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Understanding the Wireless Medical Band
and the Wireless Medical LAN
TABLE OF CONTENTS
OVERVIEW... 3
Objectives... 3
Intended Audience ... 3
INTRODUCTION ... 3
CURRENT MEDICAL TELEMETRY SYSTEMS ... 4
WIRELESS MEDICAL TELEMETRY SERVICE ... 6
Strengths of the WMTS ... 6
Limitations of WMTS ... 7
THE INDUSTRIAL, SCIENTIFIC, AND MEDICAL (ISM) BAND... 7
Strengths of the ISM Band ... 9
Limitations of the ISM Band... 10
Spread Spectrum Modulation ... 10
Frequency Hopping Spread Spectrum ... 11
Direct Sequence Spread Spectrum ... 11
Advantages of using FHSS for Medical Monitoring ... 12
Potential Clinical Applications of ISM Band Wireless LANs ... 12
CHOOSING A NEW TELEMETRY SYSTEM... 13
UNRESOLVED ISSUES ... 14
2.4 GHz vs. 5 GHz ... 14
Wireless LANs in Mixed Environments... 14
802.11 FHSS and DSSS ...14
Bluetooth ...14
SUMMARY ... 15
GLOSSARY ... 15
OVERVIEW
More and more hospitals and healthcare facilities are using mobile monitoring and computing technologies based on wireless local area networks (WLANs) to improve the quality and reduce the cost of patient care. This study guide explains why medical telemetry systems currently in use are becoming obsolete, and describes alternative technologies that will make wireless communication more reliable. Limitations of systems operating as primary users in the Wireless Medical Telemetry Service (WMTS) band designated by the Federal Communications Commission (FCC) are discussed. Advantages of standards-based systems operating in the Industrial, Scientific, and Medical (ISM) band are outlined.
Objectives
After completing this educational program, the participant should be able to:
1. Explain why hospital telemetry systems need to move from their existing VHF & UHF bands to alternative bands, based on the Federal Communications Commission (FCC) ruling.
2. Differentiate between WMTS and ISM options.
3. Describe how information technology (IT) and biomedical engineering departments can work together to find mutually satisfactory wireless telemetry solutions in the hospital. 4. Discuss unresolved issues for the future of wireless medical telemetry.
Intended Audience
This program is intended for use by IT professionals (including chief information officers or CIOs), hospital administrators, clinicians, and biomedical engineers involved in identifying the best wireless telemetry systems for their facilities.
INTRODUCTION
Recently released technologies now make it possible for hospital healthcare systems around the world to take advantage of the mobility, flexibility, and speed of mobile monitoring and computing technologies based on WLANs to improve the quality and reduce the cost of patient care. Using integrated WLAN system solutions to support a variety of hospital-wide clinical and nonclinical patient-centered service applications allows hospitals to run more efficiently, effectively, and competitively.
State-of-the-art medical telemetry systems have a number of obvious advantages over hard-wired systems:
• Using medical telemetry equipment to transmit and receive patient measurement data, such as heart rate, electrocardiogram (ECG), respiration rates, SpO2, and blood
pressure, permits patients wearing small monitors to ambulate throughout the
healthcare environment. The end result is greater mobility and increased comfort for the patient.
• Displays on these new devices allow clinicians, for the first time, to view vital signs data including waveforms at the patient location.
• Personnel can move freely from room to room, without having to return to a fixed station to enter or check information. Hospital staff have real-time access to monitoring data that are accurate and up-to-date at the bedside or other point of care, thus
allowing them to make decisions and take actions accurately and efficiently.
• Because wireless LANs can be installed significantly faster and cheaper than
traditional telemetry systems, they allow unprecedented flexibility and responsiveness to organizational needs and changes. Wireless LAN-based, mobile computing
technologies increase the reach of the institution's current information technology (IT) investment and infrastructure by bringing the power of computing to the point of activity.
In the past few years there has been a dramatic growth in the demand for telemetry systems driven by rising patient acuity levels, as well as the need to ambulate patients quickly after procedures. Along with excitement about the growing applications of medical telemetry systems has come legitimate concern about interference from other
radiofrequency (RF) applications. In February 1998, an incident occurred at Baylor Medical Center in Dallas, Texas, in which interference from a high-definition television (HDTV) test by a local television station knocked out 50% of a telemetry system on one floor. This incident, along with other factors, highlighted the need for a medical telemetry band that would minimize the risk of such interference.
As the federal agency responsible for regulating the RF spectrum in the United States, the Federal Communications Commission (FCC) recently took action to address these
concerns. On July 17, 2000, the FCC published a final rule that established a new
Wireless Medical Telemetry Service (WMTS).[1] This ruling and other recent FCC actions
that affect frequencies currently used by medical telemetry systems will have far-reaching implications for the operation of current and future medical telemetry systems. Let's take a look at how telemetry systems in your institution will be affected.
CURRENT MEDICAL TELEMETRY SYSTEMS
Each band of frequencies in the RF spectrum has primary users for whom the band was set aside by the FCC. In some bands, these primary users share the band with secondary users who must not interfere with the licensed primary users. Before the FCC rule,
• VHF television channels 7 to 13 (174-216 MHz). To aid in the transition to digital television service, the FCC has assigned an additional HDTV channel for each currently operating VHF TV station. Many HDTV stations are online and by May 1, 2003 all stations will broadcast on both their existing signals, as well as the new HDTV signal. (Monitoring the FCC web page for license allocations can assist the hospital in identifying possible conflicts with HDTV stations that are initiating transmission in their areas.) In addition, new low-power community TV station applications that have been received by the FCC are now being approved. (Note, however, that the signals
transmitted by these "low-power" TV stations are 2500 times stronger than a medical telemetry signal.) New high-power and low-power primary users will consume
spectrum that many hospitals are now using for medical telemetry, causing existing telemetry systems in the VHF spectrum to become obsolete and unsupportable, including the costly antenna systems on which they operate.
• UHF television channels 14 to 46 (470-668 MHz). These channels suffer from the same overcrowding as described above. Existing UHF systems will minimally need to be re-tuned to operate on the WMTS. Unfortunately, mere re-tuning of analog
systems doesn’t allow the system to take advantage of bi-directional digital communications that are now allowed in the WMTS. Existing UHF systems have limited bandwidth and cannot support two-way communication. New designs for both patient-worn devices and the antenna system will be required to support two-way communication. Such changes will be costly to the hospital since existing transmitters and antenna infrastructures will need to be partially or entirely replaced.
• UHF Private Land Mobile Radio, or PLMR (450-470 MHz), such as that used by police, firemen, taxicabs, delivery trucks and the pervasive walkie-talkies. A freeze on new applications for operation of high-power land mobile services in the 450-460 MHz band has been lifted and the freeze on applications for operation of these services in the 460-470 MHz band will be lifted in 2003. The potential for interference of such systems with medical telemetry systems will make operation of medical telemetry systems in these bands unfeasible.
Effective October 16, 2002, equipment approval may no longer be obtained for medical telemetry equipment operating in the UHF PLMR band, or in VHF or UHF television channels, except for UHF channel 37 as allowed under the WMTS. Manufacturers may continue to support, and hospitals may continue to use, such equipment approved before October 16, 2002, but no new medical telemetry products will be approved for operation in these bandwidths.
As HDTV and PLMR applications continue to crowd medical telemetry's traditional spectrum of operation, the amount of telemetry dropout will potentially increase, making traditional biomedical telemetry bands very risky. Given that safer alternatives now exist, continuing to use a band that is recognized as risky could make the hospital liable for damages if patient incidents should occur.[2] Fortunately, alternatives do exist, including
systems that use either the Industrial, Scientific, and Medical (ISM) band or the WMTS frequencies.
WIRELESS MEDICAL TELEMETRY SERVICE
The main purpose of the FCC's July 2000 rule is to set aside the following three bands of the RF spectrum for primary use by biomedical applications:
608-614 MHz (UHF channel 37), which is not used for television stations and was reserved for radio astronomy. Users of this band will be required to coordinate their operation with radio astronomy facilities, which will operate on a co-primary basis with medical telemetry, and to design equipment to provide sufficient protection from interference from adjacent televisions stations operating on channels 36 and 38. To prevent users from monopolizing the 608-614 MHz band, wireless medical telemetry devices utilizing broadband technologies such as spread spectrum must be capable of operating within one or more 1.5 MHz channels within this band (608.0-609.5, 609.5-611.0, 611.0-612.5, and/or 612.5-614.00 MHz), Note that the ability of spread spectrum to reject noise diminishes as the available bandwidth decreases.
1395-1400 MHz and 1429-1432 MHz are former government bands that were reallocated for non-government use. Military radar operations in those bands may continue at certain sites around the country for a number of years (until 2003 for 1395-1400 MHz and 2006 for 1429-1432 MHz).
The FCC ruling states “physiological parameters and other patient-related information” are allowed. A WMTS station may transmit any biomedical emission appropriate for
communications, except voice and video. Waveforms, such as ECGs, are not considered video.
Prior to operation, health care providers who want to use the WMTS must register the following information with a designated "frequency coordinator": specific frequencies or frequency ranges; modulation scheme; effective radiated power; number of transmitters in use, including manufacturer's name and model numbers; legal name of provider; location; and a point of contact. The frequency coordinator also must be notified of any changes in these parameters.
The FCC will coordinate WMTS frequency allocations with the Canadian and Mexican governments, as appropriate; however, given the low power of this equipment, no inter-ference issues are anticipated at border areas.
Strengths of the WMTS
The WMTS provides medical telemetry with primary status in the three bands. For the first time, medical telemetry is not a secondary user and therefore should be relatively free of intentional emissions from non-medical transmitters. Ultimately, the WMTS will allow two-way communication and higher data rates than are provided by current medical telemetry.
Limitations of WMTS
Despite its advantages, WMTS has a number of limitations. For example:
The WMTS uses a restricted bandwidth with only 6 MHz of contiguous bandwidth and 14 MHz total. The entire allocation is unlikely to be available in any individual market, and the bandwidth provided is significantly less than the amount of RF spectrum currently available to medical telemetry on an unprotected basis.
Neither video nor voice transmission is permitted. Video is excluded because it could occupy a significant portion of the spectrum allocated to the WMTS. Voice transmission is excluded because of concerns that allowing voice transmissions could encourage the equipment to be used as a form of wireless intercom.
Because the WMTS is broken up into three bands, a single facility will find it difficult to use the entire spectrum. The options are either to use complicated and expensive
trans-ceivers, or to bear the expense and trouble of installing multiple network transceivers to cover the various frequency bands.
Allocation of primary status precludes most intentional transmitters from creating inter-ference, but the possibility remains of "friendly" interference from conflicting or interfering systems operating within the band. In addition, the FCC is not requiring television broad-casters to protect WMTS from adjacent band interference.
Because WMTS operation is authorized only within a health care facility, operation in ambulances or other moving vehicles or in patients' homes is specifically excluded.
The FCC ruling forbids non-medical applications on the WMTS band, but does not prevent vendors from implementing proprietary, incompatible, and potentially interfering systems within the band. It is up to hospitals and manufacturers to prevent interference among incompatible devices from different vendors.
No standards of interoperability exist. Vendors will tend to develop their own propriet-ary communication infrastructure and protocols, making it challenging to coordinate multiple vendors' equipment operating in the WMTS. For example, a monitor from one manufacturer may not be able to communicate over the infrastructure installed by a
second manufacturer and could cause interference and dropouts. Using separate systems for each vendor will increase cost, system complexity and RF interference.
THE INDUSTRIAL, SCIENTIFIC, AND MEDICAL (ISM) BAND
The FCC ruling does not require that all medical devices use WMTS bands. An alternative spectrum for medical telemetry use is the 2.4 GHz Industrial, Scientific, and Medical (ISM) band. In fact, the FCC ruling specifically states that it will continue to allow medical
The 2.4 GHz ISM band is available worldwide on a secondary use basis as long as spread spectrum modulation techniques are used at relatively modest power levels. The international standards bodies have identified this band for wireless local area network applications. The Institute of Electrical and Electronics Engineers (IEEE) has developed an open standard for two-way wireless communications (IEEE 802.11)[3]. Ratified in June
1997, the original IEEE 802.11 specification is the wireless extension of the 802.3 communications standard for hard-wired LANs that use physical cable and the Ethernet protocol. Anyone who has worked in a networked office and used a file server is familiar with 802.3-compliant technology, which allows hundreds of vendors to offer network interface cards, hubs and other products that all cooperate with each other without problems due to interference.
Implementation of the IEEE 802.11 protocol is broken up into several layers:
• The physical layer describes how fast the data that go into and out of the computer are transmitted and the method of communication (e.g., radio, infrared, or wired). IEEE 802.11 defines three physical layers: frequency hopping spread spectrum (FHSS) in the 2.4 GHz band, direct sequencing spread spectrum (DSSS) in 2.4 GHz and infrared.
• Above the physical layer are the TCP/IP, MAC (medial access control), and link layers, which deal with things such as how many bytes to send at a time, how to determine if a transmission fails, what to do if a transmission fails, and when to transmit. These processes are invisible to the user, but they affect the data rate, which the user can see. The MAC layer is the same for all three 802.11 physical layers. The physical and MAC layers work together to determine if the channel is clear.
The 802.11 MAC is very similar to 802.3 (Ethernet standard) in that it is designed to support many users whose transmissions might experience interference without losing any data. One of the pieces of the Ethernet that supports this is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). CSMA/CA works by having the sender listen to the airways before attempting a transmission. The sender defers transmission until later if the communication channel is busy. After transmission, the sender waits for an ACK (acknowledge) message from the recipient. If data are not received or are received with an invalid CRC (cyclic redundancy check), then the transmitter will not receive an ACK from the receiver and automatically re-transmits the data.
Another feature of the 802.11 MAC layer is packet fragmentation. Packet fragmentation allows large packets to be broken into smaller units when sent over the air. This is useful in very congested environments or when interference is a factor, since smaller packets have a lower probability of being corrupted. This technique reduces the needs for re-transmission in many cases and thus improves overall wireless network performance. The MAC layer is responsible for reassembling fragments received, rendering the process transparent to higher-level protocols.
Strengths of the ISM Band
Medical telemetry systems that utilize the ISM band have a number of advantages over WMTS:
• The ISM band occupies 83 MHz of bandwidth, including guard bands to protect
against adjacent-channel interference. This is 14 times more than the 6 MHz currently occupied by the WMTS, and WMTS has no guard bands to protect against
interference from TV channels 36 and 38.
• Unlike the WMTS, the ISM band allows useful voice and/or video transmissions.
• 802.11-compliant medical telemetry devices can communicate with other wireless or hard-wired devices using bi-directional access points (APs). Thus, a single, open, 802.11-compliant wireless network infrastructure can support a wide range of mobile computing and communications devices. Such devices may include ambulatory biomedical telemetry and portable monitors, wireless laptop and desktop personal computers (PCs), wireless LAN pagers, wireless PDAs (personal digital assistants such as Palm Pilots™), and Internet protocol (IP) telephones. In addition, the wireless medical LAN can link to the institution's 802.3-compliant hard-wired network. The limitation on the number of connected devices is determined by the throughput of the access point infrastructure. Each access point is capable of supporting a given number of devices. Adding access points is simple, inexpensive, and allows more devices to be supported as needs grow.
• The ISM band is available and compatible worldwide for wireless applications, and competition drives costs down while performance increases. Conformance to IEEE 802.11 ensures that devices from different vendors can coexist on the same network. This allows the wireless network infrastructure to be utilized by multiple applications from multiple vendors.
• The propagation characteristics of the 2.4 GHz frequency make it ideal for in-building use where structures attenuate the signal between floors. Low power,
spread-spectrum technology, and attenuation characteristics within building structures allow reuse of the spectrum within large facilities.
• Conformance to 802.11 allows devices to be transportable between facilities with no need to retune the device. Monitors are not tied to a particular receiver; rather, the monitor can associate with whichever AP it determines will provide the best commun-ication channel.
• Although spectrum management is necessary, no frequency management is required, even for large, multi-hospital applications.
• Widely used SNMP network management tools can be used to monitor traffic on the network to determine loading and utilization factors and to warn when wireless network loading exceeds a preset threshold.
• Scalability permits flexible, cost-effective solutions to any wireless LAN needs, whether the installation is in a single laboratory, a multi-floor building, or a healthcare campus with thousands of wireless devices.
• The IEEE 802.11 standard specifies a security mechanism that provides secure point-to-point access and communication. The wired equivalency privacy (WEP) algorithm inhibits other stations from decrypting data sent over the wireless LAN. Under the WEP algorithm, both the access point and mobile unit share an encryption/decryption key. If an AP is set to only allow WEP communication (shared key authentication), then only mobile units (MUs) with the same key settings can associate with that AP.
• Unlike traditional telemetry, patient monitors are not tied to a particular receiver. If one AP goes down or experiences immense amounts of interference, patient monitors automatically roam to a different AP and no clinical data are lost.
• Since all unlicensed transmitters in this band are required to use low-power spread spectrum communications, there are no uncontrolled devices outside the hospital that are of concern.
Limitations of the ISM Band
Networks that operate in the ISM band require careful site surveys before installation and whenever new applications are added, in order to optimize the placement of APs so that the network has adequate capacity to serve the intended applications. Long-range
planning in anticipation of new applications, followed by good wireless network design, will ensure that RF resources are reliable and dependable. Limiting or controlling patient and visitor-owned wireless devices, in much the same way they are controlled today, will further ensure network reliability.
Other radiators in the ISM band such as other mobile devices, some wireless telephone networks, and microwave ovens can cause interference (but not necessarily a telemetry dropout, since 802.11 automatically re-sends the data). However, a proper site survey identifies these sources and places APs at locations to ensure continuous RF coverage. Also, with an 80 MHz band, spread spectrum modulation significantly enhances the device’s ability to successfully transmit in the presence of interference.
Spread Spectrum Modulation
Developed at the end of World War II, spread spectrum modulation "spreads" the RF signal over a range of frequencies, which enhances the ability of the RF communication system to reject interfering noise and provides security against eavesdropping. The IEEE 802.11 Standard specifies two types of spread-spectrum modulation: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) at 1 and 2 Mbps. IEEE 802.11b extended the DSSS definition to include 5.5 and 11 Mbps
Frequency Hopping Spread Spectrum
FHSS accomplishes "spreading" by moving the center frequency of the broadcast. To decode FHSS, the receiver must "hop" from frequency to frequency at the same time as the transmitter.
The carrier frequency of the transmitter changes (or hops) in accordance with a pseudo-random hop pattern. The hop sequence dictates the frequency order used by the AP and its associated MUs. Stations in a cell using FHSS techniques hop or change the carrier frequency at synchronized intervals. The AP transmits synchronization beacons to the MUs, which allows them to synchronize their hopping times to the AP’s. Each hop is at least 6 MHz away from the previous frequency and has a 1 MHz bandwidth.
In the United States, there are 79 channels spaced 1 MHz apart. 802.11 devices hop through these channels in a pseudo-random order as defined by IEEE 802.11, which allows up to 15 APs to be co-located without significant interference problems. This allows the wireless medical LAN designer to scale the system for higher net data rates.
Direct Sequence Spread Spectrum
DSSS multiplies the signal by a spreading function (an exclusive OR operation in the digital world) and keeps the center frequency of the broadcast constant. For 1 and 2 Mbps DSSS, the data stream is exclusive OR'd with the Barker code to generate a series of data objects called chips. Each bit is "encoded" by the 11-bit Barker code or, in other words, each group of 11 chips encodes one bit of data. Since the chipping sequence has 11 times the bandwidth of the original data, the resulting product has a higher bandwidth than the original data, i.e., it has been spread in frequency. The receiver recovers the data using this same encoding pattern. For DSSS, IEEE 802.11 (and 802.11b) specify 14 overlapping channels of 22-MHz bandwidth and 5-MHz spacing between center
frequencies. In the United States, channels 1–11 are available, so there exist 3 non-overlapping channels (1, 6 and 11). The number of chips per bit of data is referred to as the spreading ratio or the processing gain, and the FCC requires a minimum processing gain of 10. The higher the processing gain, the more immune the system is to
interference. To increase the processing gain, the bandwidth of the transmitted signal must be increased or the data rate must be decreased. This means that the data transmission rate can be traded off for noise immunity.
IEEE 802.11b defines the physical layer for the 11 Mbps DSSS communication and uses CCK (Complementary Code Keying) which uses an 8-bit code. The reason that CCK achieves a higher data rate with the same 22 MHz channel is that CCK’s 8-bit code word can represent 6 bits of data (as opposed to the single data bit represented by the Barker code). The algorithm is more complex than the 1 and 2 Mbps DSSS algorithm and is beyond the scope of this paper.
Advantages of using FHSS for Medical Monitoring
Most implementations of wireless LANs on the market today use spread spectrum. FHSS has the following advantages for this purpose:
• Because vital signs monitors each require a relatively small bandwidth and many FHSS systems can be co-located, FHSS implementations can be easily scaled to support more patient monitors than can a DSSS implementation.
• Potential for interference from other radio signals is less for FHSS than for DSSS. The probability of an interference source blocking all 79 channels is extremely low and the FHSS algorithm allows communication to continue even when a significant number of the channels are blocked. DSSS has only 3 non-overlapping channels.
• With DSSS, the time duration of each chip is quite short. Because of this, multipath signals (those that bounce off reflective surfaces in route to the receiver) are more problematic than with FHSS. Fewer problems with interference means greater reliability for FHSS systems.
• The advertised data rate for DSSS is only supported for very short distances. Both FHSS and DSSS use dynamic rate shifting, allowing data rates to be automatically adjusted to compensate for the changing nature of the radio channel or for
transmission to more distant mobile units. The lower the data rate, the better the signal-to-noise ratio. Only about 20% of an 11 Mbps DSSS AP’s coverage area has 11 Mbps coverage, and this number decreases in the presence of noise and/or multi-path propagation. This number decreases further if the MU is moving, or if there are multi-path propagation effects.
Potential Clinical Applications of ISM Band Wireless LANs
Examples of potential clinical applications of wireless LAN technology utilizing the ISM band include[4]:
• Multi-parameter ambulatory medical telemetry
• Clinical documentation at bedside, with interactive range checking
• Immediate access to the hospital information system at the point of care
• Improved efficiency for respiratory care, physical therapy and other mobile professionals
• Bedside admitting, discharge and transfer
• Medication administration process improvements, including bedside drug use evaluation
• Point-of-care documentation of diagnoses, interventions, and outcomes for ambulatory care providers
CHOOSING A NEW TELEMETRY SYSTEM
To help you decide which medical telemetry system is right for your hospital, Table 1 compares the features of systems operating in traditional bandwidths, WMTS frequencies, and the ISM band.
Table 1. Wireless Communications Options for Medical Telemetry
Characteristic Current Telemetry WMTS ISM
Frequencies VHF TV, UHF TV, UHF PLMR UHF Channel 37 (608-614 MHz);1395-1400 MHz and 1429-1432 MHz 2.4-2.4835 GHz
Protection None; medical is a
secondary user
Legal protection from intentional non-medical transmissions. Not pro-tected from adjacent channel interference nor “friendly fire” from
competing WMTS product vendors.
Unlicensed users use spread spectrum to reduce interference and increase RFI immunity
Usable spectrum Adequate in rural
settings,
overcrowded in large urban areas
Up to 14 MHz available, not all frequencies are available in all areas.
83.5 MHz; shared by all users in LAN
World wide use? No No Yes
Spectrum utilization Dedicated, fixed,
25-kHz channels
No restrictions within four 1.5-MHz subchannels
Spread spectrum required
Two-way communication
No Allowed, but not used by
most WMTS systems. Yes Supports nonmedical IT uses? No No Yes Standards-based transmission etiquette? No No Yes, IEEE 802.11
Network scalability Low Low/Moderate High
Usable in MD office? Yes, but improbable No Yes
Leverages hospital IT infrastructure?
No; requires separate isolated antenna system
No; requires isolated antenna system; two-way comms different from current telemetry
Yes; with appropriate network design
Infrastructure cost High High/Moderate Low
Supports voice and PDAs?
No No; specifically forbidden
by FCC rule
Yes
Management tools? No No standard network
management tools available.
Yes, management tools available that use SNMP network management protocol.
UNRESOLVED ISSUES
A number of issues regarding wireless medical telemetry remain unresolved in the current environment.
2.4 GHz vs. 5 GHz
In September 1999, the IEEE adopted 802.11a, which defines a high-speed (up to 54 Mbps) standard in the 5 GHz ISM band that uses orthogonal frequency division
multiplexing (OFDM) technology. Some 802.11a radio modules were available in sample quantities in early 2001, but the technology is not yet available for deployment of WLANs. OFDM is superior to DSSS in term of resistance to multipath propagation effects, but one can expect shorter range due to the higher frequency and higher data rate.
Wireless LANs in Mixed Environments
802.11 FHSS and DSSS
It is possible for FHSS and DSSS systems to coexist in a given facility. In fact, the IEEE standards bodies anticipated this when the standards were developed. Careful RF infra-structure design, e.g., avoiding co-locating APs with different communication protocols, is required to minimize interference between the spread-spectrum technologies if they need to coexist in the same geographic area. Generally, a separation of 2 meters or more makes interference issues inconsequential.[5]
The IEEE and the computer/communication industry are studying and developing ways to further enhance reliability and reduce the chance of interference of communication in mixed environments.
Facilities need to make a strategic deployment plan for WLANs that considers the applica-tions the WLAN is to support, and the placement of different WLAN devices with respect to each other.
Bluetooth
Bluetooth is a low-bandwidth, short-range wireless networking technology designed primarily to replace cables for communication between small personal computing/ communication devices, such as desktop computers, laptops, PDAs, and cell phones. This network is often called a Personal Area Network (PAN) or piconet., Each PAN is limited to 8 devices, usually with a maximum range of 30 feet. Piconets can be linked when one device is a member of 2 piconets; however this type of networking has a large overhead cost and does not offer an adequate scale for enterprise installations. As with 802.11 devices, Bluetooth devices have sleep modes and only transmit when there is a need, so most of the time the transmitter is inactive.
Bluetooth technology transmits both voice and data on the 2.4 GHz ISM band. It uses FHSS technology (1600 hops/second) to increase the reliability of the communication channel. The IEEE 802.15 working group for PANs has announced that it will adopt Bluetooth as the IEEE 802.15 standard. Bluetooth and IEEE 802.11-based wireless LANs are complementary, rather than competing, technologies. Bluetooth is not primarily
intended for use in networking. Its low data rate and other limitations will probably prohibit its use for most networking purposes except home networking and small, ad hoc wireless networks.[6]
SUMMARY
To avoid wasting money by prematurely replacing wireless infrastructure and telemetry systems with newer systems that will quickly become obsolete, or upgrading to newer systems that are already obsolete (e.g. unidirectional systems), hospitals must work out a long-term (i.e., 10-year) plan. Biomedical engineering and IT departments must work together to define, purchase, and maintain the most appropriate and cost-effective
wireless medical telemetry system for their facility. One approach is to define the facility's "musts" and "wants" with regard to medical telemetry, identify several possible vendors, make site visits to evaluate their systems in action, and define the roles of both
departments during both installation and operation. Throughout the whole process, communication is key!
Each wireless technology has certain advantages and limitations. The benefits and pitfalls of various options must be weighed against installation and performance requirements. In patient monitoring applications, the most important feature must be reliability: patient contact must be maintained at all times. Bandwidth, flexibility, expandability, ease of implementation, and cost are important, but secondary considerations. With these criteria in mind, FHSS in the ISM band is an ideal candidate for wireless connectivity. Using this band, a single open network can reliably support fixed, ambulatory, transport, and
portable patient monitors, along with a variety of caregiver tools, such as IP telephones, PDAs, and laptop and desktop PCs.
GLOSSARY
AP (access point): A MAC-layer bridge that may be thought of as a wireless hub; many devices can connect or "associate" with an AP, and the AP allows one device at a time to either communicate with another device associated to the AP or to the hard-wired LAN. Channel: A section of the radio spectrum, such as a television channel.
Chip: One bit of a sequence of bits used to increase the bandwidth of a signal for direct sequence spread spectrum. Also, the process of applying a chipping sequence.
Chipping sequence: A sequence of bits (11 for 1 and 2 Mbps DSSS) used to encode a single data bit. Used in Direct Sequence Spread Spectrum.
DSSS (Direct Sequence Spread Spectrum): A modulation technique where a high-frequency chipping sequence is XOR’d with each bit of data to produce a wide-band stream of encoded data that is then used to modulate an RF carrier. In 802.11 and 802.11b implementations, direct sequence signaling technique divides the 2.4 GHz band into 14 twenty-two MHz-wide channels. The channel separation is 5 MHz, so most of the channels overlap. Exclusive use of channels 1, 6, and 11 precludes frequency overlap. Data are sent across one of these 22 MHz channels without hopping to other channels. FHSS (Frequency Hopping Spread Spectrum): A modulation technique that spreads the signal by transmitting a short, narrowband burst on one frequency, then hopping to a different frequency for another short burst, and so forth.
GHz (gigahertz): A unit of frequency; 1 GHz = 1,000,000,000 cycles per second.
ISM Band: A set of frequency bands including 2.400 – 2.483 GHz set aside for industrial, scientific, and medical use. The 2.400 – 2.483 GHz band is available for use in almost every country of the world.
IEEE (The Institute of Electrical and Electronics Engineers): As one of its functions, the IEEE acts as a standards body to define uniform ways for electronics and computers to communicate. The IEEE developed the standard (IEEE 802.3) that defines how
computers communicate over the Ethernet.
IEEE 802.3: The Ethernet – the worldwide standard for computer networking.
IEEE 802.11: A standard for wireless networking in the 2.4 GHz ISM band – the wireless extension of the Ethernet. IEEE 802.11 defines the PHY and MAC layers of the
communication standard; all other aspects of wireless communication under 802.11 are identical to the Ethernet. Three physical layers are defined: FHSS, DSSS, and Infrared. The FHSS and DSSS PHY layers have transmission rates of 1 or 2 Mbps.
IEEE 802.11a: A standard for wireless networking in the 5 GHz ISM band with data rates of up to 54 Mbps.
IEEE 802.11b: A standard for wireless networking in the ISM band using DSSS at 5.5 and 11 Mbps.
IEEE 802.15: A standard for wireless networking in the 2.4 GHz ISM band. IEEE 802.15 and the Bluetooth standard are synonymous.
MAC Address (Media Access Control address or IEEE address): A unique hardware encoded address that is available for all Ethernet devices to help determine the device that is sending or receiving data.
Multi-path effect: Phenomenon that occurs when radio signal reach a destination via various paths, which can occur because of reflections. Radio waves that travel different paths and different distances arrive at the source at different times. They can add
constructively (e.g. when both paths result in the waves arriving in phase so the positive peaks line up, resulting in a larger signal) or destructively (e.g. when the paths result in the waves arriving 180 degrees out of phase so the positive and negative peaks align, resulting in a smaller signal).
PAN: Personal area network. A small network, often ad hoc, consisting of devices that connect to each other, but not to a larger network.
Piconet: Term used to describe a Bluetooth PAN.
PLMR (Private Land Mobile Radio): A broadcast band for radios that operate in the 450 – 470 MHz band with 6.25 kHz channel spacing. Some traditional telemetry systems
operate in this band. Previously, PLMR operated on 25 kHz channel spacing. With the refarming of this band to 6.25 kHz, there could be “interference to medical telemetry equipment, possibly making it to be unusable at times.”[1]
Receiver: A device that receives signals sent by a transmitter.
RF (radio frequency): The range of frequencies, between about 3 kHz and 300 GHz, over which electromagnetic radiation is used in radio broadcasts. It is subdivided into multiple bands, each whose maximum frequency is 10 times its lowest frequency: very low frequency (VLF, 3 kHz – 30 kHz), low frequency (LF, 30 kHz – 300 kHz), medium frequency (MF, 300 kHz – 3 MHz), high frequency (HF, 3 MHz – 30 MHz), very high frequency (VHF, 30 MHz – 300 MHz), ultra high frequency (UHF, 300 MHz – 3 GHz), super high frequency (SHF, 3 GHz – 30 GHz), and extremely high frequency (EHF, 30 GHz – 300 GHz).
Spread spectrum: A radio frequency modulation technique where the radio energy is spread over a wider bandwidth than required to transmit the data. The resulting transmission is more resistant to interference and more difficult to intercept than is a narrowband transmission.
Telemetry: Transmitting measurements of physical phenomena (e.g., patient monitoring data) to a distant recorder or observer.
Transceiver: A 2-way (bi-directional) device that can both transmit and receive signals. Medical telemetry is moving rapidly from using transmitter/receiver pairs to using
transceivers.
Transmitter: A device that sends information to a receiver in one direction.
UHF (ultra high frequency): A radio frequency band in the range of 300 MHz to 3 GHz. VHF (very high frequency): A radio frequency band in the range of 30 MHz to 300 MHz.
WMTS (Wireless Medical Telemetry Service): The frequency bands 608-614, 1395-1400, and 1429-1432 MHz designated by the FCC for the transmission of physiological
parameters and other patient-related information via radiated bi- or uni-directional electromagnetic signals.
WLAN (Wireless Local Area Network): Network that is connected by radio instead of wires.
REFERENCES
[1.] Federal Communications Commission. Wireless Medical Telemetry Service. Federal Register. July 17, 2000; 65(137): 43995-44010.
[2.] New Changes in FCC Rules Affects Everyone—Exploring Your Options. MSP Industry Alert. Lincroft, NJ: Medical Strategic Planning; 2000: 1-12.
[3.] Available by subscription at http://standards.ieee.org. Accessed on January 4, 2001. [4.] The Wireless Hospital: Extending the Reach of Your Information System Directly to
the Point of Activity. White Paper No. CJ0498. Holtsville, NY: Symbol Technologies Inc.;
April 1998.
[5.] Bluetooth Central. Bluetooth FAQ. Available at
http://www.bluetoothcentral.com/faq_plain.html. Accessed on January 3, 2001.
Shoemake, Matthew B., Wi-Fi (IEEE 802.11b) and Bluetooth Coexistence Issues and
Solutions for the 2.4 GHz ISM Band, Texas Instruments White Paper, February 2001,
Version 1.0.
[6.] Merritt, Rick, EE TIMES, Conflicts Between Bluetooth and Wireless LANs Called
REVIEW QUESTIONS
Multiple choice. Please choose the word or phrase that best completes the following statements.
1. The federal agency responsible for regulating the radio frequency spectrum in the United States is:
A. FTC. B. NASA. C. EPA. D. FCC.
2. The VHF and UHF bands traditionally used by medical telemetry systems are becoming risky for hospitals because of the potential for:
A. Increased interference from TV and PLMR signals. B. Health effects of electromagnetic radiation.
C. Electrostatic shock. D. Power failure.
3. WMTS will operate on UHF channel 37 on a co-primary basis with: A. HDTV stations.
B. PLMR.
C. Radio astronomy. D. Military radar.
4. A WMTS station may transmit: A. Voice.
B. Patient Data. C. Video.
5. IEEE 802.11 is an international standard for: A. Wide area networks.
B. Hard-wired local area networks. C. Wireless local area networks. D. Personal area networks.
6. Wireless devices that operate in the ISM band: A. Are transportable between facilities without retuning.
B. Can co-exist and can be compatible, even if manufactured by different vendors. C. Are bi-directional.
D. All of the above.
7. FHSS, DSSS and Bluetooth systems can co-exist with little interference as long as the devices are
A. Out of RF range of each other. B. Spaced 2 feet apart
C. Spaced 2 meters apart
D. None of the above, spacing is not important
8. Medical telemetry has primary user status in: A. Designated WMTS frequencies.
B. The ISM band.
C. VHF TV channels 7 to 13. D. All of the above.
9. Voice transmission and PDAs are supported by: A. Conventional medical telemetry systems.
B. WLANs operating in the WMTS. C. WLANs operating in the ISM band. D. All of the above.
10. Bluetooth-based wireless technology can be expected to compete with 802.11-compliant LANs.
A. True. B. False.
Answers to Review Questions 1. D 2. A 3. C 4. B 5. C 6. D 7. C 8. A 9. C 10. B
