Simplex vs Duplex Fiber

Simplex and duplex describe how many fiber strands a link uses. A simplex link uses one strand. A duplex link uses two strands, one for each direction. That is the whole distinction, but it has practical consequences for how much fiber a site needs, what a link costs, and which modules can be used at each end.

Most ordinary optical links are duplex. A standard transceiver has a transmitter and a receiver, and a duplex link gives each its own strand: one strand carries traffic in one direction, the other carries traffic back. The connector for a duplex link is therefore a pair, and the patch cords and panels are built around pairs. When a site plans fiber counts, it plans in pairs for duplex links, and a 24-fiber cable serves twelve duplex links. That pairing is so standard that it is easy to forget it is a choice rather than a law of nature.

A simplex link uses one strand for both directions, and it does so by using different wavelengths in each direction. A bidirectional module, often called a BIDI module, transmits at one wavelength and receives at another, and the module at the other end does the reverse. The two wavelengths share the single strand without interfering, because they are far enough apart in the spectrum. The result is that a simplex link needs half the fiber of a duplex link, which matters where fiber strands are scarce, where a single-strand plant is already installed, or where leasing fiber is charged per strand.

The catch with simplex links is matching. Both ends must be a matched pair: one end transmits at the wavelength the other receives, and vice versa. Two identical bidirectional modules will not talk to each other, because each would transmit at the wavelength the other also transmits at. A buyer ordering bidirectional modules must therefore order them as a pair, stating which end is which, or order two complementary variants. Stating both ends during the inquiry is the way to avoid a pair that cannot link.

Which should a site choose? If fiber is plentiful and the plant is already built in pairs, duplex is simpler, because standard modules are available in every speed and reach, and both ends are identical. If fiber is scarce, if the plant is single-strand, or if the link crosses a boundary where strands are charged, simplex saves fiber and can save money. The decision is rarely about speed or distance, which are the same for both; it is about the plant and the budget. The series pages on this site list bidirectional variants where the catalogue carries them, and a buyer who needs a bidirectional option should state it by name during the inquiry.

Before ordering, confirm quantity, price and delivery time during the inquiry. Certificates are available upon request.