LANs: Topologies and Wiring systems

The topology of a network is an abstract representation of how the devices in this network interact. One can think of different kinds of topology (also depending whether the network is a point-to-point or broadcasting one); for LANs bus and ring topologies are most important. The bus topology has the general advantage that it is more flexible to extensions and offers a bigger reliability. Unlike this, the ring topology on principle has a shorter over-all length of communication paths.

In both topologies data is transmitted in form of packets which contain a header which specifies the destination of the packet. For each packet a system wants to send, it waits for its next possibility (determined by the used MAC technique), then sends the packet. The destination system copies the packet, as it is passing by. All other systems let the packet just pass by. With bus topology a packet is absorbed by a terminator at the end of the bus. With ring topology it is removed when it reaches the sending system a second time.

The topology has to be distinguished from the wiring system, i. e. the actual path the cable follows. Although this seems to be an issue that can easily be neglected, it is one of the most difficult problems in data communications. The actual wiring is constrained by physical needs like walls and floors in buildings. Therefore linear wiring (wiring exactly following topology) often is impossible. Alternatively a star wiring can be used, which concentrates all the wiring in one room or closet and uses an individual cable to each device within the network. This method also increases the reliability of ring topologies, but it increases path length as well.
A given topology can be transposed in a star wiring system as follows:

There also exists a general wiring standard (EIA/TIA-568) for commercial buildings that supports a multiprotocol, multivendor environment. The goal of this standard is to enable laying out of wiring systems without exactly knowing what telecommunication products will finally be installed.