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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96
CHAPTER 3 Wireless Metropolitan Area Networks (MANs)
Are you thinking about connecting network sites within the same metropolitan area? Organizations often have requirements for communications between facilities in a semi-local area, such as a city block or metropolitan area. A hospital, for example, might consist of several buildings within the same general area, separated by streets and rivers. A utility company also might have multiple service centers and office buildings within a metropolitan area.
Traditionally, companies utilize physical mediasuch as buried metallic wire or optical fiber, or leased 56 Kbps or T1 circuitsto provide necessary connections. These forms of media, however, might require a great deal of installation time and can result in expensive monthly service fees. A cable installation between sites several thousand feet apart can cost thousands of dollars or more, and leasing fees can easily be hundreds of dollars per month. In some cases, leased communications lines might not even be available.
This chapter explains the applications of wireless MANs and covers the following wireless MAN types and applicable products:
- Radio-Based Wireless MANs
- Laser-Based Wireless MANs
Wireless MANs use technologies very similar to wireless LANs described in Chapter 2, Wireless Local Area Networks (LANs); therefore, this chapter will concentrate on technological aspects differing from wireless LANs.
Before getting into the technologies and products, you should understand what drives the need for wireless MAN connectivity. A wireless MAN can provide communications links between buildings in these situations, avoiding the costly installation of cabling or leasing fees and the downtime associated with system failures. The city of Macon, Georgia, for example, uses Cylinks wireless products to provide links for traffic control at ten consecutive intersections over a four-mile stretch of state highway. This system avoids the installation of wiring along the roadway. Other organizations, such as hospitals and government centers, use wireless MAN components to avoid digging trenches and routing cabling around rivers and roads.
A wireless MAN can result in tangible cost savings rather quickly. ATCO Products in Ferris, Texas, for example, installed a wireless MAN between its existing plant and a new plant under construction about 12 miles away. ATCO will recover all the installation costs in less than two years and will then begin a positive return on its investment. Many other companies are also realizing these types of benefits.
Another application of wireless MAN components is to facilitate a backup in case a primary leased line becomes inoperative. An organization can store the wireless equipment at a strategic location, such as the computing center, to have on hand if a primary link goes down. If a primary link fails, an organization can quickly deploy a wireless link to restore operations.
A wireless MAN, as illustrated in figure 3.1, utilizes either radio waves or infrared light as a transport for the transmission of data up to 30 miles. These systems work in a point-to-point configuration, much like that of leased lines. Wireless MANs interface easily and match the data rates of existing LANs. The cost of connecting two sites in a wireless MAN ranges from $1,500 to $20,000, depending on data rate and type of transport. The following sections explain each of these techniques.
Figure 3.1 The wireless MAN concept.
Radio-based Wireless MANs
A radio-based wireless MAN is currently the most common method for providing connectivity within a metropolitan area. These products have highly directional antennas to focus the signal power in a narrow beam, maximizing the transmission distance. As a result, spread spectrum products operating under one watt of power can reach single-hop transmission distances of 30 miles. The actual transmission distance of a particular product, though, is dependent on environmental conditions. Rain, for example, causes resistance to the propagation of radio signals, decreasing the effective range.
Radio-based wireless MAN data rates are four to five Mbps for the shorter range products operating over two to three mile links. Most products operating over a 30 mile link, however, will transmit at much lower data rates. Wireless MAN products use either spread spectrum or narrow band modulation.
Radio-based Wireless MAN Components
As shown in figure 3.2, radio-based wireless MANs consist of transceivers that modulate the data being sent across the link with a carrier that will propagate the signal to the opposite site. As with wireless LANs, the modulation transposes the computers digital data into a form suitable for transmission through the air. Radio-based wireless MAN products often include an interface to ethernet or token ring networks, as well as bridging or routing functionality.
Figure 3.2 A radio-based wireless MAN.
Wireless MAN bridges, also called wireless remote network bridges, segment data traffic by filtering each packet according to its final destination address. This form of segmentation blocks packets from crossing the wireless link unless they need to reach a destination on the opposite side. As with local bridges, this makes better use of bandwidth and increases network performance. The router versions of these products work very much like traditional routers that are dependent on physical mediathey forward packets based on the final destination address. This makes it possible to create a more intelligent network having alternate routes. In fact, a collection of these components would constitute the functionality of a WAN limited to a large metropolitan area. The greatest difference, though, is that the wireless MAN will not support mobile usersit only provides wireless connections between fixed sites.
Spread Spectrum Wireless MANs
As with wireless LANs, wireless MANs using spread spectrum in the ISM bands do not require user licensing with the FCC (refer to Chapter 2, Wireless Local Area Networks, for an explanation of spread spectrum). The advantage of not dealing with licenses is easy and rapid installation. For instance, the installation time of spread spectrum products can take a few hours, saving the two month wait for FCC licensing.
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