Wireless LAN Components
Wireless LANs consist of components similar to traditional
Ethernet-wired LANs. In fact, wireless LAN protocols are similar to Ethernet and
comply with the same form factors. The big difference, however, is that wireless
LANs don't require wires.
User Devices
Users of wireless LANs operate a multitude of devices, such as
PCs, laptops, and PDAs. The use of wireless LANs to network stationary PCs is
beneficial because of limited needs for wiring. Laptops and PDAs, however, are
commonly equipped with wireless LAN connectivity because of their portable
nature. User devices might consist of specialized hardware as well. For example,
bar code scanners and patient monitoring devices often have wireless LAN
connectivity.
Radio NICs
A major part of a wireless LAN includes a radio NIC that
operates within the computer device and provides wireless connectivity. A
wireless LAN radio NIC, sometimes referred to as a radio card, often implements
the 802.11 standard. The cards generally implement one particular physical
layer, such as 802.11a or 802.11b/g. As a result, the radio card must utilize a
version of the standard that is compatible with the wireless LAN. Wireless LAN
radio cards that implement multiple versions of the standard and provide better
interoperability are becoming more
common.
Radio cards come in a variety of form factors, including: ISA,
PCI, PC card, mini-PCI, and CF. PCs generally utilize ISA and PCI cards; but
PDAs and laptops use PC cards, mini-PCI, and CF adapters.
Access Points
An access point contains a radio card that communicates with
individual user devices on the wireless LAN, as well as a wired NIC that
interfaces to a distribution system, such as
Ethernet. System software within the access point bridges together the wireless
LAN and distribution sides of the access point. The system software
differentiates access points by providing varying degrees of management,
installation, and security functions. Figure
5-1 shows an example of access-point hardware.
In most cases, the access point provides an http interface that
enables configuration changes to the access point through an end-user device
that is equipped with a network interface and a web browser. Some access points
also have a serial RS-232 interface for configuring the access point through a
serial cable as well as a user device running terminal emulation and Telnet
software, such as hyper terminal.
|
Look at the basic radio configuration settings for a Cisco 350
access point. These types of settings are common for other access points as
well.
One parameter that you should set is the service set identifier
(SSID). The SSID provides a name for the specific wireless LAN that users will
associate with. For security purposes, it's a good idea to set the SSID to
something other than the default value.
For most applications, set the transmit power of the access
point to the highest value, which is typically 100 milliwatt (mW) in the United
States. This will maximize the range of the wireless LAN. The actual maximum
effective power output is 1 watt, but the lower transmit power allows enough
margin to allow the use of a higher-gain antenna and still remain within
limitations.
In the United States, set the access point to operate on any
one of the channels in the range from 1 through 11. When installing only one
access point, it doesn't matter which channel you choose. If you install
multiple access points, or you know of a nearby wireless LAN within range of
yours, you need to select different non-overlapping channels (such as channels
1, 6, and 11) for each access point within range of one another.
As a minimum, activate wired equivalent privacy (WEP)
encryption as a minimum level of security. You'll need to assign an encryption
key that all user devices will need in order to interface with the access point
with encrypted data. If you choose to implement 40-bit keys, enter 10
hexadecimal characters, with each character having the value of 1 through 9 or A
through F. The 104-bit keys require 26 hexadecimal characters. Keep in mind that
40-bit keys correspond with 64-bit encryption and 104-bit keys correspond with
128-bit encryption to the addition of a 24-bit initialization vector in both
cases. |
Routers
By definition, a router transfers packets between networks. The
router chooses the next best link to send packets on to get closer to the
destination. Routers use Internet Protocol (IP) packet headers and routing tables, as
well as internal protocols, to determine the best path for each packet.
A wireless LAN router adds a built-in access point function to
a multiport Ethernet router. This combines multiple Ethernet networks with
wireless connections. A typical wireless LAN router includes four Ethernet
ports, an 802.11 access point, and sometimes a parallel port so it can be a
print server. This gives wireless users the same ability as wired users to send
and receive packets over multiple networks.
Routers implement the Network
Address Translation (NAT) protocol that enables multiple network devices
to share a single IP address provided by an Internet
service provider (ISP). Figure 5-2
illustrates this concept. Routers also implement Dynamic Host Configuration Protocol (DHCP)
services for all devices. DHCP assigns private IP addresses to devices.
Together, NAT and DHCP make it possible for several network devices, such as
PCs, laptops, and printers to share a common Internet IP address.
Wireless LAN routers offer strong benefits in the home and
small office setting. For example, you can subscribe to a cable modem service
that provides a single IP address through DHCP to the router, and the router
then provides IP addresses through DHCP to clients on your local network. NAT
then maps a particular client on the local network to the ISP-assigned IP
address whenever that client needs to access the Internet. As a result, you need
a router if you plan to have more than one networked device on a local network
sharing a single ISP-assigned address. Instead of having one box for the router
and another box for the access point, a wireless LAN router provides both in the
same box. Routers, however, are seldom used in larger implementations, such as
hospitals and company headquarters. In these cases, access points are best
because the network will have existing wired components that deal with IP
addresses.
Repeaters
Access points, which require interconnecting cabling, generally
play a dominant role for providing coverage in most wireless LAN deployments.
Wireless repeaters, however, are a way to
extend the range of an existing wireless LAN instead of adding more access
points. There are few standalone wireless LAN repeaters on the market, but some
access points have a built-in repeater mode.
A repeater simply regenerates a network signal to extend the
range of the existing network infrastructure. (See Figure 5-3.) A wireless LAN repeater does not physically
connect by wire to any part of the network. Instead, it receives radio signals
from an access point, end-user device, or another repeater; it retransmits the
frames. This makes it possible for a repeater located between an access point
and distant user to act as a relay for frames traveling back and forth between
the user and the access point.
As a result, wireless repeaters are an effective solution to
overcome signal impairments such as RF attenuation. For example, repeaters
provide connectivity to remote areas that normally would not have wireless
network access. An access point in a home or small office might not quite cover
the entire area where users need connectivity, such as a basement or patio. The
placement of a repeater between the covered and uncovered areas, however, will
provide connectivity throughout the entire space. The wireless repeater fills
holes in coverage, enabling seamless roaming.
A downside of wireless repeaters, however, is that they reduce
performance of a wireless LAN. A repeater must receive and retransmit each frame
on the same radio channel, which effectively doubles the amount of traffic on
the network. This problem compounds when using multiple repeaters, because each
repeater will duplicate the data sent. Therefore, be sure to plan the use of
repeaters sparingly.
Antennae
Most antennae for wireless LANs are omnidirectional and have
low gain. Nearly all access points, routers, and repeaters come standard with
omnidirectional antennae. Omnidirectional antennae satisfy most coverage
requirements; however, consider the use of optional directive antennae to cover
a long, narrow area. In some cases, the antenna is integrated within a radio
card or access point and there is no choice to make. If a need exists to use a
directive antenna (higher gain), ensure that the radio card or access point has
an external antenna connector.
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