Digital Communications Midterm :D

DIGITAL TRANSMISSIONCONCEPTS, DATA REPRESENTATION AND OTHER TERMINOLOGIES

DIGITAL TRANSMSSION

• IS THE TRANSMITTAL OF DIGITAL PULSES BETWEEN TWO OR MORE POINTS IN THE COMMUNICATION SYSTEM.

BINARY TRANSMISSION CONCEPTS AND PRINCIPLES

• BIT CONCEPT- Is a unit of information represented

by either a one or zero.- The bit therefore has one of the two

possible states at any moment in time.

BINARY DATA REPRESENTATION

1. BINARY DATA WAVEFORMSa. UPNRZ

Minimum bandwidth

fn=fb/2

Where: fb= input bit rate

0 1 1 10 0


1. BINARY DATA WAVEFORMSb. BPNRZ

Minimum bandwidth

fn=fb/2


0 1 1 10 0


1. BINARY DATA WAVEFORMSc. UPRZ

Minimum bandwidth

fn=fb


0 1 1 10 0


1. BINARY DATA WAVEFORMSd. BPRZ

Minimum bandwidth

fn=fb


0 1 1 10 0


1. BINARY DATA WAVEFORMSe. BPRZ- AMI

Minimum bandwidth

fn=fb /2


0 1 1 10 0


1. BINARY DATA WAVEFORMSf. Digital Biphase, Manchester or Diphase

Minimum bandwidth

fn=fb


0 1 1 10 0

BINARY TRANSMISSION CONCEPTS AND PRINCIPLES

• BINARY INFORMATION CODESI. Data Link Control Characters – are used to facilitate the orderly flow of data from a source to a destination.II. Graphic Control Characters – involve the syntax or presentation of the data at the receive terminal.III. Alphanumeric characters- are used to represent the various symbol used for letters, numbers and punctuation in the English language.

Information codes or character sets

• Baudot Code- It was developed by French postal

engineer, Thomas Murray in 1875.- is used for low-speed teletype

equipment such as TWX/Telex system


• ASCII Code- It is a 7-bit character set which has

128 codes.


• EBCDIC- It is an 8-bit character code

developed by IBM and used extensively in IBM and IBM-compatible equipment. It has 256 codes making it the most powerful character set.

Binary Transmission Conventions

• Parallel Transmission- In Parallel Transmission, bits are

transmitted character at a time, and each time bit of the set of bits that represent a character has its own wire or line

- An additional wire called Strobe or clock lead notifies the receiver unit that all the bits are present on their respective wires.

Binary Transmission Conventions

• Serial Transmission- In Serial Transmission, bits are

transferred over a single line one bit at a time

Synchronization on Digital Signals

• Synchronous- The digital transitions in the signals

occur at exactly the same rate- There may, however, be a phase

different between the transitions of the two signals.


• Plesiochronous- There transitions occur at “almost”

the same rate. For example if two networks have their clocks derived from two different PRC’s.


• Asynchronous- The transitions of the signals do not

necessarily occur the same nominal rate.

Baseband, Data Rate, and Error Rate

• Baseband- Is defined as the band of frequencies

occupied by the data signal before it modulates a carrier frequency to form the transmitted line or radio signal.

- It is therefore the data signal as it leave or enters a data processing or presentation device.


• Data Rate- It is defined as the transmission rate

in bits per second(bps).

- It is constrained by the capability of the transmitting and receiving devices, the bandwidth of the media and a number of electrical impairments typical of each medium.


• Error Rate- It is a measure of data connectivity’s

performance.

- Often the unit of measurement is bit error rate (BER), which is the probability of bit error.

Digital Transmission Modes

• Simplex- Data transmission is unidirectional;

information can be sent only in one direction


• Half Duplex(HDX)- Data transmission is possible in both

directions but not at the same time


• Full Duplex(FDX)- Data transmission is possible in both

directions at the same time but they must be between the same two stations.


• Full/Full Duplex(F/FDX)- Data transmission is in both

directions at the same time but not between the same two stations

Digital Transmission Operation

• Two-Wire Operation- It involves a transmission medium

that either uses two wires (a signal and a reference lead) or a configuration that is equivalent to having only two wires

- With two-wire operation, simplex, half duplex, or full duplex transmission is possible

Digital Transmission Operation

• Four-Wire Operation- It involves a transmission medium

that uses four wires (two are used for signals that are propagating in opposite directions and two are used for reference leads) or a configuration that is equivalent to having four wires.

Digital Transmission over Voice-Grade Circuits

• Voice-Grade Circuit consists of one or more facilities connected to provide a path for communicating at voice frequency between two or more points.


• Signal Power and Level- Level in voice circuits is used to

express the relative amount of power at various points in a circuit.

- Power in electrical terms is the rate at which electric energy is taken from or supplied to a device.


• Transmission Level Point(TLP)- Transmission Level Point is the

ratio(in db) of the power of a signal at the point to the power of the same signal at the reference point

- CCITT recommends-13dBm0 at 0TLP


• Line Conditioning- Line Conditioning is preparing a line suitable for

transmitting either by adding amplifiers or equalizers to the circuit

- Lines suitable for transmission of high-speed data are provided by conditioning the basic channel

- C Conditioning it is provided on an unconditional channel control attenuation and envelope distortion

- D Conditioning it is provided on an unconditional channel control the S/N ratio and nonlinear distortion

Voice-Channel Transmission Impairments

• Amplitude Distortion- Amplitude distortion is called

frequency response by audio enthusiasts

- It is the distortion caused by the variation of transmission loss with frequency


• Phase Distortion- Phase distortion is the resultant of

difference velocities of propagation at different frequencies across the voice channel.


• Noise - It is a spurious or extraneous signal that interferes

with the wanted signala. Thermal Noise it is noise occurring in electric

conductors and resistors resulting from random movement of free electrons contained in the conducting material

b. Intermodulation Noise it is an extraneous signal energy derived from some portion of the wanted energy when the signal is passed through a nonlinear device

c. Impulse Noise it is defined as any burst of noise which exceeds the rms noise level by a given magnitude


• Crosstalk- It is defined as the undesired energy

appearing in one signal path as a result of coupling from another signal path


• Echo- It is the return of a talker’s voice or

other end-user signal- To be an impairment, the returned

signal must suffer some delay


• Singing- It is the result of sustained

oscillations due to positive feedback in amplifying circuits

- Circuits that sing are unusable and promptly overload certain types of multichannel carrier equipment

PERIODIC SIGNALS

Frequency Domain Concepts

• Signal usually made up of many frequencies• Components are sine (or cosine) waves• Can be shown (Fourier analysis) that anycontinuous signal is made up of componentsine waves

ANALOG TRANSMISSION OF DIGITAL DATA

• Analog Transmission occurs when the signal sent over the transmission media continuously varies from one state to another in a wave-like pattern.e.g. telephone networks, originally built for human speech rather than data.

• Advantage for long distance communications: much less attenuation for analog carrier than digital

ILLUSTRATE ANALOG TRANSMISSON OF DIGITAL DATA

Information Capacity

- Information capacity is a measure of how much information can be propagated through a communication system.

M-ary Encoding

- M-ary is a term derived from the word binary. M simply represents a digit that corresponds to the number of conditions, levels, or combinations possible for a given number of binary variables.

- It is therefore the data signal as it leave or enters a data processing or presentation device.

Baud and Minimum Bandwidth

• Baud- Is a unit of transmission rate, modulation

rate, or symbol rate and, therefore, the terms symbols per second and baud are often used interchangeably.

- Mathematically, baud is the reciprocal of the time of one output signaling element, and a signaling element may represent several information bits.

Baud and Minimum Bandwidth

• Minimum Bandwdith- The minimum theoretical bandwidth

necessary to propagate a signal is called the minimum Nyquist bandwidth.

ERROR DETECTION AND CORRECTION TECHNIQUES

Error Detection

- It is the process of monitoring the received data and determining when a transmission error has occurred.

Error Detection

• Redundancy- It involves transmitting each

character twice.

Error Detection

• Exact-Count Encoding- The number of 1’s in each character

is the same to determine if a transmission error has occurred.

Error Detection

• Parity- Equivalence of equality. It is the simplest

error detection scheme used for data communications systems and is used with both vertical and horizontal redundancy checking

- A single bit (parity bit) is added to each character to force the total number of 1’s in the character, including the parity bit, to be either an odd number or an even number

Error Detection

• VRC- It is an error detection scheme that

uses parity to determine if a transmission error has occurred within a character.

Error Detection

• HRC or LRC- It is an error detection scheme that

uses parity to determine if a transmission error has occurred in a message.

Error Detection

• CRC- It is the most reliable scheme for

error detection. It is generally used with 8-bit codes such as EBCDIC or 7-bit codes when parity is not used

Error Correction

Error Correction

• Symbol Substitution- It was designed to be used in a

human environment.

Error Correction

• Retransmission- When a message is received and the

receive terminal automatically calls for retransmission of the entire message.

Error Correction

- FEC- It is the only error correction scheme

that actually detects and corrects transmission errors at the receive and without calling for retransmission

Protocols

Protocols

• Bit-Oriented Synchronous Protocols Synchronous Data-Link Control (SDLC) by IBM

I. SDLC Transmission States

- Transient State - an SDLC state that exists before and after the initial transmission and after each line turnaround

- Idle State - - an SDLC state that is presumed after 15 or more consecutive 1’s have been received

- Active State - - an SDLC state exists whenever either the primary or a secondary station is transmitting information or control signals

Protocols


I. SDLC Frame Fields- Information Field - contains all the transmitted information in an

SDLC frame. The number of bits in this field must be a multiple of 8.- Flag Field - used with SDLC in the same manner that SYN characters

are used with BiSync, to achieve character synchronization. - Address Field – contains 8 bits for unique addresses of each

secondary station. The primary station has no address because all transmissions from secondary stations go to the primary

- Control Field – is an 8-bit field that identifies the type of frame it is. It is used for polling, confirming previously received information frames, and several other data link management functions

- FCS – contains the error detection mechanism for SDLC. This is equivalent to BCC used with BiSync. SDLC uses CRC-16

Protocols


I. SDLC Frame Formats- Information Frame – the frame used in SDLC that

contains the information field for transmitting sequenced information

- Supervisory Frame– this cannot be used to transfer information but are used to assist in the transfer of information and are also used to confirm previously received information frames, convey ready or busy conditions, and to report frame numbering error

- Unnumbered Frame – are used to send network control and status information

Protocols

• Bit-Oriented Synchronous Protocols High-Level Data-Link Control (HDLC) by ISO

I. Standards Combined to form HDLC- ISO 3 309-1976 – this standard defines the frame structure,

delimiting sequence, and transparency mechanism used with HDLC

- ISO 1335-1979 – this standard defines the elements of procedure for HDLC. The control field, information field, and supervisory format have increased capabilities over SDLC

- ISO 7809-1985 – this standard combines previous standards 6159(E) (unbalanced) and 6265(E) (balanced) and outlines the class of operation necessary to establish the link-level protocol

Protocols

• Bit-Oriented Synchronous ProtocolsHigh-Level Data-Link Control (HDLC) by ISO

I. HDLC Classes of Operation

- Unbalanced Operation – this class of operation is logically equivalent to a multi-point private line circuit with a polling environment.

- Balanced Operation – this class of operation is logically equivalent to a multi-point private line circuit. Each station has equal data link responsibilities, and channel access is through contention using the asynchronous response mode

Protocols

• Bit-Oriented Synchronous ProtocolsHigh-Level Data-Link Control (HDLC) by ISO

I. HDLC Data Transfer Modes- NRM – in this mode a primary station initiates

data transfer to a secondary station. - ARM – in this mode a secondary station my

initiate transmission without receiving a poll from primary station.

- ABM – this is a balanced mode that provides symmetric data transfer capability between combined stations.

Protocols

• Byte-Oriented Synchronous ProtocolsSynchronous Data-Link Control

(SDLC) by IBM- With BiSync, each transmission is

preceded by a unique SYN character: 16H for ASCII and 32H for EBCDIC

SYNCHRONIZATION

Bit Synchronization

- It is achieved through a received clock signal which is coincident with the received serial data stream.

- It is also called clock synchronization

Character Synchronization

- It is accomplished by recognizing one or two phasing characters often called SYN or sync characters.

- The receiver senses the SYN’s and phases its receive logic circuits to recognize the beginning and end of each subsequent character by bit count

Message Synchronization

- To ensure character synchronization throughout a message, SYN sequences are sometimes inserted in the transmitted data stream

- This technique is called message synchronization and permits receiving stations to verify that they are in synchronization

Digital modulation

Understanding different modulation technique

Amplitude shift keying• The amplitude of the carrier is changed

in response to information and all else is kept fixed.

• Bit 1 is transmitted by a carrier of one particular amplitude. To transmit 0, we change the amplitude keeping the frequency constant.

• ON-OFF KEYING – is a special form of ASK, where one of the amplitudes is zero .

Amplitude shift keying

frequency shift keying

• The frequency of the carrier is changed in response to the information signal, one particular frequency for a 1 and another frequency for a 0.

• FSK signal is represented by the eqn.

frequency shift keying

Frequency Shift Keying(FSK)

• Minimum Shift-Keying FSK- MSK is binary FSK except that the

mark and spaces frequencies are synchronized with the input bit rate.

- With MSK, the mark and space frequencies are selected such that they are separated from the center frequency by an odd exact multiple of one-half of the bit rate.

Digital Communications Midterm :D

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