etwork. Figure 1.4, for example, has four nodes and six matching one-way links. Because our focus is on communications networks, we restrict attention to two-way networks unless specified otherwise. A basic example of a directed network is a direct current electric circuit, as shown in Figure 1.5. Node 1 is the source of the electric power, with a positive and negative terminal. Node 2 is the load, which is a device that is powered by electricity, Figure 1.4. A one-way network with matching links forms a two-way network. Networks in Telecommunications Source (Battery) (Electric light) Figure 1.5. A direct current electric circuit. such as a light bulb or toaster. The direction of the arcs shows the direction of the flow of the current of electrons. Another way to think about networks is in terms of the capacity of the links. A natural gas pipeline can only transmit a certain amount of gas per period of time. An electric power line can only carry a certain amount of power per unit of time. For each link in a network the transmission or transportation capacity can be specified. For example, in the network in Figure 1.6, there are four links, three of which have a capacity of 12 and one of which has a capacity of 8. Thus, within any given amount of time, more can be transported from node 1 to node 4 than from node 3 to node 4. Transporting more than 8 units from node 1 to node 3 could be faster if it went through node 2 than if it went through node 4. The segment of the trip from node 3 to node 4 is a bottleneck, because the capacity is less than for other segments. However, that segment can be bypassed for transportation between node 1 and node 3 that passes through node 2. The nodes are also subject to capacity limitations. For example, a switch in a telecommunications network can be limited in the number of calls it can direct per unit of time; see Figure 1.7. Switches and routers within data networks are subject to capacity limitations. The interaction between the capacity of switches and other devices at the no