Data Comm Net

8/13/2019 Data Comm Net

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1

DATA

COMMUNICATIONS

Departemen Teknik Fisika

Institut Teknologi Bandung2001

DCS Training Series



2

Data Communications Model

Transmitter : encode and modulate the data

Receiver : demodulate and decode data

Encoder/Decoder : transform analog/digital data into digital code

Modulation/Demodulation : convert digital signal into analog

signal ready for transmission

Transmission Medium : allows signal to propagate

Signal transmission is in form of electromagnetic or optical waves

propagation

TRANSMITTER medium

data

RECEIVERAmplifier/

Repeater medium

signal datasignal

Encoder Modulator analog/

digital

analog

signal

Decoder Demodulator

analog/

digitalanalog

signal

Asource

Bdestination



3

Analog & Digital Data Signaling

1 2 3

4 5 6

7 8 9

* 8 #

R S C S T R R D T D C DTALK / DATA

TALK

Analog

Digital

Analog

Digital

telephone

Digital

modem

codec

digital

transmitter

Analog

Digital

Analog

DATA SIGNAL



4

Information/Data Representation

Analog Standard Signal

4-20 mA

1 – 5 V

Represents 0-100% full-scale for range specified

Digital Signal

Byte or word oriented (bit resolution dependent)

Grouped bit oriented (status signal) Pulse sequence

Many standards : RS 232 , RS 485 , SP50 Field

Bus



5

Digital Data Encoding

Non-return-to-Zero-Level (NRZ)

Nonreturn to Zero

Level Inverted (NRZI)

Bipolar

Pseudoternary

Manchester

Differential

Manchester

……..

Encoder Decoder

digital

data

digital

signaldigital

data



6

Digital Signal Modulation

Involve operation on

amplitude, frequency,

and phase

Modulation techniques

on digital signal :

Amplitude Shift Keying

(ASK)

Frequency Shift

Keying (FSK)

Phase Shift Keying

(PSK)

Modulator Demodulator

digitalsignal

analogsignal

digitalsignal

f c



7

Channel Capacity

Baud rate (= line modulation

rate):

the number of decision per

seconds

Bits per seconds (bps)(= data rate):

transmission speed of

digital signal

Bandwidth (Hz) :

the allowed maximum band

of signal frequency to pass

through the communication

medium with attenuation

less than 3 dB



8

Transmission Media

Guided

Twisted pair

(shielded/unshielded)

Coaxial cable

Fiber Optic

Unguided/Wireless

Broadcast Radio

Terrestrial Microwave

Satellite Microwave

Design Factor Consideration

Bandwidth

Transmission Impairments: Attenuation, Delay distortion, Noise

Interference Number of Receivers

data rate

distance



9

Twisted Pair

two conductors separately

insulated and twisted overeach other

often “bundled” into

cables

applications : communications within

buildings : instrument

wiring, local

telephone system, LAN

The least expensive and

most widely used

limited in data rate and

distance

Types:

Unshielded (UTP) , 100

(RJ45)

Shielded (STP) , 150 :

one or more twisted pair

enclosed by metallicsheath (shield)

lower noise level than

UTP

higher capacitance to

ground



10

Coaxial Cable

Less susceptible to

interference and crosstalk

than twisted pair

Characteristic impedance:

50 (RG8), 75 (RG9),

91 (RG62)

Support longer distance and

more drop line than twisted

pair

Applications :

cable TV, long distance

telephone, LAN

two concentric

conductors separated by

an uniform insulator

outer conductor isbraided shield

inner conductor is solid

metal

covered by padding

outer protectivesheath

insulator

outer conductor

dielectric(insulator)

center conductor



11

Optic Fiber

Based on the principles ofrefraction and reflection onthe boundary of twomaterials with different indexof refraction

Immune to electricalenvironment interference

Low attenuation

Large data rate and distance

Small size and lighter weight

Expensive installation andmaintenance

Applications :

LAN backbone, long-haultrunks, rural-exchangetrunks

consists of three concentric

sections : core, cladding, and

jacket

core : made of glass or plastic

cladding : glass or plastic

coating with higher index of

refraction than core

jacket : surrounding one or a

bundle of cladded fibers



12

Fiber Optic Size

Single-mode : good transmission throughput,

requires precise mechanical alignment

Multi-mode : cheaper ; e.g. plastic fiber

Bandwidth

Fiber type Core Cladding (MHz/km)

( m) ( m) 850 nm 1300 nm 1500 nm (max.)

Single Mode 5.0 85 or 125 2.3 5000 @ 850 nm

8.1 125 0.5 0.25

Multi Mode 50 125 2.4 0.6 0.25 600 @ 850 nm

(graded-index) 1500 @ 1300 nm

62.5 125 3.0 0.7 0.3 200 @ 850 nm

1000 @ 1300 nm

100 140 3.5 1.5 0.9 300 @ 850 nm

500 @ 1300 nm

Attenuationdiameter

(dB/km) (max.)



13

Summary of Guided Media (for long distance application)

TransmissionMedium

Total DataRate

BandwidthRepeater Spacing

Twisted Pair 4 Mbps 3 MHz 2–10 km

Coaxial Cable 500 Mbps 350 MHz 1–10 km

Optic Fiber 2 Gbps 2 GHz 10–100 km



14

Broadcast Radio

30 - 1000 MHz (FM, VHF, and UHF)

Omni-directional (broadcast)

Line-of-sight transmission

Multipath interference caused by reflectionfrom land, water, and other objects

Less sensitive to attenuation from rainfall

Require frequency allocation license Applications :

commercial radio, television, data-networking

applications



15

Terrestrial Microwave

Microwave relay towers is

used to achieve long-

distance transmission (every

10-100 km)

Noticeable attenuation with

rainfall

Applications :

long-haul trunks, short data

link between LANs

2 - 40 GHz frequency range

Directional

Line-of-sight transmission

Parabolic”dish” antennafixed rigidly and focuses a

narrow beam

Transmitter Receiv er Relay

Tower

F1 F2



16

Satellite Microwave

1 - 10 GHz frequency range

Acts as a microwave relay station

Linking two or more ground-basedmicrowave transmitter/receivers(earth/ground station)

Consists of several frequencybands called transponderchannels

Noticeable propagation delay dueto long distance transmission

Applications : long-distance telephone

transmission,

private business networks

television distribution

Transmitter

Satellite

Receiver

Transmitter

Receiver C

Receiver B

Receiver A

Satellite

(a) Point-to-point

(a) Multi-point



17

Summary of Wireless Media

Modulation Data rate

30-300 kHz LF (low fr.) ASK, FSK, MSK 0.1 - 100 bps Navigation

300-3000 kHz MF (medium fr.) ASK, FSK, MSK 10 - 1000 bps Commercial AM Radio

3-30 MHz HF (high fr.) ASK, FSK, MSK 10 - 3000 bps Shortwave radio

VB radio

30-300 MHz VHF (very high fr.) FSK, PSK To 100 kbps VHF television

FM radio

300-3000 MHz UHF (ultra high fr.) PSK To 10 Mbps UHF television

Terrestrial microwave

3-30 GHz SHF (super high fr.) PSK To 100 Mbps Terrestrial microwave

Satellite microwave

Frequency band Name Principal ApplicationsDigital data



18

Communication of 2 Computers(point- to-point communication)

Simplex

unidirectional

Half Duplex bi-directional

not simultaneously

Duplex

bi-directional

simulataneously

Simplex

Half Duplex

Full Duplex

A B



19

Methods of Transmission

Serial

1 data line

low speed data

transfer long distance (longer

distance if repeater is

used)

Paralel

n data line

high speed data transfer

short distance (less than1 m)

A B

Serial

0 1 0 1 0

A B

Parallel

0

1

10

1



20

Asynchronous Transmission

Asynchronous

data are transmitted in

frame of one character at a

time (5-8 bits long)

timing/synchronization mustonly be maintained within

each character

simple and cheap

requires overhead for each

character

data

signal groun d

transmit

clock

receive

clock

t r a

n s m

i t t e r

r

e c e i v e r



21

Synchronous Transmission

Synchronous

data are transmitted as a long

uninterrupted streams of bits

require clock synchronization

between two stations by :

separated clock line, or embed the clocking

information in the data

signals

more efficient for large data

size

FLAG CONTROL DATA CRC FLAG

01111110 1101000110011...0011010001 01111110

data

signal ground

m

a s t e r

s

l a v e

clock



22

Multi-points Communications

(a) Bus

(b) Tree(c) Ring

(d) Star(a)

(c)

(b) (d)

LAN Topology: - physical connection

- logical connection



23

Communication Protocol

Data exchange within two or more station isfollowing a pre-defined set of rules called

communication protocol

Key elements of protocol : Syn tax , includes such things as data format,

encoding and signal levels

Semant ics , includes control information for

coordination and error handling Tim ing , includes speed matching and

sequencing



24

Mail Analogy

SPEC

Mailbox

P.O

P.O

User

Standardized

Purchase

Order

PO in

envelopeMail

Room

Postman

Local Post Office

Bulk Mail

Center

Local Post Office

Postman

Mailbox

PO out of

envelope

Mail Room

Standardized

Purchase

Order

Airplane

User



25

Mail Analogy

L7 - Application: The

contents of a letter

inside of an envelope.

L6 - Presentation:

Format and language

style of the letter.

L5 - Session: name,

address, zip code of

both receiver and thesender

L4 - Transport : certified or

registered mail, verification

that the letter arrived at the

correct destination.

L3 - Network : from one

postal system to a system

in another city or country.

L2 - Data Link : within the

same postal system.

L1 - Physical : postman,

truck, ....



26

ISO/OSI Reference Model

Standardize end-to-end communication

Defines only a set of functional layers (7 layers)

Does not specify any standard equipment,

connectors, medium, etc Defining the layer boundaries on some systems

can be quite difficult

Allows interchangeability between vendors

Two OSI compliants is not always able to

communicate each other



27

ISO/OSI Layers (1)

H a r d

w a r e

S o f t w a r e

F i r m w a r e O

peratingSystem

Userspace

Application

Transport

Internet

LLC

MAC

Physical

ISO / OSI

TCP/IP



28

ISO/OSI Layers (2)

Physical provides the actual means of

connection to the media (copper,

fiber, or wireless)

examples : EIA-232/485, Network

Interface Card (NIC)

Data Link provides packetizing of data and “error

free” transmission to the network layer

examples : HDLC, CSMA/CD Network

Network

perform end-user to end-user messagerouting

examples :CCITT X.25, IP and IPX fromTCP/IP suite

Transport responsible for reliable end-user to end-

user communication

example : TCP or UDP from TCP/IPsuite

Session concerned with job at hand

and the scheduling of jobsfrom application layer

interface between applicationand data communication

Presentation ensuring that the

communications data is typed

correctly for the applications

layer

example : translation between

ASCII and EBCDIC code

Application provides user interface to the

system

examples : File transfer

(FTP), electronic mail,

hypertext transfer (HTTP)



29

Physical Layer Interface

Provide actual means of connection to communication medium(copper, optic fiber, wireless)

establish the data signals conversion into suitable form, level,

and power for longer distance transmission

provide line termination/impedance matching & synchronization

RS-232 (EIA-232) standard, defines: mechanical specification (ISO 2110)

electrical specification (V.28)

functional specification (V.24)

procedural specification (V.24)



30

RS232 Mechanical & Electrical

MechanicalSpecification

DB9

connect

or

DB25

connect

or

Electrical

Specification

'0' logic

not valid

'1' logic

'0' logic

'1' logic

TRANSMITTER RECEIVER

+25 Vdc

+3 Vdc

-3 Vdc

not valid

-25 Vdc

+15 Vdc

+5 Vdc

-15 Vdc

-5 Vdc



31

RS232 Functional & Procedural

Type Pin# In/Out Name Function

Data 3 Out -Tx or -TxD Transmitted data

2 In -Rx or -RxD Received data

Control 7 Out RTS Request to send

8 In CTS Clear to send

4 Out DTR Data terminal ready

6 In DSR Data set ready

1 In CD Carrier detect

9 In RI Ring indicator

Electrical 5 - SG Signal ground

DTE(computer)

TxD

RxD

RTS

CTS

DSR

DTR

CD

RI

SG

(3)

(2)

(7)

(8)

(6)

(4)

(1)

(9)

(5)

(3)

(2)

(7)

(8)

(6)

(4)

(1)

(9)

(5)

DCE

(modem)

TxD

RxD

RTS

CTS

DSR

DTR

CD

RI

SG

logik 1

logik 0RTS

CTS

TxD d a t a v a l i d

Functional Specification

Procedural Specification

(handshaking example)



32

RS485 / EIA 485

Define only the electrical standard balance transmission

every data signal requires a pair of twisted cable

signal level: 200 mV to 6 V

distance up to 1 km

TI

TE

Tx

Rx

RTS

RE

RO

TO

TO

RI

RI

RS485 transceiver RS232 signals

up to

32 transceiver



33

Data Link Layer

Functions:

Frame synchronization: to recognize the beginning and

end of each transmitted frame

Flow control: to accomplish communication betweenfaster and slower stations

Error detection and control: to avoid error

Addressing: to specify and identify the communicated

stations

Link management: to establish initiation,

maintenance, and termination

Command/control and data on same link



34

Error Detection

LRC/VRC (Longitudinal/Vertical Redundancy Check)

Odd parity: the number of ”1” + parity-bit = odd

CRC (Cyclic Redundancy Check)

Word P U M P LRC Parity

Bit 1 0 1 1 0 1

Bit 2 0 0 0 0 1Bit 3 0 1 1 0 1

Bit 4 0 0 1 0 0

Bit 5 1 1 0 1 0

Bit 6 0 0 0 0 1

Bit 7 1 1 1 1 1

Vertical 1 1 1 1 0

Parity Bit



35

Flow & Error Control

Flow Control

Echo checking

Sliding window

Error Control

Stop & Wait (Xon/Xoff)Stop & Wait ARQ

(automatic repeat

request)f r ame 1

Xo n

f r ame 2

Xo f f

Xo n

.

.

.

.

.

f r ame 3

Xo n

A B f r ame 0

Ac k 1

f r ame 1

Ac k 0

.

.

.

.

.

.

f r ame 0

f r ame 0

timeout

Ac k 1

f r ame 0

Ac k 1

f r ame 1

.

.

.

.

.

.

time

out

A B

Go back ARQ

Selective reject ARQ



36

Link Management Concepts

Each Station/Device acts as:

Controller (arbitter, commander, primary station)

Talker (secondary station master, farmer)

Listener (secondary station slave, worker)There is only 1 controller and 1 talker in communication network in

each time instant. The other stations/devices act as listener.

Arbitration: rule & procedure to determine which stations will

act as controller, talker, listener

Arbitration schemes:

Polling : Round Robin , Daisy chain

Reservation

Contention



37

Data Link Control Frame

HDLC (High-level Data Link Control)

LAPD (Link Access Procedure, D-channel), for ISDN network

LAPF (Link Access Procedure for Frame-mode bearer service),for Frame Relay

Flag Address Control Data/

Information FCS Flag

8 8 n 8 or 16 var 16 or 32 8



8 16 16 var 16 8



8 16 or 32 16 var 16 or 32 8

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