Multiplexing Techniques

Q.1 What is Multiplexing?

Multiplexing refers to the ability to transmit data coming from several pairs of equipment (transmitters and receivers) called low-speed channels on a single physical medium (called the high-speed channel). Whereas, a multiplexer is the multiplexing device that combines the signals from the different transmitters and sends them over the high-speed channel.

A demultiplexer is the device which separates signal received from a high-speed channel into different signal and sends them to receivers.

There are four basic multiplexing techniques:

• Frequency Division Multiplexing (FDM)
• Time division Multiplexing (TDM)
• Code division Multiplexing (CDM)
• Space-division Multiplexing (SDM)

FDM (Frequency division multiplexing) (FDM) is the technique used to divide the available bandwidth into a number of smaller independent logical channels with each channel having a small bandwidth. The method of using a number of carrier frequencies each of which is modulated by an independent speech signal is in fact frequency division multiplexing.

Frequency division multiplexing (FDM) is also referred as the Wavelength division multiplexing (WDM), where we are using the optical communications focusing on the wavelength rather than the frequency.

Advantages of FDM:

1. The users can be added to the system by simply adding another pair of transmitter modulator and receiver demodulators.

2. FDM system support full duplex information (Both side simultaneous Communication) flow which is required by most of application.

Disadvantages of FDM:

1. In FDM system, the initial cost is high. This may include the cable between the two ends and the associated connectors for the cable.

2. A problem with one user can sometimes affect the others.

3. Each user requires a precise carrier frequency for transmission of the signals.

TDM (Time division multiplexing) is a technique used to transmit a signal over a single communication channel by dividing the time frame into slots – one slot for each message signal.

Time-division multiplexing is primarily applied to digital signals as well as analog signals, wherein several low speed channels are multiplexed into high-speed channels for transmission.

Time Division Multiplexing (TDM) is another popular method of utilizing the capacity of a physical channel effectively.

Each user of the channel is allotted a small time interval during which it may transmit a message. Thus the total time available in the channel is divided and each user is allocated a time slot. Data from each user is multiplexed into a frame which is transmitted over the channel.

CDMA  (Code Division Multiple Access) CDMA uses spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code generally pseudorandom code) to allow multiple users to be multiplexed over the same physical channel.

CDMA is a form of spread-spectrum signaling.

The modulated coded signal has a much higher data bandwidth than the data

being communicated.

This allows more users to communicate on the same network at one time than if each user was allotted a specific frequency range.

CDMA is a digital technology, so analog signals must be digitized before being transmitted on the network.

SDM: A method by which metallic, radio, or optical transmission media are physically separated by insulation, waveguides, or space in order to maintain channel separations. Within each physically distinct channel, multiple channels can be derived through frequency, time, or wavelength division multiplexing. Some Passive Optical Network (PON) implementations employ space division multiplexing, with the downstream transmissions occurring over one fiber of a duplex fiber optic cable and upstream transmission occurring over the other fiber.