is frequency (GHz). (52.3) P = 10 exp Figure 52.7 A typical pole mounted outstation with power As an example, the parameters K (climatic factor), Q, B and C for and junction pack, shown on the bottom left (Photo North-West Europe are given by K = 1.4 X 10 8 , Q = 1, B = 1 and C courtsey of SR Telecom Inc. Canada) = 3.5. For this example, P = 1.4 x 10 X 1.5 X 243.5 X 10 o-21.5/10 or the availability is given by [(1-P) X 100] or 99.999% for a path of 24km. The error performance and availability objectives for digital radio Two wire telephony, and low bit rate data interfaces are generally systems used in the local grade portion (between the subscriber and provided at the CS and the OSs. At the CS critical circuits such as the local exchange) of an ISDN connection is given in CCITT transceivers are duplicated for increased reliability. Recommendation G.821 (CCIR, 1990b). For a 64kbit/s circuit, the A typical link budget for such systems is given in Table 52.4. At bit error ratio (BER) should not exceed 1x10 -6 during more than the relatively low operating frequency, rainfall attenuation is less 1.5% of any month, and 1x10-3 during 0.015% of any month. significant. However, the hop lengths can be long (up to about Furthermore, the total errored seconds should not exceed 1.2% of 50km) and therefore atmospheric multipath propagation becomes any month. significant. Since these systems have relatively low bandwidths the The unavailability allowance is generally divided between equip- narrow bandwidth fading model (CCIR, 1990a) has generally been ment failure and propagation effects (rainfall for systems above used to calculate the worst month outage. about 10GHz and multipath for lower frequencies). The relationship The system fade margin, M, is used to determine the non-availa- between errored seconds objectives at 64kbit/s and at the system bility of the system due to propagation effects. The probability (P) aggregate rate is given in CCIR (1987). Economics of urban and rural systems 52/11 Table 52.3 Link budget for a 