Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

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Terminology and Definitions The terminology and definitions used in the article are explained here. Some ot the terms are in general use, others are specific to the article. The definitions of reliability concepts are based on the terminology in the report of the Nordic Working Group 3 with certain simplifications made for the relevant application. Availability Instantaneous availability is the probability that a unit is functioning at a certain moment of time. Complete failure A failure that means total loss of the required function. Constant failure rate period A period of time, between the early failure period and the wear-out failure period, when failures occur at an approximately uniform rate. Degradation fa/lure A characteristic exceeds the given tolerance limits without constituting complete failure, but the process is so slow that the failure could have been anticipated by prior examination. Down time The time during which a unit should be working but is faulty or being repaired. The down time comprises fault detection time, administrative time, waiting time and repair time. Net down time includes only fault detection time and repair time. Early failure period Early period with a noticeable decrease in the failure rate. Failure Undesirable deviation of a certain characteristic. In practical applications failures should be defined in detail with respect to cause, consequences and extent. Failure rate Instantaneous failure rate, z(t), is the limit value of the ratio of the probability of failure during an interval of time to the length of the interval when the latter tends to zero. The mean failure rate, z, is the average value of the failure rate during an interval of time, i.e. the ratio of the integrated instantaneous failure rate during the interval to the length of the interval and is given in 1/h. In the US the unit FIT = 10~ 9 failures/hour is used. Failure that prevents operation A failure which means that the unit does not function. However, the failure does not necessarily prevent transmission. Function block A part of the equipment with a specific function, often in the form of a magazine or a shelf (e.g. 60-channel multiplexer, 30-channel PCM, 565 Mbit/s line terminal). Intermittent failure The function returns without any repair having been made. The failure can be recurrent. MTBF Mean time between failures, which is the mean value of the time intervals between failures. During the constant failure rate period MTBF = 1/z. MTTR Mean repair time. In practical applications it should be specified whether the Mean Time To Repair is meant, i.e. net down time (fault location, net repair and function test time), or Mean Time To Restore, i.e. total down time (fault location, administrative, waiting and repair time). The mean time observed by different administrations is usually given as a ratio of the total duration of the failures to the number of times failures occur during a given period. In these cases the MTTR is the mean time to restoration of function after a failure and can be used direct in the calculation of the mean availability. Observed failure rate A quantitative observation result, which during the constant failure rate period (with an approximately constant failure rate) is obtained as the ratio of the number of occurring relevant failures to the observation time. Observed MTBF A quantitative observation result, which during the constant failure rate period is obtained as the ratio of the observation time to the number of relevant failures occurred. Operating time The time during which a unit is in operation. Permanent failure A failure that remains until it has been repaired. Predicted failure rate or MTBF Calculated failure rate or MTBF based on Component failures can affect the function of a unit in different ways. Conestimated values from experiments or observation of other or similar units. Primary failure Failure that is not caused by the failure of another item. Probability of failure The probability that a failure will occur during a given interval of time undergiven operating and environmental conditions. Redundancy Redundancy means that a given function mode is maintained by more than one means (e.g. several parallel routes). Reliability The probability that a unit will work properly during a certain interval of time under given operating and environmental conditions. Repair time Time for fault location, fault correction and functional test. Shortage risk The probability that a spare unit is not available when needed. Sudden failure A failure that occurs so suddenly that it could not have been anticipated by prior examination. Telecommunication network All lines and equipment used to set up communication between a number of different places. A network contains nodes (exchanges) and links (transmission systems) between the nodes. Transmission equipment Equipment (hardware) in the form of different function blocks that form part of a system for the transmission of information. Transmission network A network of links between nodes, i.e. transmission systems between exchanges. Transmission system Various equipments or transmission media connected together in a configuration that makes it possible to transmit information (e.g. multiplexer, lineequipment, cable, line system). Wear-out failure period Late period with a noticeable increase in the failure rate. rates for all components in the unit are added. This means that from the point of view of reliability all components in the unit in question are considered as being series connected without any internal redundancy. Practical calculations are usually made for an operating temperature of +40° C (104° F) and standardized stress models. The operating temperature is considered to be the result of a room temperature of +25° C (77° F) with an addition of 15° C (27° F) caused by dissipated heat. However, both temperature and stress models can be varied within the limits of the operating range if necessary. sequently, different failure modes with different failure rates arise, which can be related to different functions in a complex unit. Furthermore, each unit can affect the function block in different ways depending on its own failure modes, the structure of the block, the position of the unit in the magazine and other units in the block. For practical reasons certain assumptions, conditions and limitations are introduced when making predictions, resulting in the following model: A failure is considered to be primary (not caused by another failure), total, sudden and preventing function but not necessarily preventing transmission. It is permanent, and manual action is required to restore function, it has occurred under
206 Relative occurrence for F Fig. 3 Histogram where the factor F = observed/ predicted failure rate for printed board assemblies • 52% of the printed board assemblies better than predicted (F1) i r 0,2 0,3 0,5 1 normal operating conditions and affects the transmission function in different ways. The failure concept thus excludes the human factor and any external influences, such as mechanical and electrical damage or intermittent interference. Such factors are almost completely dependent on user and applications and are therefore beyond the control of the manufacturer. After a few months of operation (burn-in period) the equipment is in its "constant failure rate period", and the failure rate is considered to be constant with time. All data refer to this period. Trustworthiness of prediction The trustworthiness of the prediction is mainly dependent on the trustworthiness of the failure rates of the components used. The component failure rates are stored in a data base which is available to the whole Ericsson Group. These failure rates are based on operational experience and have been obtained by means of long-term follow-up and analysis of repair activities. Experience with newer components is insufficient, however, and the amount of operational data is limited. In such cases the new component must be compared with a similar, older component. If there is no such component available for comparison, the manufacturer's data are studied, tested and verified, and laboratory tests are made. The use of advanced technology with integrated circuits in modern system designs increases the reliability of the system. The follow-up of the behaviour of the equipment in operation has shown that there is good agreement between the predicted and the observed reliability of equipment in construction practices M4 and M5. This conclusion is based on analysis of the predicted and the observed failure rates for 138 types of printed board assemblies from the 29 most common function blocks. Fig. 3 shows the result of a comparison of 79 different types of printed board assemblies with more than four relevant failures. It can be seen that 51 % of the observed failure rates lie between "2 times better" and "2 times worse" than the predicted value. The function blocks show even better correlation, fig. 4. As many as 62% of the analyzed function blocks have been better than predicted, 17% "considerably better", and no function block has proved to be "considerably worse" than predicted 2 . Table 1 shows a comparison between the predicted and observed reliability for some of the most frequent equipments. Presentation of reliability data Reliability parameters Reliability parameters describe quantitatively the reliability performance o the equipment. The parameters used a chosen with regard to the object ot
Page 1 and 2:
ERICSSON REVIEW 4 1985 Field Trial
Page 3 and 4: Trial of Optical Fibre Cable-TV Sys
Page 5 and 6: Head end (HC] 280 Mbit/s Farsta aut
Page 7 and 8: 158 Switching equipment Fig. 7 Futu
Page 9 and 10: 160 Fig. 10 The teleconference room
Page 11 and 12: GERHARD GOBL Public Telecommunicati
Page 13 and 14: Fig. 5, above left Two-channel A/D
Page 15 and 16: Digital input signals Digital outpu
Page 17 and 18: Power ZAV 280/4 has individual powe
Page 19 and 20: Wavelength Division Multiplexing fo
Page 21 and 22: 172 Subscriber side BP-890 Controll
Page 23 and 24: Fig. 8 Transmission characteristics
Page 25 and 26: Most computer cabinets are 19" wide
Page 27 and 28: 178 Fig. 5 Modems in Series 7 Modem
Page 29 and 30: Computer Aided Production of Plasti
Page 31 and 32: Fig. 4 Product geometry displayed o
Page 33 and 34: Rectifier for Mobile Telephone Syst
Page 35 and 36: I OR | OFU OS I CMUP HlsHi W±±±h
Page 37 and 38: 188 Fig. 6 The rectifier with the o
Page 39 and 40: Fig. 9 Rectifier BMJ A01 001 with t
Page 41 and 42: The Renewal of the London Undergrou
Page 43 and 44: 194 Rickmansworth, and the Loughton
Page 45 and 46: 196 Fig. 5 Optical fibre cable rout
Page 47 and 48: 198 In planning the optical fibre n
Page 49 and 50: Fig. 9 The digital telephone DIAVOX
Page 51 and 52: 202 Fig. 11 Transmission equipment
Page 53: 204 Fig. 2 Automatic testing of mul
Page 57 and 58: Fig. 5 Field testing of a 140 Mbit/
Page 59 and 60: 210 Fig. 7 The system down time as
Page 61 and 62: MTBF (years) Equipment 2-5 Transmul
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