An optical add/drop multiplexing unit (16, 116) includes a demultiplexer
(53, 153) which optically isolates component signals of an optical input
signal from an optical input terminal (22, 122). A plurality of switching
units (61, 161) can optically couple either of two outputs to either of
two inputs. One input of each switching unit receives a respective component signal
from the demultiplexer, and the other is coupled to a respective protection input
terminal (42, 142) of the multiplexing unit. One of the output terminals
of each switching unit is coupled to a respective protection output terminal (43,
143), and the other is coupled to a respective input of a multiplexer (82,
182). The multiplexer has an output coupled to an optical output terminal (23,
123) of the multiplexing unit, and optically combines the signals at its inputs
into an optical signal for its output. The multiplexing unit is operable to cause
one of the component signals from the input signal to be routed to the further
terminal, or to cause a component signal present at the further terminal to be
included in the optical signal at the optical output terminal.
