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

HVAC Control in the New MillenniumThe modularity of the ISO model design means that each individualmodule can be replaced with a module of a different protocol.The new module must provide the same functionality as the module itreplaced. This is accomplished without any interruption in the functionof the other layers. This approach allows multiple-vendor operation andencourages technology changes.Functional LayersThe functions performed at each layer of the ISO model define thedifferent types of hardware and software needed to install or enhancethe network. The physical layer defines the lowest or first level. Thislayer defines the methods used to transmit and receive data on thenetwork. It consists of the wiring and devices that are used to connecta station’s network interface controller to the wiring and the signalsneeded to transmit and receive data on the network.The data-link layer is the second layer. This layer synchronizes thetransmission and handles the packet error control and recovery. Frameformatting and redundancy checking to check for errors in the data areaccomplished at this layer. This layer defines the network access methodsuch as Ethernet.The network layer controls the forwarding of messages betweenstations. This layer allows the data to flow sequentially between stationsin the most economical path both logically and physically. This layeralso allows packets to be transmitted over devices known as routers.The transport layer provides end-to-end or station-to-station transmissionof the data. This layer is responsible for the transmission of databetween communicating stations.The session layer establishes, maintains and disconnects the communicationslink between two stations. This layer is also responsible forthe station physical addresses.The presentation layer is responsible for the data format and dataencryption. It is not always implemented.The application layer is the seventh layer. This layer is used forapplications that run over the network. These applications include filetransfers.Since each layer has its particular role, it provides structure in whatis a static model. Each layer has its place and cannot be moved to replaceanother layer above or below it.©2001 by The Fairmont Press, Inc. All rights reserved.