for operation in either direction. The incisive reader will observe that there is some fair probability that the same trunk can be selected from either side of the circuit. This is called double seizure. It is highly undesirable. One way to reduce this probability is to use normal trunk numbering (from top down) on one side of the circuit (at exchange A in the figure) and to reverse trunk numbering, from the bottom up at the opposite side of the circuit (exchange B). Exchange Exchange Exchange Exchange Exchange Exchange Figure 1.8 Two-way and one-way circuits: two-way operation (a), one-way operation (b), and a hybrid scheme, a combination of one-way and two-way operation (c). 7 Called both-way in the United States and in ITU-T documentation. INTRODUCTORY CONCEPTS Figure 1.8b shows one-way trunk operation. The upper trunk group is assigned for the direction from A to B; the lower trunk group is assigned for the opposite direction, from exchange B to exchange A. Here there is no possibility of double seizure. Figure 1.8c illustrates a typical hybrid arrangement. The upper trunk group carries traf- fic from exchange A to exchange B exclusively. The lowest trunk group carries traffic in the opposite direction. The small, middle trunk group contains two-way circuits. Switches are programmed to select from the one-way circuits first, until all these circuits become busy; then they may assign from the two-way circuit pool. Let us clear up some possible confusion here. Consider the one-way circuit from A to B, for example. In this case, calls originating at exchange A bound for exchange B in Figure 1.8b are assigned to the upper trunk group. Calls originating at exchange B destined for exchange A are assigned from the pool of the lower trunk group. Do not confuse these concepts with two-wire and four-wire operation discussed in Chapter 4, Section 4.4. 1.3.7 Network Topologies The IEEE (Ref. 2) defines topology as "the interconnection pattern of nodes on a net- work." We can say that a telecommunications network consists