A first optical data signal is transmitted on a first data carrier from a first network element. First service information is transmitted from a first service by means of a first optical service signal on a first service carrier. A second optical data signal is transmitted on a second data carrier from a second network element, and second service information is transmitted by a second optical service signal on a second service carrier. An offset between a frequency of the first data carrier and a frequency of the first service carrier is substantially equal to an offset between a frequency of the second data carrier and a frequency of the second service carrier.