 care must be taken to avoid circular routings ("ring-around-the-rosy"), which return the call to the point at which blocking previously occurred during call setup. Originating Terminating exchange exchange Blocked Blocked Figure 8.12 The automatic rerouting (ARR) or crankback concept. Note: Blocking from B to D activates signal S1 to A. Blocking from D to F activates signal S2 to A. (From Figure 4/E.170 of Ref. 3.) 8.4 TRAFFIC ROUTING IN A NATIONAL NETWORK A 13% Traffic stream to Four outgoing a particular routing patterns destination to the destination (e.g., area code) Figure 8.13 An example of preplanned distribution of load sharing. Note: Each outgoing routing pattern (A, B, C, D) may include alternative routing options. 8.4.4.3 Load Sharing. All routing schemes should result in the sharing of traffic load between network elements. Routing schemes can, however, be developed to ensure that call attempts are offered to route choices according to a preplanned distribution. Figure 8.13 illustrates this application to load sharing, which can be made available as a software function of SPC exchanges. The system works by distributing the call attempts to a particular destination in a fixed ratio between the specified routing patterns. 8.4.4.4 Dynamic Routing. Let us look at an example of state-dependent routing. A centralized routing processor is employed to select optimum routing patterns on the basis of actual occupancy level of circuit groups and exchanges in the network which are monitored on a periodic basis (e.g., every 10 sec). Figure 8.14 illustrates this concept. In addition, qualitative traffic parameters may also be taken into consideration in the determination of the optimal routing pattern. This routing technique inherently incorporates fundamental principles of network man- agement in determining routing patterns. These principles include: Avoiding occupied circuit groups. Not using overloaded exchanges for transit. In overload circumstances, restriction of routing direct connections. Now let's examin