![]() |
|||
![]()
|
![]() |
![]() Click Here! |
![]() |
Twisted Pair Cable Twisted pair cable is the most common transmission medium forLANs. It is comprised of copper wires individually surrounded bya PVC insulating layer and twisted around each other in a spiral.The wires are twisted to improve the transmission characteristics.There is an installed base of twisted pair cable in the majorityof buildings because twisted pair is the medium for analog voice(i.e., telephone). Twisted pair is relatively inexpensive and easyto install and terminate. There are two types of twisted pair cable, shielded twisted pair(STP) and unshielded twisted pair (UTP). In STP, a band of groundedmetal surrounds the conductors to prevent outside electricalsignals from interfering with the transmission. The shielding alsoensures that the electromagnetic field generated in one pair willnot interfere with the signal in an adjacent pair. Both STP and UTPare used for 4M- and 16M-bps Token Ring LANs; UTP is used for10Base-T Ethernet. Standards have been or are being developed foreven higher bandwidth applications over UTP. Among these are100M-bps Fiber Distributed Data Interface (FDDI) over UTP andasynchronous transfer mode(ATM)at lower speeds (25M and 50M bps). Twisted pair wire is also specified by its American Wire Gauge(AWG) number, which refers to the diameter of the copper wire.Almost all twisted pair wiring used in communications cableinstallations is between 23 and 26 AWG, but 24 AWG has become thede facto standard. Different classes or types of twisted pair exist to supportdifferent LAN types, connector form factors, and transmissionspeeds. IBM, for example, specifies several different twisted-paircabling types. The specifications developed by the EIA for UTPcables for operation at different transmission speeds, the IBMcable system categories, and EIA distance specifications for themain cable applications are listed in Exhibit 1-3-1.
Fiber-Optic Cable Fiber-optic cable consists of bundled fiber strands. Each fiberstrand has a thin, inner core of optical fiber and a cladding,which is a concentric glass covering, surrounding the core. Thecore and cladding are surrounded by a protective covering.Fiber-optic cable is able to transmit signals over long distancesat very high bandwidth. Fiber-optic cable has advantages over coaxial cable and twistedpair; it is not susceptible to electromagnetic or radio frequency interference, and it is difficult to tap into so it is atremendously secure medium. Fiber-optic cable shows a great dealof promise as a LAN transmission medium, as is evidenced in thegrowing deployment of FDDI LANs. As high-bandwidth LAN applications(e.g., the transmission of radiological images and multimediatraffic) are deployed, fiber will be increasingly installed to thedesktop. Fiber is already in widespread use as the backbone mediumof choice. Fiber-optic cable is currently more expensive to installthan copper, not because the transmission medium itself is morecostly, but because splicing and termination and the electronicsrequired at each end are more expensive. There are two types of fiber-optic transmission, single mode andmultimode. A multimode fiber accepts and propagates light thatenters the core at different angles. The mode entering the core atthe largest angle relative to the fibers axis reaches the end ofthe fiber last. Modal dispersion is the delay between when thefastest mode and the slowest mode reach the end of the fiber. Modal dispersion is eliminated by using single-mode fiber, inwhich only one mode of light is propagated through the fiber.Single mode fiber has a very small core and the cladding forces thelight signal to propagate in a focused, straight pathway throughthe core. Single-mode fiber has a higher bandwidth than multimodebut has higher equipment and installation costs. TOPOLOGY Another factor in a wiring system is the topology, which refersto the logical layout of the LANs workstations and how end pointsare connected. A network in which each end point is connected toevery other end point (a type of mesh topology), would soon becomevery cable intensive. But a mesh topology provides multiple pathsbetween end stations and is used in long-distance voice networks.A mesh network is inherently reliable because of its multiplepaths, but routing is required within the network, which can be acomplication. This topology is not common in LANs. The major LANtopologies are star, ring, and bus. In a star topology, each end node is directly connected to acentral switch, which routes data and thus provides the connectionbetween end nodes. In a ring topology each device is connected tothe next in a continuous closed loop. Each device receives data romone link, regenerates the signal, and retransmits it over the linkconnecting it to the next end station. Typically, a ring transmitsin one direction only. An example of a LAN using the ring topologyis Token Ring. A bus topology is a continuous transmission line towhich all end stations connect. A transmission signal sent from onedevice is received at all other devices. The line sharing in a busmandates that only one device transmits at a time. Ethernet is anexample of a LAN that transmits over a bus topology. A distinction must be made between logical network topologiesand physical network topologies. The description above refers tothe logical network topology, or how electrical signals flowthrough the network. The actual physical layout of a structuredwiring system is generally a star, although some older LANs arewired in physical buses and rings. A LAN that is physically wired in a start derives its logical configuration from the internalconstruction of the hub or media access unit (MAU).
|
![]() |
|
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. |