A digital voice channel operates at 64 kilobits per second. Most fiber-optic communication systems are capable of transmitting at a rate of more than 10 Gbps. 40 Gbps and 100 Gbps are very common now with researches are heading to practically implement 400 and 800 Gbps. At least a few of the pessimists are engaged in spreading rumors on inability of optical fibers and capacity crunch. To utilize the optical transmission system capacity fully, we need to transmit many channels simultaneously through multiplexing.
Multiplexing can be accomplished through time-division multiplexing (TDM) or frequency-division multiplexing (FDM). In Time Division Multiplexing, bits associated with different channels are interleaved in the time domain to form a composite bit stream.
In Frequency Division Multiplexing, the channels are spaced apart in the frequency domain. Each channel is carried by its own carrier wave. The carrier frequencies are spaced more than the channel bandwidth so that the channel spectra do not overlap.