Describes a model developed as a tool to assist the physical layer committee of the gigabit ethernet (IEEE 802.3z)standard to understand potential trade-offs between various link penalties associated with laser-based backbone links. This technical report documents the model as used by gigabit ethernet and identifies how it was used to develop the standard specifications. This technical report presents the theoretical model. Experimental verification is also presented for links operating at valid gigabit ethernet wavelengths of 780 nm, 850 nm and 1 300 nm.