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

HVAC Control in the New MillenniumEvery transceiver on the network reads the address information.When a packet destination address matches the Ethernet address of anode, the transceiver for that destination node accepts the Ethernet signalinto the receive buffers on its Ethernet controller. The signal wavecontinues traveling down the cable until absorbed by the terminators.All transceiver units listen to the coax broadcast signal. When thedestination address matches the Ethernet address of a receiving node,that transceiver reads the signal and copies the transmission into its ownbuffer without changing or damaging the signal wave.The controller verifies that the length of the data message matchesthe value in the length field. Any discrepancies are passed to the softwareprotocols. TCP/IP, for example, would request a rebroadcast of thedamaged packet. TCP/IP also reports receipt confirmation to the transmittingnode. If the transmission has been successful, the data fieldcontents are passed to the software protocols and eventually transferredto the application.Ethernet DrawbacksThe nodes contend for network access, much like users of party-linetelephones compete for a free line. This contention method of broadcastingallows more than one transceiver to transmit at the same time. Two ormore transceivers may simultaneously sense a quiet network. In that timeinterval, each will begin to transmit packets. The signals will collide, corruptingeach transmission. One unit may recognize the collision andbroadcast a jam signal. As a consequence two fragmentary packets aretransmitted in each direction for each corrupted transmission.When a collision has been sensed and acknowledged, the Ethernetcontrollers create a random number. This is usually provided by the nodecomputer equipment and is used to delay further retransmission attempts.The random delay serves to restart the contention for the network.These collisions on Ethernet create fragmentary packets and lowerthe possible transmission rate. The jam signal takes a minimum of 32bits and all colliding packets must be retransmitted.An administrative layer directs the packets and deals with trafficproblems from packet content. This software provides the ability toknow what nodes exist. Some systems map network station names into©2001 by The Fairmont Press, Inc. All rights reserved.