1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU42!.3-)33)/.G0G0-%$)!G0G0#(!2!#4%2)34)#32%,)!“),)49G0G0!.$G0G0!6!),!“),)49G0G0/2) one in which the pilot level drops by 10 dB below nominal value;3) when the total unweighted noise power, measured or calculated
2、 with an integrating time of 5 msexceeds 1 million pW (106pW) on the 2500 km hypothetical reference circuit (seeRecommendation G.222).In all instances, this condition must last at least 101)seconds.b) availability in analogue cable transmission systemsThe availability of an analogue transmission sys
3、tem is defined as the ability of the system to be in a stateto perform adequately (operating) at a given instant of time or at any instant of time within a given timeinterval. In this Recommendation, the availability of an analogue transmission system is quantified by theratio of the time during whi
4、ch the system is operating to a specified total time.Four factors influencing availability are:- reliability of the equipment;- automatic protection switching;- maintenance procedures;- cable routing and protection.In considering the importance to be attached to the individual factors, economic aspe
5、cts should play animportant role.Note - Experience has shown that in many cases the cable faults are dominating (in the order of 95% of theunavailability time) over the equipment faults and that the length of the line section and the kind of route (runningalong roads with heavy traffic, etc.) have a
6、 decisive influence on the achievable availability values._1)This value should be considered as being provisional.2 Fascicle III.3 - Rec. G.6023 Objectivesa) ReliabilityAs indicated in the definition of availability, reliability is but one of the factors involved in obtaining anavailability objectiv
7、e. Therefore, no specific objective for reliability is recommended.b) Availability1) Hypothetical reference circuit (2500 km)The objective for the availability of a 2500 km hypothetical reference circuit in one direction shouldbe greater than 99.6% for a one year duration. This takes into account ou
8、tages for both translatingand line equipment and the cable and associated powering equipments. To achieve this objective,appropriate protection switching may be required.2) Translating equipmentThe design objective for the availability of translating equipment in the Annex and in Figure A-1/G.602, f
9、or a 2500 km hypothetical reference circuit as recommended for the differenttransmission systems, should be greater than 99.9% measured for a period of one year for onedirection of transmission.3) Line sectionThe design objective for the availability of a 280 km homogeneous section for one direction
10、 shallbe derived from the overall requirement for the hypothetical reference circuit. The exact value isdependent on the network design.ANNEX A(to Recommendation G.602)Calculation exampleExample of Reliability and Availability calculations for a line section in one direction based on the followingas
11、sumptions:1) Line repeater MTBF = 2 x 105hours (one way);2) 100 line repeaters in section;3) Each failure lasts 4 hours;4) 12 tube cable with 1 : 5 protection switching.a) Reliability (MTBF)- 100 repeaters will have failure in = 2000 hoursb) Availability (A)- This is approximately 4 failures per yea
12、r 4 hours = 18 hours outage per year (0.2%)- Without protection switching A1= 99.8%Non-available X0= 2 10-3- With automatic protection switching:Fascicle III.3 - Rec. G.602 3whereN = 5 (number of systems in service)M = 1 (number of protection systems)Note - Calculations are for electronics only and do not take into account cable cuts.
