Data Communications Networking Devices - 4th Ed.pdf

292 ________________________________________________________ LOCAL AREA NETWORKSare usually unnecessary. Similarly, ®ber optic cable can be installed through areaswhere the ¯ow of electricity could be dangerous since only light ¯ows through suchcables.Since most ®bers only provide a single, unidirectional transmission path aminimum of two cables is normally required to connect all transmitters to allreceivers on a network built using ®ber optic cable. Due to the higher cost of ®beroptic cable than coaxial or twisted-pair, the dual cable requirement of ®ber cablescan make them relatively expensive in comparison to other types of cable. In addition,it is very dif®cult to splice such cable, which usually means skilled installersare required to implement a ®ber optic-based network. Similarly, once this type ofnetwork is installed, it is dif®cult to modify the network.Currently, the cost of the cable, dif®culty of installation and modi®cation makethe utilization of ®ber optic-based local area networks impractical for manycommercial applications. Today, the primary use of ®ber optic cable is to extendthe distance between workstations on a network or to connect two distant networksto one another. The device used to connect a length of ®ber optic cable into theLAN or between LANs is a ®ber optic repeater. The repeater converts the electricalenergy of signals ¯owing on the LAN into light energy for transmission on the®ber optic cable. At the end of the ®ber optic cable, a second repeater convertslight energy back into electrical energy. With the cost of the ®ber optic cabledeclining and improvements expected to simplify the installation and modi®cationof networks using this type of cable, the next few years may witness a profoundmovement toward the utilization of this transmission medium throughout local areanetworks.Access methodIf the topology of a local area network can be compared to a data highway, then theaccess method might be viewed as the set of rules that enable data from oneworkstation to successfully reach its destination via the data highway. Withoutsuch rules, it is quite possible for two messages sent by two differnet workstationsto collide, with the result that neither message reaches its destination. Twocommon access methods primarily employed in local area networks are Carrier-Sense Multiple Access with Collision Detection CSMA/CD) and token passing.Each of these access methods is uniquely structured to address the previouslymentioned collision and data destination problems.Prior to discussing how access methods work, let us ®rst examine the two basictypes of devices that can be attached to a local area network to gain an appreciationfor the work that the access method must accomplish.Listeners and talkersWe can categorize each device by its operating mode as being a listener or a talker.Some devices, like printers, only receive data, and thus operate only as listeners.Other devices, such as personal computers, can either transmit or receive data and