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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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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.


Figure 1.3  The wireless network logical architecture.


Figure 1.4  The physical components of a wireless network.

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 e-mail, 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.


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