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

Computer Networks and Securitystructure of most high-level protocols allows a rethinking, rebuildingand rebundling of network services. This repackaged Ethernet technologyalso provides a starting point for small network installations thatcan gradually expand to a coaxial backbone connecting concentrationsof twisted-pair installations.Ethernet provides plug-and-play intercompatibility that is otherwiseunavailable. This may be sufficient to prevent the replacement ofexisting networks.Expect to see more implementations that encapsulate Ethernet.Implementations of routers, packet switches, and gateways can boostthe integration of small networks into wider enterprise type systems.Ethernet comprises layers 1 and 2 of the OSI model. Layers 3through 7 are not part of Ethernet, although Ethernet is often packagedwith the TCP/IP protocol. TCP/IP provides the network-layer functions.TCP/IP was commissioned by the Department of Defense for theInternet wide area network (WAN) network. It was standardized by thegovernment, but is supplied in a rapidly growing list of products. Theadvantage of TCP/IP is its simple structure and its ready implementationwithin the memory and speed limits of most computer platforms.The disadvantage of TCP/IP as implemented is that it does not conformrigorously to the OSI protocol.Ethernet is represented by hardware but software protocols buildupon the basic hardware that defines Ethernet. Ethernet is physical, althoughmost Ethernet networks include network software such as TCP/IP. This difference between the OSI model and Ethernet TCP/IP is apt tofade.The physical layer acts as a conduit or data path and the data-linklayer is implemented by software, controller and transceiver units. Thesoftware function is often serviced by TCP/IP and is represented in thedata-link layer by communication control protocols.Ethernet VariationsEthernet hardware has several variations and there are physicaldifferences among them (See Table 7-2). There are standards for thevarious transmission media, including baseband, twisted-pair, radio frequencyand infrared, broadband coaxial cable and optical fiber.©2001 by The Fairmont Press, Inc. All rights reserved.