Engineering the wireless hospital: RF issues

This post originally ran on the Laird Technologies blog

In our recent blog post, Engineering the Wireless Hospial: Client Device Requirements, we explored the different types of client devices that typically connect to a Wi-Fi network in a hospital. Because Wi-Fi is a shared medium, every Wi-Fi device must share the airwaves with every other Wi-Fi device. Those same airwaves also may be used by other wireless technologies. When a Wi-Fi client is operating on a particular channel or area of the radio frequency (RF) spectrum, other RF transmissions on that same channel are considered interference because they interfere with transmissions from and to the Wi-Fi client.

A Wi-Fi client connects to an access point (AP) because the Wi-Fi radio in the client receives a strong signal from the AP. This signal enables the connection between the AP and the client to support a high data rate. As the client moves away from the AP, the strength of the signal decreases and the relative impact of interference sources in the area increases. Some transmitted data packets are not received, forcing the sender to retry the transmission. To maintain the reliability of data transfer over the connection, the client and AP negotiate a lower data rate. If interference becomes too heavy, then the client may lose its connection to the AP.

The IT staff in a hospital tries to design and implement Wi-Fi networks that support secure and reliable connectivity to all Wi-Fi devices. Establishing such networks requires careful planning. IT staff must have a comprehensive understanding of the hospital day-to-day operations in order to position APs optimally to ensure sufficient coverage in all areas of the hospital. A thorough site survey is a critical first step. Important RF factors that must be considered include the number and types of devices that will be used in each area, the workflow in each area, and the sources of interference in each area.

The results of a site survey are not valid forever, because hospitals are very dynamic environments. Hospital wireless networks require continuous network management and monitoring in order to perform at top efficiency. And even when a Wi-Fi network is well-designed, there still will be RF issues.

RF issues are compounded in many hospitals because of an overuse of the 2.4 GHz frequency band and an underuse of the 5 GHz band. The 2.4 GHz band supports only three frequency channels, but the vast majority of Wi-Fi devices operate there, as do Bluetooth devices, Zigbee devices, some cordless phones, baby monitors, and other wireless devices. The 5 GHz band, in contrast, has up to 23 Wi-Fi channels, few Wi-Fi devices, and few competing wireless technologies. The greater network capacity and relatively uncluttered airwaves of the 5 GHz band make it very attractive for mobile medical devices.

Utilizing the 5 GHz band, however, does present some unique challenges to hospital IT staff. First and foremost, most Wi-Fi clients do not support the band. 5 GHz radios tend to offer less range than 2.4 GHz radios, so coverage patterns are different. The abundance of 5 GHz channels presents challenges to mobile devices as they attempt to roam from one AP to another. Many mobile medical devices do not yet support 5 GHz. For more information about using the 5 GHz band in a healthcare setting, please refer to our white paper on this topic, located here.

In order to solve some connectivity problems, a recent Wi-Fi Alliance whitepaper called Wi-Fi® in Healthcare: Improving the user experience for connected hospital applications and devices, suggests that hospitals should utilize both the 2.4 and the 5 GHz band. The most feasible way to do this is to segregate BYODs and hospital-managed devices so that they are not operating on the same Wi-Fi bands. Where possible, hospital-managed devices should operate in the 5 GHz band, and BYOD devices should operate in the 2.4 GHz band. This will grant wireless medical devices that transmit metrics that are critical to patient care, access to greater bandwidth and relatively open airwaves.