HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Computer Networks and Securitywork. When transmitting, a transceiver unit lowers the voltage level to-0.7 V. The transmitting node propagates a digital signal wave outwardin both directions down the coaxial cable.Manchester encoding is used where the digital signal represents a0 as a high voltage with a transition to a low voltage and a 1 as a lowvoltage with a transition to a high voltage.Twisted-pair concentrators interpret and regenerate the signals forretransmission to other nodes. This integrity check is not available oncoaxial networks. A short circuit or other electrical malfunction willcause the failed node to be removed.Packet TransmissionIndividual bits are not transferred from location to location withouta preamble or explanation as they are in modem-connected lines.The information to be transferred is packaged and checked upon receiptfor proper delivery.The packet contains the following information:• destination address,• return or source address,• length field,• data field and• frame check sequence field.Handshaking between transmitting and receiving nodes prevents packetsfrom crashing into each other and being scrambled. When the networkis not busy, an idle signal is transmitted by all source-destinationnodes, which is the -0.7 V carrier sense. This base voltage is sometimescalled the heartbeat.Once the data set is packaged into a frame, the Ethernet controllerpolls the transceiver for the network state. If the network is busy, it waitsuntil the network is free and then transmits the packet. The packet signalpropagates in both directions over the coaxial cable.Each transceiver on the network hears the transmission preamble,synchronizes its clock, and awaits the packet. This clock synchronizationis needed to distinguish the beginning and the end of the packet. Thepreamble provides this frame alignment.©2001 by The Fairmont Press, Inc. All rights reserved.