xample is shown in Figure 3.65. 3.8.1 Loop Protection Loop protection is one of the most robust techniques. When one link fails in a loop, traffic can be routed through to the other side. In Figure 3.66 the link between site 1 and site 2 becomes unavailable; hence the traffic from site 1, which was being routed (to BSC) through sites 2, 3 and 4, will in the present conditions be routed directly to the BSC. The failure of the link between site 1 and site 2 can be due to a power cut, equipment failure, human errors, fading, obstacles, etc. The switching of the traffic flow is performed in such a short duration that there are no drop calls. Another major advantage of the loop is increased availability. The availability is increased by more than 10 times when compared to that of a single link. The unavailability in a loop can be calculated as (3.75) Figure 3.66 Loop protection Equipment Planning Figure 3.67 Availability in the the loop where PM = site unavilability N = number of hops in the loop M = consecutive number of hops from the hub p = probability of outage or unavailability of a hop in absolute terms (equipment, propa- gation, human errors) Consider site 2 in Figure 3.67. The unavailability of site 2 in a condition that it was in a chain with site 1 would be: Unavailability of the site 2 (in chain with site 1) = 0.1% + 0.2% = 0.3% Using the formula given in Equation (3.75), the unavailability of the site 2 in the present scenario = (0.1% + 0.2 %) (0.1% + 0.2 % + 0.1%) = 0.3% X 0.4% = 0.0012 %. Thus 0.0012% is a huge advantage over 0.3 % (or 0.4 % if connected via site 3 and site 4). This increased availability helps in designing links of longer hop lengths. Also, antenna sizes can be decreased. However, as seen in the section above, there is always an increased cost for the additional link. 3.9 Equipment Planning 3.9.1 BSC and TCSM Planning The number of BSC required in a network depends upon the BSC features and its capacity to support: Number of TRXs Number of BTSs Number of PCMs Number of PCU cards (only i