People love Wi-Fi access to the Internet. More and more, they are using the wireless connection technology at Starbucks caf¿s, in airport lounges and at home. Wi-Fi seems irresistible because it makes the Net available to users anytime, anywhere. It provides fast communications links that allow e-mail messages to appear almost instantly and Web pages to paint computer screens quickly--all with the mobility and freedom that has made cell phones nearly ubiquitous.
Pyramid Research, a communications industry research firm, predicts the global number of Wi-Fi users could top 271 million by 2008, with 177 million of them in the U.S. Today's Wi-Fi community already supports a vibrant international business in Wi-Fi equipment, estimated at about $3 billion annually, according to extrapolations of figures produced by In-Stat, another market research company. But the very popularity of Wi-Fi also brings problems. As Wi-Fi networks become ever more heavily used, they may be unable to handle the expanded traffic, causing clients' devices to become bogged down with slow service and long delays.
Wireless LANs are subject to problems because the technology relies on radio, which has drawbacks.
Even when the technology is working properly, wireless access is not as swift as that provided by high-speed wired connections to the Internet, such as digital subscriber line (DSL) or cable modem links, for example. Radio signals cannot hope to match the transmission speeds that copper wires or fiber-optic cables make possible. Nor can Wi-Fi, or other wireless technologies that rely on radio, supply the same degree of security; the transmissions can be intercepted by nearby radio receivers.
Many of these problems were evident even in 1993, when I led a team at Carnegie Mellon University to build Wireless Andrew, the first large-scale wireless local-area network (LAN) and a precursor to today's Wi-Fi networks. Completed in 1999, Wireless Andrew now connects the entire campus [see "Terrestrial Wireless Networks," by Alex Hills; Scientific American, April 1998].
A dozen years since the inception of our wireless network at Carnegie Mellon, much has happened in the world of wireless. Some difficult problems have arisen because of markedly increased Wi-Fi use, but substantial progress has also been made in solving them. Before considering these developments, however, we must discuss how Wi-Fi operates.
Wi-Fi networks comprise Wi-Fi-equipped mobile computers (laptops or handhelds) or special Wi-Fi telephone handsets, as well as access points (APs). APs are base stations that communicate by radio and by wire with both mobile systems and the networks that ultimately provide entr¿e to the Internet. Each AP can send and receive signals within a limited range, typically 20 to 50 meters inside a building. The coverage area of an AP forms a three-dimensional spherelike cell (analogous to a mobile telephone cell but much smaller) that can serve many mobile devices within it simultaneously.
Wi-Fi networks were originally called wireless LANs. Before 1997 wireless LAN equipment did not interoperate--systems made by one manufacturer did not talk to those produced by other companies. But in 1997 the Institute of Electrical and Electronics Engineers adopted the IEEE 802.11 standard, which eliminated that incompatibility. Today most wireless LAN equipment conforms to this standard, which is popularly called Wi-Fi (for wireless fidelity). Although it does not dictate all aspects of network operation, the standard does assure that different equipment types can work together.
Four major concerns face the designers of Wi-Fi networks: ensuring reliability by making certain that service is not disrupted by poor-quality radio transmissions; maintaining performance by avoiding slow link speeds and overlong delays; designing AP networks that can completely blanket the coverage area; and providing security against unfriendly wireless eavesdroppers or unauthorized users.
The main reason that wireless LANs are subject to these problems is that the technology relies on radio transmission, which has its own specific operational drawbacks. A signal received by a client or an AP can be degraded in various ways:
- A wireless transmission is attenuated--that is, weakened by distance even when there are no obstructions (which can cause additional reductions in radio signal strength).
- A radio wave can suffer multipath distortion by reflecting off walls and building structures, furniture, equipment or other objects in the near environs. Signals may then follow multiple paths from transmitter to receiver, which causes numerous copies of the same transmission to arrive at the receiver, each at a slightly different time. The delayed duplicates can corrupt the direct (line-of-sight) signal, creating reception problems.
- A third kind of signal degradation results from interference and noise effects. Interference is generated by conflicting radio transmissions. One common source of Wi-Fi network interference is the microwave oven, which can release stray radio signals. Fortunately, modern microwave ovens are well shielded to keep these emissions to a minimum. Radio noise occurs in nature but also comes from man-made sources such as electrical machinery, automobile engines and fluorescent lighting.
Communications engineers are accustomed to overcoming these difficulties, but unfortunately their methods can slow transmission speeds. Whereas wired Ethernet networks provide service at speeds from 100 to 1,000 megabits per second (Mbps), many wireless LANs employ the IEEE 802.11b standard and so operate at rates up to 11 Mbps. Newer IEEE 802.11a and 802.11g equipment can run at speeds up to 54 Mbps--still a bit sluggish compared with Ethernet operations. A soon-to-be-introduced version of IEEE 802.11 will allow communications as fast as 108 Mbps, however.
These numbers in fact overstate Wi-Fi transmission rates. Wi-Fi automatically drops from the maximum speed (11 or 54 Mbps) to a lower rate to cope with radio signal attenuation, multipath, interference and noise conditions. Hence, an IEEE 802.11b link may step down from a data rate of 11 Mbps to 5.5, 2 or even 1 Mbps. In addition, overhead bits--extra digital bits that are added to each transmission to control network operation and reduce errors--further reduce the effective data rate.
Since the introduction of the initial Wi-Fi technology, my colleagues and I at Carnegie Mellon and Airespace (now a part of Cisco Systems), as well as engineers at other universities and companies, have worked to solve its shortcomings in the areas of reliability, performance, design and security. The resulting second-generation Wi-Fi equipment (called smart Wi-Fi technology here) embodies various new capabilities intended to overcome existing problems. These enhancements rely on greater intelligence in the Wi-Fi systems.
Amart Wi-Fi technology will improve a user's experience with a wireless network by dealing with the issues of congestion, the changing radio environment and security in several ways.
Network congestion--when an AP is called on to serve many users and thus becomes overloaded--is likely to cause delays and degrade service significantly. Because a cell's AP and the clients using it must share a single radio channel (segment of the radio spectrum) and only one station (an AP or a client) can successfully transmit at a time, conflicts can occur. Wi-Fi networks currently resolve clashes between competing stations within a cell by using a technique called carrier sense multiple access with collision avoidance: the CSMA/CA protocol.
Under CSMA/CA, each station listens before sending a signal. If a station hears another one in the process of sending, it defers and waits until the channel is free. If two stations attempt to send at about the same time, neither will hear the other and their transmissions will collide. When this happens, neither transmission is received correctly, and repeat transmissions must be made. When many computers are using a single AP, collisions occur frequently, so multiple repeat transmissions are needed and all the users face delays.
The problem of overloaded APs can be quite severe in areas of high user density. At Carnegie Mellon we first experienced this problem in our large lecture halls and classrooms. My team quickly realized that we could not even approach wired network performance in these crowded spaces, which can, at times, hold hundreds of mobile computer users.
The CSMA/CA protocol can also cause special difficulties among distant APs and mobile devices that are operating on the same radio channel. If one AP or mobile device can hear a far-off (co-channel) AP or client, it will defer, just as it would to a station transmitting within its own cell. This co-channel overlap produces another kind of performance degradation.
Suppose, for instance, that Jane and Joe are using devices operating on the same radio channel but located in different parts of a building and associated with different APs. If Joe's system can hear Jane's, it will defer every time Jane's system transmits, delaying messages waiting to be sent by Joe's. Similarly, if Jane's system can hear Joe's, it will be unable to send whenever Joe's is transmitting, degrading her communications service. This problem would be particularly noticeable if either is using a voice handset.
Designers can mitigate these situations by assigning channels carefully and by using a new feature called load balancing, which helps to reduce the chance of overtaxing an AP. Load balancing relies on the fact that clients may be within radio range of two or more APs. Smart Wi-Fi networks attempt to relieve congestion by distributing clients among APs more or less uniformly so that no one AP gets swamped, streamlining performance considerably.
A link between a client and an AP is called an association. It begins when a client initiates an association request. When an AP receives a request, it can either accept or deny it. Although the IEEE 802.11 standard does not specify a software algorithm for making this kind of determination, a second-generation AP (or the intelligent switch that controls it) considers the AP's current load and also those being carried by nearby APs to help make a decision. A heavily loaded AP might not be the best one to associate with a new client. If such a request is received and the system knows that a lightly loaded AP is also within radio range of the requesting client, the AP may deny the association request, thus boosting the overall performance of the network. Along with other techniques, load balancing will allow future Wi-Fi networks to perform well, even in high-density locations.
Changing Radio Environments
The aforementioned radio-related problems of attenuation, multipath, interference and noise can be alleviated substantially by good network design. A Wi-Fi network designer has to decide where to place APs within a target space to provide for adequate coverage and performance. He or she must also choose which radio channels to assign to which APs. A designer needs to consider the characteristics of the radio environment and the geometry of the building in which the wireless LAN will be deployed to implement what is actually a three-dimensional radio network.
In selecting AP locations, a network designer aims to avoid coverage gaps, but he or she must simultaneously space the APs as far apart as possible to minimize the cost of equipment and installation. Another reason to separate the APs is that coverage overlap between APs operating on the same radio channel (known as co-channel overlap) degrades performance. Channel assignment, which is the second part of the design process, is typically carried out so as to minimize co-channel overlap, which reduces inter-action between stations in different co-channel cells.
Another new smart Wi-Fi feature, automatic cell-size control, allows cell sizes to expand and contract to adjust for changing radio conditions. The technique can also compensate for a less than careful design or for AP failures.
Even for a very carefully configured network, the local radio environment can change from time to time. Thus, the original conditions may no longer exist. When metallic equipment is moved in a factory, for example, a shift in the electromagnetic conditions can lead to coverage gaps. In this case, it is appropriate to expand or contract cell sizes to compensate. Cell sizes can be altered by adjusting the transmitter power output of Wi-Fi APs. If the modifications accurately reflect the new radio environment, continuous network coverage can be maintained throughout the target space without undue cell overlap. (Currently APs can modify only their own transmit power levels, but pending additions to the IEEE 802.11 standard will permit APs to instruct clients to increase or decrease their transmit power as well.)
Automatic cell-size control also has the potential to reduce the effort required to design a wireless LAN. This feature makes possible an abbreviated design process that places APs in reasonable, if not optimal, locations. Moreover, APs break down from time to time. Depending on the particular positions of the APs and the antenna types employed, automatic cell-size control can temporarily fill in coverage holes caused by AP breakdowns.
Dynamic Channel Assignment
APs may also use dynamic channel assignment in smart Wi-Fi networks to change radio channels automatically. Designers traditionally do channel assignment so as to minimize co-channel overlap based on the radio propagation environment. After the channel assignments have been made, they are typically static. The environment can change, however, so there is no guarantee that these assignments will remain valid.
A second-generation Wi-Fi network senses the radio environment at intervals and then dynamically reassigns channels accordingly. This capability eliminates the need to execute channel assignment during the original design process. If furniture is removed from an office area, for example, it might cause a cell's coverage region to enlarge. Should this expansion result in coverage overlap with another cell operating on the same channel, performance could drop off. It may be appropriate in this case to switch the second cell to a different channel. Channel-switching algorithms assure that co-channel coverage overlap is minimized across the entire network.
When many computers use an access point, collisions occur and all the users face delays.
Smart Wi-Fi systems usually activate a channel-switching algorithm periodically to ensure that channel assignments reflect the current radio environment. Dynamic channel assignment techniques can also help improve performance by allowing APs to choose channels that are not experiencing local noise or interference.
Probably the most widely discussed Wi-Fi problem is security. No users want strangers to monitor their e-mail exchanges or gain unauthorized access to their system. The original IEEE 802.11 standard provided for transmission encryption through a feature called Wired Equivalent Privacy (WEP). Encryption is a way of converting one bit stream to another (encrypted) bit stream such that the original bit stream can be reproduced only with the use of a key, the special cipher that was originally used to do the coding. But many wireless users never bother to activate the encryption feature and so send their transmissions "in the clear," which permits easy interception.
Even with WEP in use, clever people seeking to point out its vulnerabilities found ways to discover the keys and then decrypt messages. It became widely known in 2001 that WEP was flawed, and since then developers have worked to bolster Wi-Fi network security.
Authorized access is also an issue for Wi-Fi networks. Users can identify themselves through an authentication process involving a user ID and password, but if malicious people can easily eavesdrop on others' transmissions, they can readily "snoop" a user ID and password and thereby gain access to the network.
In 2003 and 2004 the IEEE 802.11 working group and the Wi-Fi Alliance (the industry group that coined the term "Wi-Fi") completed work on their related standards, IEEE 802.11i and Wi-Fi Protected Access (WPA), which make much stronger security measures available. These include enhanced encryption techniques and substantially more secure methods for APs and clients to gain access to the keys needed to encrypt and decrypt transmissions.
WPA (which uses another standard, IEEE 802.1X) also provides a considerably stronger authentication process than was previously available. The combination of these new standards dramatically betters overall security for smart Wi-Fi networks.
Some Wi-Fi equipment makers have added other security measures as well. One example is intrusion detection. Wireless networks differ from wired networks in that eavesdropping devices (and even APs) can be anywhere in or near a wireless network's coverage area. (Wired intruders may attack from a distance.) This is why some Wi-Fi equipment uses position location technology to detect the presence of a malicious station. Using this feature, the network can track down the offending station and remove it.
With the development of smart Wi-Fi techniques, wireless networks are beginning to behave more like their wired counterparts, and wireless users are starting to notice the difference. More remains to be done in this regard, though, and research to take Wi-Fi further is continuing. Work, for example, is now under way to automatically find a mobile device in a Wi-Fi network. This capability would allow network operators to quickly locate people (say, physicians in a hospital) or objects (products moving through a factory assembly line) as needed.
Smart Wi-Fi networks are beginning to behave more like their wired counterparts, and users are starting to notice.
Wi-Fi and other wireless communications technologies are growing--and changing--dramatically. More and more people in the U.S. and elsewhere are abandoning landline telephone service in favor of wireless cell phones, and municipal governments such as Philadelphia's are creating citywide Wi-Fi coverage areas. Meanwhile the use of third-generation (3G) cellular telephony is on the upswing, and a new wireless technology called WiMAX may soon have a strong presence in the market. Increasingly, we are living in a wireless world.