EE302 Lesson 20: Transmission of Binary Data in Communication Systems

EE302 Lesson 20:Transmission of Binary Data in Communication Systems

Topics Covered in Chapter 11 11-1: Digital Codes 11-2: Principles of Digital Transmission 11-3: Transmission Efficiency 11-4: Basic Modem Concepts 11-5: Wideband Modulation 11-7: Error Detection and Correction

11-1: Digital Codes

The proliferation of applications that send digital data over communication channels has resulted in the need for efficient methods of transmission, conversion, and reception of digital data.

Digital codes have evolved as technology has advanced.

11-1: Digital CodesEarly Digital Codes

The first digital code was developed by Samual Morse.

The Morse code was originally designed for wired telegraph, but was later adapted for radio communication.

The Morse code consists of a series of “dots” and “dashes” that represent letters of the alphabet, numbers, and punctuation marks.

Figure 11-1 The Morse Code

11-1: Digital CodesBaudot Code The Baudot (baw dough) code

was one of the first alphanumeric codes developed in the early days of teletype machines.

The Baudot code is a 5-bit code giving it 25 or 32 possible values (it actually had 52 symbols using a control character).

It is obsolete and of historical interest only.

11-1: Digital CodesBaudot Code

11-1: Digital CodesASCII Binary representation of alphanumeric symbols

(letters, numbers, punctuation, etc.) are given by American Standard Code of Information Interchange (ASCII) code.

Each ASCII codeword is 7-bits long yielding 27 or 128 possible characters.

ASCII has remained the international standard in data communications.

ASCII Figure 11-3 The ASCII Code

11-1: Digital Codes

Modern Binary Codes: Extended Binary Coded Decimal Interchange Code The Extended Binary Coded Decimal Interchange

Code (EBCDIC) was developed by IBM. The EBDIC is an 8-bit code allowing a maximum of

256 characters to be represented. The EBCDIC is used primarily in IBM and IBM-

compatible computing systems and is not widely used as ASCII.

11-2: Principles of Digital Transmission

Serial Transmission As discussed earlier, data can be transmitted in two

ways:1. Parallel: all bits transmitted simultaneously2. Serial: all bits transmitted one after another

Data transfers in long-distance communication systems are made serially. Parallel data transmission is not practical.

The LSB is transmitted first and the MSB is transmitted last.

Each bit is transmitted for a fixed interval of time, t.


Figure 11-4: Serial transmission of the ASCII letter M.


Serial Transmission: Expressing the Serial Data Rate The speed of data transfer is usually indicated as number of bits

per second (bps or b/s). The speed in bps is the reciprocal of the bit time, t.

bps = 1/t. Example: if bit time is 104.17 µs, bps=1/104.17µs = 9600 bps Another term used to express the data speed in digital

communication systems is baud rate. Baud rate is the number of signaling elements or symbols that

occur in a given unit of time. A signaling element is simply some change in the binary signal

transmitted. In many cases it is a binary logic voltage level change, either a 1 or a 0.


Serial Transmission: Expressing the Serial Data Rate With the new modulation schemes (discussed later),

multiple bits can be transmitted with one symbol.

Now, Bit rate = baud rate x bits per symbol

or

Bit rate = baud rate x log2S,

where S = number of states per symbol. These modulation schemes were developed to

improve transmission rates over bandwidth-limited communication channels, such as the telephone lines.


Asynchronous Transmission In asynchronous transmission each data word is

accompanied by start and stop bits that indicate the beginning and ending of the word.

When no information is being transmitted, the communication line is usually high, or binary 1.

In data communication terminology, this high level is referred to as a mark.

To signal the beginning of a word, a start bit, a binary 0 or space is transmitted.

The change from ‘mark’ to ‘space’ indicates the beginning of a word.


Figure 11-6: Asynchronous transmission with start and stop bits.


Asynchronous Transmission Asynchronous transmissions are extremely reliable. Most low-speed digital transmission (the 1200- to

56,000-bps range) is asynchronous. The primary disadvantage of asynchronous

communication is that the extra start and stop bits effectively slow down data transmission.

The extra start and stop bits are called ‘overhead’ and reduce efficiency


Synchronous Transmission The technique of transmitting each data word one

after another without start and stop bits, usually in multiword blocks, is referred to as synchronous data transmission.

To maintain synchronization between transmitter and receiver, a group of synchronization bits is placed at the beginning and at the end of the block.

Each block of data can represent hundreds or even thousands of 1-byte characters.


Synchronous Transmission The special synchronization codes at the beginning and end of a

block represent a very small percentage of the total number of bits being transmitted, especially in relation to the number of start and stop bits used in asynchronous transmission.

Synchronous transmission is therefore much faster than asynchronous transmission because of the lower overhead.

An error detection code usually appears at the end of the transmission (discussed later).

Synchronous transmission uses a precise clock to track the individual bits.


Figure 11-8: Synchronous data transmission.


Encoding Methods Whether digital signals are being transmitted by

baseband methods or broadband methods, before the data is put on the medium, it is usually encoded in some way to make it compatible with the medium.


Encoding Methods In the nonreturn to zero (NRZ) method of encoding, the signal

remains at the binary level assigned to it for the entire bit time. Normally used at slow speeds, when asynchronous transmission is

being used. Since there is no voltage change when there are long strings of 1’s

and 0’s transmitted, it is difficult for the receiver to determine where one bit begins and ends.

In return to zero (RZ) encoding the voltage level assigned to a binary 1 level returns to zero during the bit period.

Because there is clearly one discernible pulse per bit, it is extremely easy to derive the clock from the transmitted data.


Encoding Methods Manchester encoding, also referred to as biphase

encoding, is widely used in LANs. In this system a binary 1 is transmitted first as a positive

pulse, for one half of the bit interval, and then as a negative pulse for the remaining part of the bit interval.

A binary 0 is transmitted first as a negative pulse, for one half of the bit interval, and then as a positive pulse for the remaining part of the bit interval.

The choice of an encoding method depends on the application


Figure 11-9Serial binary

coding methods

Unipolar NRZ

Bipolar NRZ

Bipolar RZ

Unipolar RZ

Bipolar RZ-AMI

Manchester

11-3: Transmission Efficiency Transmission efficiency is the accuracy and

speed with which information, whether it is voice or video, analog or digital, is sent and received over communication media.

It is the basic subject matter of the field of information theory.

11-3: Transmission EfficiencyTransmission Media and Bandwidth

The two most common types of media used in data communication are wire cable and radio.

The two types of wire cable used: Coaxial cable: usable bandwidth 200 MHz-3 GHz depending

on the size. Bandwidth decreases with length. Twisted-pair cable: usable bandwidth 2 KHz-100 MHz.

Coaxial cable has a center conductor surrounded by an insulator over which is a braided shield. The entire cable is covered with a plastic insulation.

A twisted-pair cable is two insulated wires twisted together.

11-3: Transmission Efficiency

Figure 11-10 Types of cable used for digital data transmission

Coaxial Cable

Twisted Pair

11-3: Transmission Efficiency The radio channel bandwidth must be wide enough to

pass all harmonics and preserve the waveshape. If the higher harmonics are filtered out, the signal will be

distorted.

Hartley’s Law The amount of information that can be sent in a given

transmission is dependent on the bandwidth of the communication channel and the duration of transmission.

Mathematically, Hartley’s law isC = 2B

Where C is the channel capacity (bps) and B is the channel bandwidth (Hz). Assuming there is no noise in the system.


Hartley’s Law The greater the number of bits transmitted in a given

time, the greater the amount of information that is conveyed.

The higher the bit rate, the wider the bandwidth needed to pass the signal with minimum distortion.

Example: The maximum theoretical bit capacity for a 10 kHz bandwidth channel is

C = 2B = 2(10,000 Hz) = 20,000 bps

11-3: Transmission Efficiency The encoding method used also effects the required

bandwidth for a given signal. The bandwidth requirement for an RZ scheme is twice that

for an NRZ scheme.

Multiple Coding Levels Channel capacity can be increased by using multiple-

level encoding schemes that permit more bits per symbol to be transmitted (Section 11-4).


Impact of Noise in the Channel Increasing bandwidth increases the rate of

transmission but also allows more noise to pass. Shannon-Hartley Theorem determines channel

capacity in the presence of noise.

Shannon-Hartley Theorem

C = B log2(1 + S/N)

C = Channel capacity, bps

B = bandwidth, Hz

S/N = signal-to-noise ratio (power)


Example Problem 1

Find the channel capacity for a voice grade telephone line with a bandwidth of 3100 Hz and a S/N ratio of 30 dB (dB = 10 log P)?

11-3: Transmission Efficiency This answer conflicts with Hartley’s Law

C = 2B = 2(3100 Hz) = 6200 bps Shannon-Hartley Theorem determines what is

theoretically possible. But, multilevel coding is required to achieve these higher rates.

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EE302 Lesson 20: Transmission of Binary Data in Communication Systems