Wireless local area network in a prehospital environment
© Chen et al; licensee BioMed Central Ltd. 2004
Received: 02 December 2003
Accepted: 31 August 2004
Published: 31 August 2004
Wireless local area networks (WLANs) are considered the next generation of clinical data network. They open the possibility for capturing clinical data in a prehospital setting (e.g., a patient's home) using various devices, such as personal digital assistants, laptops, digital electrocardiogram (EKG) machines, and even cellular phones, and transmitting the captured data to a physician or hospital. The transmission rate is crucial to the applicability of the technology in the prehospital setting.
We created two separate WLANs to simulate a virtual local are network environment such as in a patient's home or an emergency room (ER). The effects of different methods of data transmission, number of clients, and roaming among different access points on the file transfer rate were determined.
The present results suggest that it is feasible to transfer small files such as patient demographics and EKG data from the patient's home to the ER at a reasonable speed. Encryption, user control, and access control were implemented and results discussed.
Implementing a WLAN in a centrally managed and multiple-layer-controlled access control server is the key to ensuring its security and accessibility. Future studies should focus on product capacity, speed, compatibility, interoperability, and security management.
The development of the Internet has encouraged doctors to use computers and hospitals to use wireless communications , since wireless technology offers many benefits over its wired counterpart, including ease of installation and access to network information [2–6], and higher productivity and convenience . One study using personal digital assistants (PDAs) connected to a network showed that the device was of limited use in transmitting data in prehospital stroke management . Another study showed that cellular phones, pagers, or other radio-based devices will remain an important communication mode in the near future . The advancement of wireless local area network (WLAN) technology provides the potential to allow physicians to obtain a patient's information anywhere, even before the patient reaches the emergency room (ER) (Orthner, personal communication). Timely access to a patient's information may fundamentally improve patient care  in both pre- and in-hospital settings, due to earlier doctor interventions.
At present, patient data such as electrocardiograms (EKG) and demographics are seldom sent from the prehospital environment to the ER before the ambulance arrives . As a result, some preventive measures have to be given, regardless of need (e.g., aspirin  or thrombolysis  for presumed acute myocardial infarction. However, despite the potential benefits of wireless technology in prehospital settings, the application of this technology has been slow and few related studies have been carried out.
The objective of this study was to assess the ability of wireless technology to facilitate data communication between a prehospital setting and an ER (Orthner, personal communication). The idea was for all the data collected by paramedical personnel to be transmitted to an ER server from the patient's home, on the way to the ER, or upon arrival at the ER. Thus the transmission rate is crucial to the usefulness and applicability of the technology. To test the feasibility of wireless data transmission under the various scenarios, two separate WLANs were created, one around our office and another in a house. In this report, we discuss our testing of the wireless technology, and its potentials and limitations in simulated prehospital settings.
The WLAN products used (Aironet 340 and 350 series wireless client adaptors and access points (APs); Cisco) offered 11-megabits-per-second (Mbps) transmission rates, built-in security features (including 40- to 128-bit encryption) and Web-based management. The transmission rates of files of different sizes were measured with different APs, patch antennae, clients, and transfer methods. The security of patient data was ensured using a centrally managed Access Control Server (ACS). Other issues such as standards, roaming, and cost are also discussed here.
Within a WLAN, data are transmitted between a server and its wireless clients via an AP antenna. Both workstations and laptops were used here as servers, and file transfer rates were measured for both systems. We used Gateway Select series, Dell Inspiron series, and a Toshiba Satellite laptop computer as clients. The computers had CPU operating at 0.8–1.2 GHz, 256–1024 MB of RAM, and 10–40 GB hard drives, and all ran the Microsoft Windows 2000 Professional operating system. PDAs (Ipaq Pocket PCs, models 3550 and 3570, 200-MHz CPU, 32–64 MB RAM, Compaq) were also tested as wireless clients. The wireless coverage was tested using two APs (Aironet 340 and 350 series, Cisco) and a patch antenna (S2406P, Cushcraft Corporation). The feasibility of using cellular phones (StarTac 7868, Motorola) in data transmission in the area not covered by the WLAN was also tested.
The various software used in the study for wireless client management, file transfer, and access control included the Aironet Client Utility (Cisco), Link Status Meter (LSM, Cisco), the ACS (V3.0, Cisco), Phone Tools (BVRP Software) for faxing, and file transfer protocol (FTP) for measuring the file transmission rate. The software LSM classifies the link status as the percentage of maximum signal strength and quality: "excellent" (>75%), "good" (40–75%), "fair" (20–40%), or "poor" (<20%); where signal strength and quality refer to the client adapter's radio signal at the time packets are being received, quantified as bytes transmitted and received and the errors that occur. Detailed descriptions of the mentioned software are available from the manuals provided by the vendors.
The WLAN and its configuration
The Aironet 340 and 350 series APs were tested by a two-step approach. In the first step, one AP was connected directly with the server that was not connected to the campus Ethernet backbone. In the second step, the AP was assigned a public Internet Protocol (IP) address and connected to the Ethernet backbone in the Susan Mott Webb Nutrition (Webb) Building at the University of Alabama at Birmingham. The IP address was assigned to an AP through either HyperTerminal or a Web console using a Web browser. An administrator ID and password were then created to enhance the Web console security. The client computer required a type II PCMCIA (Personal Computer Memory Card International Association) card slot. Every client needed a functional IP address to become associated with the AP. The Wired Equivalent Privacy (WEP) keys were enabled for both the AP and the clients to ensure two-way authentication.
Comparison of different coverage of APs and patch antennas
The coverage of the WLAN was found to highly structure dependent. The floor of the Webb building measures about 60 by 25 meters. A single Aironet 340 AP was unable to cover the entire floor with a "good" link status. This was achieved using two (more powerful) Aironet 350 APs. Achieving the "excellent" link status on the floor required the use of the S2406P (Cushcraft) patch antenna. The wireless clients associated with the AP had a "fair" or "good" status one floor up and one floor down from the floor where the AP was located. There was a small area outside the 5-floor building in which the clients could associate with the AP with a "poor" status.
Link statuses around a simulated patient's home
File transfer rate with laptops
File transfer directions and rates (kbps, mean ± SD, n = 4)
A. File transfer rate in an open area between two laptops for different file sizes and link statuses
B. Transfer rates for a 50-MB file between a server and clients under different link statuses
I. Server to one client
3090 ± 70
5540 ± 180
2870 ± 280
5530 ± 130
630 ± 500
2070 ± 550
II. One client to server
2600 ± 110
5210 ± 410
2710 ± 330
4350 ± 1030
340 ± 80
750 ± 440
III. Two clients to server
2160 ± 60
3130 ± 320
2140 ± 370
3230 ± 40
IV. Client 1 to client 2
2400 ± 70
2560 ± 130
V. Server to one client while moving
2500 ± 500
1200 ± 410
Rate of file transfers involving multiple clients (kbps, mean ± SD, n = 4)
A. Transferring files individually
5830 ± 1340
Toshiba Satellite 350
5250 ± 600
5260 ± 990
4890 ± 490
5960 ± 910
B. Transferring files simultaneously
1330 ± 180
1430 ± 140
1740 ± 490
Toshiba Satellite 350
1600 ± 340
1580 ± 160
1610 ± 240
1400 ± 190
1470 ± 140
1500 ± 130
1490 ± 200
1580 ± 240
1590 ± 260
1240 ± 440
1540 ± 190
1560 ± 280
1690 ± 200
1900 ± 400
7140 ± 110
Toshiba Satellite 350
1850 ± 60
1860 ± 350
6830 ± 120
1730 ± 170
1820 ± 220
4360 ± 2800
1690 ± 190
2000 ± 470
3780 ± 1060
1840 ± 310
1770 ± 400
3160 ± 1400
File transfer rate with Pocket PC and cellular phone
Pocket PC and cellular phone file transfers (SD: 10~25%, n = 4)
A. Pocket PC to laptop
B. Cellular phone to a fax machine
File size (kB)
File size (kB)
Enhancing the WLAN security using an ACS
A private local area network
Cost of a small-scale WLAN
Cost of a WLAN of similar scale to the one implemented here
Unit price (US $)
Workstation and accessories
Laptop and accessories
Pocket PC and accessories
Palm Pilot and accessories
Cellular phone and network costs
Total (US $)
After collecting all patient data at a patient's home, the data must ultimately be transferred to the ER. This involves two critical synchronization steps: (1) from the patient's home to a server on the ambulance, and (2) from the ambulance to the ER (while in transit or upon arrival). Here the WLAN was employed for both of these steps, using Microsoft Briefcase and Windows Workgroups. Automatic synchronization with the destination server and batch synchronization were desired. The ultimate objective is, however, to link the two synchronization steps using a long-range antenna that reaches up to 25 miles (e.g., a yagi antenna from Cisco). This would significantly shorten the time needed to transfer data from a patient's home to the ER, since the data will reach a ER WLAN earlier. We are currently performing the associated experiments. We also tested the use of a cellular phone and other types of PDA (e.g., a Palm Pilot) with network capabilities in transmitting a small (up to 50 MB) but critical file. The results suggested that cellular phones or PDAs with network cards can be effective alternatives to the use of a long-range antenna to transmit data from a patient's home to the ER.
Wireless transfer of EKG data
EKG data are considered very valuable in the early detection, early intervention, and possibly better outcome of heart attack patients . The use of a wearable device with sensors to monitor specific physiological signals and communicate with a personal server has been reported . Land-based telephone lines have also been employed to transmit EKG data and for monitoring by clinical personnel . In our study, we showed that files up to 5–10 MB (the size of a typical high-quality digitalized EKG image) could be transferred using FTP or other file transfer methods within minutes. Handheld devices such as a Pocket PC and cellular phone may be useful in transmitting EKG files when the ambulance is still at the patient's home, as shown in Table 3. The time required to transmit a file is proportional to its size in the case of a Pocket-PC-to-laptop transfer, but this was found not to be the case between a cellular phone and a fax machine. The reason for this discrepancy is unknown, and needs to be further investigated. The use of a long-range antenna may ultimately be needed to increase the transmission capacity and speed.
The WLAN operates at 2.42 GHz with an output power of 100 mW, which may pose a risk of interference with medical devices using similar frequencies. Previous studies have shown that a WLAN may interfere with medical devices in close proximity  but is unlikely to be interfered with by such devices . Further studies are needed to clearly address the possibility. In another study, infrared modems exhibited a similar performance to a wired system even in an electrically noisy environment , indicating that infrared wireless connectivity can be safely and effectively used in operating rooms. These studies suggest that a WLAN can be acceptable for use in prehospital settings if careful interference testing is conducted.
Security and privacy
The major concern over a WLAN is its security [17–20], especially when personal information is involved. It has been reported that the open-air clear-text transmission of WEP keys and MAC addresses increases network vulnerability [13, 27–29]. One approach to minimizing the risk is to control the access of remote and/or wireless clients through the Remote Access Dial-in User Service and AP management using the Extensible Authentication Protocol. The regulation by the Healthcare Insurance Portability and Accountability Act may further delay an organization's decision to adopt WLAN technology, although both the Institute of Electrical and Electronic Engineering (IEEE) 802.3 and the OpenAir standard specifications offer security protection (these are the two major standards in the unlicensed commercial 2.4-GHz WLAN market). According to our experiences, the following steps are required to implement a secure WLAN. First, anonymous access should be disabled and the Service Set Identifier of an AP and data encryption key (WEP key) should be enabled. Secondly, a Web console should be used to designate an administrator and manage APs and clients. Thirdly, an ACS server such as Cisco Secure ACS should be implemented to work with Active Directory in order to offer both device- and user-dependent AAA services. Digital certificates should be applied whenever possible for mutual authentication to protect sensitive information through secure server access and secure Web access. In addition, the physical security of the APs, client, and server computers can never be overemphasized.
Standards and interoperability
The IEEE 802.11 specification addresses both the physical and MAC layers (Orthner, personal communication), and the OpenAir 2.4 interface standard is derived from the Wireless LAN Interoperability Forum  and needs to be interoperable with the IEEE 802.11 standards. The 5-GHz band WLAN standard (IEEE 802.11a) will become more popular once its cost decreases and the required components become more widely available. The use of standardized compliant devices facilitates communication and interoperability.
Limitations of the study
The present study was mainly based on the Windows operating system and Cisco wireless products. IEEE 802.11a products for the next generation of WLANs are emerging quickly from various vendors. Hence the stability, compatibility, and interoperability with other vendors require further evaluation. Although currently it is relatively expensive to implement a WLAN using this new protocol, the prices and capabilities are expected to improve within the near future.
Application of WLAN technology will help both paramedics and other health-care professionals in their daily acquisition of information in a localized area such as within a patient's home, an office, a small clinic, or an ER. Implementing a WLAN in a centrally managed and multiple-layer-controlled ACS is the key to ensuring its security and accessibility. Future studies should focus on product capacity, speed, compatibility, interoperability, and security management.
This project has been funded in part with US federal funds from the National Library of Medicine, National Institute of Health, under Contract No. N01-LM-0-3524 and under Fellowship No. F38LM07185.
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