STANAG 3910 High Speed Data Transmission Under STANAG 3838 or Fibre Optic Equivalent Control is a protocol defined in a NATO Standardization Agreement for the transfer of data, principally intended for use in avionic systems. STANAG 3910 allows a 1 Mb/s STANAG 3838 / MIL-STD-1553B / MoD Def Stan 00-18 Pt 2 (3838/1553B) data bus to be augmented with a 20 Mb/s high-speed (HS) bus, which is referred to in the standard as the HS channel: the 3838/1553B bus in an implementation of STANAG 3910 is then referred to as the low-speed (LS) channel. Either or both channels may be multiply redundant, and may use either electrical or optical media. Where the channels use redundant media, these are individually referred to as buses by the standard.
History The original STANAG 3910, i.e. the NATO standard, reached, at least, draft version 1.8, before work on it was abandoned in the early 1990s in favour of its publication through non-military standardization organizations: the foreword to Rev. 1.7 of the STANAG from March 1990 stated "The main body of this document is identical to the proposed Rev 1.7 of prEN 3910". Following this, several provisional, green-paper versions, prEN 3910 P1 & P2, were produced by working-group C2-GT9 of the Association Europeene des Constructeurs de Materiel Aerospatial (AECMA) (now ASD-STAN), before its development also ceased in 1996-7 (following the withdrawal of the French delegation, who held the chair of AECMA C2-GT9 at the time). As a result, the standard remains (as of Aug. 2013) in green paper form: the latest draft version is prEN3910-001 Issue P1, the front sheet of which states, 'This "Aerospace Series" Prestandard has been drawn up under the responsibility of AECMA (The European Association of Aerospace Industries). It is published on green paper for the needs of AECMA-Members.' However, despite this disclaimer, the document is offered for sale by ASD-STAN, currently (August 2013) at €382.64.
Utilisation The incomplete nature of the standardization process (as of Aug. 2013) has not prevented at least two versions of STANAG 3910 being implemented: one for the Eurofighter Typhoon and one for the Dassault Rafale. The Eurofighter version, known as EFABus, is standardized by an internal Eurofighter document (SP-J-402-E-1039). The standardization documentation for the Dassault version is unknown. The EFABus version of STANAG 3910 is known to use an electrical low speed (3838/1553B) control channel and a fibre optic HS channel. The version specified for the Dassault Rafale uses electrical media for both channels. There are a number of manufacturers of avionic equipment that supply both flight and ground (e.g. test) equipment to this protocol standard.
Media The (draft) standard contains annexes, known as slash-sheets, that specify a number of different media types for the high-speed and low-speed channels, implementations identifying a specific slash-sheet with the relevant specifications.
Optical Versions of STANAG 3910 using optical media for the HS channel component require an additional passive component, in the form of an optical star coupler either reflective or transmissive, to interconnect the remote terminals. This limits the number of remote terminals that may be connected to the HS media, through the effect of the optical star on the optical power (determined by the number of "ways" of the star). Therefore, it may not be possible for all the (up to) 31 RTs (and 1 BC) that may be connected to the LS channel to have HS channel connections. The optical media types include 200 and 100 μm diameter core (280, 240, or 140 μm cladding) Step-index profile (depressed cladding) optical fibre. These are much larger-core fibres than are commonly used in short-haul commercial applications, which are more normally 50/125 or 62.5/125 μm. This is, in part at least, to reduce the problems associated with contamination of the optical connectors – a given size of particle between the end faces of the fibre in a connector or misalignment of such a connector has significantly less effect on the larger fibre – which is seen as a significant issue in avionic applications, especially where contaminating environments, high vibration, and wide temperature ranges can apply. The major difference between the transmissive and reflective star coupled fibre networks is that two fibres are needed with the transmissive star coupler to connect a line-replaceable item (LRI), but with the reflective star, and a "Y" coupler internal to the LRI, only a single fibre is required: a "Y" coupler is a three-port optical device that connects the simplex transmitter and simplex receiver to a single fibre that carries the optical signals transmitted and received by the LRI in opposite directions (half duplex). However, while the use of the reflective star reduces the cabling in the aircraft, and thus weight, the excess losses involved in the use of the "Y" couplers and reflective star coupler makes meeting the power budget requirements, given a transmitter power and receiver sensitivity, more difficult. Whilst it is explicitly stated that the LS buses may be a fibre optic equivalent to STANAG 3838, e.g. MIL-STD-1773, there are no known implementations of this approach.
Electrical Versions using an electrical HS channel require an additional active component, in the form of a "central repeater", with multi-tap collector and distributor lines (which use directional couplers to connect to the LRIs) and a buffer memory, to allow for small differences in data rates. The standard and the electrical media slash sheet it contains specify a 100-ohm characteristic impedance cable for both collector and distributor lines. A maximum cable length is not given for either, and neither are limits on the numbers of directional couplers and thus RTs. However, the losses in the directional couplers, etc., especially for the RT furthest from the central repeater, and the limitations on dynamic range between the furthest (and most attenuated) and nearest (and least attenuated) RT, will limit the number of RTs operating to the standard that may be connected to the HS media.
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