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Twisted-Pair

Recently twisted-pair has become very popular as a LAN medium. Although its performance characteristics are less appealing, its low cost and high availability certainly are attractive. Unshielded Twisted-Pair (UTP) performs nicely at low data rates using the same cable plant for LANs as is used for telephone terminals, hence the current tendency to pull multiple Category 5 UTP pairs to each jack—voice and data terminals can share a common wiring system. Additionally, UTP has been proved to perform at very high data rates (100 Mbps) over short distances.

The advantages of UTP include its low costs of acquisition, deployment, and reconfiguration; its durability is moderate. The disadvantages of UTP include its relatively low bandwidth and poor error performance, at least over long distances. Additionally, UTP offers little security as the radiated electromagnetic field is considerable at high speeds (frequencies).

Shielded twisted-pair (STP) sometimes is used in LAN applications, although it is unusual outside of Token Ring LANs. STP might be used in an environment in which UTP data transmission might be especially susceptible to EMI/RFI, or might cause interference on adjacent pairs. Generally, however, another transmission medium, such as ThinNet, would be employed.

The disadvantages of UTP have been mitigated to some extent, and the LAN applications have increased through the development and use of Category (Cat) or Level 3, 4, and 5 UTP. By way of example, consider 10Base-T, 1Base5 and CDDI.

10Base-T, or twisted-pair Ethernet
uses Cat 3, 4, or 5 UTP. 10Base-T translates to 10 Mbps, Baseband, Twisted-pair. The maximum segment length between the 10Base-T hub and the attached device (workstation, printer, etc.) is 100 meters or less. 10Base-T actually is a wire hub that serves as a multiport repeater, as well as a central point of interconnection.
1Base5
a variation on the theme of 10Base-T, uses Cat 3, 4, or 5 UTP. 1BaseT translates to 1 Mbps, Baseband, 500 meters or less.
CDDI, or Cable Distributed Data Interface
also is known as TPDDI (Twisted-pair Distributed Data Interface). CDDI employs Cat 5 UTP as an inexpensive means of connecting workstations and peripherals to FDDI fiber optic backbone LANs. Transmission rates up to 100 Mbps are supported. Distance is not specified, but it is generally less than 20 meters between the hub and the device.
Additionally, Category 3 (Cat 3) UTP often is used for 4 Mbps Token Ring LANs; Category 4 (Cat 4) UTP has a bandwidth of 20 MHz and commonly is used for 16 Mbps Token Ring LANs.

Fiber-Optic Cable

Due its outstanding performance characteristics, fiber-optic cable is also used in LANs. However, its cost and fragility generally relegate it to use as a backbone technology. FDDI (Fiber Distributed Data Interface) is the current LAN standard (IEEE and ANSI) for such a network. FDDI can be extended to the desktop, either directly or through the use of twisted-pair in a CDDI application.

The advantages of fiber include its high bandwidth, which is 100+ Mbps in the LAN world, and excellent error performance (Bit Error Rate (BER) [identically equal to] 10-14). Additionally, fiber is capable of transmitting data over very long distances and with excellent security. The disadvantages of fiber include its high costs of acquisition, deployment, and reconfiguration. Fiber also is very fragile; it must be protected carefully. Additionally, few users truly require 100 Mbps at the desktop—at least for the moment.

Wireless Technology

Wireless LANs offer the obvious advantage of avoidance of cabling costs, which can be especially important in a dynamic environment where there is frequent reconfiguration of the workplace. Additionally, wireless LANs provide LAN capabilities in temporary quarters, where costly cabling would have to be abandoned.

Each workstation is fitted with a low power transmit/receive radio antenna in the form of a card. Frequency assignments are in the 900 MHz, 2 GHz, and 5 GHz bands. A hub antennae is located at a central point (Figure 9.1), such as the center or a corner of the ceiling, where line-of-sight can be established with the various terminal antennae. The antenna are then connected to other hub antennae and to the servers, peripherals, and hosts via cabled connections, which also connect together multiple hub antennae for transmission between rooms, floors, and buildings. In order to serve multiple workstations, spread-spectrum radio technology is used to make effective use of limited bandwidth. Spread spectrum involves scattering packets of a data stream across a range of frequencies, rather than using a single transmission frequency. A side benefit of spread-spectrum is that of increased security, as the signal is virtually impossible to intercept [9-5]. While the raw aggregate bandwidth of a wireless radio LAN is generally described as 4 Mbps, the effective throughput is more in the range of 1 to 2 Mbps per hub [9-6]. Some wireless LANs also use direct sequence transmission—a signal is sent simultaneously over several frequencies; increasing the chances that the signal will get through to the access hub [9-7].


Figure 9.1  Wireless LAN configuration.

Some wireless LANs use unlicensed frequencies (900 Mhz) to avoid expensive and lengthy licensing by the regulatory authorities (FCC). However, other systems in close proximity could cause interference and additionally, a wide variety of other devices (e.g., garage door openers and bar code scanners) use the same frequencies. The systems which use licensed frequencies (2 GHz and 5 GHz), avoid the potential for interference, but do require that the manufacturer carefully police the deployment of such systems under the terms of an omnilicense.

Although it still is somewhat unusual, infrared light can be used as the transmission medium, rather than radio. As described in Chapter 3, an infrared LAN system generally requires line-of-sight between the light source and receiver. Within a room, however, the light signal can bounce off of walls, ceilings and other surfaces until it reaches the receiver. As the light signal cannot penetrate solid objects, it will bounce around until it loses power. This infrared transmission technique is known as diffused propagation [9-5]. PDAs (Personal Digital Assistants) make widespread use of infrared to establish links with workstations and other PDAs for data transfer. Enhanced infrared technology recently has been demonstrated at speeds of 1.5, 4, and even 155 Mbps [9-8].


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