Data Communications Networking Devices - 4th Ed.pdf

3.2 TECHNOLOGICAL CHARACTERISTICS ___________________________________________ 277units or MAUs). In an FDDI network, a second ring can be activated automaticallyas part of a self-healing process to overcome the effect of a cable break.A tree-structured network is similar to a star-structured network in that all signals¯ow through a common point. In the tree-structured network the common signalpoint is the headend. Failure of the headend renders the network inoperative.This network structure requires the transmission of information over relatively longdistances. For example, communications between two stations located at oppositeends of the network would require a signal to propagate twice the length of thelongest network segment. Due to the propagation delay associated with the transmissionof any signal, the use of a tree structure may result in a response time delayfor transmissions between the nodes that are most distant from the headend.Signaling methodsThe signaling method used by a local area network refers to both the way datais encoded for transmission and the frequency spectrum of the media. To a largedegree, the signaling method is related to the use of the frequency spectrum of themedia.Broadband versus basebandTwo signaling methods used by LANs are broadband and baseband. In broadbandsignaling, the bandwidth of the transmission medium is subdivided by frequency toform two or more subchannels, with each subchannel permitting data transfer tooccur independently of data transfer on another subchannel. In baseband signalingonly one signal is transmitted on the medium at any point in time.Broadband is more complex than baseband, because it requires information tobe transmitted via the modulation of a carrier signal, thus requiring the use ofspecial types of modems.Figure 3.2 illustrates the difference between baseband and broadband signalingwith respect to channel capacity. It should be noted that although a twisted-pairwire system can be used to transmit both voice and data, data transmission isbaseband since only one channel is normally used for data. In comparison, abroadband system on coaxial cable can be designed to carry voice and severalsubchannels of data, as well as fax and video transmission.Broadband signalingA broadband local area network uses analog technology, in which high frequencyHF) modems operating at or above 4 kHz place carrier signals onto the transmissionmedium. The carrier signals are then modi®edÐa process known asmodulation, which impresses information onto the carrier. Other modems connectedto a broadband LAN reconvert the analog signal block into its originaldigital formatÐa process known as demodulation.