Originally Published MDDI June 2004

Electronics

Subjective comparison of industry standard wireless technologies. Devices = Maximum devices in a network. Adoptees = Number of implementers (click to enlarge).

With many wireless options available, manufacturers face the task of comparing the strengths and weaknesses of each to find the one most suitable for a given application. The chart below represents the wireless options available to medical device designers today. The assessments shown are the author's subjective, qualitative view. A basic overview of each technology is presented in alphabetical order here.

Bluetooth

Bluetooth technology is discussed in detail in the main text of the article. See www.bluetooth.org for more information.

Cellular

Cellular is an extremely broad topic that includes both voice and data transmission. Cellular offers several modalities for data transmission. There are two main camps competing globally—global system for mobile communications (GSM) and code division multiple access (CDMA)—with other technologies (used primarily in the United States) on the wane. Both of these camps have voice and data options, as well as high speed variants for use in 2.5-, 3-, and 4-generation data communications.

Cellular technologies vary in frequency usage by country, as well as bandwidth and distance. Distance is governed by terrain and location of cellular infrastructure (towers and antennas), as well as building materials when used inside. Cellular technology is relatively high priced and includes a connection fee for usage. Power usage for data is much higher than that of IrDA and Bluetooth and may be more comparable to that of 802.11g (but varies widely depending on technology and speed).

The technology has been used in the medical field for many years for voice communication. For data, it is seeing deployment in mobile health applications such as emergency medicine and home healthcare. Usage in a hospital environment can be problematic due to regulations governing usage stemming from concern about interaction between cellular and medical devices. Several groups are working to provide better guidance on this issue to promote appropriate usage scenarios.

Bluetooth is designed as the wireless linkage for data to the cellular WAN, and demonstrations of this capability have been proposed for use in medical applications. It is likely that both technologies will be found in tandem for several types of telemedicine applications. See www.gsmworld.com and www.cdg.org for additional information.

DECT

Digital electronic cordless telephony (DECT) is primarily a European technology. Designed for cordless telephony, its primary use is in roaming handsets within a corporate environment. Its data transport capability has seen less deployment, but it is still of interest.

A U.S. version of DECT called wireless digital cordless telephony (WDCT) is used for 2.4-GHz wireless phones sold commercially. It is simply a DECT technology that is modified for use in the ISM band. See www.dect.org for additional information.

IEEE 802.11

IEEE 802.11 was developed specifically for LAN replacement, using standard 802.3 (Ethernet) protocols. It is designed for computing devices contained within a building that is covered by one or more cells. It offers structured networks with cellularlike roaming for devices moving between access points.

There are several types of 802.11 technologies currently using the ISM band. The frequency-hopping spread-spectrum technology is used in several patient-worn medical devices currently on the market, but is declining in its supply base and usage. The direct sequence 802.11b (also called Wi-Fi) is experiencing great acceptance in hospital, office, and home computing environments. Because 802.11b currently offers a relatively high 11-Mb/sec data rate, it is being used in many fixed and mobile applications requiring high speed.

Even higher data rates are provided by the 802.11a and 802.11g technologies. Both variants offer a top data rate of 54 Mb/sec, but while 802.11a occupies a higher frequency spectrum (5.2 GHz), 802.11g offers backward compatibility with the installed base of 802.11b in the 2.4 GHz band.

The cost of 802.11 implementations in terms of power is much higher than IrDA and quite a bit higher than Bluetooth (this varies with supplier and application, so it must be evaluated on a case-by-case basis). Examples of usage in PDAs indicate that current 802.11b implementations yield 1¼4 to 1¼8 the operating time of those using Bluetooth. Chip set costs for 802.11 implementations are higher than the current generation of Bluetooth.

The 802.11 technology has wide support and implementation. It is easy to integrate into existing wired networks and standard enterprise computing infrastructures. Issues for usage in medical devices are its power, software support, and IT administration. See www.
wi-fi.org for additional information.

IrDA

IrDA refers to the Infrared Device Association that oversees the specifications for technology using optical transmitters and receivers for the transmission of data. IrDA was developed from the same technology used in television remote controls and shares some of the advantages and disadvantages of those devices.

As the first widespread cable replacement technology, it has an important place in the wireless landscape. It is implemented in almost every laptop computer and palm-sized organizer on the market.

IrDA has a limited range, with high-speed (16-Mb/sec) transmission requiring separations as short as 1 ft for reliable operation and no further than 4 ft for its slower rates. It requires that two devices are in line of sight, and it only works between one pair of devices at a time. IrDA is very low in cost and is relatively simple to design into products.

While its line-of-sight operation and range limitations can be a drawback in some applications, they do provide a secure link that can be relatively immune to eavesdropping or other attempts to intercept data being transferred.

However, IrDA been called the most successful failure in recent computing history. Getting it to operate between devices can be quite difficult, and application support has been limited. The IrDA group has recently instituted new interoperability testing standards to help, but concerns regarding interoperability with the technology remain.

Even with its drawbacks in some applications, IrDA will continue for the future due to its low cost, ease of implementation, and security. It is currently implemented in several medical devices from a number of manufacturers and has been discussed by several medical communications standards bodies. See www.irda.org for additional information.

Ultrawide Band

Ultrawide band (UWB) has been generating quite a bit of buzz in the wireless industry as the next contender for everything from PANs to replacing high-data-rate audio and digital video cabling in home entertainment equipment. It is not listed in the chart because it lacks an industry standard. IEEE standardization activities surrounding UWB have been very contentious with two main camps. Implementations have been demonstrated and are being tested by several suppliers, but no clear decisions have been made.

While no standard exists today, UWB is already being promoted for use in hospital settings, beginning with systems for location of devices and personnel. See www.uwb.org for additional information.

Wireless Telemetry

The wireless medical telemetry system (WMTS) is not included in the chart because it is not an industry standard technology. However, it is one of the most widely deployed wireless technologies used in hospital devices today and certainly bears discussion here.

The WMTS band was created to protect hospital ambulatory telemetry systems from interference created by the new digital television transmissions. It exists in the United States from 608–614 MHz with additional spectrum to be assigned when military users at the 1.394 GHz band free up that segment.

Data rates and distance vary widely among manufacturers as do modulation and frequency utilization methods. Some use traditional analog FM modulation methods, while others use frequency-hopping techniques. Particular specifications and methods are proprietary to each medical device manufacturer.

Additional devices operate in the 902–928 MHz band and 2.4 GHz ISM bands, although neither of these two ISM bands is exclusive to medical uses.

Solutions in telemetry have typically been custom, beginning with the original one-way FM analog transmissions of waveform. The newest implementations use this band to transmit digital and two-way information to facilitate new devices and use models. Most implementations are quite low in cost due to their specialized nature. Costs are usually lower than all of the other standardized technologies with the exception of IrDA.

While WMTS is used widely in hospitals for ambulatory patient monitoring, most of these implementations are custom, and there is little or no interoperability across manufacturers. In addition, there is no uniform international support for this band, requiring changes and requalification for countries outside of the United States. FCC regulations prohibit use of WMTS outside of the hospital setting.

ZigBee

ZigBee is the popular name for IEEE 802.15.4. It is the standard for low-rate, low-power PANs. It is primarily used to enable communication with wireless sensors and networks of sensors such as those used in meters and heating and cooling systems.

There are three frequency bands defined for usage with ZigBee: one channel at 868 MHz, 10 channels at 915 MHz, and 16 channels at 2.4 GHz. The trade-offs between the bands are data rate, power usage, distance, and global availability. Rates vary from 250 Kb/sec in the 2.4 GHz band to 20 Kb/sec at 868 MHz. This technology allows the use of several topologies, including both star and mesh, with minimal complexity.

ZigBee is in the very early stages of adoption with chip sets and software stacks now beginning to become available, so little is known about implementation and product-level issues. Few, if any, end products have been released to date based on ZigBee. But, it is clear that it may offer advantages, particularly low power consumption, for low-data-rate devices. See www.zigbee.org for more information.

Copyright ©2004 Medical Device & Diagnostic Industry