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WLAN Components

This section briefly reviews some key points in WLAN product evolution. In the beginning, all WLANs were very vendor-specific and proprietary systems. Data rates started at 10 kbps and approached 1 Mbps with the advent of spread spectrum, with radios transmitting in the 900-MHz ISM band. Product availability was limited to APs, ISA cards, and to a lesser degree, PCMCIA cards. For PCMCIA cards, the overall power consumption of the WLAN client was high, and in many cases exceeded the capability of the device it was being used in. The high power consumption also reduced available battery run time, which therefore limited the degree to which a user could be truly untethered from an AC power source.

These early WLANs were used mainly by retail and warehouse systems for bar coding and inventory control. The required bandwidth for such application was, and in many cases still is today, comparatively low, as were overall transaction rates. The total number of users on this type of system is typically low, on the order of several to perhaps 30 for an entire system, so the limited availability of bandwidth was generally acceptable.

The convenience of being untethered prompted users to develop new ideas about how wireless might be used. Initially, the radio devices that were attached to the network were all proprietary, both in the RF protocol area and on the network connection side. Many devices used special remote transceiver antenna assemblies, attached back to some form of protocol converter over RS 232 or RS 485 interfaces. Proprietary cabling was used to provide connection and permit the radio device to be located in the area that the users needed connection.

As time moved on, demand increased to move the network-side RF device to a standard networking interconnection such as Ethernet or Token Ring. (Yes, there were loud cries for Token Ring devices at the time.)

A basic WLAN consists of a device that is attached to the network (AP), an antenna, and a device that is portable (commonly referred to as the client) and its associated antenna. The next sections examine these devices a little closer.

Access Points

The device that provides access to the network by the remote or portable radio devices is called the access point (AP); some vendors call it an access port. For the small office, or home use, the AP is often referred to as a wireless gateway. This device typically uses a standard network connection (Ethernet being the most common) that ties back to a network switch, router, or hub.

Figure 1-5 shows some of the popular APs available on the market today.

Figure 1-5. Access Point Examples

Chapter 2, "Understanding RF Fundamentals," and Chapter 9, "Discovering Wired Network Requirements."

Client Devices

The client device is the remote or portable device that communicates to the AP. Many of the early devices were specialty devices, such as bar code scanners, lift-truck mounted terminals, and point-of-sale devices. Virtually all of these devices were proprietary. Figure 1-6 shows some of the popular form factors of wireless clients, including bar code scanners, PCI cards, PCMCIA cards, compact Flash radios, and standalone USB or Ethernet radios.

Figure 1-6. Client Device Examples

ISA cards, available since in the early 1990s, were the first industry-standard devices geared for general computer use. PCMCIA cards followed, as did Ethernet standalone devices (sometimes called workgroup bridges or wireless hubs).

More recently, a form factor called miniPCI has been introduced; it is a style used in many laptop computers. This permits the client radio device to be embedded inside the computer, leaving the USB and Ethernet ports and the PCMCIA slots available for other devices, and making the WLAN client an integral part of the computer (see Figure 1-7).

Figure 1-7. MiniPCI Radio Card

Bridges

The term bridge in the WLAN lexicon usually indicates a wireless device that connects a group of computers or devices to another group of computers or devices over a single RF link. Most commonly found in building-to-building connections, bridges many times follow the 802.11 specification, even though they are not actually included in the specification. As depicted by Figure 1-8, bridges enable you to connect multiple buildings (or networks) together, eliminating the need for cable runs or leased lines.

Figure 1-8. Wireless Bridging

Note

The 802.11 specification was intended for wireless local-area networks, with the imperative word here being local. Because of certain laws in the physics of RF, and certain timing constraints invoked to keep performance at a maximum, the local distance is set for approximately 1000 feet. Although it may work fine at distances beyond this, longer distances are not covered under the 802.11 specification.

Some devices enable you to alter timing to provide for longer distances by stretching timing parameters, such as ACK wait times and slot times, beyond the specification.

Bridges come in two main architectures or topologies, as follows:

Point-to-point (PTP)

Point-to-multipoint (PMP)

PTP systems permit connection between only two points, whereas PMP systems permit a central-site communication to multiple remote sites. Any PMP system will function as a PTP system as well.

Figure 1-9 and Figure 1-10 show the differences between a PTP and PTM system.

Figure 1-9. Point-to-Point Link

Figure 1-10. Point-to-Multipoint Link

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When you are setting up a bridge, many things play an important part. Features that you need to research include available antennas and cables, indoor/outdoor transmitter design, transmitter power, and so on.

Accessories

With any type of networking, there are the main devices, and then there are the accessories that make design and installation possible. The same is true for WLAN systems.

Accessories such as mounting brackets, inline power injectors, lightning arrestors, proper RF cables, and weatherproof enclosures can make the difference between a system that you can just design and one that you can design and install. For example, Figure 1-11 shows a mounting bracket for an Oronoco AP1000. Although mounting brackets are not necessarily thought of as technical, a bracket similar to the one shown (with multiple features and mounting options) can make the installation task much simpler.

Figure 1-11. Access Point Mounting Bracket

Per the FCC (and most approval agencies), WLAN device antennas must be certified with the particular transmitter. Some vendors offer a wide variety of antennas, whereas others offer only a very limited selection (leaving the feasibility of using certain types of antennas in question).

When using an RF cable on a WLAN system, a significant amount of loss occurs in the cable. To offset this, the cable has to be physically large. In some cases, the connection to the radio may be a PCMCIA card antenna port, limiting the cable size, which can affect performance as well as offer a point of failure.

Utilities that come with the WLAN systems are also a vital part of the product. If you select an AP that lists SNMP support, verify the Management Information Base (MIB). A few APs on the market do support SNMP, but the MIB has a total of only four parameters supported.

Some devices provide utilities for measuring RF link quality, verifying status of the devices, and displaying historical data concerning the devices. Some even include site survey tools. Figure 1-12 shows one such utility. The more feature rich your devices, the easier the installation and support.

Figure 1-12. Site Survey Utility

Make certain that the devices you select have an assortment of accessories that enable you to use the device in your situation.