It is advisable to start priority assignment with priority = 89 at the routes to the local exchange level (=0). To differentiate between local exchange routes and internal connections to the clock master start here with the value = 85

(see Figure 64). Interfaces of the same value must be assigned the same priority. Steps of 1 or 2 are recommended for interfaces with different values. Connections with the same priority are selected automatically according to their quality (REFTA error counter). In case of a failure the next connection is selected, but never the last one. If you want to return to a certain interface after it has been proven OK it must be assigned a priority value which is 1 higher than all others. A return to the interface is possible after a minimum of 45 minutes and if the error counter is lower than 42,000 points. Figure 1 shows the graduation of priorities as a suggestion for a network structure. The graduation of the values in the reference clock table according to the hierarchy levels simplifies the assignment of the individual REFTA printouts and therefore are processed for the Network Management Center. It is also helpful to enter the individual interface layers of the REFTA in the network synchronization plan for easier assignment.

With the same interface type priorities should be assigned in the order alternate routing, wire and radio and then, if necessary, graduated by distance and number of repeaters. Interfaces with different bit rates provide different quality. This fact should be taken into consideration when assigning priorities. The order is:

in ROW: STM1, E1, PCM30, BA (U-k0, S0, U-2B1Q), X.21;

in USA: OC3, T1, T1.5 Accunet, BA (U-2B1Q)

For the assignment of priorities for tie lines the systems of one clock hierarchy level are sorted. The system with the most paths to the higher level is designated System 1. For this system the disable function is set to YES at the tie trunk interfaces to System 2 and the system is assigned a lower priority than provided for in System 2 for this connection. The system with the lower values corresponds to the zero. This allows turning the synchronization direction for this important system in the network. For all other tie trunk connections the priority is set to 0. The procedure is the same for System 2 and all other systems.

However, all possible network errors must be considered. Particular attention should be paid to the fact that clock loops do not occur in the event of errors or automatic changes - not even in scenarios with three or four systems.