CH02 Basic Concepts of Data Communications

8/4/2019 CH02 Basic Concepts of Data Communications

1/75

TJ 2013 Data Communications

Basic Concepts of

Data CommunicationsChapter 2


2/75


Objectives

At the end of this chapter, studentsshould: Be able to differentiate between host

and terminal Be able to differentiate between bit and

byte

Be able to explain various types of

character code Be able to explain the different types of

transmission modes.


3/75


Contents

Host vs. Terminal

Bits vs. Byte

Character Code Serial vs. Parallel Transmission

Asynchronous vs. Synchronous

Transmission Simplex, Half-Duplex & Full-Duplex

Communications


4/75


Host vs. Terminal


5/75


Host

A computer on a network thatprovides services to other computerson the network

Accessed by a user working at aremote location.

System that contains data is called

the host, while the computer atwhich the user sits is called theterminal.


6/75


Host

Types of host:

(revision of OSK topic)

Super Computer Mainframe

Mini Computer

Micro Computer


7/75


Terminal

Some definitions of terminal:

A device that allows users to sendcommands to a computer somewhere

else.

A monitor and keyboard attached to acomputer (usually a mainframe), used

for data entry and display. Unlike a personal computer, a terminal

does not have its own CPU or hard disk


8/75


Terminal

Some definitions of terminal:

A computer Workstation linked to aServer or other computer over a

network on which a user may displayinformation

Machine that allows you to send

commands to a remote computer A device that works as a client of a

central computer or host in a network


9/75


Terminal

Types of terminal

Dumb

Smart

Intelligent


10/75


Terminal

Dumb Terminal A computer terminal with no processing

or programming capabilities, generally

used for simple data entry or retrievaltasks.

Consist of a keyboard and displaymonitor

Keyboard is used to sent data to the CPU Display monitor acts as output device

that accepts data from the CPU.


11/75


Terminal

Smart Terminal

A terminal that can do some processing,usually to edit data it receives

Consist of keyboard, display monitorand memory

Has the capability of sending additional

information to host such as terminaladdress, error control etc.


12/75


Terminal

Intelligent

A terminal that has both memory anddata processing capabilities

Contains not only a keyboard andscreen, but also has built-in processingcapabilities and storage devices

Programmable terminal to perform newtasks such as, write data to the storagedevices.


13/75


Host vs. Terminal


14/75


Bits vs. Bytes


15/75


Bits

A bitis the smallest unit of informationthat can be stored or manipulated on acomputer

It consists of either zero or one. Depending on meaning, implication, or

even style it could instead be described asfalse/true, off/on, no/yes, and so on.

We can also call a bita binarydigit,especially when working with the 0 or 1values


16/75


Bytes

A byte also happens to be how manybits are needed to represent lettersof the alphabet and other characters.

For example, the letter "A" would be0100 0001


17/75


Bytes

Revision

Hexadecimal

0100 = ?

1000 = ?

1010 = ?


18/75


Bit vs. Byte

The Basic Data Transfer

kilo (k)* = 10 ^ 3 = 1,000 (thousand)

mega (M) = 10 ^ 6 = 1,000,000 (million)

giga (G) = 10 ^ 9 = 1,000,000,000 (billion)

tera (T) = 10 ^ 12 = 1,000,000,000,000 (trillion)

* Note:k = kilobitK = Kilobyte


19/75


Bit vs. Byte

Data Storage / Memory Math

1 byte (B) = 8 bits (b)

1 Kilobyte (K / KB) = 2^10 bytes = 1,024 bytes

1 Megabyte (M / MB) = 2^20 bytes = 1,048,576 bytes

1 Gigabyte (G / GB) = 2^30 bytes = 1,073,741,824 bytes

1 Terabyte (T / TB) = 2^40 bytes = 1,099,511,627,776 bytes


20/75


Character Code


21/75


Data Representation

Information comes in different forms Text, numbers, images ,audio, video, etc.

With few exceptions, digital computerscommunicate through a series of 1s and 0sknown as bits.

This binary representation can also bethought of as being on and off.


22/75


Data Representation

Groups of bits are referred to as bytes In most systems, a byte consists of 8 bits

Usually each byte represents a single character

A-Z, a-z, 0-9 punctuation characters (e.g., @, #, %)

special characters (LF, CR, ESC)

Bits and bytes are closely related to

the binary number system.


23/75


Character Codes The relationship of bytes to characters is

determined by a character code

Each time a user presses a key on aterminal/PC, a binary code is generated

for the corresponding character.


24/75


Character Codes

Various character codes have been used indata communication including:

Morse, Baudot

EBCDIC, ASCII Unicode

Regardless of the character code, both theterminal/ host or sender/receiver must

recognize the same coding scheme


25/75


Morse Code

First character code developed

For transmitting data over telegraphwires

telegrams

Used dots (short beep) and dashes(long beeps) instead of 1s and 0s

More frequent the character, thefewer the beeps


26/75



27/75


Morse Code

Problems:

variable length characterrepresentation

required pauses between letters

no lower case, few punctuation orspecial characters

no error detection mechanism


28/75


Baudot Code

One of first codes developed for machineto machine communication

Uses 1s and 0s instead of dots and

dashes For transmitting telex messages (punch

tape)


29/75


Baudot Code

Fixed character length (5-bits) 32 different codes

increased capacity by using two codes

for shifting11111 (32) Shift to Lower (letters)

11011 (27) Shift to Upper (digits,punctuation)

4 special codes for SP, CR, LF & blank Total = 26 + 26 + 4 = 56 different

characters


30/75



31/75


Badout Code

Problems: required shift code to switch between

character sets

no lower case, few special characters no error detection mechanism

characters not ordered by binary value

designed for transmitting data, not fordata processing


32/75


Badout Code

International Baudot

Added a 6th bit for parity

Used to detect errors within a single

character


33/75


EBCDIC

Extended Binary Coded DecimalInterchange Code

8-bit character code developed by IBM

used for data communication, processing andstorage

extended earlier proprietary 6-bit BCD code

designed for backward compatibility ormarketing?

still in use today on some mainframes andlegacy systems.


34/75


EBCDIC

Allows for 256 different characterrepresentations (28)

includes upper and lower case

lots of special characters (non-printable)

lots of blank (non-used codes)

assigned to international characters in

various versions used with/without parity (block

transmissions)


35/75



36/75


ASCII Code American Standard Code for Information

Interchange 7-bit code developed by the American

National Standards Institute (ANSI) most popular data communication character

code today Allows for 128 different character

representations (27) includes upper and lower case

lots of special characters (non-printable) generally used with an added parity bit better binary ordering of characters than

EBCDIC


37/75


ASCII Code

Extended ASCII uses 8 data bitsand no parity

Used for processing and storage of data

Allows for international characters

8th bit stripped of for transmission ofstandard character set


38/75



39/75



40/75


UNICODE Designed to support international

languages:Latin; Greek; Cyrillic; Armenian; Hebrew; Arabic;Syriac; Thaana; Devanagari; Bengali; Gurmukhi;Oriya; Tamil; Telegu; Kannada; Malayalam;Sinhala; Thai; Lao; Tibetan; Myanmar; Georgian;

Hangul; Ethiopic; Cherokee; Canadian-AboriginalSyllabics; Ogham; Runic; Khmer; Mongolian; Han(Japanese, Chinese, Korean ideographs);Hiragana; Katakana; Bopomofo and Yi


41/75


UNICODE

Uses a 16-bit code for total of 65,536possible char.

Incorporates ASCII in first 128 codes

Incorporates LATIN in first 256 codes

Support found in newer hardware &software, especially web technologies

(e.g., JAVA, XML, HTML)

For more see www.unicode.org


42/75


Summary of Character Codes

Morse = .-

Baudot = 5 bit (no parity)

Int. Baudot = 6 bit (5 data + 1 parity)

ASCII = 8 bit (7 data + 1 parity)

or

= 8 bit (no parity)

EBCDIC = 9 bit (8 data + 1 parity)

or

= 8 bit (no parity)

UNICODE = 16 bits (no parity)


43/75



Normally terminals and hosts mustuse the same code

However, code conversionhardware/software can be used toallow different machines tocommunicate

Bits per character affect storage requirements

throughput of information


44/75



Use of larger codes became feasibledue to

higher transmission speeds

denser storage mediums

Choice of character coding schemeis a trade off between

simplicity & brevity

expressivity


45/75


Serial vs. Parallel

Transmission


46/75


Data Transmissions


47/75


Transmission Mode


48/75


Parallel

Multiple bits travel down individualwires in a parallel mode

Faster than serial

Shorter distances than possible withserial


49/75


Parallel Transmission


50/75


Serial

Bits travel along a single wire, one ata time

Slower than parallel

Longer distances possible than withparallel

Examples

USB (universal serial bus) - high-speed,multipoint serial connection standard

IEEE-1394 (Firewire) - higher speed (thanUSB), multipoint serial connection standard


51/75


Serial

Serial transmission can be dividedinto two modes:

Synchronous

Asynchronous


52/75


Serial Transmission


53/75


Asynchronous

Timing of devices independentlyestablished

Start and stop bits are used to

establish timing for each charactertransmitted

Character-at-a-time transmission

Overhead includes start bit and oneor more stop bits per charactertransmitted


54/75


In asynchronous transmission, we

send 1 start bit (0) at the beginningand 1 or more stop bits (1s) at the end

of each byte. There may be a gap

between each byte.

Note:


55/75



56/75


Asynchronous here means

asynchronous at the byte level, butthe bits are still synchronized; their

durations are the same.

Note:


57/75


Synchronous

Timing is established by theexchange of a clocking signalsupplied by a device or embedded in

the carrier Block-at-a-time transmission

Synchronization characters precedeand follow the data block

Overhead includes bits insynchronization characters


58/75


In synchronous transmission,

we send bits one after another withoutstart/stop bits or gaps.

It is the responsibility of the receiver to

group the bits.

Note:


59/75



60/75


Transmissions Efficiency


efficiency=data transmitted X 100

total bits sent


61/75



Example

Compare a 10K Byte data transmission

usingi. Asynchronous (1 start & 1 stop bit)

ii. Synchronous (10 bytes for wholetransmission)

Determine the efficiency (10 kBytes = 80kbits).


62/75



Solution

Asynchronous:Add 2 bits (1 Start and 1 Stop bits) for every byte transmitted.

80 kbits + 20 kbits = total of 100 kbits transmitted

efficiency= data transmitted X 100total bits sent

= 80 X 100100

= 80 %


63/75



SolutionSynchronous

Add 10 bytes (80 bits) for the complete 10K bytedata packet.

80 kbits + 80 bits = total of transmitted

efficiency = data transmitted x 100

= 80 000 X 10080 080

= 99.9%


64/75



65/75


Simplex, Half-Duplex&Full-Duplex Transmission

Directions


66/75


Transmission Characteristics

A character code determines what bits we willsend between a terminal and host

But how will those bits be sent:

Direction of Transmission Path

Parallel vs. Serial Transmission

Serial Transmission Timing

Line Topology

Others which well look at later speed

organization of data (protocol)

transmission media


67/75


Flow of Transmission Path

Simplex

Half duplex

Full duplex


68/75


Simplex

Data in a simplex channel is alwaysone way.

Simplex channels are not often used

because it is not possible to sendback error or control signals to thetransmit end.

It's like a one way street.

An example of simplex is Television,or Radio


69/75


Simplex


70/75


Half-Duplex

A half-duplex channel can send andreceive, but not at the same time.

It's like a one-lane bridge where two waytraffic must give way in order to cross.Only one end transmits at a time, theother end receives.

In addition, it is possible to perform errordetection and request the sender toretransmit information that arrivedcorrupted.


71/75


Half-Duplex

In some aspects, you can think ofInternet surfing as being half-duplex,as a user issues a request for a web

document, then that document isdownloaded and displayed before theuser issues another request.

Another example of half-duplex istalk-back radio


72/75


Half Duplex


73/75


74/75


Full-Duplex

An example can be a consumerwhich uses a cable connection to notonly receive TV channels, but also

the same cable to support theirphone and Internet surfing. All theseactivities can occur simultaneously.


75/75

Full Duplex

Documents

CH02 Basic Concepts of Data Communications