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

Computer Networks and SecurityEthernet ProtocolsSince Ethernet is not sufficient in itself to run a network other protocolsneed to be used. The transmission control protocol (TCP) and Internetprotocol (IP) are the protocols of choice for dealing with issues of routingand end-to-end data integrity. These are the network and transportlayers in the Open Systems Interconnect (OSI) protocol model.While TCP and IP are transport and network layer protocols, theyusually come bundled with application layer protocols like file transferprotocol, simple mail transport protocol, and simple network managementprotocol (SNMP). Several control systems vendors use some ofthese protocols (such as SNMP) in their hardware, but they are not idealfor control systems. The MODBUS application protocol can be used. Thisis the application layer from the MODBUS protocol suite, bundled withTCP/IP.Both TCP and MODBUS are widely supported. However, MODBUSdoes not support an object-based communications model like most of thenew fieldbus technologies. When you read a MODBUS device over a network,you only get the data without any details about its function or format.It is no longer enough to consider a device as a group of registers.The network must be able to see an object. The IEEE 1451 standard providestechniques for self-describing measurement and control devices. Itis an object technology that specifies a common object model for devices,how they communicate, either client/server or publish/subscribe andhow they are managed.These network-neutral interfaces can be implemented on Ethernetand they provide a way of integrating multiple networks and using commonapplication software.The Fieldbus Foundation uses fast Ethernet as the base-level protocolfor its H2 network. The foundation maps the Layer 2 technologiesfrom its H1 standard into Ethernet. This includes the object model andscheduling and publisher/subscriber services. Other systems encapsulatetheir protocols inside of an Ethernet frame. This technique is knownas tunneling.The utility of Ethernet has grown with the wide acceptance of theTransaction Control Protocol and Internet Protocol (TCP/IP) suite. Partof this success has been due to the standardization and the multipleplatform support provided by Ethernet. This success is in contrast to the©2001 by The Fairmont Press, Inc. All rights reserved.