1. Introduction

A subsea supply contract quotes one capacity figure per fiber pair, and that figure is never the Shannon capacity of the fiber pair. On the reference system worked through this article, a 4.5 THz C-band passband delivering a generalized signal-to-noise ratio (GSNR) of 11.0 dB at the receive terminal, the linear Shannon bound evaluates to 33.88 Tb/s (theoretical limit, computed from the stated GSNR). The figure a supplier commits to on the same fiber pair is 20.5 Tb/s (computed from the four-term accounting derived in Section 6). The 13.4 Tb/s between those two numbers is 39.5% of the bound, and it is not measurement error, vendor conservatism, or a rounding convention. It is four named engineering allowances, each with a physical mechanism behind it, each owned by a different party to the contract, and each recoverable or forfeitable by specific design decisions.

Those four allowances are the subject of this article. The gap-to-Shannon allowance, conventionally about 2 dB on a modern coherent transceiver, covers the shortfall between an ideal capacity-achieving code operating on a Gaussian input distribution and a deployed forward error correction (FEC) engine operating on a probabilistically shaped quadrature amplitude modulation (QAM) constellation. Spectrum occupancy, conventionally about 0.95, covers the fraction of the amplifier passband that carries symbols rather than guard band, dead band, or monitoring channel. System margin, conventionally 2 dB on a transoceanic system, covers ageing of the pump lasers, splice loss added by cable repairs over a 25-year design life, and the spectral spread between the average and worst channel. Terminal noise, expressed as a modem signal-to-noise ratio SNR_m near 20 dB and an impairment signal-to-noise ratio SNR_i near 24 dB, covers everything the transceiver adds that the wet plant did not.

The reason these four terms need naming rather than lumping is that they do not act on the same axis of the capacity formula. Two of them divide the signal-to-noise ratio inside the logarithm, one multiplies the bandwidth outside it, and one enters by reciprocal addition before either. A single lumped penalty in decibels reproduces the right answer only at one operating point, and a subsea system moves across operating points every time the design changes fiber pair count, repeater output power, or span length. The linear Shannon bound for a fiber channel sets the ceiling; the accounting below sets the contract.

The commercial pressure on this arithmetic has increased sharply. Open cable procurement separates the wet plant from the terminal equipment, so the supplier who builds the repeaters is not the supplier who supplies the modems, and the capacity number has to be predicted from a wet plant metric before the modems exist. Open cable architectures formalise that split through GSNR, which describes the cable alone. Spectrum sharing agreements go further and sell a slice of the passband rather than a set of wavelengths, with the allocation frequently expressed as a percentage of the Shannon capacity of the slice. When a fraction of a bound becomes the unit of trade, the terms that convert the bound into deliverable throughput stop being an internal design detail and become a contract term.

Spatial division multiplexing (SDM) has raised the stakes again. Once cable capacity became limited by the electrical power that can be fed from shore rather than by the capacity of any single fiber, the design question changed from maximizing capacity per fiber to maximizing capacity per volt. Power-constrained subsea transmission pushes systems toward more fiber pairs at lower GSNR, which moves the operating point down the logarithm to where each decibel buys more capacity per decibel spent. A 2 dB allowance that looked like a modest deduction on a 20 dB GSNR turnkey system is a large deduction on an 8 dB GSNR SDM design. The accounting has to travel with the design, and the only way it travels correctly is if each term keeps its own identity.

This article defines each of the four terms from first principles, states the axis it acts on, builds the complete capacity expression, works one reference fiber pair from 33.88 Tb/s to 20.5 Tb/s with every intermediate value shown, and then extends the same accounting to three line designs at 12, 20 and 32 fiber pairs under a fixed power feed budget. It covers C-band repeatered transoceanic systems using single-mode fiber and coherent terminal equipment. It does not cover unrepeatered systems, multiband transmission beyond C+L, or the nonlinear modelling that produces GSNR in the first place, all of which are treated as inputs here rather than derived.