Handbook of Local Area Networks, 1998 Edition:Advanced LAN Issues and Solutions Click Here! Search the site:   ITLibrary ITKnowledge EXPERT SEARCH Programming Languages Databases Security Web Services Network Services Middleware Components Operating Systems User Interfaces Groupware & Collaboration Content Management Productivity Applications Hardware Fun & Games EarthWeb sites Crossnodes Datamation Developer.com DICE EarthWeb.com EarthWeb Direct ERP Hub Gamelan GoCertify.com HTMLGoodies Intranet Journal IT Knowledge IT Library JavaGoodies JARS JavaScripts.com open source IT RoadCoders Y2K Info Previous Table of Contents Next WIRELESS NETWORK ARCHITECTURE The functional ability of a wireless network, as illustrated in Exhibit 2-6-2, applies to the lower layers of a network architecture. The lowest layer—the physical layer—provides the transmission of raw bits of data that comprise the information being sent from one location to another. For the physical layer, wireless networks use either radio waves or infrared light for the transmission of data, instead of wireline type media consisting of twisted-pair, coaxial cable or optical fiber. The physical layer of wireless networks modulates the data with either a radio or infrared light signal for transmission through the atmosphere. Exhibit 2-6-2.  Wireless Network Logical Architecture The data link layer of the network architecture is responsible for synchronization and error control of the link between sender and receiver. For synchronization, wireless networks use a carrier sense protocol (e.g., carrier sense multiple access [CSMA]), which is similar to the common Ethernet standard. This protocol allows a local group of computers to share the same frequency and space. With CSMA, wireless stations take turns sending data by first listening to see if another station is transmitting. This avoids having a distorted signal that will occur if two or more stations transmit at the same time. This is similar to having a discussion with a group of people—if one person would like to speak, they wait until the other person stops talking. Each wireless station handles error control by checking incoming packets for errors. If the station receives a data packet without errors, it will send back an acknowledgment. If the station detects errors in the packet, the data link protocol ensures the transmitting station resends the packet. There are several types of wireless networks—wireless LANs, point-to-point networks, and wireless WANs. The following sections of this chapter describe each of these types of wireless networks. WIRELESS LANS A wireless LAN applies to the same confines of a traditional wireline LAN, typically the coverage of a room or small building. The use of wireless LANs is effective for satisfying mobility requirements within a facility (e.g., hospitals and warehouses), and installing within difficult-to-wire environments. Most wireless LAN products use carrier sense protocols over omnidirectional radio waves or diffused infrared light to transmit data between computers. Other products use point-to-point infrared technologies to replace the cabling in Token Ring networks. Another quasi-wireless medium that some wireless LAN products take advantage of is the electrical wiring within a home or office to provide a medium for the transmission of data. Most of the wireless LAN adapters are compatible with popular network operating systems (e.g., Netware and Lantastic), and they integrate well into existing Ethernet and Token Ring networks. The following sections explain these wireless LAN approaches. Radio-Based LAN Techniques The most common transmission media used in wireless LANs are radio waves. As shown in Exhibit 2-6-3, a radio LAN consists of a user-installable wireless network interface card (NIC) installed on the bus of a desktop computer, or a small credit card-sized adapter inserted into the Personal Computer Memory Card International Association (PCMCIA) slot of a portable computer. The wireless cards have an external antenna that can be attached to a wall or laid on a desk. These radio-based cards, ranging from $400 to $700 each, are more expensive than Ethernet boards. The benefits of having a wireless connection, however, may outweigh the additional cost. Exhibit 2-6-3.  Radio-Based Wireless LAN Most radio-based wireless LAN vendors also sell a local bridge that will interface their wireless products to a wireline network (e.g., Ethernet). These bridges convert the vendor's proprietary wireless protocol to 802.3 (Ethernet) and filter packets based on destination addresses. Wireless local bridges cost approximately $1,500 each. The advantage of using radio waves is that transmitted signals propagate fairly well through the atmosphere, despite the presence of obstructions. A disadvantage, however, is that radio waves must be managed and supported in an environment along with other possibly intrusive electromagnetic propagation. Most radio-based LANs are designed to operate with an output power under one watt in the shared industrial, science, and medicine (ISM) bands (902-928 MHZ, 2400-2483.5 MHZ, and 5725-5850 MHZ, respectively). The ISM bands require the use of spread spectrum modulation that spreads a signal's power over a wider band of frequencies, making the data signal much less susceptible to electrical noise and resulting in less interference with other radio-based systems. Radio-based LANs using the ISM bands are capable of operating at near-Ethernet speeds (10M bps) and as far as 1,000 feet apart in an open office area, or distances of 100 feet within buildings having fixed walls. In addition, commercial users who purchase ISM band products do not need to manage special FCC licenses. Some radio LANs operate using narrowband modulation (nonspread spectrum), but this requires users to obtain special FCC licenses. Licensing can be a management problem, however, the licensing ensures other nearby systems will not be licensed to operate in the same frequency band. Proxim's RangeLAN2 is an example of a radio-based wireless LAN network interface card. RangeLAN2 is available in both ISA and PCMCIA formats and provides data rates of 1.6M bps over the 2.4 Ghz ISM frequency bands. Previous Table of Contents Next Use of this site is subject certain Terms & Conditions. Copyright (c) 1996-1999 EarthWeb, Inc.. All rights reserved. Reproduction in whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Please read our privacy policy for details.