Let’s say you send an email from your PC to a friend in another city. Your email leaves your house and joins up with other messages being transmitted in your neighborhood. The messages from your neighborhood feed into a larger transmission line and join together with other messages from your city. Eventually, your email gets dropped off at the correct destination in the correct city.

How do all of these messages get joined together and transmitted without getting mixed up? It’s done using a technique called multiplexing. Several different types of multiplexing are employed in telecommunications applications. Let's cover the basics of how multiplexing works and the different types of multiplexing that are used.

## Multiplexing Basics

Multiplexing basically involves taking multiple signals and combining them into one signal for transmission over a single medium, such as a telephone line. The input signals can be either analog or digital. The purpose of multiplexing is to enable signals to be transmitted more efficiently over a given communication channel, thereby decreasing transmission costs.

A device called a multiplexer (often shortened to "mux") combines the input signals into one signal. When the multiplexed signal needs to be separated into its component signals (for example, when your email is to be delivered to its destination), a device called a demultiplexer (or "demux") is used.

Multiplexing was originally developed in the 1800s for telegraphy. Today, multiplexing is widely used in many telecommunications applications, including telephony, Internet communications, digital broadcasting and wireless telephony.

## Time Division Multiplexing

In time division multiplexing (TDM), each input signal (or data stream) is assigned a fixed-length time slot on a communication channel. Each sender transmits a block of data during its assigned time slot.

For example, let’s say that input streams from three sending devices are being multiplexed into one signal for transmission over a single physical channel. Device 1 transmits a block of data during time slot 1, device 2 transmits a block of data during time slot 2, and device 3 transmits a block of data during time slot 3. After device 3 transmits, the cycle begins again with each device transmitting in turn in its assigned time slot.

A drawback to standard TDM is that each sending device has a reserved time slot in each cycle, regardless of whether it is ready to transmit. This can result in empty slots and underutilization of the multiplexed communication channel.

Statistical TDM (STDM) represents an improvement over standard TDM. In STDM, if a sender is not ready to transmit in a cycle, the next sender that is ready can transmit. This reduces the number of wasted slots and increases the utilization of the communication channel. STDM data blocks are known as packets and must contain header information to identify the receiving destination.

Applications that use TDM include long-distance telephone service over a T-1 wire line and the Global System for Mobile Communications (GSM) standard for cellular phones. STDM is used in packet-switching networks for LAN and Internet communications.

## Frequency Division Multiplexing

In frequency division multiplexing (FDM), each signal is assigned its own frequency range (or channel) within a larger frequency band. Frequency ranges for channels cannot overlap. Frequency bands are often separated by an unused block of the frequency spectrum to reduce interference.

FDM is used mainly for analog transmissions. It can be used over both wired and wireless mediums.

An example of an application that uses FDM is FM radio. FM is a band that occupies the frequency range from 88 MHz to 108 MHz within the larger radio frequency spectrum. Each radio station transmits at the frequency assigned to its channel (for example, 95.7 MHz, 98.3 MHz, and so on).

Another application that uses FDM is cable TV. The TV transmission cable carries all available channels at their assigned frequencies. When you choose a cable channel with your remote control, the set-top box processes the signal at the frequency assigned to that channel.

## Code Division Multiplexing

In code division multiplexing (CDM), signals from multiple senders are transmitted in an assigned frequency band. CDM uses a principle known as spread spectrum, in which transmitted signals are spread out over all frequency channels in the assigned band.

In simplest terms, each signal in a CDM system is multiplexed by means of a spreading code assigned to the sender. This spreading code modulation increases the bandwidth required for the signal. The receiver is aware of the spreading code and uses it to demultiplex the signal.

Although it increases the bandwidth needed for transmission, CDM has the advantage of being more secure than other types of multiplexing. In CDM transmissions, an individual user’s signal is mixed in with the signals of other users in the frequency band. Without the spreading code required for demultiplexing an individual signal, CDM transmissions appear merely to be noise to a receiving device.

CDM is used in cellular telephone systems.

## Just One Layer of the OSI

As you can see, transmitting an email from your PC or a picture from your phone is a complicated affair. We’ve only scratched the surface of the complexities of multiplexing; there are other types of multiplexing and there are multiple variations of the types we’ve discussed. And multiplexing is only one task in one layer of the Open Systems Interconnection (OSI) model, which describes the architecture for enabling data communications between systems. (Get some background information in An Introduction to the OSI Model.)