MULTIPLEXING

In a simple way, a medium can carry only one signal at any instant in time. Usually, it is desirable that a medium should carry multiple signals at the same time. A common example is multiple channels on cable TV. This is achieved by a technique called MULTIPLEXING. A channel is an assigned set of frequencies that is used to transmit the required signal. For multiple signals to share single medium, the medium must be divided to give each signal a portion of the total bandwidth.

In other words:

• Multiplexing Breaks up a higher speed circuit into several slower (logical) circuits so that several devices/ users can use it at the same time.

• Multiplexing: sharing some amount of bandwidth by many different users or channels

• Main advantage: cost, multiplexing is cheaper because fewer network circuits are needed

Dividing a link or a medium into channels

4-INPUTS

ChannelisationMUX/ DEMUX

4-OUTPUTS

SINGLE LINKA Link or Medium can be Copper, Optical Fiber or Air

• Categories of multiplexing:– Frequency division multiplexing (FDM)– Time division multiplexing (TDM)– Statistical time division multiplexing

(STDM) – Wavelength division multiplexing (WDM)

Categories of multiplexing DIAGRAM

Frequency Division Multiplexing (FDM)

• FDM works by making a number of smaller channels from a larger frequency band. FDM is sometimes referred to as dividing the circuit “horizontally”.

• In order to prevent interference between channels, unused frequency bands called GUARD BANDS are used to separate the channels. Because of the GUARDBANDS, there is some wasted capacity on an FDM circuit.

• FDM was commonly used to multiplex telephone signals before digital transmission became common and is still used on some older transmission lines.

FDM is an analog technique that combines signals

Example Example

Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands.SolutionSolution

Shift (modulate) each of the three voice channels to a different bandwidth, as shown in the next slide.

Solution:

KHz

KHz

KHz

KHz

Frequency Division MultiplexingMakes a number of smaller channels from a larger frequency band

3000 Hz available bandwidth

circuit

FDMFDM

Four terminals

Host computer

FDM is an analog technique

ExampleExample

Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?SolutionSolution

For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x 100 + 4 x 10 = 540 KHz, as shown in the next slide

SOLUTION

TelephoneMultiplexed signal

Telephone

Telephone

DIAGRAM SHOWS MULTIPLEXNIG PROCESS IN FDM

DIAGRAM SHOWS DEMULTIPLEXING PROCESS IN FDM

Multiplexed signal

Telephone

Telephone

Telephone

ExampleExample

An analog Cellular Service Provider uses two bands. The 1st band ( 824 to 849 MHz) is used for sending; and 2nd band (869 to 894 MHz) is used for receiving. Each user has a bandwidth of 30 KHz in each direction. The 3-KHz voice is modulated using FM, creating 30 KHz of modulated signal. How many people can use their cellular phones simultaneously?SolutionSolution

Each band is 25 MHz. If we divide 25 MHz into 30 KHz, (25 MHz/30 KHz) we get 833.33. Practically, the band is divided into 832 bidirectional channels.

ANALOG HIERARICHY IN FDM To increase efficiency, Telephone HIGHER-

BANDWIDTH LINES. Such an arrangement is called HIERARICHY. companies multiplex signals from LOWER-BANDWITH LINES onto

The HIERARICHY used by AT&T is made up of GROUPS, SUPERGROUPS, MASTERR GROUPS and JUMBO GROUPS as shown in the next slide

FDIAGRAM: FDM/Analog hierarchy (AT&T)

Time Division Multiplexing (TDM)

• TDM allows multiple channels to be used by allowing the channels to send data by taking turns. TDM is sometimes referred to as dividing the circuit “vertically”

• With TDM, Time on the circuit is shared equally with each circuit getting a specified time slot, whether or not it has any data to send.

• TDM is more efficient than FDM, since TDM doesn’t use guardbands, so the entire capacity can be divided up between the data channels.

Time Division MultiplexingDividing the circuit “vertically”

• Allows multiple channels to send data by taking turns

4 terminals sharing a circuit, with each terminal sending single or multiple characters at a time. 1 character = 8 bits

Diagram in the next slide gives further explanation of the TDM technique. 4 computers are sending signals to the TDM multiplexer. The data flow of each computer is divided into units and the link combines one data unit of each connection to make a frame. The size of one unit can be 1 bit or several bits. In the diagram, portions of signals 1,2,3, and 4 occupy the link sequentially.

TDM

TDM DATA UNITS, TIME SLOTS & FRAMESNext slide shows the concept of time slots and frames.

There are three input devices or connections sending data. Therefore data rate of the link or medium should be equal to (or greater than) three times the data rate of each connection. Conversely, the duration of a connection should be 3 times the duration of a data unit on the link (because the link is 3 times Faster).The duration of a data unit on the link is called a TIME SLOTTime slots are grouped into FRAMES. A frame consists of one complete cycle of time slots, with one slot dedicated toeach sending device. In a system with n input lines, each frame has n slots, with each slot allocated to carry data from a specific input line.

TDM DATA UNITS, TIME SLOTS & FRAMES

A1, A2, A3 are Data Units or simply unitsB2,B3 and C1, C3 are also Data units. One Time Slot is dedicated to carry data from a specific input line. So each frame has 3 slots

REMEMBERREMEMBERIn a TDM, the data rate In a TDM, the data rate

of the link is n times of the link is n times faster, and the unit faster, and the unit duration is n times duration is n times

shorter. shorter.

ExampleExampleFour 1-Kbps connections are multiplexed together. A data unit is 1 bit. Find (1) the duration of 1 bit before multiplexing, (2) the transmission rate of the link, (3) the duration of a time slot, and (4) the duration of a frame?

SolutionSolution

Answers as follows:1. The duration of 1 bit is 1/1 Kbps, or 0.001 s (1 ms).2. The rate of the link is 4 Kbps.3. The duration of each time slot 1/4 ms or 250 s. 4. The duration of a frame 1 ms.

EXAMPLE• A multiplexer combines four 100-Kbps

channels using a time slot of 2 bits.• Show the output with four arbitrary inputs.

What is the frame rate? What is the frame duration? What is the bit rate? What is the bit duration?

See next slide for solution

SOLUTION

Bit rate =Frame rate =

Bit Rate of the Medium = Sum of bit rates of each input = 4 (100 Kbps) = 400 Kbps

ExampleExample

Four channels are multiplexed using TDM. If each channel sends 100 bytes/s and we multiplex 1 byte per channel, show the frame traveling on the link, the size of the frame, the duration of a frame, the frame rate, and the bit rate for the link.

SolutionSolution

See next slide

SOLUTION

Frame rate =

Bit rate =

Interleaving& Synchronization between MUX & DEMUX The sequence of data received at the DEMUX must coincide with the sequence at the MUX. The process of picking data units from inputs and sending them on a medium is called INTERLEAVING. Fig shows two rotating synchronized switches rotating at the same speed but in opposite direction. Left switch sends data of 1st device, the right switch receives the same data first. This is Synchronization. In practice, Sync bits are added in the beginning of each frame for DEMUX