Abstract: In an optical switching network, a master optical space switch is interconnected with remote slave equipment for switching block-multiplexed messages among the remote equipment. The master optical space switch responds to a frequency reference clock and steps through a sequence of states, or configurations, for establishing various routes among the remote equipment for appropriately routing the messages to correct destinations. The frequency reference clock also is distributed to the remote equipment. During one of the states per frame, each remote slave equipment transmits a distinct signature signal. All of the slave equipments are connected through the master optical space switch to return to themselves their own signals transmitted during that time slot. Each remote slave equipment compares the returned signal with the originally transmitted signature signal to determine how much the round trip delay has varied from a nominal value.