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DATA
COMMUNICATIONS
Departemen Teknik Fisika
Institut Teknologi Bandung2001
DCS Training Series
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Multi-points Communications
(a) Bus
(b) Tree(c) Ring
(d) Star(a)
(c)
(b) (d)
LAN Topology: - physical connection
- logical connection
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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
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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
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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, ....
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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
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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
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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)
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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)
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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
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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)
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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
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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
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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
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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
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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
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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
Flag Address Control Data/
Information FCS Flag
8 16 16 var 16 8
Flag Address Control Data/
Information FCS Flag
8 16 or 32 16 var 16 or 32 8