Clients | Basics of Wi-Fi

A client is the typical end-user device. Unlike access points, which are strategically placed for coverage, clients are almost always mobile (or potentially so).
Wi-Fi clients can be general networking interface devices, such as those in laptops, or can be part of a purpose-built mobile voice handset. Either way, these clients appear to the network as endpoints, just as Ethernet devices do.
From the user's perspective, however, Wi-Fi clients add an extra complication. Unlike with wireline connections, where the user is assigned a port or cable and has the expectation that everything will work once the cable is plugged in and the process has settled down (which, for administrators, generally means that Dynamic Host Configuration Protocol (DHCP) automatic IP address discovery has completed), wireless connections have no one cable to solve all problems. The user must be involved in the connection process, even when the reason for connection or disconnection is not readily apparent. As mentioned previously, the user must learn about SSIDs. When a wireless interface is enabled, the user is normally interrupted with a list of the available networks to connect to. Knowing the right answer to this question requires an unfortunate amount of sophistication from the user, not because the user does not understand the technology, but because they usually do understand the power of mobility, and have learned to strategically hunt out wireless networks for casual email access. This is clearly evidenced by the pervasive nature of the "Free Public WiFi" ad hoc (Independent Basic Service Set, or IBSS) SSID that tends to be on so many laptops.
Ultimately, the user is responsible for knowing what the appropriate network is to connect to at any given location. Most devices do remember previous connections—including authentication credentials, in many cases—and can make the connection appear to be automatic. However, because of that caching, installations that run multiple SSIDs are often forced to deal with users not knowing exactly which network they are connected to.
Once the connection is established, the interface comes up much as a plugged-in Ethernet link does. Any automatic services, such as DHCP or Universal Plug and Play (UPnP), that run on interface startup will get kicked off, and the users will be able to communicate as if they had plugged directly into the network.
The last wrinkle comes, however, with mobility. Once the user leaves the coverage range of the one access point that it is on, the client will perform its list gathering activity (scanning) again. If it can find an SSID that it already has in its list—especially if the SSID is the same as the one the client was already associated to—the client will try to hand over to the new access point without user intervention. However, if the handoff does not succeed, or there are no more known networks in range, the client will disconnect and either warn the user with a popup or just break the connection without warning. This can come as quite a shock to the user, and can lend negative impressions about the network.

Access Points | Basics of Wi-Fi

The access point (abbreviated as "AP"; see Figure 1) serves as the base station. The concept is common, from cordless phones to the large wireless carriers: the access point is what provides the "network," and the clients connect to it to gain access. Each Wi-Fi radio, whether it be in the access point or the client, is designed to send its wireless signals across a limited range, far enough to be useful but not so far as to violate the limits set by the regulations and to grossly exceed the bounds of the building the network is deployed within. This range is in the order of 100 feet, though. To set apart which device connects to the network, the access point must take on a role as some sort of master.
Figure 1: A typical Access Point
An access point often looks like a small brick, but with antennas and an Ethernet cable. The Ethernet cable provides the connection to the wired network, and, if power over Ethernet (PoE) is in use, the access point receives its power over the same cable. Access points are normally independent physical devices. Commonly, they are placed along walls, or above or below a false ceiling, to provide the maximal amount of wireless coverage with the least amount of physical impediments to the signals (see Figure 2). 

 Figure 2: Typical Access Point mounting locations
Access points make their networks known by sending frequent wireless transmissions, known as beacons. These beacons describe to the client devices what capabilities the access point has, and most importantly, what network the access point is providing access to. The way the network is designated is by an arbitrary text string provided by the administrator, known as a service set identifier (SSID). This text string is sent in the beacons, and other transmissions, to the clients, which then provide a list of SSIDs seen to the user. Thus, when the user brings up a list of the networks that his or her laptop sees and can connect to, the list contains the SSIDs of the access points.
Because the SSID is the only way users can select which network they wants to connect to, we need to look into it a bit deeper. There are very few technical restrictions on the SSIDs except for the length, which must be less than 32 characters. However, the SSID needs to be meaningful to the user, or else he or she will not connect to it. Because SSIDs are supposed to name the network that the user is connecting to, rather than the individual access point, multiple access points can and do share the same SSID. That being said, there is nothing stopping someone else from giving an access point the SSID that belongs to your network. There is no security in the SSID itself. Eavesdroppers can trivially discover what the SSID is that your network is using (even if you use a feature known as SSID hiding or SSID broadcast suppression) and use it to either gain entry into your network or spoof your network and try to fraudulently get your clients to connect to them instead. In fact, there is nothing that prevents SSIDs from being used for nearly any purpose at all. Most of what applies to SSIDs are in the form of best practices, of which the important ones are:
  • The SSID should be meaningful to the user: "employees" and "guest" are good examples of meaningful names. They may be based on the role of the user, the device the user has (such as "voice" for phones), or any other words that help the user find the network.
  • When the installation shares the air with neighboring networks from other organizations, the SSID should also include text to highlight to the user what the right network is; "xyz-employees" is an example of an SSID for an organization named XYZ.
  • The SSID should be able to be easily typed by the user. Although most devices show SSIDs in a list from what already are being broadcasted, allowing the user to select the SSID with minimal effort, there are many occasions on which when the user may need to type the SSID. This is especially true for mobile devices, with small keyboards or limited keys.
  • Again, do not rely on obscurity of the SSID to restrict access to your network. Use real security mechanisms, as described later, instead.

Devices | Advantages of Wi-Fi

Wi-Fi was initially thought of as a data network only. Partially, this was because of an attempt to avoid the bad image that cordless phones also projected, as users were far too used to static and interference on cordless phones. But mostly, the original iterations of Wi-Fi occurred when Wi-Fi itself was struggling to find a place, and allowing users to check email or surf the Web while moving from room to room seemed to be enough of an application to motivate the fledgling industry.
But when mobile data networking took off, and people became addicted to remote email over the cellular network, the seeds were sown for device vendors to want to integrate Wi-Fi into their mobile devices. And because those devices are primarily phones, the connection of mobility to voice over Wi-Fi was natural.
Broadly, there are two categories of voice mobility devices that use Wi-Fi as a connection method. The first are Wi-Fi-only devices. These devices are often dedicated for a specific application in mind. For example, Vocera Communications makes a Wi-Fi-based communicator that is often used in hospitals to allow doctors and nurses to communicate with each other using voice recognition, rather than a keypad, to determine whom to call.
This device looks and acts more like a Star Trek communicator than a phone, but is an excellent example of voice mobility within a campus. Polycom, through its SpectraLink division, Cisco, and Ascom all make handsets that look more like a traditional mobile phone. In all of these cases, single-mode networking—using just Wi-Fi, in these examples, as the only means of connectivity—makes sense for the environment and the application.
The second type is made of mixed-mode, or integrated devices. These devices are mobile phones, made to be used with the cellular network as well as Wi-Fi. Nearly every mobile handset manufacturer is selling or is planning on selling such a device, including Research in Motion, Nokia, Samsung, and Apple with its iPhone. These devices can be made to place voice calls directly over the Wi-Fi network, rather than the cellular network, thus unlocking the entire fixed-mobile convergence (FMC) industry.
In both cases, the push from Wi-Fi networks originally designed for data allows for voice to become a leading, if not the dominating, purpose for many networks, as the maturity and variety of Wi-Fi-enabled voice devices make voice mobility over Wi-Fi possible.

Universal Presence | Advantages of Wi-Fi

Even though the focus—and of so many people—is with enterprise and large-scale deployments, in explaining what makes Wi-Fi compelling, we must not lose track of the consumer, and how consumer demands have pushed the entire Wi-Fi industry forward, inevitably benefiting the enterprise.
The major contribution the consumer space has given Wi-Fi is that is has driven people to demand wireless. Three historic events changed the landscape of mobility and connectivity: the Internet moved into the home; laptops replaced desktops and were being issued by corporate IT for usage everywhere; and darkly roasted coffee came onto the scene. Or rather, for the last one, people began to find reasons to want to work and live outside of the home and office. All three demanded a simpler solution than having to drag oversized telephone cables around with each user. And that gap was filled with Wi-Fi.
Wi-Fi is now in many places that mobile users are expected to show up in. In the home, it is difficult now to find a consumer-level gateway that does not include wireless. Just as television once was the centerpiece of the living room, but contention over control of the remote and the drop in prices lead televisions to spring up in nearly every room of the house, the Internet has migrated from being connected to one prized home computer in the living room to being spread throughout the house by Wi-Fi. In the enterprise, the advantages of unwiring the network edge has lead to IT organizations peppering the office with access points. And on the road, hotels, airports, cafes, and even sporting arenas have outfitted with Wi-Fi, to try to encourage their customers to get back with their online selves as often as possible, and maybe make each one be a little more "sticky" in the meanwhile.
What this means for voice mobility is that the cycle of demand drives the technology to get ever better. Consumers' demand and expectations "pull" advanced wireless into the home, just as enterprises "push" laptops onto their employees, encouraging them to be used outside the office, therefore increasing the number of hours employees think and do their work far beyond the amount of time each employee spends in the office.
And with this cycle of demand also comes maturity of the underlying technology. Wi-Fi has gone through a number of iterations, getting faster, more powerful, and less prone to mistakes. Now, it is nearly impossible to find laptops without wireless built in. It is even an option on many desktop systems, not considered to be traditionally mobile, yet eager to be joined in on the wireless bandwagon to help company's save on cabling costs.

Unlicensed Spectrum | Advantages of Wi-Fi

Generally, the ability to transmit radio signals over the air is tightly regulated. Government bodies, such as the U.S. Federal Communications Commission (FCC), determine what technologies can be used to transmit over the air and who is allowed to operate those technologies (see Figure 1). They do this latter part by issuing licenses, usually for money, to organizations interested in transmitting wirelessly. These licenses, which are often hard to obtain, are required in part to prevent multiple network operators from interfering with each other.

Figure 1: The United States Spectrum Allocation. Wi-Fi operates in the circled bands
The advantage of Wi-Fi, over other wireless technologies such as WiMAX (which we will cover in Chapter 7), is that no licenses are needed to set up and operate a Wi-Fi network. All that it takes to become a network operator is to buy the equipment and plug it in.
Clearly, the array of allocations within the spectrum is bewildering. And network operators for licensed wireless technologies must be aware of the rules for at least the part of the spectrum that their technology works in, to avoid violating the terms of the license. But, thankfully, all of this is taken care of automatically when 802.11 technology is used. Wi-Fi operates in two separate stretches (or "bands") of the radio spectrum, known in the United States as the Industrial, Scientific, and Medical (ISM) bands, and the Unlicensed National Information Infrastructure (U-NII) bands. These bands have a long history, and it is no coincidence that voice lead the way. Many people first became familiar with the concept of unlicensed radio transmissions when 900MHz cordless telephones were introduced. These phones require no licenses, but have a limited range and do only one thing—connect the call back to the one and only one base station. However, the power from using wireless to avoid having to snake cables throughout the house and allowing callers to walk from room to room revealed the real promise of wireless and mobility.
For enterprises, the benefits of the freedom from using unlicensed spectrum are clear. Removing the regulatory hurdles from wireless brings the requirements for setting up wireless networks down to the same level as for wireline networks. Expanding the network, or changing how it is configured, requires no permission from outside authorities (ignoring the physical requirements such as building codes necessary to pull cables). There is no concern that a regulatory agency might reject a Wi-Fi network because of too many neighboring allocations. Enterprises gain complete control of their air, to deploy it how they see fit.
Because being unlicensed gave the potential for every user to be her own network operator, wireless networking settled into the hands of the consumer, and that is where we will continue the story.

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