page 81 for a list of a few devices currently on the market).
Cable Replacement. Bluetooth was designed for cable replacement. It replaces relatively low-data-rate connections such as the traditional RS-232 used in many medical devices with data rates in the 9600–115,200-baud range. It provides a wireless connection and eliminates the need for expensive isolation and potentially hazardous cabling.
Connectivity. Using IEEE standardized technology (802.15.1), Bluetooth allows connection with off-the-shelf components and enables connectivity not only to personal-area networks (PANs), but also to local-area networks (LANs) and wide-area networks (WANs) through access points and cellular handsets. By using a standard interface, issues associated with incorporating quickly changing technology into a medical device are no longer a concern.
Power. Bluetooth offers considerable power savings compared with other wireless standards that are also designed for highly mobile, battery-powered devices. Power consumption is only 10–25% of that of 802.11b. For devices that do not continuously transmit data, Bluetooth provides several power-saving modes to decrease power further.
Security. With 128-bit SAFER+ encryption and authentication, the technology provides security. Controllable discovery and connectability modes control access. This is particularly important for safe connections needed for mobile data devices in the medical environment. The SAFER+ encryption has yet to be broken; however, successful attacks on devices with incorrectly implemented security policies have been reported. The security provided meets the requirements for medical device and patient information data.
Reliability. The frequency-hopping, spread-spectrum technology is highly tolerant of ambient radio-frequency (RF) noise and retains good bandwidth even in the presence of devices such as electrosurgical units and home appliances.
Coexistence. Testing performed to date shows low levels of interference with other technologies, including 802.11b, in the ISM band.
Bluetooth Technology Primer
This section provides a basic review and update on Bluetooth technology. An in-depth discussion of the technology was provided previously in MD&DI and in Medical Electronics Manufacturing.1,2
A basic Bluetooth radio covers 10 m in open air (although current implementations operate well in a significantly larger range). An optional higher-power version allows 100-m operation. Bluetooth is designed for moderate speed, with a total bandwidth of 1 Mb/sec, and a theoretical 720-Kb/sec payload, dividing the bandwidth between devices using data and voice channels. It can support eight devices on a piconet.
Bluetooth was developed as a result of lessons learned from many other wireless technologies. It borrows shamelessly from digital enhanced cordless telecommunications (DECT), Infrared Data Association (IrDA), global system for mobile communications (GSM), and others to enable it to provide relatively simple implementation with high levels of functionality and interoperability.
Bluetooth radio technology was designed to be highly immune to noise and easy to implement on silicon. Its major base-band portions are implemented in either hardware or firmware to optimize cost, power, and size. Some implementations incorporate the entire RF and base-band processing section on a single chip with few external components.
To achieve robust connections, Bluetooth employs several techniques: frequency hopping, adaptive power control, and short data packets. Frequency hopping enables secure and robust connections. The Bluetooth protocol (for data channels) automatically retransmits corrupt data packets (most likely having hopped away from an interfering source). The pseudorandom hopping sequence is designed to maximize frequency spacing between sequential channel hops.
Bluetooth, along with other ISM radio technologies, operates in the 2.4–2.485-GHz band, and its radio meets the set of power and spectral emissions specifications defined by ETSI ETS300-328 in Europe and CFR 47 Part 15 in the United States. Bluetooth uses the following set of parameters:
• Frequency-hopping, spread-spectrum 79 channels, 1600 hops/sec.
• Gaussian frequency shift modulation (GFSK).
• 1-Mb/sec raw data rate, realizable throughput >700 Kb/sec.
• 83.5 MHz of spectrum, divided into 1-MHz channels.
• Power control based on received signal strength indicator (RSSI) feedback from receiving device (Class I requirement).
• 0 dBm (1 MW) without power control (Class III, 10-m range).
• 20 dBm (100 MW) with power control (Class I, 100-m range).
Transmission Channels. Two types of transmission channels are defined in Bluetooth: asynchronous communications link (ACL) and synchronous connection oriented (SCO) link. The ACL link is used for data communications and is set up for every link between two Bluetooth devices. It is a packet-switched transmission method that provides error detection, forward-error correction, packet tracking (numbering), and packet retransmission. It is an error-free link to the application; it will either give good data or keep trying to get good data until it signals that the link is down. It is important to note that it is the retransmission of data in the presence of interference that increases latency and slows down net data rates.
The SCO link is defined for the transmission of voice data. It is a circuit-switched transmission method that provides known latency and assured transmission rates. It has only error detection and forward-error correction. Each SCO channel is given time slots that are predefined in the transmission sequence. A maximum of three SCO channels are allowed in each piconet. Little bandwidth is left over for ACL data when all three channels are in use.
The SCO and ACL channels share the total bandwidth, and so a combination of the two types of channels can be used in a piconet. The total bandwidth can include multiple SCO and ACL channels. The availability of two types of channels allows users to select the packet length and to determine the amount of forward-error correction. These parameters are often automatically controlled depending on the desired data throughput along with the amount of interference the device encounters.
Piconets and Scatternets. A Bluetooth piconet consists of at least one master and one slave; this is defined as a point-to-point connection. A full piconet consists of one master and up to seven slaves (eight total devices). The master controls all timing, including the clock and hopping sequence (to which all slaves synchronize). Each master has a slightly different clock, or skew, and hopping sequence, which is based on both its device address (48-bit IEEE address, as used for Ethernet) and when the device is powered on. These differences allow for multiple piconets to be established and used in the same physical space.
A Bluetooth master is responsible for controlling all data traffic in a piconet. All transmissions go through the master in a star network topology. This topology does affect the realizable bandwidth for any given application and configuration. The PAN profile allows devices to talk to each other without knowing that they are in the configuration; the master seamlessly coordinates the transfer of data.
The Bluetooth specification defines the ability to exchange the master and slave relationship between two devices. It also allows a device to be both a master on one piconet and a slave on another, or to be a slave on more than one piconet if the hardware and base-band implementations support it.
A scatternet is formed from two or more piconets that share a common member. This shared member may either be a slave on both piconets or a master on one and a slave on another. Each of these configurations has architectural trade-offs, and the Bluetooth 1.1 specification does not define the preferred one, nor does it completely define the operation of scatternets.
Profiles. Collections of features and functionality required to perform a given function are called profiles for Bluetooth devices. The required profile gives all devices some level of interoperability—the ability to function automatically with other devices with little user intervention. These required functions are called the generic access profile. Additional optional profiles depend upon the application requirements and implementation details. The following list defines the profiles currently specified for version 1.1. It is important to note that a device can support multiple profiles. Additional profiles continue to be released independently of the main specification. The following profiles are currently available:
• Generic access: provides access in all devices.
• Service discovery: allows devices to determine capabilities of other devices.
• Cordless telephony: provides audio and dialing functions for cordless telephone handsets.
• Intercom: allows voice connections and calling between two devices.
• Serial port: enables functions required and methods for establishing a virtual serial connection between two devices; it is used in many of the higher-level profiles.
• Headset: provides functionality to implement a hands-free headset for cell phones and computers.
• Dial-up networking: enables functions and methods required to establish a remote Internet connection.
• Fax: allows faxing between devices.
• LAN access profile: allows Bluetooth to be used as the transport to a standard LAN for use in computers and access points.
• Generic object exchange (OBEX): specifies how to pass high-level objects (e.g., files). OBEX is the basis for the next three profiles described; it was developed for IrDA and, with minimal changes, it allows Bluetooth to be used by software applications developed for IrDA.
• Object push (OBEX based): allows transmission of named objects containing data.
• File transfer (OBEX based): allows transfer of files between devices.
• Synchronization (OBEX based): synchronizes computers, PDAs, and cell phones.
Since the release of version 1.1, new profiles include hands free, PAN, basic printing, human interface device, and several others.
Bluetooth has been designed to facilitate easy setup of small groups of devices. It provides discovery and ad hoc network support, which both play major roles in its design, and security features that protect these connections.
Service Discovery Protocol (SDP). SDP allows for automatic recognition and configuration between two different types of devices from different manufacturers. There are two types of discovery within the Bluetooth specification. Device discovery allows one device to query devices within range and acquire, in turn, key information about their general capabilities. Fundamental information includes full address, human-readable name, and general type (e.g., cell phone, laptop, headset).
Service discovery enables a device to obtain details about supported profiles and actually enables the device to browse those profiles to determine how to access certain features. The service discovery concept allows for even more information and access methods to be exchanged.
Ad Hoc Networks. Ad hoc networking enables a device to quickly establish and dissolve small groups of devices with very little user involvement and no permanent address assignment. Several devices establish the network with little interaction and retain the relationship only for the desired period of interaction. If security is desired, users can type in passwords or PINs to allow bonding and encryption to be set up.
Security. Bluetooth supports several security features, depending on the application and user requirements. Such capabilities include the inherent protection from eavesdropping by other frequency-hopping spread-spectrum devices and the use of keys or PIN and password combinations to provide layers of security.
With the use of PINs (alphanumeric strings of up to 16 characters), the 128-bit SAFER+ encryption algorithm creates strong security and encryption between devices. Additional security can be added at the application level if desired. Current encryption levels meet the requirements of healthcare systems.
The Bluetooth Special Interest Group released version 1.2 of the specification in November 2003. This specification includes resolution of several errata as well as new functionality. In addition, since the release of revision 1.1, several new profiles have been released.
Bluetooth 1.2 is completely backward compatible, meaning that all 1.1 devices will operate correctly with 1.2 devices, but without the newer devices’ improved features and functionality. Several of the improvements outlined below may be of particular interest to medical device manufacturers.
Adaptive Frequency Hopping. Adaptive frequency hopping (AFH) is the technique used to allow improved coexistence with other RF technologies, such as 802.11b and 802.11g, that operate in the same part of the ISM band. AFH features allow the Bluetooth master to select at least 20 of the 79 available channels for use in the hopping sequence for its piconet. The 20 are chosen either through a communication interface between chip sets when the two technologies are used in the same device, or by sniffing for available channels. The Bluetooth master will then dictate the hopping sequence to AFH-capable slaves to avoid interference.
Quality of Service. Quality of service (QoS) allows Bluetooth devices to request minimum throughput and latency requirements for their communication channel. The master device is then responsible for allowing and negotiating these parameters, then prioritizing data flow to maintain them or to report potential issues to the host or to the device that negotiated its QoS requirements. This is an optional feature and may not be supported (especially initially) on all devices.
Workable Scatternet. Although version 1.1 includes scatternets, the specifications did not go far enough to ensure that the networks could be implemented, and that once implemented, they would interoperate. These issues have been resolved in version 1.2.
Audio Communications. An enhanced version of the synchronous communications oriented channel, known as eSCO, is used for audio data. It gives lower effective error rates (for higher audio quality) primarily through error correction techniques.
As for most advanced technologies, the cost of implementing wireless technology is highly dependent on the application.
The $5 solution touted in the early days of Bluetooth is a reality, but only if the product volumes are in the range of 100,000 units per year. In addition, system and design resources must be available. Modular solutions can decrease time, effort, and development costs.
It is also important to be aware of the additional
approvals and testing required when incorporating any wireless technology, and specifically for Bluetooth. In addition to FDA requirements, Bluetooth-enabled devices must meet Federal Communications Commission and Bluetooth testing requirements. They must also obtain the relevant foreign approvals if the device will be marketed outside the United States. Wireless manufacturers can
assist with some of these requirements, and at the modular level some developers provide preapproved devices. However, it is essential that medical device manufacturers review the entire process to fully assess development time and costs.
When developing a Bluetooth medical device, it is important to consider issues involved with the medical device approval process for both the U.S. and foreign agencies and factor in the associated costs. Issues that should be addressed include interference and coexistence with other medical and wireless devices as well as security and reliability. A thorough failure modes and effects analysis should be conducted that addresses these issues as well as any others that are critical for the particular wireless implementation.
Wireless technologies have made significant progress. Bluetooth, in particular, has entered the mainstream for many types of applications. Bluetooth is being used in a variety of medical applications ranging from home-healthcare devices to operating room equipment.
Medical devices also face many development issues that can be addressed with wireless technologies. Medical systems are using wireless technologies to eliminate cables and to integrate new architectures into their devices.
The different wireless technologies provide varying benefits. For cable replacement and for use in highly mobile, battery-powered devices, Bluetooth provides an ideal solution. Its excellent security and reliability are critical to medical device manufacturers.
1. WE Saltzstein, “Bluetooth: The Future of Wireless Medical Technology?” Medical Device &Diagnostic Industry 24, no. 2 (2002): 44–52; available on Internet: www.devicelink.com/mddi/archive/ 02/02/001.html.
2. WE Saltzstein, “Exploring a Wireless Future for Medical Electronics,” Medical Electronics Manufacturing 10, no. 1 (2002): 40–51; available on Internet: www.devicelink.com/mem/archive/ 02/10/saltzstein.html.
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