Wireless Network Technologies and Standards (2)

Wireless Network Architecture
In general, networks perform many functions to transfer information from source to destination.
1. The medium provides a bit pipe (path for data to flow) for the transmission of data.
2. Medium access techniques facilitate the sharing of a common medium.
3. Synchronization and error control mechanisms ensure that each link transfers the data intact.
4. Routing mechanisms move the data from the originating source to the intended destination.
A good way to depict these functions is to specify the network’s architecture. This architecture describes
the protocols, major hardware, and software elements that constitute the network. A network
architecture, whether wireless or wired, may be viewed in two ways, logically and physically.
Logical Architecture of a Wireless Network
A logical architecture defines the network’s protocols—rules by which two entities communicate.
People observe protocols every day. Individuals participating in a business meeting, for example,
interchange their ideas and concerns while they avoid talking at the same time. They also rephrase a
message if no one understands it. Doing so ensures a well-managed and effective means of
communication. Likewise, PCs, servers, routers, and other active devices must conform to very strict
rules to facilitate the proper coordination and transfer of information.
One popular standard logical architecture is the 7-layer Open System Interconnection (OSI) Reference
Model, developed by the International Standards Organization (ISO). OSI specifies a complete set of
network functions, grouped into layers. Figure 1.2 illustrates the OSI Reference Model.
The OSI layers provide the following network functionality:
• Layer 7—Application layer. Establishes communications with other users and provides
services such as file transfer and e-mail to the end users of the network.
• Layer 6—Presentation layer. Negotiates data transfer syntax for the application layer and
performs translations between different data types, if necessary.
• Layer 5—Session layer. Establishes, manages, and terminates sessions between applications.
• Layer 4—Transport layer. Provides mechanisms for the establishment, maintenance, and
orderly termination of virtual circuits, while shielding the higher layers from the network
implementation details.
• Layer 3—Network layer. Provides the routing of packets from source to destination.
• Layer 2—Data Link layer. Ensures synchronization and error control between two entities.
• Layer 1—Physical layer. Provides the transmission of bits through a communication channel
by defining electrical, mechanical, and procedural specifications.
NOTE:
Each layer of OSI supports the layers above it.
Does a wireless network offer all OSI functions? No. As shown in figure 1.3, wireless LANs and MANs
function only within the Physical and Data Link layers, which provide the medium, link synchronization,
and error control mechanisms. Wireless WANs provide these first two layers, as well as Network Layer
routing. In addition to the wireless network functions, a complete network architecture needs to include
functions such as end-to-end connection establishment and application services.
Physical Architecture of a Wireless Network
The physical components of a wireless network implement the Physical, Data Link, and Network Layer
functions (see fig. 1.4). The network operating system (NOS) of a network, such as Novell Netware,
supports the shared use of applications, printers, and disk space. The NOS, located on client and server
machines, communicates with the wireless Network Interface Card (NIC) via driver software, enabling
applications to utilize the wireless network for data transport. The NIC prepares data signals for
propagation from the antenna through the air to the destination comprised of the same set of components.
End-User Appliances
As with any system, there must be a way for users to interface with applications and services. Whether
the network is wireless or wired, an end-user appliance is a visual interface between the user and the
network. Following are the classes of user appliances:
• Desktop workstations
• Laptops
• Palmtops
• Pen-based computers
• Personal Digital Assistants (PDA)
• Pagers
The desktop workstation is currently the most common type of network user appliance. The personal
computer (PC), developed initially by IBM and based on Intel’s microprocessor and Microsoft’s
Windows, is found in many organizations and appears to be the industry standard for the office. Some
companies employ Apple’s Macintosh (Mac) computers as well. The Mac seems to suit artisans because
of its excellent graphics support.
Smaller computers have been effective in satisfying portable computing needs of business executives and
other professionals. Laptops, which measure roughly 8×10×3 inches, can run the same type of software
as the desktop computers, but fit in a briefcase and include rechargeable batteries to sustain operations
where electricity is not present. Palmtops fit in the palm of your hand, but they generally do not perform
as well as the leading laptop and desktop computers. For some applications, such as electronic patient
record keeping, pen-based computers are handy because they enable you to enter data into a portable device via a pen. More and more mobile professionals also are turning to PDAs that enable them to keep
track of contacts, schedules, and tasks. Apple’s Newton PDA, for instance, combines contact
management software and electronic book creation software for salespeople. Pagers are also available as
user appliances. Pagers get your attention when someone calls a special telephone number. These
devices, however, are more than just beepers. Some pagers are now capable of receiving and sending
limited alphanumeric text due to narrowband PCS.
Because wireless network appliances are often put into the hands of mobile people who work outside, the
appliance must be tough enough to resist damage resulting from dropping, bumping, moisture, and heat.
Some companies now offer more durable versions of the portable computer. Itronix, for example, sells
the X-C 6000 Cross Country 486 portable computer. The X-C 6000’s case is built from strong,
lightweight magnesium and includes an elastomer covering that protects the unit from weather and
shock. The unit is impervious to rain, beverage spills, and other work environment hazards.
Some of these vendors also produce pen-based and handheld computers well-suited for wireless
applications. Telxon has a PTC-1184 full-screen 486 pen-based computer that combines pen-based
technology with bar code scanning and AIRONET’s MicroCellular radio-based wireless network
interface card. This system makes it possible to use the unit in environments such as hospitals, factory
floors, and warehouses.
Network Software
A wireless network supports the NOS and its applications, such as word processing, databases, and email,
enabling the flow of data between all components. NOSs provide file and print services, acting as a
platform for user applications. Many NOSs are server-oriented, as shown in figure 1.5, where the core
software resides on a high-performance PC. A client, located on the end user’s appliance, includes server
software that directs the user’s command to the local computer resources, or puts it out onto the network
to another computer. Some wireless networks may also contain middleware that interfaces mobile
applications to the wireless network hardware.
Wireless Network Interface
Computers process information in digital form, with low direct current (DC) voltages representing data
ones and zeros. These signals are optimum for transmission within the computer, not for transporting
data through wired or wireless media. A wireless network interface couples the digital signal from the
end-user appliance to the wireless medium, which is air, to enable an efficient transfer of data between
sender and receiver. This process includes the modulation and amplification of the digital signal to a
form acceptable for propagation to the receiving location. Modulation is the process of translating the
baseband digital signal to a suitable analog form. This process is very similar to the common telephone
modem, which converts a computer’s digital data into an analog form within the 4 KHz limitation of the
telephone circuit. The wireless modulator translates the digital signal to a frequency that propagates well
through the atmosphere. Wireless networks employ modulation by using radio waves and infrared light.
Amplification raises the amplitude of the signal so it will propagate a greater distance. Without
amplification, you would have a difficult time talking to a crowd of 1,500 people outside in an open area.
But, add amplification, such as a PA system, and everyone can hear you.
Figure 1.5 The server-based network operating system.
NOTE:
Chapter 2, “Wireless Local Area Networks (LANs),” covers modulation techniques in more detail.
The wireless network interface also manages the use of the air through the operation of a
communications protocol. For synchronization, wireless networks employ a carrier sense protocol similar
to the common ethernet standard. This protocol enables a group of wireless computers to share the same
frequency and space. As an analogy, consider a room of people engaged in a single conversation in
which each person can hear if someone speaks. This represents a fully connected bus topology (where
everyone communicates using the same frequency and space) that ethernet and wireless networks, especially wireless LANs, utilize. To avoid having two people speak at the same time, you should wait
until the other person has finish-ed talking. Also, no one should speak unless the room is silent. This
simple protocol ensures only one person speaks at a time, offering a shared use of the communications
medium. Wireless networks use carrier sense protocols and operate in a similar fashion, except the
communications are by way of radio signals or infrared light. Figure 1.6 illustrates the generic carrier
sense protocol.
Figure 1.6 The operation of the carrier sense protocol.
Wireless networks handle error control by having each station check incoming data for altered bits. If the
destination station does not detect errors, it sends an acknowledgment back to the source station. If the
station detects errors, the data link protocol ensures that the source station resends the packet. To
continue the analogy, consider two people talking to each other outside. If one person is speaking and a
disruption occurs, such as a plane flying overhead, the dialog might become distorted. As a result, the
listener asks the speaker to repeat a phrase or two. The wireless network interface generally takes the
shape of a wireless NIC or an external modem that facilitates the modulator and communications
protocols. These components interface with the user appliance via a computer bus, such as ISA (Industry
Standard Architecture) or PCMCIA (Personal Computer Memory Card International Association). The
ISA bus comes standard in most desktop PCs. Many portable computers have PCMCIA slots that accept
credit card-sized NICs. PCMCIA specifies three interface sizes, Type I (3.3 millimeters), Type II (5.0
millimeters), and Type III (10.5 millimeters). Some companies also produce wireless components that
connect to the computer via the RS-232 serial port.
The interface between the user’s appliance and NIC also includes a software driver that couples the client’s application or NOS software to the card. Several de facto driver standards exist, including ODI
(Open Data-Link Interface) and NDIS.
Antenna
The antenna radiates the modulated signal through the air so that the destination can receive it. Antennas
come in many shapes and sizes and have the following specific electrical characteristics:
• Propagation pattern
• Radiation power
• Gain
• Bandwidth
The propagation pattern of an antenna defines its coverage. A truly omnidirectional antenna transmits its
power in all directions, whereas a directional antenna concentrates most of its power in one direction.
Figure 1.7 illustrates the differences. Radiation power is the output of the radio transmitter. Most wireless
network devices operate at less than 5 watts of power.
Figure 1.7 The omnidirectional versus directional antennas.
A directional antenna has more gain (degree of amplification) than the omnidirectional type and is
capable of propagating the modulated signal farther because it focuses the power in a single direction.
The amount of gain depends on the directivity of the antenna. An omnidirectional antenna has a gain
equal to one; that is, it doesn’t focus the power in any particular direction. A directional antenna,
however, is considered to add gain (amplification) to the signal in certain directions. Consider the
example of watering your lawn with a garden hose. Attach a circular sprayer to the end of the hose and
turn the water on. The water pressure is divided among many directions, and the resulting water spray
will reach seven or eight feet. If a more directive nozzle is attached to the hose, the spray might reach
twenty feet because the device concentrates the water pressure in one direction. Similarly, the
combination of transmit power and gain of an antenna defines the distance the signal will propagate.
Long-distance transmissions require higher power and directive radiation patterns. With wireless
networks, these signals are relatively low power, typically one watt or less. Most wireless LANs and
WANs utilize omnidirectional antennas, and wireless MANs use antennas that are more directive. Bandwidth is the effective part of the frequency spectrum that the signal propagates. For example, the
telephone system operates over a bandwidth roughly from 0–4 KHz. This is enough bandwidth to
accommodate most of the frequency components within our voices. Radio wave systems have greater
amounts of bandwidth located at much higher frequencies. Data rates and bandwidth are directly
proportional—the higher the data rates, the more bandwidth you will need.
The Communications Channel
All information systems employ a communications channel along which information flows from source
to destination. Ethernet networks may utilize twisted-pair or coaxial cable. Wireless networks use air as
the medium. At the earth’s surface, where most wireless networks operate, pure air contains gases such
as nitrogen and oxygen. This atmosphere provides an effective medium for the propagation of radio
waves and infrared light. Rain, fog, and snow, however, can increase the amount of water molecules in
the air and can cause significant attenuation to the propagation of modulated wireless signals. Smog
clutters the air, adding attenuation to the communications channel as well. Attenuation is the decrease in
the amplitude of the signal, and it limits the operating range of the system. The ways to combat
attenuation are to either increase the transmit power of the wireless devices, which in most cases is
limited by the FCC, or to incorporate special amplifiers called repeaters that receive attenuated signals,
revamp them, and transmit downline to the end station or next repeater.