Gigabit Ethernet Notes

Notes from Greg Ferro on cable lengths for Gigabit Ethernet, based on research from Nortel Networks Accelar manuals.

1000BaseLX (Laser)

The worst case scenario is when launch power is -9.5 dBm and receiver sensitivity is -20 dBm, giving a worst case of 11.5 dB of headroom for guaranteed function. Launch power is affected by fibre connector quality and the laser installed. Receiver sensitivity is degraded by patch leads, fibre length, fusion splice or mechanical splice, and connector types.

Even on circuits where loss exceeds 11.5 dB, things may still work due to good launch characteristics (up to -3 dBm launch giving up to 6.5 dB extra headroom). The recommended length of 550m for 1000BaseLX on 62.5um multimode could be longer depending on end-to-end cabling performance.

1000BaseSX

For 1000BaseSX, the figures result in a less power budget, which is why shorter cable lengths come into play. In a given situation it may be possible to exceed the standard, but at risk of intermittent faults.

Environmental Considerations

The overall power budget can fluctuate with temperature and other environmental issues, which may cause intermittent errors. The risk of overrunning standard lengths is intermittent and indeterminate faults, often with no apparent cause. These are particularly difficult to troubleshoot because the link may work perfectly under normal conditions but fail under temperature extremes or increased load.