MODULATION IS THE TECHNIQUE THAT CONVERT DIGITAL ELECTRONIC SIGNAL TO ANALOG SIGNAL. THERE ARE THREE TYPES OF MODULATION SIGNAL EXIST.

AMPLITUDE MODULATION

FREQUENCY MODULATION

PHASE MODULATION

WITH AM THE AMPLITUDE OR HEIGHT OF THE WAVE IS CHANGED.

ONE AMPLITUDE IS DEFINE TO BE ZERO, AND ANOTHER AMPLITUDE IS DEFINE TO BE A ONE.

AM EFFECTED BY NOISE AND INTERFERENCE.

FREQUENCY MODULATION (FM)

FREQUENCY SHIFT KEYING (FSK)

IS A MODULATION TECHNIQUE WHERE EACH 0 OR 1 IS REPRESENTED BY A NUMBER OF WAVE PER SECOND.

ONE FREQUENCY (A CERTAIN NUMBER OF WAVES PER SECOND) IS DEFINE TO BE A ONE, AND A DIFFERENT FREQUENCY (A DIFFERENT NUMBER OF WAVES PER SECOND) IS DEFINED TO BE A ZERO. FREQUENCY MODULATION IS MORE RESISTANT TO EXTERNAL INTERFERENCE THAN AM.

A PHASE SHIFT OCCURS TO INDICATE A CHANGE IN THE TYPES OF BITS BEING TRANSMITTED.

CIRCUITS CAN BE DESIGNED TO PERMIT DATA TO FLOW IN ONE DIRECTION OR IN BOTH DIRECTIONS ACTUALLY, THERE

ARE THREE WAYS TO TRANSMIT:

SIMPLEX

HALF DUPLEX

FULL DUPLEX

SIMPLEX IS ONE-WAY TRANSMISSION, SUCH AS THAT IN RADIO OR TV TRANSMISSION.

HALF DUPLEX IS TWO-WAY TRANSMISSION, BUT YOU CAN TRANSMIT IN ONLY ONE DIRECTION AT A TIME A HALF DUPLEX COMMUNICATION LINK IS SIMILAR TO A WALKIE-TALKIE LINK.

FULL DUPLEX TRANSMISSION, YOU CAN TRANSMIT IN BOTH DIRECTIONS SIMULTANEOUSLY. EX. TELEPHONE

HUMAN ERRORS, SUCH AS A MISTAKE IN TYPING A NUMBER, USUALLY ARE CONTROLLED THROUGH THE APPLICATION PROGRAM.

NETWORK ERRORS, SUCH AS THOSE THAT OCCUR DURING TRANSMISSION, ARE CONTROLLED BY THE NETWORK HARDWARE AND SOFTWARE.

LINE NOISE AND DISTORTION

1. WHITE NOISE

2. IMPULSE NOISE

3. CROSS-TALK

4. INTERMODULATION NOISE

5. ECHOES

6. ATTENUATION

7. ATTENUATION DISTORTION

8. DELAY DISTORTION

9. JITTER

10. HARMONIC DISTORTION

11. LINE OUTAGE

THE FAMILIAR BACKGROUND HISS OR STATIC ON RADIOS AND TELEPHONES IS CAUSED BY THE THERMAL AGITATION OF ELECTRONS AND THEREFORE IS INESCAPABLE.

THE AMOUNT OF NOISE IS DIRECTLY PROPORTIONAL TO THE TEMPERATURE OF THE MEDIUM.

WHITE NOISE USUALLY IS NOT A PROBLEM UNLESS IT BECOMES SO STRONG THAT IT OBLITERATES THE TRANSMISSION.

WHAT CAN WE DO TO MINIMIZE THE WHITE NOISE?

THE MEDIUM SHOULD BE KEPT AS COOL AS POSSIBLE.

IMPULSE NOISE SOMETIMES CALLED SPIKES IMPULSE NOISE IS THE PRIMARY SOURCE OF ERRORS IN DATA COMMUNICATIONS.

IMPULSE NOISE IS HEARD AS A CLICK OR A CRACKING NOISE AND CAN LAST AS LONG AS 1/100 OF A SECOND.

SUCH A CLICK DOES NOT REALLY AFFECT VOICE COMMUNICATIONS, BUT IT CAN DAMAGE A GROUP OF DATA, CAUSING ERRORS IN DATA.

EX. AT 300 BITS PER SECOND, THREE BITS WOULD BE CHANGED BY A SPIKE OF 1/100 OF A SECOND, WHEREAS AT 14400 BITS PER SECOND, 144 BITS WOULD BE CHANGED.

SOME OF THE SOURCES OF IMPULSE NOISE ARE

VOLTAGE CHANGES IN ADJACENT LINES OR CIRCUITRY SURROUNDINGTHE DATA COMMUNICATION LINE

TELEPHONE SWITCHING EQUIPMENT AT THE EXCHANGE BRANCH OFFICERS

MAINTENANCE EQUIPMENT USED FOR LINE TESTING

LIGHTING FLASHES DURING THUNDERSTORMS

FLUORESCENT LIGHTS

POOR ELECTRICAL CONNECTIONS IN THE DATA COMMUNICATION EQUIPMENT

INTERMODULATION NOISE IS A SPECIAL TYPE OF CROSS-TALK. THE SIGNALS FROM TWO CIRCUITS INTERMODULATE AND FORM A NEW SIGNAL THAT FALLS INTO A FREQUENCY BAND DIFFERENT FROM BOTH THAT IS RESERVED FOR ANOTHER SIGNAL. THIS TYPE OF NOISE IS SIMILAR TO HARMONICS IN MUSIC.

EX. ASSUME TWO TRANSMISSIONS OCCURS, ONE AT 1000 HZ AND THE OTHER ONE AT 2500 HZ ON A VOICE -GRADE TRANSMISSION LINE. IF INTERMODULATION NOISE OCCURS, THE SIGNAL MAY COMBINE, PRODUCING A SIGNAL OF 3500 HZ WHICH IS OUTSIDE THE ACCEPTABLE RANGE OF THE TRANSMISSION LINE( VOICE GRADE RANGE 300-3400).

CROSSTALK OCCURS WHEN ONE CIRCUIT PICKS UP SIGNALS IN ANOTHER.

BROKEN SHIELDING ON A CABLE CAN CAUSE THE CABLE TO COME TOO CLOSE TO ANOTHER CABLE AND CAUSE INTERFERENCE.

IN MULTIPLEXED LINKS CARRYING MANY DISCRETE SIGNALS

IN MICROWAVE LINKS IN WHICH ONE ANTENNA PICKS UP A REFLECTION FROM ANOTHER ANTENNA

IN ANY TELEPHONE CIRCUITS THAT ARE TOO CLOSE TO EACH OTHER OR ARE NOT ELECTRICALLY BALANCED.

WET OR DAMP WEATHER CAN ALSO INCREASE CROSS-TALK.

ATTENUATION IS THE LOSS OF POWER A SIGNAL SUFFERS AS IT TRAVELS FROM THE TRANSMITTING DEVICE TO THE RECEIVING DEVICE. SOME POWER IS ABSORBED BY THE MEDIUM OR IS LOST BEFORE IT REACHES THE RECEIVER. AS THE MEDIUM ABSORBS POWER, THE SIGNAL BECOMES WEAKER, AND THE RECEIVING EQUIPMENT HAS LESS AND LESS CHANCE OF CORRECTLY INTERPRETING THE DATA.

THIS POWER LOSS IS A FUNCTION OF THE TRANSMISSION METHOD AND CIRCUIT MEDIUM. ATTENUATION INCREASES AS FREQUENCY INCREASES OR AS THE DIAMETER OF THE WIREDECREASES.

DATA MAY ARRIVE AT DIFFERENT TIMES, DEPENDING ON WHAT FREQUENCY IS USED.

THEN DELAY DISTORTION REFERS TO THE DISTORTION OF THE SIGNAL THAT OCCURS BECAUSE DIFFERENT FREQUENCIES TRAVEL AT SLIGHTLY DIFFERENT SPEEDS THROUGH THE MEDIA.

IF DATA ARE TRANSMITTED AT TWO DIFFERENT FREQUENCIES, THEN THE BITS TRANSMITTED AT ONE FREQUENCY MAY TRAVEL SLIGHTLY FASTER THAN THE BITS TRANSMITTED AT THE SECOND FREQUENCY.

HOW DO WE SOLVE THIS PROBLEM?

A DEVICE CALLED AN EQUALIZER IS USED TO ENSURE THE DATA ARRIVES AT ITS DESTINATION AT THE SAME TIME.

JITTER MAY AFFECT THE ACCURACY OF THE DATA BEING TRANSMITTED BECAUSE THE PACKETS BETWEEN A GIVEN SOURCE AND DESTINATION MAY VARY IN LENGTH, MAY TAKE DIFFERENT ROUTES, AND MAY BE SUBJECT TO VARYING DELAY IN THE SWITCHES THEY ENCOUNTER, THE OVERALL PACKET DELAY CAN VARY SUBSTANTIALLY.

HARMONIC DISTORTION USUALLY IS CAUSED BY AN AMPLIFIER ON A CIRCUIT THAT DOES NOT CORRECTLY REPRESENT ITS OUTPUT WITH WHAT WAS DELIVERED TO IT ON THE INPUT SIDE.

LINE OUTAGES ARE A CATASTROPHIC CAUSE OF ERRORS AND INCOMPLETE TRANSMISSION. OCCASIONALLY, A COMMUNICATION CIRCUIT FAILS FOR A PERIOD. THIS TYPE OF FAILURE MAY BE CAUSED BY FAULT TELEPHONE-END OFFICE EQUIPMENT, STORM, LOSS OF THE CARRIER SIGNAL, AND ANY OTHER FAILURE THAT CAUSES A SHORT CIRCUIT.

THERE ARE MANY ERROR PREVENTION TECHNIQUES, SOME OF THEM ARE:

1. SHIELDING

2. MOVING CABLES

3. CHANGING MULTIPLEXING TECHNIQUES

4. IMPROVING CONNECTION QUALITY

5. AMPLIFIERS AND REPEATERS

6. EQUALIZATION

7. CONDITIONING

SHIELDING IS PROTECTING WIRES BY COVERING THEM WITH AN INSULTING COATING.

SHIELDING IS ONE OF THE BEST WAYS TO PREVENT IMPULSE NOISE AND CROSS-TALK.

MOVING CABLES RELOCATING CABLES AWAY FROM SOURCES OF NOISE (ESPECIALLY POWER SOURCES) CAN ALSO REDUCE IMPULSE NOISE AND CROSS-TALK.

FOR IMPULSE NOISE, THIS MEANS AVOIDING LIGHTS AND HEAVY MACHINERY.

LOCATING COMMUNICATION CABLES AWAY FROM POWER CABLES IS ALWAYS A GOOD IDEA.

FOR CROSS-TALK, THIS MEANS PHYSICALLY SEPARATING THE CABLES FROM OTHER COMMUNICATION CABLES.

CROSS-TALK AND INTERMODULATION NOISE IS OFTEN CAUSED BY IMPROPER MULTIPLEXING.

CHANGING TECHNIQUES (FROM FDM TO TDM) OR CHANGING THE FREQUENCIES OR SIZE OFTHE GUARD BANDS IN FREQUENCY DIVISION MULTIPLEXING CAN HELP.

IMPROVING CONNECTION QUALITY MANY TYPES OF NOISE (ECHOES,WHIT NOISE, JITTER, HARMONIC DISTORTION) CAN BE CAUSED BY POORLY MAINTAINED EQUIPMENT OR POOR CONNECTIONS AND SPLICES AMONG CABLES.

AMPLIFIERS AND REPEATERS

EQUALIZATION

CONDITIONING

A CONDITIONED CIRCUIT IS ONE THAT HAS BEEN CERTIFIED BY THE CARRIER TO EXPERIENCE FEWER ERROR AT INCREASINGLY HIGHER COST. CONDITIONED CIRCUIT EMPLOY A VARIETY OF THE TECHNIQUES (SUCH AS EQUALIZATION, SHIELDING) TO PROVIDE LESS NOISE.

1, PARITY CHECKING

2. LONGITUDINAL REDUNDANCY CHECKING (LRC)

3. POLYNOMIAL CHECKING

EVEN PARITY

ODD PARITY

LONGITUDINAL REDUNDANCY CHECKING (LRC) LRC ADDS ONE ADDITIONAL CHARACTER, CALLED THE BLOCK CHECK CHARACTER (BCC), TO THE END OF THE ENTIRE MESSAGE OR PACKET OF DATA.THE VALUE OF THE BCC IS DETERMINED IN THE SAME MANNER AS THE PARITY BIT, BUT BY COUNTING LONGITUDINALLY THROUGH THE MESSAGE, RATHER THAN BY COUNTING VERTICALLY THROUGH EACH CHARACTER.

THE FIRST BIT OF THE LRC IS DETERMINED BY COUNTING THE NUMBER OF 1'S IN THE FIRST BITS OF ALL CHARACTERS IN THE MESSAGE, AND SETTING THE FIRST BIT OF THE LRC TO A 1 OR 0 DEPENDING UPON WHETHER THE SUM IS ODD OR EVEN. THE SECOND BIT OF THE BBC IS DETERMINED BY COUNTING THE NUMBER OF 1'S IN THE SECOND BITS OF CHARACTERS IN THE MESSAGE, AND SO ON FOR ALL BITS IN THE BCC.

POLYNOMIAL CHECKING A 98 PERCENT ERROR DETECTION RATE IS REASONABLY GOOD, BUT IT IS STILL NOT PERFECT. LIKE LRC, POLYNOMIAL CHECKING ADDS A CHARACTER OR SERIES OF CHARACTERS TO THE END OF THE MESSAGE BASED ON A MATHEMATICAL ALGORITHM.

ONE OF THE MOST POPULAR OF POLYNOMIAL ERROR CHECKING SCHEMES EDUNDANCY CHECK (CRC).

CRC PRODUCING ERROR DETECTION RATE ABOVE 99 PERCENT IS CYCLICAL

ERROR CORRECTION VIA RETRANSMISSION

ONCE ERROR HAS BEEN DETECTED, IT MUST BE CORRECTED. THE SIMPLEST, MOST EFFECTIVE, LEAST EXPENSIVE, AND MOST COMMONLY USED METHOD FOR ERROR CORRECTION IS RETRANSMISSION.

WITH RETRANSMISSION, A RECEIVED THAT DETECTS AN ERROR SIMPLY ASKS THE SENDER TO RETRANSMIT THE MESSAGE UNTIL IT IS RECEIVED WITHOUT ERROR.

THIS IS OFTEN CALLED AUTOMATIC REPEATER QUEST (ARQ)

1. STOP AND WAIT ARQ

2. GO-BACK-N ARQ

3. CONTINOUS

STOP AND WAIT ARQ WITH STOP AND WAIT ARQ THE SENDER STOPS AND WAITS FOR A RESPONSE FROM THE RECIEVER AFTER EACH MESSAGE OR PACKET OF DATA.

STOP AND WAIT IS A HALF DUPLEX TRANSMISSION TECHNIQUE.

GO-BACK-N ARQ IN THIS TECHNIQUE, THE SENDER STARTS TRANSMITTING NUMBERED PACKETS, ONE AFTER ANOTHER. THE RECEIVER CHECKS FOR ERRORS AND SENDS ACKS OR NAKS.

CONTINUOUS ARQ WITH CONTINUOUS ARQ THE SENDER DOES NOT WAIT FOR AN ACKNOWLEDGMENT AFTER SENDING A MESSAGE; IT IMMEDIATELY SENDS THE NEXT ONE.

FORWARD ERROR CORRECTION FORWARD ERROR CORRECTION USES CODES CONTAINING SUFFICIENT REDUNDANCY TO PREVENT ERRORS BY DETECTING AND CORRECTING THEM AT THE RECEIVING END WITHOUT RETRANSMISSION OF THE ORIGINAL MESSAGE. THE REDUNDANCY, OR EXTRA BITS REQUIRED, VARIES WITH DIFFERENT SCHEMES.

1. HAGELBARGER CODE, CORRECTS UP TO SIX CONSECUTIVE BIT ERRORS.

2. HAMMING CODE A FORWARD ERROR CORRECTION (FEC) TECHNIQUE NAMED FOR ITS INVENTOR.

3. SLIDING WINDOW IN THIS TECHNIQUE, THE RECEIVER HAS A CERTAIN NUMBER OF CREDITS. THEN THE SENDER CAN TRANSMITS THAT NUMBER OF PACKETS BEFORE A POSITIVE OR NEGATIVE ACKNOWLEDGMENT MUST BE RECEIVED. IF THE NUMBER OF PACKETS IS EXCEEDED, THE SENDER CEASES TRANSMISSION.

1. SERIAL TRANSMISSION

2. PARALLEL TRANSMISSION

1. ASYNCHRONOUS TRANSMISSION

2. SYNCHRONOUS TRANSMISSION