# How does FDM and TDM work in relation to Circuit Switching?

I am currently trying to understand what exactly is FDM and TDM. I understand the definitions, but I am having trouble visualizing it. First, in Circuit switching a connection between host A and host B must be made where we have created a path for the connection to travel.

Now, if we are talking about FDM, and we have 4 users is that saying that the 4 users are going to use that very same path created but at different frequencies?

Also, if we are talking about TDM, and we have 4 users is that also saying that for that same path users will get to use it at different times?

• You got it right. What is the question? – Everton Feb 20 '16 at 21:42
• Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 7 '17 at 19:11

In circuit switching, resources remain allocated during the full length of a communication, after a circuit is established and until the circuit is terminated and the allocated resources are freed. Resources remain allocated even if no data is flowing on a circuit, hereby wasting link capacity when a circuit does not carry as much traffic as the allocation permits. This is a major issue since frequencies (in FDM) or time slots (in TDM) are available in finite quantity on each link, and establishing a circuit consumes one of these frequencies or slots on each link of the circuit.

You are essentially correct. Think of TDM as a conveyor belt with alternating colors of buckets... blue, red, green, yellow... blue, red, etc. Each bucket might have a corresponding input queue which will collect outgoing data until the next bucket for that color arrives. Naturally, this is only effective if the individual buckets are each a fraction of the overall capacity of the transmission media.

Originally circuit switching worked with physical circuits. When a connection was established a complete physical circuit would be tied up by each "hop" of the connection.

However this is an inefficient way of running a network. An analog phone call needs about 4kHz of bandwidth. An uncompressed digital phone call needs about 64kbps of data rate. That isn't very much. We can easilly build circuits with a far greater capacity than that and a small number of high capacity circuits are cheaper than a larger number of low capacity circuits.

Therefore there is a desire to split each physical circuit into multiple "virtual circuitS". For this we can use frequency division multiplexing (FDM) or time division multiplexing (TDM). Generally frequency division multipexing is associated with analog systems while time division multiplexing is associated with digital systems. The circuit switching can then work with virtual circuits instead of physical circuits.

With frequency division multiplexing the signal for each virtual circuit is modulated onto a carrier (in the same way that radio systems modulate signals onto carriers). The modulated signals are then combined and sent down the circuit. At the far end the signals are seperated using frequency filters and then demodulated (again much like a radio system).

With time division multiplexing the data for each virtual circuit is sent down the physical circuit at a different time. The physical circuit will switch rapidly between virtual circuits to avoid causing undue latency. There will be headers of some sort to identify which data belongs to which virtual circuit. The virtual circuits may be either be fixed data rate with garuanteed delivery (e.g. a phone call) or variable data rate with the possibility of dropping data (e.g. a "broadband" internet connection).