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1 Over time, wireless services at airports have expanded from a voice communication tool for radio communications between air traffic control and pilots or between airport public safety agencies such as police, fire, and airport operations, to a ubiquitous and complex system of voice, data, and video services usually supported through a shared network infrastructure. Today the dominant wireless services utilize the wireless fidelity (WiFi) and cellular networks. Wireless services, particularly WiFi, are in transition from being high-end amenities, used by relatively few travelers, to technologies commonly used by a majority of travelers and, sig- nificantly, also by the airport to support core operational functions. However, these networks often are not viewed or funded as an essential component of an airportâs core infrastructure. They are funded and managed in a variety of creative ways, which are understandable given their history, but are proving insufficient for their emerging role at airports. The complexity of the highly technical task of managing these networks requires funding and management structures that are equal to the task. The management structures used at airports must also be appropriate for the larger ecosystem of the WiFi and cellular communities. The business structures and ecosystem of WiFi and cellular networks, and the companies that create and support them, are a study in contrast. The service quality gives evidence of the differences that result from these contrasting business ecosystems. Complaints about performance and radio frequency interference are increasingly rare from cellular users. However, complaints are common and increasing among users of the WiFi network for data communications. Both WiFi and cellular are wireless service networks that are susceptible to radio frequency (RF) interference; however, business and other non-technical differences are resulting in very different technical outcomes related to RF interference. A key finding of the research performed to create this Guidebook was that solving RF interference problems often requires dealing with several different issues. An example is the RF interference created by travelersâ choices. The market is giving consumers a variety of choices for accessing the Internet. They can use the airportâs WiFi service, they might prefer to use the free WiFi provided by an airport coffee shop or airline VIP lounge, or they can access the Internet through their cellular network. Some of these choices have the potential for creating interference to the airportâs WiFi network. The need for a good WiFi plan is critical for an airport to provide wireless capabilities that meet the current and future growth of its use by travelers and employees. Airports must first understand the basic business models of service providers. For example, the cellular network operates in dedicated frequency bands which the cellular operators purchase rights to through a competitive bidding process. These cellular companies have paid billions of dollars for the right to utilize licensed spectrums. In addition, the companies have funded the cost of establishing and operating their own networks. Funding comes from subscribers S U M M A R Y A Guidebook for Mitigating Disruptive WiFi Interference at Airports
2 A Guidebook for Mitigating Disruptive WiFi Interference at Airports who each month contribute to a portion of the initial cost of setting up the network and for the ongoing network operating cost. Network operators decide what devices they will allow onto their networks and subscribers must choose a device from among those approved for use on the network. To ensure the quality of their network, the cellular operators have cre- ated a device certification through their trade association, CTIA, and rigorously test devices before allowing them onto their network. In contrast, the WiFi network providers operate in unlicensed spectrums and networks which are built ad hoc, and in some cases with no single entity responsible for the entire network. Travelers have come to expect free WiFi service at an airport, just as they expect lights, air conditioning, and well-maintained bathrooms. Travelers bring their own equip- ment such as smartphones, laptops, and tablets, and commonly expect to pay no fees to use the airportâs network. In contrast to the cellular network, the revenue stream generated by the WiFi network is much more complex, indirect, and difficult to quantify. Further, there are often discontinui- ties between the cost of network maintenance and the revenues generated by the network. Users may pay for content, but services like video and voice are only effective if the network provides enough bandwidth. The traveler may be willing to pay for Netflix but then expects the airport to provide the bandwidth to watch that video stream. The result is that a patch- work of revenue sources may be used to fund WiFi networks, including relatively unpopular user fees, advertising, and a variety of commercial support services. Although the WiFi Alli- ance has a WiFi certification program, no entity has the power to approve devices before they are allowed onto the network. The result is that funding is problematic and the power to manage the network is limited. It is important to understand how airports install and operate their WiFi networks, because any interference problem must be resolved within the structure used to manage the network and within the limits of available resources. Simply put, time and money are limited and solutions to network problems are only viable to the degree that they conform to these realities. One of the emerging realities is conflicting expectations between some airport managers and travelers. Service expectations of WiFi networks vary widely among both airport net- work managers and travelers. Some airport managers and travelers have come to view WiFi as part of the necessary infrastructure. Others view WiFi as a free service and fringe benefit. With the rapidly changing environment surrounding WiFi, expectations on all sides extend across a significant range. Research conducted at airports for this Guidebook uncovered a number of important findings. One is that nobody really understands the range and variation of indoor RF envi- ronments. The spectrum measurements made during this study represent one of the few and current broad surveys of the RF environments that exist at airports. A direct conse- quence of this effort is the initiation of a separate effort led by researchers at the National Institute of Standards and Technology (NIST) to gather spectrum data on an even wider basis so that product developers and network managers can have a better understanding of RF environments they must plan for. This additional effort is being coordinated through the ANSI ASC C63.27 standards committee. Another finding is that the dominant source of WiFi interference is from other WiFi devices. WiFi operates in shared use bands and a large variety of other types of equipment operate in these bands, particularly in the 2.4 GHz band. Historically there have been many documented cases of interference from other kinds of equipment. For example, microwave ovens, Bluetooth, and cordless phones have all created interference problems. However, in the current research the most common source of interference to WiFi at airports was from other WiFi devices.
Summary 3 The study also showed that there is a strong correlation between band crowding and inter- ference. Repeatedly it was documented that the most significant interference problems occur in the most spectrally congested locations and on the most heavily used channels. It could be said that interference problems largely can be attributed to crowding in the 2.4 GHz band, where data rates averaged under 6 MBs, compared to channels in the less crowded 5 GHz band, where data rates were significantly higher, nearly reaching 24 MBs (Figure 1). When channels become congested and suffer interference, their data rates slow down and the error rates go up and result in data retransmissions. Although packet retransmis- sions occur routinely in WiFi communications, the common metric is that retransmissions greater than 1% are a symptom of interference in the channel. Overcrowding in the 2.4 GHz band results in lower data rates and a higher level of transmission errors, which require a large number of packet retransmissions, in contrast to the 5 GHz band (Figure 2). Average Data Rates by Channel (Mbits/s) 7. 86 3 8. 48 8 2. 72 3 6. 45 5 7. 43 5 5. 98 0 3. 86 6 3. 80 7 5. 43 2 3. 43 4 9. 27 9 14 .6 82 13 .8 40 17 .3 76 14 .4 12 14 .4 33 22 .4 39 18 .8 92 16 .8 55 1 9. 32 4 23 .4 97 10 .8 88 10 .3 05 17 .0 67 12 .3 17 13 .9 18 12 .5 60 14 .9 79 9. 07 1 1 2 3 4 5 6 7 8 9 10 11 36 40 44 48 52 56 60 64 10 0 10 4 10 8 11 2 13 6 14 9 15 3 15 7 16 1 16 5 WiFi Channel Figure 1. WiFi data rates by channel observed at 28 airports. Figure 2. Packet retransmission rates observed at 28 airports. Percent Retries by Channel 0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 20% (r et ry p ac ke ts /t ot al p ac ke ts ) Channel 1 Channels 2-5 Channel 6 Channels 7-10 Channel 11 All 5 GHz traffic
4 A Guidebook for Mitigating Disruptive WiFi Interference at Airports The crowding is compounded by WiFi traffic that is crowded into two channels of the 2.4 GHz band that together account for 57% of distribution (Figure 3). To make the situa- tion even worse, the 2.4 GHz band is used not only by WiFi but also by Bluetooth, ZigBee, and a wide range of other devices. The 2.4 GHz industrial, scientific, and medical (ISM) band is clearly a victim of its own success. For reasons perhaps not unlike those that require workers to drive downtown in the morning and to the suburbs in the evening, the 2.4 GHz band is host to high levels of congestion and interference, but it nevertheless continues to be where the bulk of the WiFi traffic resides. Radio frequency interference should be addressed strategically, taking into consideration how much wireless networks are used by travelers and by airport operations. A cost-effective RF interference solution is critical for viable service implementation. An emerging source of interference is largely being created by the need to support two networks in an increasing number of frequency bands. The result is that antennas for the WiFi and cellular networks are often placed close to each other, and at times they share the same antenna in a shared distributed antenna system. Strong RF signals can create inter- modulation products and a variety of related problems. These issues are well-known to the military, where many RF sources are often crowded close together on aircraft and ships. The same problems are starting to emerge in airport systems. Another class of interference problem is created by technology changes. In some cases, innovations made to reduce interference actually end up causing more interference. An example of technology changes causing interference is Institute of Electrical and Electronics Engineers (IEEE) 802.11b. Newer versions of the IEEE 802.11 standard have moved away from direct sequence spread spectrum modulation to orthogonal frequency-division multi- plexing (OFDM) modulation. However, support for the older IEEE 802.11b was necessary for backward compatibility. In the research for this Guidebook, it was not uncommon to still find IEEE 802.11b devices operating. However, they often become sources of interfer- ence because they are mismatched with how most WiFi devices currently operate. This Guidebook describes the issues surrounding interference and congestion, and options for solving these problems, including solutions that build on cooperation among all stakeholders and reduce RF interference in ways that provide a good return on investment. Figure 3. Traffic distribution among WiFi channels. WiFi Traffic Distribuon by Channel All 5GHz traffic 20% Channel 11 44% Channels 7 10 5% Channel 6 9% Channels 2 5 9% Channel 1 13%
