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.
The AP is essentially a bridge between the wired network
protocol (such as 802.3) and the RF protocol (802.11). It also provides the
802.11 protocol RF connection requirements, providing all features defined by
the standard such as 802.11 association and 802.11 authentication, packet
acknowledgment, handoff notification, and so on.
APs communicate on a one-to-many basis with wireless clients.
For all purposes, they are the wireless hub for that particular RF area.
APs come with different features, functions, and performance
levels, as well as in various physical form factors. The differences between
these are discussed throughout various chapters of this book.
The AP also requires connection to power and an antenna of some
sort. These two features in particular can make a difference in the product you
choose for the network. The particulars of this topic are discussed in 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.
Wireless 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).
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.
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:
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.
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.
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.
Make certain that the devices you select have an assortment of
accessories that enable you to use the device in your situation.
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