PCM Principles,PCM Principles,Digital multiplexingDigital multiplexingDigital hierarchy, Digital hierarchy,

Principles of MUX and Principles of MUX and Higher order MUXHigher order MUX

RTTC AM RTTC AM

MULTIPLEXING MULTIPLEXING TECHNIQUESTECHNIQUES

• The technique used to provide a number of circuits The technique used to provide a number of circuits using a single transmission link is called using a single transmission link is called Multiplexing.Multiplexing.

• There are basically two types of multiplexing There are basically two types of multiplexing techniquestechniques

• i.i. Frequency Division Multiplexing Frequency Division Multiplexing (FDM) (FDM) • ii Time Division Multiplexing ii Time Division Multiplexing (TDM)(TDM)

Frequency Division Frequency Division Multiplexing Multiplexing (FDM) (FDM) •The FDM techniques is the The FDM techniques is the

process of translating process of translating individual speech circuits individual speech circuits (300-3400 Hz) into pre-(300-3400 Hz) into pre-assigned frequency slots assigned frequency slots within the bandwidth of within the bandwidth of the transmission medium. the transmission medium.

Frequency Division Frequency Division Multiplexing Techniques Multiplexing Techniques (FDM)(FDM)

Frequency Division Frequency Division Multiplexing Techniques Multiplexing Techniques (FDM)(FDM)•FDM techniques usually find FDM techniques usually find

their application in analogue their application in analogue transmission systems. An transmission systems. An analogue transmission system analogue transmission system is one which is used for is one which is used for transmitting continuously transmitting continuously varying signals.varying signals.

Time Division MultiplexingTime Division Multiplexing( ( TDM TDM ))• Basically, time division multiplexing is sharing Basically, time division multiplexing is sharing

ofofa transmission medium by a number of circuits a transmission medium by a number of circuits in time domain by establishing a sequence of in time domain by establishing a sequence of time slots during which individual channels time slots during which individual channels (circuits) can be transmitted. (circuits) can be transmitted.

• Thus the entire bandwidth is periodically Thus the entire bandwidth is periodically available to each channel. available to each channel.

• Normally all time slots are equal in length. Normally all time slots are equal in length.

• Each channel is assigned a time slot with a Each channel is assigned a time slot with a specific common repetition period called a specific common repetition period called a frame interval. frame interval.

( PCM )( PCM )PULSE CODE MODULATION PULSE CODE MODULATION SYSTEMSYSTEM

•a Pulse Code Modulation (PCM) system a Pulse Code Modulation (PCM) system to transmit the spoken word in digital to transmit the spoken word in digital form. form.

•PCM systems use TDM technique to PCM systems use TDM technique to provide a number of circuits on the provide a number of circuits on the same transmission medium viz open same transmission medium viz open wire or underground cable pair or a wire or underground cable pair or a channel provided by carrier, coaxial, channel provided by carrier, coaxial, microwave or satellite system.microwave or satellite system.

PCMPCM

• To develop a PCM signal from several To develop a PCM signal from several analogue signals, the following analogue signals, the following processing steps are requiredprocessing steps are required ::

• 1 1 Filtering Filtering

• 2 2 Sampling Sampling

• 3 3 Quantisation Quantisation

• 4 4 Encoding Encoding

• 5 5 Line Coding Line Coding

11 FILTERINGFILTERING

• Filters are used to limit the speech signal Filters are used to limit the speech signal to the frequency band 300-3400 Hz.to the frequency band 300-3400 Hz.

22 SAMPLINGSAMPLING• Sampling Theorem StatesSampling Theorem States• "If a band limited signal is sampled at "If a band limited signal is sampled at

regular intervals of time and at a rate regular intervals of time and at a rate equal to or more than twice the highest equal to or more than twice the highest signal frequency in the band, then the signal frequency in the band, then the sample contains all the information of sample contains all the information of the original signal." Mathematically, if fH the original signal." Mathematically, if fH is the highest frequency in the signal to is the highest frequency in the signal to be sampled then the sampling frequency be sampled then the sampling frequency Fs needs to be greater than 2 fH.Fs needs to be greater than 2 fH.

• i.e. Fs>2fHi.e. Fs>2fH

• Let us say our voice signals are band Let us say our voice signals are band limited to 4 KHz and let samplinglimited to 4 KHz and let samplingfrequency be 8 KHz.frequency be 8 KHz.

• Time period of sampling Ts=1 Time period of sampling Ts=1 sec/8000sec/8000

• or or Ts = 125 micro secondsTs = 125 micro seconds

• In a 30 channel PCM system. TS i.e. 125 In a 30 channel PCM system. TS i.e. 125 microseconds are divided into 32 parts.microseconds are divided into 32 parts.

• The 30 time slots are used for 30 speech The 30 time slots are used for 30 speech signals, one time slot for signalling of all signals, one time slot for signalling of all the 30 chls, and one time slot for the 30 chls, and one time slot for synchronization between Tx & Rx.synchronization between Tx & Rx.

• The time available per channel would be The time available per channel would be Ts/N = 125/32 = 3.9 microsecondsTs/N = 125/32 = 3.9 microseconds

• Thus in a 30 channel PCM system, time Thus in a 30 channel PCM system, time slot is 3.9 microseconds and time period slot is 3.9 microseconds and time period of sampling i.e..the interval between 2 of sampling i.e..the interval between 2 consecutive samples of a channel is 125 consecutive samples of a channel is 125 microseconds. This duration i.e. 125 microseconds. This duration i.e. 125 microseconds is called Time Frame.microseconds is called Time Frame.

Time Division Time Division MultiplexingMultiplexing

QUANTISATIONQUANTISATION

• In FDM systems we convey the speech In FDM systems we convey the speech signals in their analogue electrical form. signals in their analogue electrical form. But in PCM, we convey the speech in But in PCM, we convey the speech in discrete form. The sampler selects a discrete form. The sampler selects a number of points on the analogue number of points on the analogue speech signal (by sampling process) and speech signal (by sampling process) and measures their instant values. The measures their instant values. The output of the sampler is a PAM signal output of the sampler is a PAM signal

• The finite number of amplitude intervals The finite number of amplitude intervals is called the "quantizing interval". is called the "quantizing interval".

QUANTISATIONQUANTISATION

• Thus, Thus, quantizing means to divide quantizing means to divide the analogue signal's total the analogue signal's total amplitude range into a number amplitude range into a number of quantizing intervalsof quantizing intervals and and assigning a level to each intervals. assigning a level to each intervals.

• these levels are given a binary code. these levels are given a binary code. This is called encoding. In practical This is called encoding. In practical systems-quantizing and encoding are systems-quantizing and encoding are a combined process. a combined process.

: QUANTIZING - SIGNAL WITH + Ve & - Ve VALUES

ENCODINGENCODING• Conversion of quantised analogue levels to Conversion of quantised analogue levels to

binary signal is called encoding. To represent binary signal is called encoding. To represent 256 steps, 8 level code is required. The eight 256 steps, 8 level code is required. The eight bit code is also called an eight bit "word".bit code is also called an eight bit "word".

• The 8 bit word appears in the formThe 8 bit word appears in the form• P ABCP ABC WXYZ WXYZ• Polarity bit ‘1’Polarity bit ‘1’ Segment CodeSegment Code Linear Linear

encodingencodingfor +Ve, 'O' for -Ve. for +Ve, 'O' for -Ve. in the segmentin the segment• The first bit gives the sign of the voltage to The first bit gives the sign of the voltage to

be coded. Next 3 bits gives the segment be coded. Next 3 bits gives the segment number. There are 8 segments for the number. There are 8 segments for the positive voltages and 8 for negative voltages. positive voltages and 8 for negative voltages. Last 4 bits give the position in the segment. Last 4 bits give the position in the segment. Each segment contains 16 positions. Each segment contains 16 positions.

Structure of FrameStructure of Frame

• A frame of 125 microseconds duration has 32 A frame of 125 microseconds duration has 32 time slots. These slots are numbered Ts 0 to Ts time slots. These slots are numbered Ts 0 to Ts 31.31.

• Information for providing synchronization Information for providing synchronization between trans and receive ends is passed between trans and receive ends is passed through a separate time slot. Usually the slot Ts through a separate time slot. Usually the slot Ts 0 caries the synchronization signals. This slot is 0 caries the synchronization signals. This slot is also called Frame alignment word (FAW).also called Frame alignment word (FAW).

• The signaling information is transmitted The signaling information is transmitted through time slot Ts 16.through time slot Ts 16.

• Ts 1 to Ts 15 are utilized for voltage signal of Ts 1 to Ts 15 are utilized for voltage signal of channels 1 to 15 respectively.channels 1 to 15 respectively.

• Ts 17 to Ts 31 are utilized for voltage signal of Ts 17 to Ts 31 are utilized for voltage signal of channels 16 to 30 respectively.channels 16 to 30 respectively.

Frame Remark

Numbers B1 B2 B3 B4 B5 B6 B7 B8

FO X 0 0 1 1 0 1 1 FAW

F1 X 1 Y Y Y 1 1 1 ALARM

F2 X 0 0 1 1 0 1 1 FAW

F3 etc X 1 Y Y Y 1 1 1 ALARM

In frames 1, 3, 5, etc, the bits B3, B4, B5 denote various types of alarms. For example, in B3 position, if Y = 1, it indicate Frame synchronisation alarm. If Y = 1 in B4, it indicates high error density alarm. When there is no alarm condition, bits B3 B4 B5 are set 0. An urgent alarm is indicated by transmitting "all ones". The code word for an urgent alarm would be of the form.

X 111 1111

SIGNALLING IN PCM SIGNALLING IN PCM SYSTEMSSYSTEMS

• In a telephone network, the signalling information In a telephone network, the signalling information is used for proper routing of a call between two is used for proper routing of a call between two subscribers, for providing certain status subscribers, for providing certain status information like dial tone, busy tone, ring back. information like dial tone, busy tone, ring back. NU tone, metering pulses, trunk offering signal NU tone, metering pulses, trunk offering signal etc. All these functions are called "signalling" in etc. All these functions are called "signalling" in PCM systems. The signaling information can be PCM systems. The signaling information can be transmitted in the form of DC pulses. transmitted in the form of DC pulses.

• The signalling pulses retain their amplitude for a The signalling pulses retain their amplitude for a much longer period than the pulses carrying much longer period than the pulses carrying speech information. It means that the speech information. It means that the signalling information is a slow varying signal in signalling information is a slow varying signal in time compared to the speech signal. Therefore, a time compared to the speech signal. Therefore, a signalling channel can be digitized with less signalling channel can be digitized with less number of bits than a voice channel.number of bits than a voice channel.

SIGNALLING IN PCM SIGNALLING IN PCM SYSTEMSSYSTEMS

• In a 30 chl PCM system, time slot In a 30 chl PCM system, time slot Ts 16Ts 16 in each frame is in each frame is allocated for carrying signalling information.allocated for carrying signalling information.

• The time slot 16 of each frame carries the The time slot 16 of each frame carries the signalling data corresponding to signalling data corresponding to two VF channelstwo VF channels only. only. Therefore, to cater for 30 channels, we must transmit 15 Therefore, to cater for 30 channels, we must transmit 15 frames, each having 125 microseconds duration. For frames, each having 125 microseconds duration. For carrying synchronization data for all frames, one carrying synchronization data for all frames, one additional frame is used. Thus a group of 16 frames (each additional frame is used. Thus a group of 16 frames (each of 125 microseconds) is formed to make a "multiframe". of 125 microseconds) is formed to make a "multiframe". The The duration of a multiframe is 2 millisecondsduration of a multiframe is 2 milliseconds.. The The multiframe has 16 major time slots of 125 microseconds multiframe has 16 major time slots of 125 microseconds duration. Each of these (slots) frames has 32 time slots duration. Each of these (slots) frames has 32 time slots carrying, the encoded samples of all channels plus the carrying, the encoded samples of all channels plus the signaling and synchronization data. signaling and synchronization data. Each sample has Each sample has eight bits of duration 0.400eight bits of duration 0.400 microseconds (3.9/8 = microseconds (3.9/8 = 0.488) each. The relationship between the bit duration 0.488) each. The relationship between the bit duration frame and multiframe.frame and multiframe.

• We have 32 time slots in a frame, We have 32 time slots in a frame, each slot carries an 8 bit word.each slot carries an 8 bit word.

• The total number of bits per The total number of bits per frame = 32 x 8 = 256frame = 32 x 8 = 256

• The total number of frames per The total number of frames per seconds is 8000seconds is 8000

• The total number of bits per The total number of bits per second are 256 x 8000 = 2048 second are 256 x 8000 = 2048 K/bits.K/bits.

• Thus, a 30 chP PCM system has Thus, a 30 chP PCM system has 2048 K bits. 2048 K bits.

DEFINITION AND DEFINITION AND DESCRIPTION OF DIGITAL DESCRIPTION OF DIGITAL HIERARCHIESHIERARCHIES

• the bit rate of the next lower level. In CCITT the bit rate of the next lower level. In CCITT Rec. G.702, the term “digital multiplex Rec. G.702, the term “digital multiplex hierarchy” is defined as follows : hierarchy” is defined as follows :

• ““A series of digital multiplexes graded A series of digital multiplexes graded according to capability so that multiplexing at according to capability so that multiplexing at one level combines a defined number of digital one level combines a defined number of digital signals, each having the digit rate prescribed signals, each having the digit rate prescribed for the next lower order, into a digital signal for the next lower order, into a digital signal having a prescribed digit rate which is then having a prescribed digit rate which is then available for further combination with other available for further combination with other digital signals of the same rate in a digital digital signals of the same rate in a digital multiplex of the next higher order”.multiplex of the next higher order”.

DIGITAL HIERARCHY BASED ON DIGITAL HIERARCHY BASED ON THE 2048 KBIT/S PCM THE 2048 KBIT/S PCM PRIMARY MULTIPLEX PRIMARY MULTIPLEX EQUIPMENTEQUIPMENT

Encoded TDM (European)

Bit rate Mbit/sec.

No. of channels

2.048 308.448 120

34.368 480139.264 1920

DIGITAL MULTIPLEXING HIERARCHY

MULTIPLEXING OF MULTIPLEXING OF SYNCHRONOUS DIGITAL SYNCHRONOUS DIGITAL

SIGNALSSIGNALS• Block interleaving :Block interleaving :• Bunch of information taken at a time Bunch of information taken at a time

from each tributary and fed to main from each tributary and fed to main multiplex output stream. The multiplex output stream. The memory required will be very large. memory required will be very large.

• Bit interleaving :Bit interleaving :• A bit of information taken at time A bit of information taken at time

from each tributary and fed to main from each tributary and fed to main multiplex output stream in cyclic multiplex output stream in cyclic order, a very small memory is order, a very small memory is required. required.

• Timing of all operations within a Timing of all operations within a multiplexer is controlled by a highly multiplexer is controlled by a highly stable oscillator called the Master stable oscillator called the Master Clock. Typically, the frequency Clock. Typically, the frequency reference used within the oscillator is reference used within the oscillator is a quartz crystal of tolerance in the a quartz crystal of tolerance in the range 10 to 50 ppm. range 10 to 50 ppm.

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