Wednesday, 6 July 2011

Wireless Network System Components


A wireless network consists of several components that support communications using radio or light waves propagating through an air medium. Some of these elements overlap with those of wired networks,

but special consideration is necessary for all of these components when deploying a wireless network.

Wireless Networks Include Computer Devices, Base Stations, and a Wireless Infrastructure




A user can be anything that directly utilizes the wireless network. One of the most common types of user is a person. For example, a business traveler accessing the Internet from a public wireless LAN at an airport is a user. In some cases, however, the user might not be human. A robot, for example, might receive instructions over a wireless network from a central computer that controls a manu-facturing process. Because the wireless network exists to serve the user, the user is the component that receives the benefits of a wireless network. As a result, users are an important part of the wireless network.

The user initiates and terminates use of a wireless network, making the term end-user appropriate. Typically, a user operates a computer device, which often performs a variety of application-specific functions in addition to offering an interface to the wireless network.

Users of wireless networks tend to be mobile, constantly moving throughout a facility, campus, or city. Mobility is one of the most prominent benefits of deploying a wireless network.

For example, a person walking through a convention center while sending and receiving e-mail from a PDA is exercising mobility. The PDA in this case must have continual or frequent connections to a wireless network


Some users might require only portability; whereby, they stay at a particular location while using the wireless network for a specific period of time. An example of this type of usage is someone operating a laptop wirelessly from a conference room. The user will turn on the laptop after sitting down in the conference

room and shut off the laptop before leaving. As a result, the wireless network doesn't need to support continual movement.

Other users might actually be stationary, which means that they operate from one place for an indefinite period of time. An example of this type of user is someone working from a wireless computer in an office. The biggest difference between a stationary and portable user is that the stationary user will not require any form of roaming functions. Roaming functions are difficult to implement in some situations.


Computer Devices


Many types of computer devices, sometimes referred to as clients, operate on a wireless network. Some computer devices might be specifically designed for users, whereas some computer devices are end systems. In generally, any computer device might communicate with any other computer device on the same wireless network.

To support mobile applications, computer devices are often small, making them practical for people to carry with them at all times. These devices generally have small screens, limited keyboards, and small batteries. The devices are mobile, but they can support only certain applications.

With portable and stationary applications, however, the computer devices are much larger. These devices generally have larger displays and keyboards, making them more suitable to use when browsing the

Internet and other applications requiring relatively high performance. The problem, however, is that these devices weigh more and are difficult to carry from one place to another.

Computer devices within a wireless network also include end systems such as servers, databases, and websites. For example,  website includes news that someone can view from a public wireless LAN connection from a hotel room. Similarly, a clerk can wirelessly interface with a warehouse management system, which acts as an end-system computer device.

Users can adapt many existing computer devices to operate on a wireless network. A user, for example, can purchase and install a wireless network interface card (NIC) within his laptop to enable operation on a particular type of wireless network. Some devices, such as a wireless bar code scanner, operate only on a wireless network.

A computer device also has an operating system, such as Windows XP, LINUX, or MAC OS. The operating system runs software needed to realize the wireless network application. In some cases, the operating system has built-in features that enhance wireless networks. For example, Windows XP has the ability to automatically identify and associate with wireless LANs.




The network interface card provides the interface between the computer device and the wireless network infrastructure. The NIC fits inside the computer device, but external network adaptors are available that plug in and remain outside the computer device. Figure 2-3 shows examples of several types of wireless NICs.

Wireless network standards define how a wireless NIC operates. For example, a wireless LAN NIC might implement the IEEE 802.11b standard. In this case, the wireless NIC will only be able to interface with a wireless network infrastructure that complies with the 802.11b standard. As a result, users must be careful to

ensure that the wireless NIC they choose matches the type of wireless network infrastructure they want to access.

Wireless NICs also comply with a specific form factor, which defines the physical and electrical bus interface that enables the card to communicate with the computer device. Again, the user must consider this to ensure that the chosen wireless NIC will fit within their computer device. The following is a summary of the different internal form factors available for wireless networks:

Industry-Standard Architecture (ISA)—ISA has been around since the early 1980s. Because of this, the proliferation of the ISA bus has been significant. Despite its limited performance, nearly all PCs manufactured up until recently had at least one ISA bus. The ISA bus has failed, however, to advance at the pace of the rest

of the computer world, and other higher-speed alternatives are now available. ISA doesn't impose too much of a performance impact on 802.11b wireless LANs. It's not advisable, however, to purchase new

ISA cards because of the possibility of them becoming obsolete.


Peripheral Component Interconnect (PCI)—The PCI bus is the most popular interface for PCs today and boasts high performance. Intel originally developed and released PCI in 1993, and it satisfies the needs of the recent generations of PCs for multimedia and graphics. PCI cards were the first to popularize "plug-and-play"

technology, which makes it easy to install the NIC. PCI circuitry can recognize compatible PCI cards and work with the computer's operating system to set the configurations for each card. This saves time and

prevents installation headaches for nontechnical users.

PC Card—The PC Card was developed in the early 1990s by the Personal Computer Memory Card International Association (PCMCIA). The PC Card is a credit-card-sized device that provides extended memory, modems, connectivity to external devices, as well as wireless LAN capabilities to small computer devices such as laptops and PDAs. In fact, they are the most widely available NICs available. They are more popular than ISA or PCI cards because of use in a growing number of laptops and PDAs.

It's possible to share a PC Card with a desktop PC by using an adaptor that converts a PC Card into a PCI card. This allows purchasing one NIC for use in both types of computers. You can take the PC Card with you on a business trip— or home from work— and utilize the same card when back in the office using a PC.

Some PDAs require a sled device that accommodates the PC Card and mounts underneath the PDA.

This is the only way to add wireless network capability to some older PDAs. The combination of the sled, PC Card and PDA, however, adds a lot of bulk and weight that depletes the usability.

Mini-PCI—A Mini-PCI card is a smaller version of a standard desktop PCI card and fits well within small, mobile computer devices. It has all the same features and functionality of a normal PCI card, but is about one quarter the size. Mini-PCI cards are integrated within laptops as an option to buyers. A strong advantage of this form of radio NIC is that it frees up the PC Card slot for other devices, such as memory extenders and graphics accelerators.

In addition, manufacturers can provide Mini-PCI–based wirelessNICs at lower costs. The Mini-PCI card is not without disadvantages,however. The replacement of a Mini-PCI card typically requires thedisassembly of the laptop, which might void the manufacturer's warranty. Mini-PCI cards might also lead to lower performance because they require the computer to do some, if not all, of theprocessing. Despite these drawbacks, the Mini-PCI card is becoming a solid technology in the wireless laptop world.

CompactFlash—SanDisk Corporation first introduced CompactFlash (CF) in 1994, but wireless NICs were not available in CF form factors until recently. A CF card is small, weighing half an ounce, and is
less than half the thickness of a PC Card. It also holds only one quarter the volume of PC Card radio card. The CF cards draw little power, which enables the batteries to last longer than devices using PC Cards. Some PDAs come with direct CF interfaces, which results in a lightweight and compact wireless PDA. If the computer device doesn't have a CF slot, you can purchase an adapter so that the CF card will fit into a standard PC Card slot. A CF radio card is definitely the way to go, especially for compact computing devices.

In addition to the internal NICs, a variety of external network interfaces connect to the computer device through parallel, serial, and USB ports. These might be suitable for stationary computers, but they certainly hinder mobility in most wireless applications. As Chapter 3, "Radio Frequency and Light Signal Fundamentals:

The Invisible Medium," discusses in detail, a wireless NIC includes an antenna that converts electrical signals to radio or light waves for propagation through the air medium. Antennae employ many structures, and they can be external, internal, permanent, or detachable.

The antenna for a PC Card, for example, generally attaches to the end of the card and protrudes out the side of the laptop.

Mini-PCI cards, however, might have an antenna that resides inside the outer edge of a laptop monitor. Some NICs have antennaes that are permanent, which have one particular propagation pattern. Other NICs allow the replacement of the antenna, which increases flexibility in choosing an antenna that best satisfies requirements.

Air Medium

Air serves many purposes, such as providing a basis for speech, enabling air travel, and sustaining life. Air also provides a medium for the propagation of wireless communications signals, which is the heart of wireless networking. Air is the conduit by which information flows between computer devices and the wireless infrastructure.

Think of communication through a wireless network as similar to talking to someone. As you move farther apart, it's more difficult to hear each other, especially when a loud noise is present.

Wireless information signals also travel through the air, but they have special properties that enable propagation over relatively long distances. Wireless information signals cannot be heard by humans, so it's possible to amplify the signals to a higher level without disturbing human ears. The quality of transmission, however, depends on obstructions in the air that either lessen or scatter the strength and range of the signals.

Rain, snow, smog, and smoke are examples of elements that impair propagation of wireless communications signals. In fact, a heavy downpour of rain can limit signal range by 50 percent while the rain is occurring. Other obstacles, such as trees and buildings, can impact the propagation and performance of the wireless network. These issues become most important when planning the installation of a wireless MAN or WAN.

With wireless networks, the air medium supports the propagation of radio and light waves that travel from one point to another. These types of signals have been in use for more than 100 years, but they are still somewhat mysterious and not well understood by most computer professionals. Chapter 3 provides details on signal characteristics and impairments that relate to the air medium.


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Protocols for WLAN

Multiple Access with Collision Avoidance (MACA) is a slotted media access control protocol used in wireless LAN data transmission to avoid collisions caused by the hidden station problem and to simplify
exposed station problem.

The basic idea of MACA is a wireless network node makes an announcement before it sends the data frame to inform other nodes to keep silent. When a node wants to transmit, it sends a signal called Request-To-Send (RTS) with the length of the data frame to send. If the receiver allows the transmission, it replies the sender a
signal called Clear-To-Send (CTS) with the length of the frame that is about to receive.

Meanwhile, a node that hears RTS should remain silent to avoid conflict with CTS; a node that hears CTS should keep silent until the data transmission is complete.

WLAN data transmission collisions may still occur, and the MACA for Wireless (MACAW) is introduced to extend the function of MACA. It requires nodes sending acknowledgements after each successful frame transmission, as well as the additional function of Carrier sense.


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infrared technology

Infrared (IR) light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.7 micrometers, and extending conventionally to 300 micrometres. These wavelengths correspond to a frequency range of approximately 430 to 1 THz,[1] and include most of the thermal radiation emitted by objects near room temperature. Microscopically,
IR light is typically emitted or absorbed by molecules when they change their rotational-vibrational movements.

Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation.

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