ABSTRACT
Communication networks consist of nodes and links. Figure 1.1 shows an example of a network. This network consists of six nodes, node 1 to node 6. An arrow between two nodes is a connection, called a link, of those nodes. The traffic has a direction from the tail to the head of the arrow. For example, the arrow from node 1 to node 2 means that node 1 and node 2 are connected and the traffic flows from node 1 to node 2. The network in which each link has a direction, represented by a corresponding arrow, as shown in Figure 1.1, is called a directed graph. A number on each link indicates its link cost. In the case that the connection is represented by just a line, instead of an arrow, the traffic can flow in both directions on the link. A network with links through which the traffic flows in both directions is called a undirected graph.
