How it works
In FDM, each channel occupies a unique frequency band allocated by assigning a different carrier frequency to each user. For N voice channels each of bandwidth B Hz, the total FDM bandwidth is N×B plus guard bands between channels. In telephone FDM systems, each 4 kHz voice channel is assigned 4 kHz bandwidth; 12 such channels form a Group (48 kHz), five Groups form a Supergroup (240 kHz), and so on up the FDM hierarchy. Guard bands prevent inter-channel interference but waste spectrum. TDM is more common in digital systems while FDM suits analog transmission media like coaxial cable and radio.
Key points to remember
Synchronous TDM assigns fixed time slots to each channel regardless of whether data is waiting — idle channels waste bandwidth. Statistical TDM (Asynchronous TDM) assigns slots only to active channels, improving efficiency at the cost of needing address headers. FDM advantage: all channels transmitted simultaneously, compatible with analog systems; FDM disadvantage: requires filters for each channel and is susceptible to crosstalk. OFDM (Orthogonal FDM) uses subcarriers spaced at 1/T apart (T = symbol period) to achieve spectral orthogonality, eliminating guard bands completely — used in LTE and Wi-Fi 802.11a/g/n at 20 MHz bandwidth with up to 64 subcarriers.
Exam tip
Every Anna University paper asks you to calculate the T1 bit rate from the frame structure, showing the 193-bit frame and 8000 frames/second calculation — never state 1.544 Mbps without deriving it, because the derivation carries most of the marks.