If my understanding is correct, when we are multiplexing, the stations are not supposed to be aware of the multiplexing.

For example, if we have computers 1, 2, 3, & 4 talking to computers a, b, c, & d, respectively, through a time-multiplexed channel, each pair must be communicating with each other as if they are directly connected. The stations must be oblivious that they are connected to a mux/demux. And if I want to add another pair, it will be just plug-and-play, that is, I will just plug the first one on any vacant port on the mux and plug the second one on the corresponding port on the demux, at anytime.

But from Principles of Electronic Communication Systems by Louis Frenzel, on the section about time-division multiplexing, it says:

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Now that's confusing. If each input will transmit on its time slot and then "halt", then the stations must be aware of the multiplexing. They must have some way to know when is their time to transmit. And if you will add a pair of stations, they cannot communicate immediately as if you have connected them directly because they must first inform other stations about their presence and ask for a time slot.

Isn't it more like a channel access protocol than multiplexing? Or my understanding is incorrect at the first place?

Follow-up questions:

1) If the highlighted statement is true, how do they know when to transmit?

2) If stations only take turns in transmitting, why is the output rate the sum of the input rates, like in T-carrier system? Shouldn't the output rate be just the rate of the highest input rate since they are just taking turns?


1 Answer 1


Time-division multiplexing is a protocol. Needs some hardware to process the packets received by the intermediate hardware. Also, the source and the final receiver and all other systems in the chain must adhere to the protocol.

Space division multiplexing is facilitated by the hardware and doesn't demand the receiver/source to be 100% adhere to the protocol. Whereas for time-division multiplexing, all systems in a network must be capable enough to adhere to the protocol. CDMA is the best example where the phone must be capable to adhere to the protocol.

  • \$\begingroup\$ I have worked with TDM systems where the transmit sources were not time slot aware and transmitting continuously (telephone system in the USA multiplexing 56 T1 lines to dual DS-3). Certainly each T1 has a specific protocol but the multiplexer took care of any time shifting. \$\endgroup\$ Commented Mar 21, 2020 at 12:47
  • \$\begingroup\$ I think it is misleading to say that TDM is a protocol. That's a very general statement, and time-division multiplexing can be used without packets or receivers. I would say that TDM, when used to communicate between systems, requires a protocol to control the access to the channel. \$\endgroup\$ Commented Mar 21, 2020 at 13:39

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