transmission switching

transmission switching

For transmission of data beyond a local area, communication is typically achieved
by transmitting data from source to destination through a network of intermediate
switching nodes; this switched network design is sometimes used to implement
LANs and MANs as well. The switching nodes are not concerned with the content
of the data; rather their purpose is to provide a switching facility that will move the
data from node to node until they reach their destination. The end devices that wish
to communicate may be referred to as stations. The stations may be computers, terminals,
telephones, or other communicating devices. We will refer to the switching
devices whose purpose is to provide communication as nodes. The nodes are connected
to each other in some topology by transmission links. Each station attaches
to a node, and the collection of nodes is referred to as a communication network.
Figure 3.3 illustrates a simple network. Signals entering the network from a station
are routed to the destination by being switched from node to node. For example, information
from station A intended for station F is sent to node 4. It may then be routed via
nodes 5 and 6 or nodes 7 and 6 to the destination. Several observations are in order:
1. Some nodes connect only to other nodes (e.g., 5 and 7). Their sole task is the
internal (to the network) switching of information. Other nodes have one or
more stations attached as well; in addition to their switching functions, such
nodes accept information from and deliver information to the attached stations.
2. Node-station links are generally dedicated point-to-point links. Node-node links
are usually multiplexed links, using either frequency division multiplexing
(FDM) or some form of time division multiplexing (TDM).
Usually, the network is not fully connected; that is, there is not a direct link
between every possible pair of nodes. However, it is always desirable to have
more than one possible path through the network for each pair of stations.
This enhances the reliability of the network.