Tuesday, December 09, 2008

Optical cabling - advice to customers

I'm not blogging much at the moment - lot on. However - this is what we're advising customers who don't use us fot their fibres;

  • Grade of cable - All current models of film and video SANs make use of multimode connection. OM1 cable is still the preferred grade (62.5 micron VCSEL-optimised glass in accordance with ISO-11801) and since current configurations are 4 gigabit (moving to 8 gigabit) more attention needs to paid to circuit loss than 1 gigabit (the standard when OM1 was introduced). OM2 and OM3 cable is still unsuitable because of the 2.5dBs of loss when going between dissimilar core sizes (62.5 vs 50 microns). This is a function of current host-bus adaptors rather than the response of the cable.
  • Bandwidth - Whereas 1 gigabit traffic will tolerate up to 8dBs of loss we are now dealing with SANs that demand at least two octaves more bandwidth and so best practise says that we now expect no more than 3dBs of loss on a SAN circuit.
  • Style of cable - Although tight-buffered cable is easy to install it is never optimal for long runs. For interconnection between equipment within a cabinet it is appropriate and between cabinets if run in protection – Copex etc. For inter-area runs a loose-tube cable is the best solution as it is an order of magnitude more robust and although has an slightly larger install-time cost has a much lower TCO.
  • Connectors - All contemporary host-bus adaptors and fibre-switches terminate runs in the LC connector. If existing cables are terminated in legacy SC or ST connectors they should either be re-terminated or re-run as adaptors introduce signal loss. SC or ST patch panels are fine so long as run-out cables are SC-LC (to equipment) as appropriate.
  • Testing – We will ascertain if circuits are suitable for proposed SAN deployment by illuminating them with a calibrated laser tester (850nM wavelength, -19dB(m) signal) and measuring circuit loss – these results will be provided to the customer.

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