Digtial Transmission

By: Srashti Vyas

4.1

4.2

ANALOG-TO-DIGITAL CONVERSIONANALOG-TO-DIGITAL CONVERSION

A digital signal is superior to an analog signal because: A digital signal is superior to an analog signal because: 1.1.It is more robust to noiseIt is more robust to noise2.2.Easily be recovered, corrected and amplified. Easily be recovered, corrected and amplified.

For this reason, the tendency today is to change an For this reason, the tendency today is to change an analog signal to digital data. analog signal to digital data.

PCM• PCM consists of three steps to digitize an analog

signal:1. Sampling2. Quantization3. Binary encoding

Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.

Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.

4.3

4.4

Figure 4.21 Components of PCM encoder

4.5

According to the Nyquist theorem, the sampling rate must be

at least 2 times the highest frequency contained in the signal.

Note

4.6

Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second.

Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second.

4.7

Figure 4.26 Quantization and encoding of a sampled signal

4.8

We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample?

SolutionThe human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:

4.9

Note:We have a low-pass analog signal of 4 kHz. If we send the analog signal, we need a channel with a minimum bandwidth of 4 kHz. If we digitize the signal and send 8 bits per sample, we need a channel with a minimum bandwidth of 8 × 4 kHz = 32 kHz.

4.10

DIGITAL CARRIER SYSTEM• Digital carrier is a digital signaling represents a

telecommunication service.• Digital Services in North America defines a four level transmission

hierarchy called “T-carrier” range from T1, T2, T3 & T4.

• In Europe and South America, there is five level transmission hierarchy called “E-carrier” range from E1, E2, E3, E4 & E5.

• Both system use the PCM technique to encode an analog signal into digital form.

• The signal is sampled 8000 times per second, and each sample value is encoded in an 8-bit value.

• TDM is used for signal transmission.

4.12

TIME DIVISION MULTIPLEXING OF PCM SIGNALS

• PCM is most prevalent technique used for TDM digital signals.

• In TDM-PCM system, two or more voice channels are sampled, quantized and encoded to PCM codes.

• The fundamental building block of TDM System begin with a digital signal level(DS-0)

4.13

• DS-0 channels used an 8 KHz sample rate and 8-bit PCM code, which resulted to 64 kbps PCM line speed.

Line Speed=(800 samples/sec) X (8 bits/sample)=64kbps

• A PCM carrier system comprised of two DS-0 channels time division multiplexed.

• One 8-bit PCM code from each channel (total 16 bits) is called a TDM Frame.• Time taken to transmit one TDM frame is called Frame time. • With two channel system, time allocated to transmit the PCM bits from each

channel is equal to one-half the total frame time. Thus, the line speed at the output of multiplexer is:

Line Speed =(2-channel/frames) X (800 samples/sec)X(8 bits/sample) Line Speed = 2 X 64kbps Line Speed = 128kbps

4.14

4.15

1.536MHz

4.16

T1 and SONET/SDH standard defines the rates and formats, the physical layer, network and the network operational layer.

4.17

• The non-local calls are routed between central offices through Inter-office trunks(IOT).

• The original IOT connections were made an analog system called the N- carrier.

• The N-carrier system deployed an analog transmission, allowing 12 voice calls to share the same line.

• The L-carrier system was designed in 1960’s for handling much higher capacity. This can carry upto 6000 calls on the coaxial cable. Repeater was needed to improve signal periodically along the line. Though the system was very expensive.

• The new digital transmission called T-carrier system was developed.

• The T-1 version can handle 24 calls on two pairs of copper wire. It is the fundamental building block of the multiplexing hierarchy.

4.18

T-1 carrier System

4.19

Pulse Trains in T-carrier rely on the B8ZS (Bipolar With 8 Zero Substitution) coding)

Frame Synchronization

• It is the process by which incoming frame alignment signals, i.e. distinctive bit sequences are distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission.

• T-1 uses two types of synchronization:

1. Super Frame(SF):Older standard. Also known as D4 or D3/D4 framing.

2. Extended Super Frame(ESF):Upgraded from of SF. Also known as D5 framing.

4.20

• A DS-1 frame contains 193 bits divided into 24 slots (one for each voice channel) of 8 bits and one extra bit called a framing bit (f-bit) which is used for synchronizing.

• 8-bit voice samples are taken each of 24 channels at a rate of 8000 times per second (64 kbps rate)

• To support this speed, T-1 must transmit a DS-1 frame every 1/8000 of a second (125 micro seconds)

• This yield the bit rate of (24 X 8 + 1)bits/125µs=1.544Mbps.

4.21

4.22

• 12 consecutive frames comprise a “SUPERFRAME”(also called D4 framing).• The 12 framing bits in this Superframe (one framing bit per frame) goes

through the 12 bit pattern 100011011100.• The signalling bits are substituted in frames 6 & 12, the most significant bit

into frame 6, and the least significant bit into frame 12.• Frames 1 to 6 are called the A-highway; with frame 6 designated the A-

channel signalling frame.• Frames 7 to 12 are called the B-highway; with frame 12th designated the B-

channel signalling frame.

• Therefore, in identifying the signalling frame, the 6th & 12th frame must also be positively identified.

• The synchronization pattern occurring in odd number frames is 101010. These are the synchronization bits for channel banks.

• The voice clock synchronization pattern occurring in the even-numbered frames has a pattern 001110.

• The combine F’s bit pattern is 100011011100 for superframe.• It is called the S bit is shared between framing and

signalling. 4.23

4.24

Extended SuperFrame• It not only provide frame synchronization but also error detection and a data

channel, all using the framing bit.• The value in every 193 bits (in bits 193, 386 & so on) are used for three

purpose:1. Every 4th bit of this 24 bit cycle (i.e. the framing bits for frames 4,8,12,16,20

and 24) goes through the pattern 001011.This provide frame synchronization.2. The framing bits for frames 2,6,10,……22 are used to send a 6 bit CRC,

generated from the data in previous 24 frames. This provide “Error detection”.

3. The Channel Service Units (CSU) can then track the error rate and generate an alarm if it gets too high. This error checking is done constantly while the links is in service and for any type of data.

4. The remaining framing bits(for frames 1,3,5,7,….,23) provide a 4 kbps supervisory data channel that is used for other functions such as remote configuration and monitoring of CSU.

5. The 8th bit in every channel of frames 6,12,18 & 24 is used for signaling between central offices.

4.25

4.26

Fractional T-Carrier Service• A Fractional T-carrier is one or more channels bundled together

and sold to a customer as a set. However less than the full set of 24 channels is available to the customer.

• This allows a customer to purchase less than a full T1’s BW at lower cost.

• The individual channels can be voice or data and a CSU/DSU is used to split the channels.

• The fractional T-carrier emerged because standard T1 carriers provide a higher capacity (higher bit rate) than most users require.

• It distributes the channels (i.e bits) in a standard T1 system among more than one user, allowing several subscribers to share one T1 line.

4.27

• Bit rates offered with fractional T1-carrier system are 64 kbps(1-channel), 128 kbps(2-channels), 256 kbps(4-channels), 384 kbps(6-channels),512kbps(8-channels) and 768 kbps(12-channels) with 384 kbps(1/4T1) and (1/2T1) being the most common.

• The minimum required data rate required to propagate video information is 384 kbps.

4.28

USER- 2USER- 2

USER-1USER-1

USER-3USER-3

USER-4

DSU/CSUDSU/CSU

128kbps

256 kbps

384 kbps

768 kbps

8-kbps Framing Bits

1.544 Mbps T1 line

Digital Signal Service• Telephone companies implement TDM through a hierarchy of digital

signal, known as Digital Signal (DS) service. DS Line Rate(Mbps) Voice

ChannelsServices

DS-0 (Single Digital Channel)

T-1

64Kbps 1 Voice & Data

DS-1 T-1 1.544 24 DS-0 Voice & Data

DS-2 T-2 6.312 4 DS-196 DS-0

Voice or Data, Picture Phone

DS-3 T-3 44.736 7 DS-2 or 28 DS-1 or 672DS-0

Voice or Data, Picture Phone, Broadcast

DS-4 T-4 274.176 4032 DS-0 Voice or Data, Picture Phone, Broadcast

DS-5 T-5 560.160 8064 Voice or Data, Picture Phone, Broadcast

TDM Hierarchy

4.30

MASTER GROUP & COMMERCIAL TELEVISION

• The master group terminal receives voice band channels which are multiplexed using FDM.

• The signal processor, shifts the master group signals frequencies, from a 564 KHz to 3084 KHz bandwidth to a 0 KHz to 2520 KHz BW.

• DC restoration is also provided to TV signals.

• The master group band is sampled at 5.1 MHz rate & for commercial TV signal sampling rate is 10.2 MHz, i.e twice of master group rate.

4.31

Block Diagram of Master group & Commercial TV Digital Terminal

4.32

Signal

Processor

Encoder

(9-bit)

Sampler

Signal

Processor

&

Sampler

Digital

Processor

TV Cahnnel

Master

Group

or

TV

Signal

Digital

Processor

Framer

PAM

Signal12

9

T-3 Signal

T3 Signal

o/p (46 Mbps)

Channel for TV(46 Mbps)

Decoder

(9-bit)

2

9

1Recovered

Master

Group

Or

TV Signal

• A 9-bit PCM code is used to digitize each sample of the master group or television signal.

• The output of digital processor is approx 46 Mbps for master group and 92 Mbps for TV signal.

As there is no 92 Mbps line speed in the digital hierarchy, the 92 Mbps digital output must be split into two 46 Mbps signals for TV terminal.

The digital terminal has three main functions:

1. Conversion of the parallel data from the output of the encoder to the serial data.

2. Frame synchronizing bits are inserted.

3. It converts the serial binary signal to form more suitable for transmission.

4.33

Picture Phone Terminal1. Used to transmit the low quality video signals for the

use of non dedicated subscribers.

2. This picture phone signal is encoded into T2 capacity of 6.312 Mbps, which is less than that for commercial network broadcast signals.

3. Thus, reduced cost and affordability are achieved.

4. It allows adequate details and contrast resolution to satisfy the average picture phone subscriber.

5. Picture phone service is ideally suited to a DPCM.

4.34

DATA TERMINAL• These are designed to transmit the signals other than voice.

• In most of the cases, the data rate generated by each individual subscriber are substantially less than the data rate capacities of digital lines.

• Therefore, it seems only logical that terminals be designed that transmit data signal from several sources over the same digital line.

• Data coding method is more efficient & it codes the transition time . The coding format is as follows:

1. It uses 3 bit code to indentify when transition occurs on the data and whether the transition is from ‘1’ to ‘0’ or vice versa.

First bit is referred as “Address bit”. Second bit indicate “Transition bit” Third bit indicate “Direction of transition or sign bit”

4.35

1. Address bit

4.36

2. Transition bit

4.37

When there are no transition in the data, a second of all ‘1’ is transmitted.

3. Direction of Transition or Sign bit

4.38

4.39

Note:

Though the transmission of address bit is sufficient

but, the sign bit provides a degree of error

protection and limit error propagations.

Note:

Though the transmission of address bit is sufficient

but, the sign bit provides a degree of error

protection and limit error propagations.

DATA CODING FORMAT4.40

CODECS

4.41

COMBO CHIPS• It replaces the older codec and filter chip combination.

• It provides analog to digital conversion and transmit & receive filtering necessary to interface a full duplex telephone circuit to the PCM highway of TDM carrier system.

Function of Combo chip:

1. Band pass filtering of the analog signals before encoding and after decoding.

2. Encoding and decoding of voice signals.

3. Encoding and decoding of signalling and supervision information.

4. Digital companding4.42

Block Diagram of Combo Chip4.43

Modes of operation

There are two types of modes of operation:1.Fixed data rate mode:2.Variable data rate mode:

4.44

Fixed Data Rate Mode• Transmit and receive data rate must be same as the

Master Clock Rate i.e. must be either 1.536 Mbps or 2.048 Mbps

• In this mode, the master transmit and receives clocks on a combo chip performs the following functions:

1. It provides the master clock for the on board switched capacity filter.

2. It provides the clock for the A/D& D/A convertors.

3. It determines the input & output data rate between the codec and the PCM highway.

4.45

Variable Data Rate Mode

• It provides the ability to vary the frequency of transmit and receive bit clocks.

• It allows for a flexible data input and output clock frequency.

• In this mode a master clock frequency of 1.536MHz, 1.544MHz or 2.048MHz is still needed for proper operations of the on board band pass filters and the A/D & D/A convertor.

• It is also known as “Shift register mode”

4.46