Definition: TDM

(1) (Target Disk Mode) A method for transferring files between two Macs. See Target Disk Mode.

(2) (Target Display Mode) An iMac feature that enables the screen to function as an external monitor for another computer. See Target Display Mode.

(3) (Time Division Multiplexing) A technology that transmits multiple signals simultaneously over a single transmission path. Each lower-speed signal is time sliced into one high-speed transmission. In the simplest example, three incoming 1,000 bps signals (A, B and C) can be interleaved into one outgoing 3,000 bps signal as ABCABCABCABC. The receiving end divides the single stream back into its original signals.

TDM enabled the telephone companies to migrate from analog to digital on all their long distance trunks, and later to the local loops. TDM is widely used to combine multiple 64 Kbps streams into the 1.544 Mbps capacity of a T1 channel. For example, a channel bank converts 24 analog voice conversations into digital and then multiplexes them via TDM onto the T1. Contrast with FDM. See TDMA, circuit switching, channel bank and DS.

From FDM to TDM to Packet Switching

TDM has replaced FDM (frequency division multiplexing) for telco backbone networks, but packet switching is expected to become the norm for voice and data in the 21st century.