Out-of-band data communication of diagnostic or other data is performed
using transceivers in a data or communication network. A light beam or
other carrier is modulated with high-speed data and out-of-band data to
create a double modulated data signal. A physical layer signal is created
that includes modulations of the double modulated signal. The physical
layer signal is transmitted onto a physical link. The diagnostic or other
data can be transmitted in the out-of-band signal without substantially
reducing or otherwise interfering with the transmission rate of the
high-speed data.