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Exploiting the 5 GHz Band for Medical Devices


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In the early 2000s, wireless local area networking became a mainstream technology known as Wi-Fi. Computing devices such as laptops and notebooks began to support the IEEE 802.11b standard, which has a top data rate of 11 Mb/s and operates in the 2.4 GHz frequency band. After 802.11b came 802.11g, which is five times faster, and then 802.11n, which boasts data rates that rival those of wired networking options. Today, Wi-Fi is found not just in computing devices but also in medical devices such as imaging systems, patient monitoring systems, and infusion pumps.

The vast majority of Wi-Fi client devices operate in the 2.4 GHz band. Only three nonoverlapping channels are available in the band, so every Wi-Fi client and infrastructure device, such as an access point (AP), must operate on one of three channels. When two Wi-Fi clients or APs on the same channel are in relatively close proximity, the transmissions of one act as interference or noise to the other. The 2.4 GHz band also is the home of microwave ovens, baby monitors, some cordless phones, and Bluetooth. A Wi-Fi client transmits on one channel, but other wireless devices can cause interference across the entire band. In many hospitals, the 2.4 GHz band is nearly saturated with wireless traffic. As Wi-Fi continues to grow in popularity ensuring reliable connectivity in the 2.4 GHz band will become more and more challenging.

Fortunately, there is another frequency band for Wi-Fi: the 5 GHz band. This band offers many more Wi-Fi channels (23 in North America). Because few devices operate at 5 GHz, the band is relatively uncluttered. A Wi-Fi deployment in the 5 GHz band, however, is different than a deployment in the 2.4 GHz band. Let’s look at some of the differences.

Multipath. When sound waves bounce off objects and arrive at their destination at different times, the result is an echo. When Wi-Fi transmissions bounce off objects and arrive at their destination at different times, the result is called multipath propagation. Multipath propagation usually has a negative impact on Wi-Fi operation because the recipient of a transmission must sort through multiple copies of transmissions, some of which arrive out of sequence. The effects of multipath propagation are more significant in the 5 GHz band than in the 2.4 GHz band.

Range. Range is the maximum distance at which a Wi-Fi client and an AP can establish and maintain a connection. Range at 5 GHz tends to be less than range at 2.4 GHz because of factors such as waveform characteristics, signal attenuation, data rates, and transmit power.

There is an inverse relationship between frequency and signal propagation indoors—the higher the frequency, the shorter the distance a signal travels. The frequencies used by Wi-Fi at 5 GHz are approximately twice as high as frequencies used by Wi-Fi at 2.4 GHz (shown in Figure 1). As a result, devices operating in the 2.4 GHz band typically provide greater range than those operating in the 5 GHz band.

Figure 1. Shown are 2.4 and 5 GHz wave forms (click image to enlarge).

Attenuation is the degree to which a signal is absorbed by physical objects. Lower frequency waves generally penetrate solid objects more than higher frequency waves. A 5 GHz wave is attenuated by common building materials to a greater degree than a 2.4 GHz wave. On the other hand, the 2.4 GHz wave form is optimally absorbed by water. A microwave oven operates at 2.4 GHz because that is the frequency at which the water in the food absorbs the microwave energy, creating heat.

Lower data rates allow for operation at greater distances than higher data rates, so lower data rates at 2.4 GHz also lead to greater range there. Wi-Fi radios that operate in the 2.4 GHz band support 802.11b (in addition to 802.11g or 802.11n), and 802.11b supports lower data rates than the 802.11a and 802.11n standards in the 5 GHz band. 802.11b’s lowest rates of 1 and 2 Mb/s, however, may be insufficient for even the most modest performance requirements of today’s network applications. Some hospitals turn off these lowest rates, even though they allow for greater range, because connections at such low data rates are of no operational benefit and can reduce performance for 802.11g devices.

The final factor that affects range is transmit power. Because transmit power at 5 GHz tends to be less than transmit power at 2.4 GHz, range at 5 GHz tends to be less than range at 2.4 GHz.

Benefits of 802.11n in the Infrastructure

By taking advantage of 802.11n in Wi-Fi infrastructure, a hospital can realize significant improvements in 5 GHz range. Two features of 802.11n, transmit beam forming (TxBF) and maximal ratio combining (MRC), actually leverage multipath propagation. By using dual-band 802.11n APs that support multiple antennas, a hospital can improve coverage patterns and even range for all Wi-Fi clients, even those that do not support 802.11n.

With TxBF, the AP sends out a different copy of a signal on each antenna. A client that is unable to receive the signal from one antenna may receive that signal from a different antenna. Without TxBF, the client is out of range of the AP; with TxBF, the client is in range. By filling in nulls or dead spots, TxBF increases transmit range.

TxBF enhances transmitting; MRC enhances receiving. With MRC, each of the AP’s antennas is a potential recipient of a signal from a client. When one antenna cannot receive a transmission from a client but another antenna can, the client is effectively in range. By filling in nulls or dead spots, TxBF increases receive range.

While TxBF and MRC increase range, interference reduces it. Radiofreqency (RF) interference, which is prevalent in the 2.4 GHz band, raises the noise floor and reduces the signal-to-noise ratio (SNR), which is the difference between the transmitted signal and the surrounding interference. As the SNR decreases, so does range. Ever-increasing activity in the 2.4 GHz band nullifies some of the range advantages of that band.

When a hospital’s Wi-Fi infrastructure supports dual-band 802.11n, the slight range penalty at 5 GHz is offset by the reliability that results from the relative lack of interference in the band. Due to the rate-shifting capabilities of all 802.11 standards, client devices in the 5 GHz band may, at worst, operate at slightly lower data rates than in the 2.4 GHz band when at the same distance from the AP.

Mobility

When a Wi-Fi client device initializes, it must find an AP to which to connect (associate). When its connection to that AP becomes tenuous, it must find an AP that offers a better connection. The process of searching for an AP is referred to as scanning. There are two types of scanning:

  • Active scanning: For each channel on which an AP may be operating, the client transmits a probe request and waits to receive a probe response. Based on probe responses, the client determines the best AP to which it can associate.
  • Passive scanning: Instead of transmitting probe requests, the client listens on each channel for beacon frames transmitted at regular intervals by APs.

An AP will respond to a probe request within 20 ms, whereas an AP may take 100 ms or longer to issue a beacon. Because a client spends less time on each channel waiting for information from APs, active scanning is more efficient than passive scanning.

While doing active or passive scanning, a client device is incapable of sending and receiving payload data. Because of this, long scans can have a negative impact on applications that require a persistent network connection.

A Wi-Fi feature unique to the 5 GHz band is dynamic frequency selection (DFS). In many parts of the world, some 5 GHz channels, known as DFS channels, are used by weather and military radar systems that have priority over Wi-Fi devices. Table 1 shows 5 GHz channels, detailing the available DFS (D) and non-DFS (√) channels by regulatory domain.

Table 1. This table shows 5 GHz channels (click on image to enlarge).

Before transmitting on a DFS channel, an AP must first listen for the presence of a radar system. If the AP detects radar, then the AP must flag the channel as unavailable, move to an unoccupied channel, and instruct all associated client devices to do the same.

Because wireless clients cannot detect the presence of radar, they first must conduct passive scans of each DFS channel to detect whether or not APs are sending out beacons on that particular channel. Once beacons are detected, the client is allowed to do active scans and connect to an AP on that channel. If the AP later detects radar and moves to another channel, then the client must move to that same channel (to maintain its connection to that AP) or roam to another AP.

Given that a passive scan may take hundreds of milliseconds per DFS channel, the use of DFS channels for devices that require a persistent network connection, such as medical devices, is discouraged, especially in the FCC and ETSI regulatory domains where there are 15 DFS channels.

Recommendations

For a hospital network administrator, the 5 GHz band presents greater challenges than does the 2.4 GHz band in areas such as range and mobility. The 5 GHz band is attractive, however, because it offers greater network capacity and relatively uncluttered airwaves. The need to incorporate 5 GHz operation into hospital Wi-Fi networks will increase over time as the 2.4 GHz band becomes more overused by a variety of devices.

Here are some recommendations for using the 5 GHz band in a hospital:

  • AP placement: Because the 802.11a and 802.11n standards that operate in the 5 GHz band provide for data rates no lower than 6 Mb/s, APs should be placed such that client devices maintain connectivity at data rates of 6 Mb/s or greater. When placing APs for operation at a given data rate, spacing between APs should be determined based on 5 GHz requirements rather than 2.4 GHz requirements.
  • 802.11n: A dual-band 802.11n infrastructure brings benefits to all Wi-Fi client radios, namely more reliable connectivity and greater range. Hospitals should upgrade to dual-band 802.11n infrastructure as resources allow.
  • DFS channels: Although the number of channels available in the 5 GHz band provides for increased network capacity, it also results in longer scan times which can impact performance and reliability. This is particularly the case when only passive scanning is allowed, as with DFS channels. Therefore, network architects should not configure APs for operation on DFS channels unless absolutely necessary and should remove these channels from the scan list on medical devices.

Chris Bolinger is vice president of engineering at Summit Data Communications (Akron, Ohio).

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Chris Bolinger
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