SIST EN 2591-100:2018
(Main)Aerospace series - Elements of electrical and optical connection - Test methods - Part 100: General
Aerospace series - Elements of electrical and optical connection - Test methods - Part 100: General
This European Standard specifies the general requirements for the methods of testing elements of
electrical, optical and data transmission system connections used in aerospace applications.
Luft- und Raumfahrt - Elektrische und optische Verbindungselemente - Prüfverfahren - Teil 100: Allgemeines
Série aérospatiale - Organes de connexion électrique et optique - Méthodes d'essais - Partie 100 : Généralités
Aeronavtika - Električni in optični spojni elementi - Preskusne metode - 100. del: Splošno
Ta evropski standard določa splošne zahteve za metode preskušanja elementov električnih in optičnih povezav ter povezav sistemov za prenos podatkov, ki se uporabljajo v aeronavtiki.
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Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.6SORãQRLuft- und Raumfahrt - Elektrische und optische Verbindungselemente - Prüfverfahren - Teil 100: AllgemeinesSérie aérospatiale - Organes de connexion électrique et optique - Méthodes d'essais - Partie 100 : GénéralitésAerospace series - Elements of electrical and optical connection - Test methods - Part 100: General49.060Aerospace electric equipment and systemsICS:Ta slovenski standard je istoveten z:EN 2591-100:2018SIST EN 2591-100:2018en,fr,de01-november-2018SIST EN 2591-100:2018SLOVENSKI
STANDARDSIST EN 2591-100:20061DGRPHãþD
SIST EN 2591-100:2018
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 2591-100
August
t r s z ICS
v {ä r x râ
v {ä r { r Supersedes EN
t w { sæ s r rã t r r wEnglish Version
Aerospace series æ Elements of electrical and optical connection æ Test methods æ Part
s r rã General Série aérospatiale æ Organes de connexion électrique et optique æ Méthodes d 5essais æ Partie
s r r ã Généralités Luftæ und Raumfahrt æ Elektrische und optische Verbindungselemente æ Prüfverfahren æ Teil
s r rã Allgemeines This European Standard was approved by CEN on
t v December
t r s yä
egulations which stipulate the conditions for giving this European Standard the status of a national standard without any alterationä Upætoædate lists and bibliographical references concerning such national standards may be obtained on application to the CENæCENELEC Management Centre or to any CEN memberä
translation under the responsibility of a CEN member into its own language and notified to the CENæCENELEC Management Centre has the same status as the official versionsä
CEN members are the national standards bodies of Austriaá Belgiumá Bulgariaá Croatiaá Cyprusá Czech Republicá Denmarká Estoniaá Finlandá Former Yugoslav Republic of Macedoniaá Franceá Germanyá Greeceá Hungaryá Icelandá Irelandá Italyá Latviaá Lithuaniaá Luxembourgá Maltaá Netherlandsá Norwayá Polandá Portugalá Romaniaá Serbiaá Slovakiaá Sloveniaá Spainá Swedená Switzerlandá Turkey and United Kingdomä
EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre:
Rue de la Science 23,
B-1040 Brussels
9
t r s z CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Membersä Refä Noä EN
t w { sæ s r rã t r s z ESIST EN 2591-100:2018
EN 2591-100:2018 (E) 2 Contents Page European foreword . 3 1 Scope . 4 2 Normative references . 4 3 Terms and definitions . 5 4 Standard test conditions . 8 5 Test main requirements . 9 5.1 Fibre end preparation . 9 5.2 Light Launch System (LLS) . 13 6 List of test methods . 20 7 Test report . 25
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 3 European foreword This document (EN 2591-100:2018) has been prepared by the Aerospace and Defence Industries Association of Europe - Standardization (ASD-STAN). After enquiries and votes carried out in accordance with the rules of this Association, this Standard has received the approval of the National Associations and the Official Services of the member countries of ASD, prior to its presentation to CEN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by February 2019, and conflicting national standards shall be withdrawn at the latest by February 2019. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 2591-100:2005. According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom. SIST EN 2591-100:2018
EN 2591-100:2018 (E) 4 1 Scope This European Standard specifies the general requirements for the methods of testing elements of electrical, optical and data transmission system connections used in aerospace applications. 2 Normative references The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
EN 2083, Aerospace series — Copper or copper alloy conductors for electrical cables — Product standard EN 2084, Aerospace series — Cables, electrical, general purpose, with conductors in copper or copper alloy — Technical specification
EN 2234, Aerospace series — Cable, electrical, fire resistant — Technical specification EN 2346, Aerospace series — Fire resistant electrical cables — Dimensions, conductor resistance and mass 1) EN 2591 (all parts), Aerospace series — Elements of electrical and optical connection — Test methods EN 3745-201, Aerospace series — Fibres and cables, optical, aircraft use — Test methods — Part 201: Visual examination EN 4641-100, Aerospace series — Cables, optical 125 µm diameter cladding — Part 100: Tight structure 62,5/125 µm core GI fibre 1,8 mm outside diameter — Product standard EN 60512-1, Connectors for electronic equipment — Tests and measurements — Part 1: General (IEC 60512-1:2001) EN 60793-1-43, Optical fibres — Part 1-43: Measurement methods and test procedures — Numerical aperture measurement EN 60793-1-45, Optical fibres — Part 1-45: Measurement methods and test procedures — Mode field diameter TR 4257, Aerospace series — Elements of electrical and optical connection — Relationship between the numbering systems for parts of EN 2591 2)
IEC 60050-581, International Electrotechnical Vocabulary — Part 581: Electromechanical components for electronic equipment 3)
1) Published as ASD-STAN Prestandard at the date of publication of this standard. (http://www.asd-stan.org/) 2) Published as ASD-STAN Technical Report at the date of publication of this standard. (http://www.asd-stan.org/) 3) Published by: IEC Commission Electrotechnique Internationale http://www.iec.ch/
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 5 IEC 61300-3-35, Fibre optic interconnecting devices and passive components — Basic test and measurement procedures — Part 3-35: Examinations and measurements — Visual inspection of fibre optic connectors and fibre-stub transceivers 3) 3 Terms and definitions For the purposes of this document, the terms and definitions given in IEC 60050-581 and EN 60512-1 and the following apply. ISO and IEC maintain terminological databases for use in standardization at the following addresses:
IEC Electropedia: available at http://www.electropedia.org/
ISO Online browsing platform: available at http://www.iso.org/obp 3.1 element of electrical or optical connection 4) component such as connector, module, etc., the purpose of which is to ensure the connection of circuits 3.2 flight cover (or protective cover) accessory designed to ensure, in flight, mechanical protection and sealing of front face of a non-coupled connector 3.3 connector with built-in protection of contacts connector with characteristics such that male or female contacts, mounted in a plug or receptacle, cannot come into contact with the front of the connector to which it is coupled (scoop-proof) and in which, in the event of accidental coupling of two parts or the connector equipped with male contacts, no electrical contact can take place 3.4 contact pressure point point at which a square ended gauge pin of the same basic diameter as the mating contact first engages the female contact spring member 3.5 initial measurement examination or measurement of characteristics carried out to determine the magnitude of the variations produced by the stress or stresses applied This examination or measurement is carried out at the end of pre-conditioning and under normal atmospheric conditions for measuring. 3.6 final measurement examination or measurement of characteristics carried out at the end of the recovery to assess the condition of the specimen after testing and to determine the magnitude of the variations in characteristics in relation to the values recorded at initial measuring
4) In test standards the term "element of connection" shall be used. SIST EN 2591-100:2018
EN 2591-100:2018 (E) 6 3.7 flammability a product is considered to be "non-flammable" when combustion due to exposure for a given duration to a standard external flame remains localized and stops spontaneously after withdrawal of the flame 3.8 fire resistance a product is considered to be "fire-resistant" when, subjected to a standard flame:
it retains its electrical role for six minutes;
the flame does not propagate to the other side of the support in the first twenty minutes. 3.9 values of alternating voltage and current unless otherwise indicated, alternating voltage and current are indicated in root mean square values 3.10 line data bus pair of twisted wires, shielded, having a specified impedance, a matched impedance at its two ends and used for data transport 3.11 branch line section of twisted wires, shielded, with a specified impedance, which connects equipment to a bus line 3.12 line coupler element of electrical or optical connection the purpose of which is to shunt the transmission signals from a bus line to equipment 3.13 line coupler, single coupler consisting of one line and one branch 3.14 line coupler, double coupler consisting of one line and two branches 3.15 in-line splice permanent element of electrical or optical connection for two-wire cables 3.16 line termination end line component the purpose of which is to match the bus line to its characteristic impedance 3.17 branch termination end branch termination the purpose of which is to eventually replace equipment SIST EN 2591-100:2018
EN 2591-100:2018 (E) 7 3.18 recovery treatment of a specimen, after conditioning, so that the properties of the specimen may be stabilized before measuring 3.19 optical fibre cable a certain number of optical fibres or bundles, coated separately and joined inside a common sheath 3.20 beam splitter device for dividing an optical beam into two separate beams 3.21 insertion loss (of an optical element) extra optical attenuation caused by the insertion of an extra optical element into an optical system 3.22 launch angle the launch angle is the angle between the wave propagation vector of the incoming light and the normal vector of an optic fibre end face 3.23 mode conditioner a device for adapting the light output from a source to produce a defined launch condition for testing an optical system 3.24 multimode fibre a multimode fibre is an optical fibre having a large core diameter dimension in relation to the wavelength of the light, and in which a large number of modes can propagate 3.25 optical port the port which radiates or accepts optical power at the interface 3.26 fibre optic branching device a device possessing three or more optical ports which shares optical power among its ports in a predetermined fashion 3.27 patchcord an assembly where the cable or fibre is terminated at each end with either a plug or receptacle connector 3.28 pigtail a pigtail is a short length of fibre between a component and a transmission fibre, often permanently secured to the component (LED, coupler, connection elements, .) 3.29 power meter a device for measuring the optical power in a fibre optic system. Power measurements are usually made in Watts or dBm. Relative power measurements are made in dB. SIST EN 2591-100:2018
EN 2591-100:2018 (E) 8 3.30 single mode fibre a single mode fibre is an optical fibre in which only one mode can propagate 3.31 passive coupler a passive coupler is a passive branching device in which power from one or more incoming optical ports is distributed to one or more outgoing optical ports 3.32 tee coupler an optical fibre tee coupler is a passive coupler or combiner with three optical ports 3.33 return loss light energy reflected back from discontinuities in a fibre optic link 3.34 Light Launch System LLS device designed to create defined and repeatable light coupling conditions in a test setup 3.35 Light Detection System LDS device designed to take repeatable measurements of light transmitted by a test setup 3.36 temporary joint non permanent optical fibre connecting devices for use on equipment 3.37 terminator a non-reflective termination of an optical fibre 3.38 test cord It is a terminated optical fibre cord used to connect the test equipment to the optical span, or to provide a suitable interfaces to the cabling under test. 3.39 For the following terms, see EN 3745-201 Optical fibre - Core - Cladding - Primary coating - Refractive index profile - Step index fibre - Graded index fibre - Quasi-step index fibre - Core diameter - Cladding diameter - Concentricity error core/cladding - Non circularity of core - Non circularity of cladding - Attenuation - Numerical aperture - Bandwidth 4 Standard test conditions 4.1 The test methods are written so that the test may be carried out either individually or included in a test sequence. When the test is carried out individually, the measurements are applicable so that the effect of the test on the performances of the specimen can be evaluated. This is why "if applicable" has been added to the titles "Initial measurements" and "Final measurements". SIST EN 2591-100:2018
EN 2591-100:2018 (E) 9 4.2 Unless otherwise indicated in the test method, technical specification or product standard, the test conditions shall be as follows: • temperature: (23 ± 5) °C; • atmospheric pressure: 86 kPa to 106 kPa (860 mbar to 1 060 mbar); • relative humidity: 45 % to 75 %. The temperature and humidity shall remain constant throughout a series of measurements. Unless otherwise indicated in the technical specification, the cables used for tests shall be in accordance with EN 2083 and EN 2084 or EN 2234 and EN 2346. 5 Test main requirements 5.1 Fibre end preparation 5.1.1 General The aim of this section is to give recommendations on the acceptable end condition of fibres, whether terminated or not. It is not intended to describe a precise method for fibre end preparation; instead it gives the information necessary to describe and quantify fibre end quality. This paragraph is applicable therefore to all tests which require the use of at least one optical interface of this type. It applies to all types of fibre, silica, plastic or a combination of these and other materials, generally up to a diameter of 125 µm. Comments are made for some other fibre sizes. 5.1.2 Parameters The quality of a fibre end can be described in terms of the end face profile and the surface condition. These terms are now described in more detail.
5.1.2.1 End face profile If terminated in a connector ferrule or contact, the fibre/ferrule end-face will be required to have a particular profile depending on the application. The most common connector profiles are listed below: The connector end face profile will determine the connector insertion loss and return loss (back reflection). Minimizing back reflection is of great importance in certain high-speed and analogue fibre optic links to prevent instability at the source. PC Polish — A Physical Contact (PC) polish results in a slightly curved connector surface, forcing the fibre ends of mating connector pairs into physical contact with each other. This produces much lower back reflection of − 30 dB to − 40 dB. The PC polish is the most prevalent connector end face in most applications. APC Polish — The Angled PC (APC) polish uses an 8 degree angle to the connector end face. Back reflections of < − 60 dB can routinely be accomplished with this type of polish.
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 10 For cases where the fibre is not terminated into a connector ferrule, it may be prepared as a nominally flat interface. This may require cleaving to produce an acceptable end although polishing to a flat profile is also possible (within a suitable bare fibre adapter). For cases where a ‘flat’ fibre end is to be produced the flatness of the end-face should be good. In quantitative terms this can be defined in terms of end face angle (see 5.1.2.2).
It is useful to describe the industry standard parameters for a PC (Physical Contact) profile. These are shown in Figure 1. This profile is characterised typically by three key parameters namely, (1) fibre/ferrule radius of curvature, (2) fibre height (undercut or protrusion with respect to the ferrule) and (3) polish offset (essentially the difference between the fibre centre and any higher polish apex of the polished end-face. If the system under test has a particular fibre interface profile then the light launch system should also have a similar profile.
a) Radius of curvature b) Fibre undercut (planar)
c) Fibre protrusion (planar) d) Polish offset Key 1 Fibre centre 2 Polish Apex Figure 1 — PC parameters for a fibre connection
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 11 5.1.2.2 Surface condition The face of the optical fibre shall be free from large scratches, defects and cracks. For cleaved fibres the face of the optical fibre shall not exhibit any nicking or lips and internal defects such as cracks.
For connectorised fibres quantitative descriptions of allowable defect and scratch sizes for different fibre types and sizes can be found in document IEC 61300-3-35. For the purposes of setting requirements on end face quality the end face of the fibre is divided into measurement regions. These are reproduced from IEC 61300-3-35 in Table 1. The standard goes on to specify allowable numbers and sizes of features in each region. Note in this standard scratches are defined as permanent linear features. Defects are all non-linear features detectable on the fibre. This includes particulates and other debris, pits, chips, edge chipping, etc. Table 1 — Measurement regions for single fibre connectors (reproduced from IEC 61300-3-35) Zone Diameter for single mode Diameter for multimode A: core 0 µm to 25 µm 0 µm to 65 µm B: cladding 25 µm to 120 µm 65 µm to 120 µm C: adhesive 120 µm to 130 µm 120 µm to 130 µm D: contact 130 µm to 250 µm 130 µm to 250 µm NOTE 1 All data above assumes a 125 µm cladding diameter. NOTE 2 Mutimode core zone diameter is set at 65 µm to accommodate all common core sizes in a practical manner. NOTE 3 A defect is defined as existing entirely within the inner-most zone which it touches.
Table 2 — Example of visual requirements (allowable surface features and sizes) for multimode single fibre connectors (reproduced from IEC 61300-3-35) Zone name Scratches Defects A: core No limit ≤ 3 µm 0 > 5 µm 4 ≤ 5 µm None > 5 µm B: cladding No limit ≤ 5 µm 0 > 5 µm No limit < 2 µm 5 from 2 µm to 5 µm None > 5 µm C: adhesive No limit No limit D: contact No limit None
·
s r µm NOTE 1 For scratches, the requirement refers to width. NOTE 2 No visible subsurface cracks are allowed in the core or cladding zones. NOTE 3 All loose particles should be removed. If defect(s) are non-removable, it should be within the criteria above to be acceptable for use. NOTE 4 There are no requirements for the area outside the contact zone since defects in this area have no influence on the performance. Cleaning loose debris beyond this region is recommended good practice. NOTE 5 The zone size for multimode fibres has been set at 65 µm to accommodate both 50 µm and 62,5 µm core size fibres. This is done to simplify the grading process. NOTE 6 Structural features that are part on the functional design of the optical fibre, such as microstructures, are not considered defects.
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 12 Table 3 — Example of visual requirements (allowable surface features and sizes) for low reflectance (return loss > 45 dB) single fibre connectors (reproduced from IEC 61300-3-35) Zone name Scratches Defects A: core None None B: cladding No limit ≤ 3 µm None > 3 µm No limit < 2 µm 5 from 2 µm to 5 µm None > 5 µm C: adhesive No limit No limit D: contact No limit None
·
s r µm NOTE 1 For scratches, the requirement refers to width. NOTE 2 No visible subsurface cracks are allowed in the core or cladding zones. NOTE 3 All loose particles should be removed. If defect(s) are non-removable, it should be within the criteria above to be acceptable for use. NOTE 4 There are no requirements for the area outside the contact zone since defects in this area have no influence on the performance. Cleaning loose debris beyond this region is recommended good practice. NOTE 5 Structural features that are part on the functional design of the optical fibre, such as microstructures, are not considered defects.
To provide clarity for automated systems, scratches are defined as being less than 4 µm wide, linear in nature, and with a length that is at least 30 times their width. Defects size is defined as the diameter of the smallest circle that can encompass the entire defect.
An example of allowable features for a multimode connector is shown in Table 2. As can be seen, no scratches or defects larger than 5 µm are allowed in the core region. In comparison for a low reflectance single-mode connector (see Table 3), no scratches are allowed in the much smaller single-mode core. The reader is referred to this standard for inspection criteria for other fibre types such as higher reflectance single-mode and also angle polished connectors. 5.1.3 Methods Any method of fibre end preparation is acceptable provided that the parameters defined in 5.1.2 are within the range specified for the fibre type and end profile required. For cleaved fibres (essentially flat), cleaving tools are widely available to produce this profile by putting the fibre under tension and then scribing the fibre so it breaks to produce a mirror-like finish.
Connectorised fibres are usually prepared by polishing the end face either by manual or mechanical means. 5.1.4 Specimen examination and acceptance Any method, optical or otherwise, which enables defects of the size quoted in 5.1.2 (or more generally in the cited standard reference IEC 61300-3-35) to be assessed, shall be acceptable. The methods may use systems which are as simple as a microscope with appropriate magnification for the measurement or as sophisticated as an interferometer system for three dimensional end face analysis. For any optical inspection system factors such as field of view, optical resolution and contrast will be important in being able to detect defects.
SIST EN 2591-100:2018
EN 2591-100:2018 (E) 13 For optical inspection (e.g. of surface condition) the simplest techniques may involve direct-view microscopy with human measurement of observed features. Video microscopy may be used where the image is displayed on a monitor (after being detected on a sensor or camera). Again measurement may be made on that image with a calibrated measuring system. Automated microscopy systems are also available that can reduce human subjectivity by performing digital image capture and software analysis (with appropriate algorithms). Pass/Fail acceptance can be performed by the software. For fibre profile measurement, interferometer (non-contact) systems are now commonly available to routinely measure the end face profile of fibre optic connectors. These can measure and analyse connectors quickly and display values for the key PC parameters. Again Pass/Fail acceptance can be performed by the software.
For cleaved fibres it is noted that simpler interferometer equipment is also available that may give simple qualitative information about the end face (e.g. a 2D optical image with fringe visibility). For a flat fibre end, the operator will in that case be looking for essentially parallel interference fringes on the fibre face to confirm a flat fibre face perpendicular to the long fibre axis.
5.1.5 Termination cleaning 5.1.5.1 General To ensure satisfactory optical coupling performance between any two elements in a link, it is essential that both interfaces are clean.
5.1.5.2 Required information The information required for cleaning the end of an optical fibre, whether terminated or not, essentially involves recommending the use of components and/or tools. A list shall therefore give the recommended components, products and/or tools, those not on the list not being allowed. These recommendations shall be specific to each fibre/cable/connector manufacturer and shall appear in the product standard. One requirement is however that where solvents are necessary, they shall not harm the ozone layer, and shall require minimum protection for the operator. It is noted that ‘traditional’ solvents for cleaning optical fibres such as IPA can give good results but may attract water (hygroscopic) and create static charge build up on the connector or fibre (that can in turn attract dust to the surface). More suitable fibre cleaning solvents are now available that can clean most contaminants. These are usually used with lint free wipes or dedicated fibre cleaning ‘sticks’ (disposable items) often in a wet-then-dry process. Cartridge cleaners with a cleaning yarn that lifts and removes dirt from the fibre surface (indexing to reveal a fresh surface for each clean) are also popular.
5.2 Light Launch System (LLS) 5.2.1 General The Light Launch System will be used to inject light into an optical system or component to measure e.g. a value for the insertion loss. Importantly however, the light distribution launched into the fibre system (often termed the launch condition) will affect the value of the measurement made. Using uncontrolled sources from different manufacturers can give different values of the loss. Clearly this is undesirable. SIST EN 2591-100:2018
EN 2591-100:2018 (E) 14 The effect of launch conditions on the loss of fibre links is especially dramatic in short haul multi-mode avionic systems that may feature short lengths of fibre as well as many connector breaks and possibly other optical components. The light in such systems does not settle to an equilibrium modal distribution as might happen in say a longer haul link. It is important therefore that launch conditions are clearly defined in order to provide repeatable and reproducible measurements on a component or system with a low spread of values.
Generally a source will have two key distributions. Firstly, the variation of power across the aperture of the source (the near field distribution). Secondly, the variation in power with angle or the far field power distribution. Optical fibres have key parameters of core size (diameter = 2a) and also numerical aperture (NA). The launch condition is effectively a comparison of the source power distribution to the parameters of the fibre system under test. For example, a launch condition of 85:85 would describe a source with a near field profile that fills 85 % of the core and has a far field angular distribution that fills 85 % of the fibre NA. The ideal launch condition distribution would lie somewhere between an overfilled launch condition (that would give an unduly pessimistic value for the insertion loss) and a grossly underfilled launch condition (that would give an optimistic value). In general, the optimum launch condition slightly underfills both the core of the fibre and the numerical aperture. This removes that part of the source’s light distribution that is most likely to be attenuated by the harness components due to manufacturing imperfections. Studies have shown that the correct launch condition lies somewhere between an 80:80 launch and 90:90 launch. Another consequence of using an ideal launch condition to make insertion loss measurements is that the difference between the loss of a complete harness and the sum of the losses of the individual harness components is minimised. It should be mentioned that the launch distribution of a source used to transmit data down the fibre in a real system will also have a given power distribution. This may well be different to the source used to characterise the fibre system although if the two are similar, then the performance of the real system link can be anticipated from the loss d
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