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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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IEEE 802.3 describes several physical layer specifications, namely 10baseT and 100baseT. The word “base” identifies the use of baseband signals—that is, digital signals. The “10” and “100” mean 10 Mbps and 100 Mbps, respectively. 10baseT and 100baseT configure network devices as shown in figure 8.7. Both enable computers to connect in a star topology, via IEEE 802.3 network interface cards (ethernet boards), to a hub or switch at a distance of 100 meters (300 feet). A hub normally deploys “shared ethernet” that constitutes a single-collision domain. As an example, when station A transmits a frame, all other stations connected to that hub (B and C) will receive it. Since ethernet operates in a half-duplex mode, station A will block all other stations connected to the hub (within the collision domain) from transmitting. A switch, though, is smarter because it physically connects the sending station directly to the receiving station only. This results in multiple collision domains that significantly increase throughput. For example, communications can take place simultaneously between stations A and H and stations D and G.


Figure 8.7  The configuration of 10baseT and 100baseT networks.

  Coaxial Cable. Coaxial cable includes a solid metallic core with a shielding as a return path. The shielding reduces electrical noise interference within the core wire. As a result, coaxial cable can extend to much greater lengths than twisted-pair wiring. The disadvantage of coaxial cable is its bulky shape, which makes it difficult to install. Also, coaxial cable does not lend itself very well to centralized wiring topologies, making it difficult to maintain.
During the 1980s, coaxial cable was very popular for wiring LANs, and you might find some still existing in older implementations. Very few—if any—new implementations will require the use of 10base2 or 10base5; however, you should be aware of these types of networks in case you have wireless users that need to interface with them. IEEE 802.3 defines two physical layer specifications, 10base2 and 10base5, based on the use of coaxial cable. 10base2 uses RG58 cable, the same used to connect your television to a cable outlet, and will operate at a distance of up to 185 meters (600 feet). 10base5 uses a much larger cable than RG58, but is capable of operating up to 500 meters (1,640 feet) without the use of repeaters. Both 10base2 and 10base5 utilize a bus topology, as shown in figure 8.8.


Figure 8.8  The configurations of 10base2 and 10base5 networks.

  Optical Fiber. Optical fiber is a medium that uses changes in light intensity to carry information from one point to another (see fig. 8.9). An optical fiber system consists of a light source, optical fiber, and light detector. A light source changes digital electrical signals into light (on for a logic “1” and off for a logic “0”), the optical fiber transports the light to the destination, and a light detector transforms the light into an electrical signal.


Figure 8.9  The optical fiber communication system.


The main advantages of optical fiber are very high bandwidth (Mbps and Gbps), information security, immunity to electromagnetic interference, lightweight construction, and long distance operation without signal regeneration. As a result, optical fiber is superior for bandwidth demanding applications and protocols, operation in classified areas and between buildings, as well as installation in airplanes and ships. IEEE 802.3’s 10baseF and 100baseF specifications identify the use of optical fiber as the medium. FDDI identifies the use of optical fiber as well.

Interbuilding Connections

The interconnection of buildings often goes beyond the range of wireless LAN technologies and standards. Therefore, you need to use other means to provide these types of connections. In general, the wireless technologies offer many advantages as noted in Chapter 1, “Introduction to Wireless Networking,” for installation of links between buildings in difficult-to-wire environments, such as across freeways, rivers, or hard soil. For wireless implementations, consider the use of radio-based or infrared-based approaches as described in Chapter 3, “Wireless Metropolitan Area Networks (MANs).” Recall that these methods use highly directive signals to focus the power in a single direction.

Choosing Interbuilding Media

The following are some suggestions for choosing a wireless interbuilding medium type:

Radio Waves

Consider the use of radio waves for the following situations:

  Buildings that are separated farther than 1 mile
  When the lowest cost solution is desired

Infrared Light

Consider the use of infrared light for the following situations:

  When high bandwidth is required
  When the potential for radio frequency interference is high
  When buildings are separated by less than one mile
  When information security is important

Optical Fiber

Optical fiber is an alternative to wireless networking. In many cases, your local city, county, or building company might have installed multiple optical fiber strands between buildings in your area. In that case, building-to-building interconnectivity would only require you to obtain access to the fiber in both of the buildings. But if no fiber exists between the buildings of interest, a wireless solution will probably be most practical.

Wide Area Networks

If requirements indicate the need for mobile users covering a large geographical area to have access to network resources, then you certainly want to focus on one of the wireless WAN approaches mentioned in Chapter 4, “Wireless Wide Area Networking (WANs).” These approaches—packet radio, analog cellular, CDPD, satellite, or meteor burst—are all based on the use of radio waves. Refer to Chapter 4 for trade-offs between these wireless methods when selecting a wireless WAN technology. In many cases, though, you may be only deploying wireless LAN components. Requirements stating the need for high bandwidth and operation from fixed sites will necessitate the design and implementation of a wire-based WAN. The remaining part of this section briefly describes traditional WAN concepts and technologies.


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