A serial bus line for data transmission within a locally restricted area is referred to as a LAN (or also an "inhouse net"). Local networks are the sole responsibility of the user and are restricted to their premises. (ISO/IEC 2382-25.). More than one local network may be connected with each other in the same private area. A gateway to the public network can be arranged to allow communication with remote local networks too.
A LAN is used for data transmission between independent devices which are connected with each other (autonomous systems). Independent devices include computers, PLCs, computerised numerical controls (CNCs) and also storage and peripheral devices (e.g. printers, terminals). The way the flow of information is controlled between the devices within a LAN is dependent on the network structure (topology). A distinction is made (Figure L 6) between the indirect transmission strategy (star structure, tree structure), whereby messages are sent via one or more relay stations responsible for forwarding information, and the direct transmission strategy, where messages are sent from the information source directly to the receiver.
Cables with a high bandwidth (coaxial, fibre-optic, copper cables) are predominantly used as the transmission medium to local networks. They can be used on an exclusive or shared basis for this purpose. Exclusive use is possible for networks with a star topology or a fully intermeshed structure. With this scenario, two stations are always connected using a transmission medium of their own. With shared use, all devices are connected to a single transmission medium (ring, bus line). To prevent signal overlap and the associated destruction of sent data, each device has to monopolise the transmission medium for the purpose of data transmission. With the shared use scenario therefore, a suitable multiplex procedure will have to be adopted to enforce coordination in respect of the transmission medium. A distinction is made between space-related, frequency-related and time-related multiplex procedures. Space-related and frequency-related multiplex procedures enable devices to use the transmission medium independently of each other. The time-related multiplex procedure requires data transmission to be coordinated between individual devices.
The collision-free time-related multiplex procedure adopts the principle of token passing for the most part. The token is a short message relayed from one bus node to its designated successor and represents exclusive sender rights for the possessor of the token.
With stochastic bus access, each bus node wishing to send something begins its transmission when things seem to be right with the bus. This may result in data collisions if more than one node wants to send something at the same time. Disrupted transmissions are interrupted when collisions are detected and then started once more after a random waiting period if the bus appears to be free again (Carrier sense multiple access with collision detection (CSMA/CD), ALOHA and the like). Local networks make it possible to access more than one node via shared databases, to split the processing of calculations or to share expensive peripheral devices.
(Literature: H. Töpfer, W. Kriesel, Funktionseinheiten der Automatisierungstechnik [Functional Units in Automation Technology], VEB Verlag Technik, Berlin 1988).
Figure L 6: Basic structures for local networks (LAN)