Introduction to Multiplexing

Multiplexing

The transmission of information (either voice or data) from more than one source to more than one destination on the same transmission medium (facility)

Multiplexing

When all the signals passing through a given channel originate form the SAME SOURCE

Multiple-Access

When signals from SEVERAL DIFFERENT SOURCES are combined on a single channel

FREQUENCY DIVISION MULTIPLEXING (FDM) / FREQUENCY DIVISION MULTIPLE-ACCESS (FDMA)

The frequency spectrum can be divided up and parts of it allocated to different users on a full-time basis.

TIME-DIVISION MULTIPLEXING (TDM) / TIME-DIVISION MULTIPLE-ACCESS (TDMA)

The whole of the available spectrum can be allocated to each user for part of the time.

CODE-DIVISION MULTIPLE-ACCESS (CDMA)

There is a third form of multiple access

FREQUENCY DIVISION MULTIPLEXING / FREQUENCY DIVISION MULTIPLE-ACCESS

THE MOST BASIC FORM OF MULTIPLEXING AND HAS BEEN USED SINCE ALMOST THE FIRST DAYS OF RADIO.

FREQUENCY DIVISION MULTIPLEXING / FREQUENCY DIVISION MULTIPLE-ACCESS

Multiple sources that originally occupied same frequency spectrum are each converted to a different frequency band and transmitted simultaneously over a single transmission medium

FREQUENCY DIVISION MULTIPLEXING / FREQUENCY DIVISION MULTIPLE-ACCESS

Many relatively narrowband channels can be transmitted over a single wideband transmission system

Frequency Division Multiplexing (FDM) / Frequency Division Multiple-Access (FDMA)

The process where multiple sources originally occupying the same frequency spectrum are each converted to a different frequency band and transmitted simultaneously over a single medium.

Narrowband channels

In FDM, many of these types of channels can be transmitted over a single wideband transmission system.

A band of frequencies (or Frequency band)

In FDM/FDMA, this is assigned to each transmission on a full-time basis.

Out-of-band interference

In FDM systems, receivers must be carefully designed to reject this specific type of interference to prevent signal encroachment.

Types of metallic cable

• Twisted pair

• Coaxial cable

• Optical fiber

Wavelength-Division Multiplexing (WDM)

This is the equivalent multiplexing technique to FDM/FDMA when it is specifically used in optical fiber.

Other applications for FDM

• Commercial FM

• Television broadcasting

• High-volume telecommunications systems

Multiplexing process

In FDM/FDMA, this process (also known as stacking) is accomplished without any synchronization between stations.

High-capacity electronic switching system

In high-volume telephone communication systems where many voice-band channels share common origins and destinations, the terminal equipment is most likely this type of system.

Synchronized

This must be done to all multiplexing and demultiplexing operations when there is a large number of narrowband channels originating and terminating at the same location.

Three factors determine the travel distance of a radio signal.

• Frequency

• Antenna height

• Transmitter power

Message Channel

The basic building block of the FDM hierarchy. It was originally intended for voice transmission, but now includes any transmissions that utilize voice-band frequencies (VB) (0 – 4 kHz).

Basic Group

The next higher level in the FDM Hierarchy and the first multiplexing step for message channels. It is comprised of 12 voice-band channels stacked next to each other in the frequency domain.

Basic Supergroup

A combination of five groups. It carries information from 60 voice-band (VB) channels or can handle high-speed data up to 250 kbps.

Basic Mastergroup

Comprised of 10 supergroups (600 voice-band channels). Its specific type depends on the system capacity and the transmission medium (coaxial cable, microwave radio, optical fiber, or satellite link).

Types of transmission mediums

• coaxial cable

• microwave radio

• an optical fiber

• a satellite link

Mastergroup banks

Used to further multiplex mastergroups to form jumbogroups, multijumbogroups, and superjumbogroups.

Basic FDM/FM microwave radio channel

Carries three mastergroups (1800 VB channels)

Jumbogroup

Has 3600 VB channels

Superjumbogroup

Has three jumbogroups (10800 VB channels)

Baseband

Modulating signal (intelligence signal)

Composite signal

Total intelligence signal prior to modulation of the final carrier

Time-Division Multiplexing / Time-Division Multiple-Access (TDM/TDMA)

Mainly used for digital communication where transmissions from multiple sources occur on the same facility but not at the same time

Interleaving

The way transmissions from various sources are arranged in the same domain

PCM

The most common type of modulation used in TDM/TDMA

Frame time

Time it takes to transmit one sample from each channel

Epoch

a fixed time slot for the PCM code for each channel to occupy within the total TDM frame

DS-1 signal

An arrangement which consists of 24 PCM voice channels, multiplexed using TDM

64 kbps

The bit rate for each voice channel (each channel is sampled at 8 kHz, with 8 bits/sample)

Frames

In DS-1, each contains the bits representing one sample from each of the 24 channels

Framing bit

Added to each frame to help synchronize the transmitter and receiver

193 bits

The total number of bits contained in each frame (24 × 8 + 1)

T1 carrier

The system known when the DS-1 signal is transmitted over twisted-pair line, using the AMI line code

Superframe

A group of twelve (12) frames

Signaling information

In one frame out of every six, this replaces part of the PCM signal in each of the twenty-four samples

A and B frames

The signaling information is different on the sixth and twelfth frames.

Time Switching

Moves PCM samples from one time slot to another in a TDM signal.

Time Switching

Switches that normally use multiples of 32 time slots (as in the North American transmission hierarchy), though a real switch might more typically have 128 time slots.

Digital Switches

Inherently "four-wire" devices that handle signals in one direction only, meaning another switch is required for the other direction.

Space Switching

A crosspoint type of switch that connects one line to another for the duration of one sample, rather than for an entire call, allowing it to switch much more quickly.

Spread-Spectrum Systems

A communication method used to reduce interference that does not require strong central control by spreading the signal over a much wider portion of the spectrum than usual.

Low Average Power Density

The characteristic of spread-spectrum systems that is responsible for their relative immunity from both interference and eavesdropping.

Spread-Spectrum Communication

A communication method that is especially effective in a portable/mobile environment.

Rayleigh Fading

Very deep fades over a narrow frequency range often caused by signal cancellation due to the reflection of signals.

Two important types of spread-spectrum systems

• Frequency hopping

• Direct sequence

Frequency-Hopping System

The simpler of the two spread-spectrum techniques.

Frequency synthesizer

A device used to generate a carrier in the ordinary way, but uses a pseudo-random noise (PN) sequence instead of operating at a fixed frequency.

Pseudo-random noise (PN)

A sequence used by a frequency synthesizer to generate a carrier that operates at varying frequencies instead of a fixed frequency.

Direct-Sequence Systems

Systems commonly used with digital communication schemes where the data to be transmitted is combined with a pseudo-random noise (PN) bit stream.

Chips

The extra bits transmitted as a result of combining the data with the high-speed PN bit stream.

Chipping Rate

The resulting bit rate of the signal after the data is combined with chips; it is typically at least ten times greater than the original information bit rate.

Processing Gain/Spreading Gain

Defined equal to the bandwidth expansion (which, for direct-sequence spread-spectrum, is also equal to the ratio of chips to information bits)

Processing Gain/Spreading Gain

also describes the amount by which the signal-to-noise ratio of the signal is reduced by the spreading process during transmission

Narrowband Receiver (Frequency-Hopping)

A relatively conventional receiver used for frequency-hopped signals that must hop in the same way as and be synchronized with the transmitter.

Synchronization Requirements (Frequency-Hopping):

The receiver must be provided with the specific frequency-hopping sequence and a synchronizing signal to track the transmitter.

Transmission Detection (Frequency-Hopping)

The specific means or signal provided to allow the receiver to identify exactly when a transmission starts.

Autocorrelation

The process of multiplying the received signal by a locally generated pseudo-noise (PN) code. The output is large when the input matches the PN code and small at all other times.

Preamble Signal

A prearranged sequence of ones and zeroes sent by the transmitter to allow the receiver to synchronize before the actual data is sent

PN Code (Pseudo-Noise)

The specific signal generated at the receiver used in the autocorrelation process to extract the original data from the spread-spectrum signal.

Code-Division Multiple Access (CDMA)

THIRD METHOD THAT IS ALLOWED BY HAVING SPREAD-SPECTRUM COMMUNICATION