Practical Industrial Data Networks:Design, Installation and ...

334 Practical Industrial Data Networks: Design, Installation and TroubleshootingB.11 Local vs wide area networksB.11.1Local area networks (LANs)LANs are characterized by high-speed transmission over a relatively restrictedgeographical area such as a building or a group of buildings. LANs consist of a commonmedium (such as a coaxial cable), interconnecting computers, printers, programmablelogic controllers, etc. Users can share resources such as printers, transfer data betweeneach other, and communicate via e-mail or chat sessions.There are many types of LANs, characterized by their topologies (the geometricarrangement of devices on the network), protocols (the rules for sending and receiving thedata), media (for example, the cable used) and media access methods (used to control theaccess of individual nodes to the medium).A typical LAN is Ethernet 10Base2, operating at 10 Mbps over a maximum distance of100 m across unshielded twisted pair wire via a hub. This concept is illustrated in FigureB.20.Figure B.20Local area networkB.11.2Wide area networks (WANs)While LANs operate where distances are relatively small, wide area networks (WANs)consist of inter-linked LANs that are separated by large distances ranging from a few tensof kilometers to thousands of kilometers. WANs normally use the publictelecommunications carriers to provide cost-effective connection between LANs. Sincethese links are supplied by independent telecommunications utilities, they are commonlyreferred to (and illustrated as) a ‘communications cloud’. Special equipment calledgateways (routers) have been developed for this type of activity, storing the message atLAN speed and transmitting it across the ‘communications cloud’ at the speed of theinterconnecting carrier. When the entire message has been received at the remote LAN,the message is reinserted at the local LAN speed. The speed at which a WANinterconnects is often slower than the LAN speed, but not necessarily so since many ofthe WAN carrier technologies such as ATM are capable of speeds far in excess of typicalLAN speeds. At the end of the day it boils down to cost.The concept of a WAN is shown in Figure B.21.