4.4 Applications 8.4.4.1 Automatic Alternative Routing. One type of progressive (irreversible) rout- ing is automatic alternative routing (AAR). When an exchange has the option of using LOCAL AND LONG-DISTANCE NETWORKS more than one route to the next exchange, an alternative routing scheme can be employed. The two principal types of AAR that are available are: 1. When there is a choice of direct-circuit groups between two exchanges 2. When there is a choice of direct and indirect routes between the two exchanges Alternative routing takes place when all appropriate circuits in a group are busy. Sev- eral circuit groups then may be tested sequentially. The test order is fixed or time- dependent. 8.4.4.2 Automatic Rerouting (Crankback). Automatic rerouting (ARR) is a routing facility enabling connection of call attempts encountering congestion during the initial call setup phase. Thus, if a signal indicating congestion is received from Exchange B, subsequent to the seizure of an outgoing trunk from Exchange A, the call may be rerouted at A. This concept is shown in Figure 8.12. ARR performance can be improved through the use of different signals to indicate congestion-S1 and S2 (see Figure 8.12). S1 indicates that congestion has occurred on outgoing trunks from exchange B. S2 indicates that congestion has occurred further downstream-for example, on outgoing trunks from D. The action to be taken at exchange A upon receiving S1 or S2 may be either to block the call or to reroute it. In the example illustrated in Figure 8.12, a call from A to D is routed via C because the circuit group B-D is congested (S1 indicator) and a call from A to F is routed via E because circuit group D-F is congested (S2 indicator). One positive consequence of this alternative is to increase the signaling load and number of call set-up operations resulting from the use of these signals. If such an increase is unacceptable, it may be advisable to restrict the number of reroutings or limit the signaling capability to fewer exchanges. Of course,