Lets start with a PSTN call

The PSTN or the public switched telephone network (PSTN) is the network of the world's circuit-switched telephone networks. You can think of it as a network where each Telephone switch in the network is connected to another switch, either directly or in an indirect way (through other switches). Also the switches are intelligent enough to route your call to a destination switch near to your friend’s or relative’s house, be it a national or an international call. Each of these switches uses the telephone number that you dial, to find the switch where your friend belongs to.

Now when you want to talk to some one over a telephone, you will first dial his/her telephone number. Then the other end’s phone started ringing and if somebody lifts the phone, you will get an answer. Do you know what happened during that tiny gap between the “end of dialing” and “Ringing”?  This is the duration where the switches are trying to establish a physical connection between you and the person you dialed through the PSTN network. This physical connection is used to convey your voice data (speech) to the other person and vice versa. Now what does this physical connection looks like? Is it the physical wires that we are talking about? You might have seen only one or two optical cables or electrical cables between the telephone switch boxes, but still it supports thousands of users from your locality. How is this possible? There comes the technology called TDM (Time division multiplexing).

Multiplexing is the method of combining many smaller units of signals carried via separate channels on to a bigger and single channel. Or in other words multiplexing helps to carry multiple users information simultaneously. The unit can be a time slice or a frequency. When it is frequency, that method is called FDM (Frequency division multiplexing) and if it is time, then it is called TDM. Consider your link has a carrying capacity (bandwidth) of 2Mbps or 2 Mega bits per second (i.e. it can carry 2 Megabits every 1 sec), and you want to carry “n” users data simultaneously through this link How do you achieve this? The TDM multiplexer will do the job for you. Each user will be allocated 1/n sec duration of the bandwidth out of this 2Mbps. All users will send there data to the Multiplexer in a parallel fashion. The multiplexer will buffer this information and will arrange them in a serial manner to make the 2Mb of information and sent these data chunks every second. And at the other end a de-multiplexer will separate the information and forward it to multiple destinations. Now you can see, how a single high capacity link can me equipped to carry multiple users data simultaneously.

Same thing is happening in a switch (Digital switch). Your telephones will be wired separately to a switch. The switch will do the multiplexing for you and send your information, as if separate cables are laid for you between the switches.