.c. Outer to screen shall Attenuation withstand 2000V d.c. for 1 minute. Nominal attenuation at 1MHz is 5.3dB/km +0.2dB/km. Attenuation above 2MHz is 0.07 + 5.15Vf + 0.005f. The lay up factor causes the attenuation of multipair cables to be about 0.5% greater per sheath km. 15.16 Appendix 15.4 Construction Inner conductor is 1.2mm diameter copper wire. Submarine optical fibre characteristics Outer conductor is 0.15mm or 0.18 copper tape. Internal diameter is 4.4mm over insulation. Core diameter = 8.4um (germanium doped). Refractive index difference = 0.004 Impedance Regularity Fibre diameter = 125um The most demanding requirement is for 140Mbit/s digital oper- Cut-off wavelength = 1.15 - 1.28pm ation. Reflections from a 10ns pulse shall be at least 49dB down. Wavelength of zero dispersion = 1.315um Attenuation at 1.310um = 0.34dB/km (b) Installed Cable Attenuation at 1.55pm = 0.19dB/km Dispersion at 1.55pm = 17ps(km.nm) The far-end crosstalk ratio between any two coaxial pairs shall not Mode field diameter = 9.8um be less than that given in Table A15.2. Alternative Dispersion Shifted Fibre Dielectric Strength The pair shall measure not less than 5000Mohm-km at 500V d.c. Core diameter = 6.9um (at base of triangular profile) after electrification for one minute at 1000V d.c. If in normal use Refractive index difference = 0.011 the outers are not earthed, outers to sheath shall withstand 2000V Cut-off wavelength = 0.84um d.c. (Isolated outers are one technique for reducing induced currents Wavelength of zero dispersion = 1.53um from railway traction and lightning strokes). Attenuation at 1.55pm slightly greater than 0.19dB/km 15.15 Appendix 15.3 15.17 Appendix 15.5 0.7/2.9mm coaxial pair (G.621) characteristics Multimode graded index optical fibres (CCITT G.651) key parameters (a) Factory Length Core diameter = 50um, deviation less than +6% Impedance The characteristics impedance at 1MHz is 75 +2.5 ohm. Cladding surface diameter = 125um, deviation less than 2.4% Typically, then Z at 0.2MHz is 77.7ohm; Z at 5MHz it i