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Selecting the Client Products

Before discussing what you need to consider when making client product selections, this section first examines a new initiative in client cards. In the past, client cards supported a single RF technology. If you moved from 802.11b to 802.11a, you had to change radio cards in your client device. In 2003, the introduction of dual-band client cards changed how to migrate from one technology to another. Several of the newly introduced cards support 802.11b or 802.11a. (Notice the or; one band operates at a time!) So, a single card can be used to operate on either system.

Shortly after that, cards started to appear that support 802.11g (and hence 802.11b by definition) and 802.11a. Many of these cards enable you to actually roam from one technology to another, and thus promote greater scalability. If you have a dual-band AP installed, the client treats them like two different APs and selects the AP (in this case, the same AP but different radios) with the best performance. Virtually all new cards are now dual-band (tri mode: a, b, and g) radios. This is the industry trend today.

Because most of the network features reside in the AP (at the edge of the network) or in the controller, you have to consider significantly fewer items on the client side. The biggest question is what type of clients will be required, and who controls the client selection.

First, look at issues regarding the type of client. Not all devices have been migrated to support 802.11a or 802.11g. This can be one crucial factor in the technology decision. You must also consider interoperability (not only for the basic 802.11 side, but also for things such as security and QoS). Many of the specialty client devices on the market today do not support the wide range of features that are supported by the standard WLAN network interface card (NIC)-type devices. Some devices even still operate under DOS environments, severely limiting their feature support. For this reason, first select the features your system needs and then search for the client devices. Sometimes you may have no choice but to use some of these "featureless" client devices; in such cases, you might want to use VLANs to segregate traffic and help keep the main network secure.

Then you must look at who determines which clients are used on the network. If this is an education or public network, the network administrator typically has minimal input on the client-side decision and is limited to a statement similar to "802.11b Wi-Fi compliance is required." Although this seems fine at first, it has a major effect on the design of the network, because not all clients' radios are created equal.

Some radio vendors provide a very typical transmitter power of 15 dBm (30 mW), whereas others provide a slightly higher transmitter power of up to 20 dBm (100 mW) or even more, and yet a few have very low power levels (such as an SDIO radio). Using a 100-mW AP with a 30-mW client card results in asymmetrical performance. (Remember, this is a two-way communication path.) The client can hear the AP, but the AP cannot hear the client. As a result, the lower-power client limits the performance and the range of the system. If you plan to permit such lower-powered clients on your network, perform survey testing with a device set to the minimum power you anticipate being used by a client. If you are installing a 100-mW, higher-end AP into the system, set the power levels of the AP to be comparable to the lowest-power client card. This provides the best overall performance from all client devices, and minimal interference between APs.

Some client-end, wireless devices support several wired devices over a single radio connection (see Figure 5-18). This is known as an Ethernet client, bridge, minibridge, or workgroup bridge. The idea behind this device is to provide RF connectivity for some small number of wired devices. Consider, for example, a hospital nursing station that has three or four wired computers at the desk. Instead of pulling three or four cables (or even a single cable) to the desk, you can install one of these Ethernet clients, add an inexpensive hub, and attach all the devices to the hub. The devices will all access the network via the single radio device.

Another example is a mobile crane in a shipping port. Such a crane likely has more than one computer device, and therefore requires more than one radio device and corresponding antennas. (Because the crane is made almost entirely of metal, the antenna has to be remote and placed outside the crane operator's suite or computer closet.) By using the Ethernet client, you can funnel all the network devices in the crane to a single radio and one antenna.

Be aware, however, that some of the Ethernet clients on the market are designed for only a single connection. The rationale behind such a device is to connect systems that have only an Ethernet connection for networking. Because the Ethernet clients run their own driver and firmware, there is usually no need for a driver. Some similar devices have USB connections, but these typically have the limitation of a single device connection as well.

For desktop-style computers, or other devices that require PCI cards, two main styles of devices are offered today, the main difference being their antenna options. Figure 5-19 shows both styles of PCI cards. In one case, the PCI card is really a PC card-to-PCI converter, and a standard PC or cardbus is used. This means that the antennas are typically attached and remote antennas are not available. The second style of PCI card is one that offers an external antenna (one that you can mount remotely). So why the two different styles? Suppose that your computer is one that you put on the floor under your desk, and that your desk is made of steel. Or perhaps your computer is in a point-of-sale device, such as a portable cash register, that you mount under the mobile cart. These typical installations can affect the antenna's capability to transmit or receive properly, unless you can position the antenna in an open area.

The most common client by far today is the embedded radio card. PC manufacturers are now putting WLAN cards into a wide range of laptops as standard or optional features. Included with such a system is an embedded antenna. However, different PCs perform at different levels. Although most vendors have tested and found optimum locations for the antennas, some devices have antennas placed in less-than-optimum locations (such as under where your hands rest when typing on the keyboard). You may find varying performance between different versions of laptops, and you need to maintain some margin to compensate for these differences.