Page 1

Introduction To Data Communication and Networking

Module 4: Digital Transmission


• In this section, we will discuss how we can represent digital data by using digital signals. • The representation involves three techniques: • line coding • block coding • scrambling.


LINE CODING • Process of converting digital data (sequence of bits) to digital signals • At the sender, digital data are encoded into a digital signal; at the receiver, the digital signal are decoded into digital data.

Figure 4.1 Line coding and decoding 4.3


• Data element : The smallest entity that can represent a piece of information (bit). • Signal element : The shortest unit (timewise) of a digital signal to carry data element. • Data elements are what we need to send; signal elements are what we can send



Figure 4.2 Signal element versus data element 4.5

DATA RATE AND SIGNAL RATE • Data rate : The number of data elements (bits) sent in 1s. Unit : bps • Signal rate : The number of signal elements sent in 1s. Unit : baud • Relation between data rate and signal rate (baud rate): S = c x N x 1/r • Example 4.1 A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is 0.5? Solution The baud rate is then



DC Components • Direct-current components • The signal that have zero frequency and the average amplitude is nonzero Self-synchronization • The method to correctly interpret the signals received from the sender



Figure 4.3 Effect of lack of synchronization 4.8


Figure 4.4 Line coding schemes


UNIPOLAR SCHEME Uses only one voltage level • Positive voltage defines bit 1 and the zero voltage defines bit 0

Figure 4.5 Unipolar NRZ scheme


POLAR SCHEME NRZ (Non-Return-to-Zero) • Have two versions of polar NRZ: i. NRZ-L (NRZ-Level) – Bit 1 is represented by negative voltage, bit 0 represented by positive voltage. ii. NRZ-I (NRZ-Invert) – Bit 1 is represented inversion of voltage, bit 0 is represented by no change.



Figure 4.6 Polar NRZ-L and NRZ-I schemes 4.12

POLAR SCHEME RZ (Return-to-Zero) • Uses three values: positive, negative, and zero • In RZ, bit 1 is represented by positive-to-zero voltage, bit 0 is represented by negative-to-zero voltage.

Figure 4.7 Polar RZ scheme 4.13

POLAR SCHEME Biphase : Manchester and Differential Manchester • Best solution for synchronization problems • Manchester (RZ + NRZ-L) – Bit 1 is represented by negative-to-positive transition; bit 0 is represented by positive-tonegative transition.

• Differential Manchester (RZ + NRZ-I) – Bit 1 is represented by no transition; bit 0 is represented by transition. 4.14


Figure 4.8 Polar biphase: Manchester and differential Manchester schemes 4.15

BIPOLAR SCHEME • Alternate Mark Inversion (AMI) – A bit 1 is represented by positive and negative voltage alternately; bit 0 is represented by zero voltage. – Advantages : DC component is zero and provide synchronization for a long strings of 1s.

Figure 4.9 Bipolar schemes: AMI 4.16

Summary of Line Coding Scheme

Table 4.1 Summary of line coding schemes


ANALOG-TO-DIGITAL CONVERSION • Sometimes, we have to sent analog data (signal) using digital signal. • In this section, we will discuss how we can represent analog signal by using digital signals. • In this section we describe two techniques, • Pulse Code Modulation • Delta Modulation.


PULSE CODE MODULATION (PCM) • Technique to change an analog signal to digital data (digitization) • PCM encoder has three process: 1. The analog signal is sampled (sampling) 2. The sampled signal is quantized (quantizing) 3. The quantized values are encoded as streams of bit (encoding)

Figure 4.21 Components of PCM encoder


SAMPLING • Sampling is the process of measuring the nonintegral amplitude of analog signal at equal intervals. • The analog signal is sampled every Ts s, where Ts is the sampling interval or period • The inverse of the sampling interval is called the sampling rate. • There are three sampling methods : ideal, natural, and flat-top • The sampling process is sometimes referred to as Pulse Amplitude Modulation (PAM). 4.20


Figure 4.22 Three different sampling methods for PCM 4.21

SAMPLING Sampling Rate • Based on Nyquist theorem ; 1. We can sample signal only if the signal is band-limited -> signal with an infinite bandwidth cannot be sampled 2. Sampling rate must be at least 2 times the highest frequency contained in the signal. Example What sampling rate needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)?

• Solution Sampling rate = 2 (11,000) = 22,000 samples/S


QUANTIZATION • The result of sampling is a series of pulses with amplitude values between the maximum and minimum amplitudes of the signal. • Set of amplitudes can be infinite with non-integral values and these values cannot be used in the encoding process. • Quantization is the method of assigning integral values in a specific range to sampled instances. • In quantization, we approximate the value of the sample amplitude to the quantized values.



Figure 4.26 Quantization and encoding of a sampled signal 4.24

ENCODING • After each sample is quantized and the number of bits per sample is decided, each sample can be changed to an nb-bit code word • A quantization code of 2 is encoded as 010; 5 is encoded as 101; etc Bit rate = sampling rate x no. of bits per sample = fs x nb • Example 4.14 We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:


TRANSMISSION MODES • The transmission of binary data across a link can be accomplished in either parallel or serial mode. • In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick.



Figure 4.31 Data transmission and modes


PARALLEL TRANSMISSION • Send data n bits at a time using n channels • Conceptually: use n wires to send n bits at one time

Figure 4.32 Parallel transmission 4.28

SERIAL TRANSMISSION • Send data one bit follows another using one channel • Serial occurs in one of three ways : asynchronous, synchronous, and isochronous

Figure 4.33 Serial transmission 4.29

ASYNCHRONOUS TRANSMISSION • The timing of signal is unimportant • A byte is sent with one start bit (0) at the beginning and one or more stop bits (1) at the end of each byte.

Figure 4.34 Asynchronous transmission 4.30

SYNCHRONOUS TRANSMISSION • The bit stream is combined into longer frames, which may contains multiple bytes • Bytes are sent one after another without start and stop bits or gap

Figure 4.35 Synchronous transmission Asynchronous vs. Synchronous 4.31

Profile for datacomm network

Module 4 Digital Transmission  

Module 4 Digital Transmission