17. Wireless Connectivity

17.5 Radio-based wireless connectivity

The most widely sold wireless LAN products use radio waves as a medium between computers and the WEB or each other. An advantage of radio waves over other forms of wireless connectivity such as infrared and microwaves is that they propagate through walls and other obstructions with little attenuation. Even though several walls might separate the user from the server or an access point to the Web, users can maintain connections to the network, thus supporting true mobility. The disadvantage for radio frequencies is that governments manage the region and not all the spectrum can be used everywhere; hence, techniques such as FHSS and DSSS (as described ) must be used.

There are three regions of the E-M spectrum utilized by these waves:

Presently Metricom is operating a two way radio based multi-user data communications system is San Francisco called Ricochet. The architecture is shown below:

The concept is to use wireless access points and network radio relays approximately one half mile apart to facilitate connectivity between users. The radios operate in the license-free 902-928 portion of the radio spectrum using FHSS. The underlying network protocol is TCP/IP, allowing it to interact with the Internet seamlessly.

An important goal for wireless communications has been to make the application layer transparent to the underlying protocol (TCP/IP) in order to have more acceptability by the Web users. To understand the kind of standards developed for wireless networks, it helps to see the affected layers in an OSI (Open System Interconnect) model. The bottom two layers are the ones of interest to us. At the very bottom is the Physical layer. This layer defines the electrical characteristics of the actual connection between network nodes. For wired networks, it covers topics like voltage levels and type of cabling. But for wireless networks, it addresses areas such as frequencies used and modulation techniques, including spread-spectrum technologies.

The next layer up is the Data Link Layer. It deals with how the network is shared between nodes. The Data Link Layer defines rules such as who can talk on the network, how long they can occupy network resources. This layer can be further divided into two separate layers (shown below).

 

The first five layers of the OSI model remains unchanged; hence, TCP and IP can be implemented in their respective layers.

Copyright © 1996 Farhood Moslehi, All Rights Reserved

Farhood Moslehi <moslehi@vt.edu>
Last modified: Dec 10, 1996