Download Internet Model

Applied
Telecommunications
Systems and Technologies
Professor Michael Khader
Tel 973-596-6078
Office: ITC 2106
Office Hours:
Lecture1
Communications Essentials
Overview
Topics
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Basic Communications Model
Data Communication Networking
Protocols and Protocol Architecture
Files, text, communication
The Public Telephone Network
Transmission Media
Data Representation and encoding
A Communications Model
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Source
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Transmitter
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Carries data
Receiver
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Converts data into transmittable signals
Transmission System
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generates data to be transmitted
Converts received signal into data
Destination
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Takes incoming data
Simplified Communications
Model - Diagram
Key Communications Tasks
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Transmission System Utilization
Interfacing
Signal Generation
Synchronization
Exchange Management
Error detection and correction
Addressing and routing
Recovery
Message formatting
Security
Simplified Data
Communications Model
Networking

Point to point communication not
usually practical
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Devices are too far apart
Large set of devices would need
impractical number of connections
Solution is a communications network
Simplified Network Model
Wide Area Networks
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Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
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Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)
Circuit Switching
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Dedicated communications path
established for the duration of the
conversation
e.g. telephone network
Packet Switching
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Data sent out of sequence
Small chunks (packets) of data at a
time
Packets passed from node to node
between source and destination
Used for terminal to computer and
computer to computer communications
Frame Relay
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Packet switching systems have large
overheads to compensate for errors
Modern systems are more reliable
Errors can be caught in end system
Most overhead for error control is
stripped out
Asynchronous Transfer Mode
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ATM
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet
switching technique
Integrated Services Digital
Network
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ISDN
Designed to replace public telecom
system
Wide variety of services
Entirely digital domain
Local Area Networks

Smaller scope
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Building or small campus
Usually owned by same organization as
attached devices
Data rates much higher
Usually broadcast systems
Now some switched systems and ATM
are being introduced
Protocols
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Used for communications between entities in
a system
Must speak the same language
Entities
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User applications
e-mail facilities
terminals
Systems
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Computer
Terminal
Remote sensor
Key Elements of a Protocol
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Syntax
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Semantics
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Data formats
Signal levels
Control information
Error handling
Timing
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Speed matching
Sequencing
Protocol Architecture
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Task of communication broken up into
modules
For example file transfer could use
three modules
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File transfer application
Communication service module
Network access module
Simplified File Transfer
Architecture
A Three Layer Model
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Network Access Layer
Transport Layer
Application Layer
Network Access Layer
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Exchange of data between the
computer and the network
Sending computer provides address of
destination
May invoke levels of service
Dependent on type of network used
(LAN, packet switched etc.)
Transport Layer
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Reliable data exchange
Independent of network being used
Independent of application
Application Layer
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Support for different user applications
e.g. e-mail, file transfer
Addressing Requirements
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Two levels of addressing required
Each computer needs unique network
address
Each application on a (multi-tasking)
computer needs a unique address
within the computer

The service access point or SAP
Protocol Architectures and
Networks
Protocols in Simplified
Architecture
Protocol Data Units (PDU)
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At each layer, protocols are used to
communicate
Control information is added to user data at
each layer
Transport layer may fragment user data
Each fragment has a transport header added
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Destination SAP
Sequence number
Error detection code
This gives a transport protocol data unit
Network PDU
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Adds network header
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network address for destination computer
Facilities requests
Operation of a Protocol
Architecture
TCP/IP Protocol Architecture
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Developed by the US Defense Advanced
Research Project Agency (DARPA) for its
packet switched network (ARPANET)
Used by the global Internet
No official model but a working one.
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Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer
Physical Layer
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Physical interface between data
transmission device (e.g. computer) and
transmission medium or network
Characteristics of transmission medium
Signal levels
Data rates
etc.
Network Access Layer
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Exchange of data between end system
and network
Destination address provision
Invoking services like priority
Internet Layer (IP)
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Systems may be attached to different
networks
Routing functions across multiple
networks
Implemented in end systems and
routers
Transport Layer (TCP)
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Reliable delivery of data
Ordering of delivery
Application Layer
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Support for user applications
e.g. http, SMPT
TCP/IP Protocol Architecture
Model
OSI Model
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Open Systems Interconnection
Developed by the International
Organization for Standardization (ISO)
Seven layers
A theoretical system delivered too late!
TCP/IP is the de facto standard
OSI Layers
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Application
Presentation
Session
Transport
Network
Data Link
Physical
OSI v TCP/IP
Standards Organizations
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Internet Society
ISO
ITU-T (formally CCITT)
ATM forum
Types of Communications
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Based on directional flow
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simplex
half duplex
full duplex
Simplex: one-way
Transmitter
Receiver
Full duplex: two-way
simultaneously
Transmitter
Receiver
Receiver
Transmitter
Half duplex: one at a time
either transmitting or receiving
Transmitter
Receiver
or
Receiver
Transmitter
Figure 1.3: types of communications, based on
information flow consideration: simplex, full
duplex, and half duplex.
Communication classification

Based on devices and links configuration
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point-to-point
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A single link with two devices
multipoint
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Multiple devices on a single link, On may by a broadcaster and
others are receivers.
Point-to-point
Multipoint
(a)
Satellite
Earth Stations
(b)
Figure 1.4: Types of communications,
considering the number of devices on
link. (a): point-to-point, and (b): multipoint.
Relationship between Files,
Data, and Signals
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Files are collection of characters from a code
set (possibly ASCII)
When a file to be transmitted:
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The characters are translated into 1’s and 0’s
An electrical signal representing the 1’s and 0’s is
placed on the transmission medium
The receiver, converts the incoming electrical
signal into 1’s and 0’s, then into characters
that make up the original file.
FILE
HAMLET.TXT
Ye a, fr om the table of m y m e m ory
I'll w ipe aw ay all tr ivial fond re cords
USER DATA
INTERNAL
REPRESENTATION
OF USER DATA
y
01011001
f
01100110
SIGNAL
10 011010
e
01100101
r
01110010
a
01100001
o
01101111
,
00101100
m
01101101
1 01001 10
LSB: Le as t Significant Bit
LSB
LSB
Figure 1.5: Files, Data, and Signals
b
00100000
b
00100000
Synchronous &
transmission

Asynchronous
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Asynchronous
A character in a message is transmitted as an individual
entity, without regard to when the previous character was
transmitted
Synchronous

All characters in a message are sent contiguously, framing
characters indicate the beginning and end of the entire
message
1
0
0 1
y
1
e
0
1 0
1
0
1
0
0 1
1
0
time
Framing
LSB
MSB
Framing
Asynchronous transmission
* Timing of eac h bit is spec if ied w ithin a c harac ter
* inter-c aharc ter time is nonunif orm
* Eac h c harac ter mus t be f ramed
1
0
0 1
y
1
e
0
LSB
1 0
1
0
1
0
0 1
1
0
MSB
Synchronous:
* Characters, within a block, are
sent contiguously
* Each block is framed
Figure 1.6: Asynchronous and
synchronous transmission

In asynchronous transmission:
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Characters are delineated by a start and a stop bit
In synchronous, framing is for the entire message.
Usually a message header and trailer are used

Frame check sequence to protect from transmission errors.
Transmission
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SIGNALS
 Analog and digital signals
 Analog is equated with continuous
 Digital is equated with discrete
A telephone set converts the sound waves into an analog
signal
The analog signal is digitized when it reaches the
telephone network
The digital waveform is converted back into an analog
signal and subsequently the sound wave at the receiving
end.
Digital Transmission

Digitization of voice signals
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Sampling
 equally spaced discrete values extracted from the analog
signal.
 The discrete values follow the amplitude of the analog
waveform
 The result is a PAM signal
quantization
 Assigning numbers approximate values to the PAM signal
Bandwidth and Passband of a
Communication Channel

Bandwidth
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A difference between two frequencies : highest and lowest
that the channel can handle
Passband

The range between the highest and lowest frequencies
CHANNEL A
8.15 MHz
8.25 MHz
CHANNEL B
12.8 MHz
Bandwidth = 100 KHz
Figure 1.11: PASSBAND AND BANDWIDTH
OF A CHANNEL
12.9 MHz
The Sine and Cosine Waveforms
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Periodic signals
Characterized by three parameters
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Amplitude
Frequency (in Hz)
Phase shift (in degree or radian)
A
time
-A
PERIOD
SINE WAVE
A
time
-A
PERIOD
COSINE WAVE
Figure 1.8: Sine and
Cosine waveforms
Fourier Series Expansion
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States: almost any periodic signal can be expresses
as a sum of sineand cosine waveforms that are
harmonically related
usually a sum of DC components and n-harmonic
terms
first harmonic is the fundamental frequency
nthe harmonic is n times the first harmonic

f (t )  a0   ( an sin nt  bn cos nt )
n 1
Noise

Types of Noise
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Thermal: heat dissipation in electronic
devices.
Crosstalk: Occurs on adjacent channels,
conversation from one channel can be
heard on another
Interference: Caused by many stimulus;
power lines, Auto ignition, …. etc
The Concept of signal to noise ratio
(K + 1)
(K)
TIME
TRANSMITTED LEVEL
(K) SIGNAL
NOISE
TIME
RECEIVED SIGNAL PLUS
NOISE
* To reduce effect of noise:
A. Increase separation between levels
B. Slow rate of transmission
C. Add error-correction information
Result: Decrease the effective data
transfer rate
Figure 1.12: The concept of signal-to-noiseratio
Transmission Media
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Guided media
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Twisted pair
Optical fibers
Coaxial cables
Unguided media
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Microwave
Wireless
Twisted Pair

One of the oldest media
 At one point AT&T owned 80% of world copper
 Typical use: subscriber’s loop, T1 carrier, and
moderate to low speed analog transmission.
 Amplification every 5 to 6 km for analog signals
 Repeaters every 2 to 3 km for digital signals
Twisted Pair
Figure 1.13: Twisted Pair
Coaxial Cables
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the cable Workhorse of industry
Used for both digital and analog transmission
Other use: Local Area Networks (Token rings)
Less susceptible to crosstalk and interference
Coaxial Cables
Solid Cylinder
Insulating Bead
Stiff Wire
Polythylene filler
Wire
Figure 1.14: Coaxial Cable
Braided
outer
conductor
Optical Fibers
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Significant breakthrough in
telecommunication
Enormous bandwidth
Billion bits/sec (Gpbs)
Low attenuation rate, 1 bit in error ever
100 billion or more bits
Immune from electrical interference
Optical Fibers
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Based on rules of physics
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Incident angle between different media
Two types:
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Single mode
multi-mode
Optical Fibers
B1
B3
B2
AIR/SILICA
BOUNDARY
a1
a2
a3
SILICA
(a)
Total internal
reflection
Light source
(b)
Figure 1.15: (a) Three examples of a light ray from inside a
silica fiber impinging on the air/silica boundary at different
angles, (b) Light trapped by total internal reflection.
Terrestrial Microwave
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Towers operation use line-of-sight transmission.
Line-of-site refers to the geographical arrangement of
the transmitting antennas and the receiving tower
such that the wave travels in straight line from the
transmitter to the receiver.
Since microwaves travel in straight line, if the towers
are too far apart, the earth will get in the way .
The higher the towers are, the further apart they can
be. For 100-m high tower, repeaters can be spaced
80 km apart (assuming there are no large hills in
between).
BROADCAST RADIATION
TO A 25 TO 75 MILE
RADIUS
Radio Tower
Satellite dish
STUDIO
An example of a microwave
configuration
Terrestrial Microwave - Continue
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Microwaves do not pass through buildings well.
In addition, even though the beam may be well focused at the
transmitter, there is still some divergence in space.
Multipath Fading is caused when some waves may be refracted
off low-lying atmospheric layers and may take slightly longer to
arrive than direct waves.
The delayed wave may arrive out of phase.
The most common type of microwave antenna is a rigidly fixed
parabolic dish-shaped antenna of approximately ten feet
diameter, Common frequencies used for microwave transmission
are in the rang of 2 GHz to 40 GHz. The higher the frequency
employed, the higher the bandwidth and data rate.
Satellite Microwave
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A communication satellite is essentially a microwave relay
station.
It links two or more ground-based transmitters/receivers
called earth stations.
The satellite receives the analog or digital signals
transmitted by an earth station on one frequency band
called the uplink
It then repeats or amplifies the signal on another frequency
band known as the downlink.
A single satellite can operate on a number of frequency
bands called a transponder.
To ensure that the satellite is in the line-of-site of earth, it is
made to rotate at a period equal of that of the earth. This is
possible when the satellite is at a distance of approximately
35,800 km.
Sate
llite di
sh
Satellite
Satellite dish
(a)
Satellite dish
Satellite
Satellite dish
ter
mit
ns
Tra
Sa
tell
ite
dis
h
Satellite dish
(b)
Satellite Microwave: (a) point-topoint, (b) Broadcast (multi-receivers
The Public Telephone Network
--- History and Evolution -




Alexander Graham Bell invented the telephone
Around 1890
Simple networks connected telephones by
manually operated switches.
In this network, as shown on next slide, the signal
is analog
To call another telephone, a customer first rings
the operator and provides the phone number of
the party.
The operator then determines the line that goes
either directly to the other party or to another
operator along a path to the other party.
The Public Telephone Network
-- History and Evolution -
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


The parties remain connected for the duration of the
conversation and are disconnected by the operator at the
end of the call.
Paths are established by means of circuit switching
“circuit” refers to the capability of transmitting one
telephone conversation along one link.
To set up a call, a set of circuits has to be connected, joining
the two telephone sets.
By modifying the connection, the operators can switch the
circuits. Circuit switching occurs at the beginning of a
telephone call. Operators were later replaced by mechanical
switches and, eventually, by electronic switches.
Telephone
Telephone
A
A
A
Telephone
Telephone
A = Analog
The telephone network as existed around
1890 (the telephone however looks like
today's telephone, I'm looking for a
template for an antique telephone
The Public Telephone Network
-- History and Evolution -





A major development in the Public Telephone Network is the
digital transmission, as shown on the next slide.
An electronic interface in the switch converts the analog
signal traveling on the link from the telephone set to the
switch into a digital signal, and from digital to analog in the
opposite direction.
The switches themselves are computers, which makes them
very flexible.
This flexibility allows the Telephone Company to modify
connections by sending specific instructions to the computer.
common channel signaling (CCS) – another major
development
CCS is a data communication network that the switches use
to exchange control information among themselves. This
“conversation” between switches serves the same function
as the conversation that took place between operators in the
manual network.
D
Telephone
Telephone
D
CCS
A
Telephone
Telephone
Telephone network around 1988.
The transmissions are analog (A)
or digital (D). The switches are
electronic and exchange control
information by using a data
network called common channel
signaling (CCS).
Current Telephone Network Structure


Currently, The telephone system is organized as a highly
redundant, multilevel hierarchy.
The present configuration, simplified:




From each telephone comes a pair of wires that goes directly to
the telephone company’s nearest end office which is also called the
local central office.
The two-wire connection between each subscriber’s telephone and
the end office is known as the local loop.
In the United States alone there are about 20,000 local central
offices. The concentration of the area code and the first three
digits of the telephone number uniquely specify a local central
office, which is why the rate structure uses this information.
If a subscriber, attached to a particular end office calls a subscriber
attached to the same end office, the switching mechanism within
the office sets up a direct electrical connection between the two
local loops. This connection remains intact for the duration of the
call.
The current Telephone Network
Structure




If the called telephone is attached to another end office, a
different procedure has to be used.
Each end office (local switch) has a number of outgoing lines to
one or more nearby switching centers, called toll offices (or if
they are within the same local area, tandem switches).
These lines are called toll-connecting trunks. If both the calling
party and the called party end offices happen to have a toll
connecting trunk to the same toll office (a likely occurrence if
they are close by), the connection may be established within the
toll office.
If the calling party and the called party do not have a toll office
in common, the path will have to be established somewhere
higher up in the hierarchy.
Very high bandwidth intertoll trunks
Intermediat
switches
offices
Toll connecting
trunks
Toll office
Toll of fice
Telephone
Telephone
End office
End of fice
Typical circuit rout for a call of a medium
distance
O F F -H O O K
S w i tc h c o n n e c ts D T M F r e c e iv e r
D ia l T o n e
S ta r t D i a l in g
T e le p h o n e N u m b e r
S w i tc h S ta r ts to C o l l e c t D i g i ts ( o n e
b y o n e u s i n g D T M F c i rc u i t)
S t o p d i a l to n e
S e l e c t p a th
S e n d r i n g i n g s i g n a l i f c a l l e d p a r ty
is n o t b u s y
R in g -b a c k
to n e
R in g in g s ig n a l
S w i tc h d i s c o n n e c ts r i n g i n g s ig n a l
fr o m c a l l e e a n d r e m o v e r i n g b a c k
to n e fr o m c a l l e r
R em ove
rin g -b a c k
R em ove
rin g in g
s ig n a l
C o n n e c ti o n P a th b e tw e e n c a l l e r
a n d c a lle e
O N -H O O K (H A N G u p )
C o n n e c ti o n p a th i s r e m o v e d a n d
r e s o u r c e s a r e fr e e d fo r a n o th e r
c a ll
A local telephone call scenario
O FFH O O K
Table 1.1: The telephone system hierarchy as it exists today
Order
Name
Comments
Class 1
Regional center
Top of the hierarchy
Class 2
Sectional Center
Class 3
Primary Center
Class 4
Toll center
Now “point of presence” (POP) where local
exchange meets IEX
Class 5
End Office
In the local exchange carrier (LEC) area
Standards


Required to allow for interoperability between
equipment
Advantages



Ensures a large market for equipment and
software
Allows products from different vendors to
communicate
Disadvantages


Freeze technology
May be multiple standards for the same thing
Five components of data communication
Components of data communications





Message: The information to be communicated – text, sound,
video, or a combination
Sender: The device that sends the message – computer,
telephone, TV, and so on.
Receiver: The device that receives the message – computer,
telephone, TV, and so on.
Medium: A path by which the a message travels from sender
to receiver – twisted pair, coaxial cables, fiber optic cable, or
radio waves (terrestrial or satellite microwave)
Protocol: Rules that governs communications among devices
Data representation





Text:
represented by a bit pattern of 0s and 1s, the number of
bits in a pattern depends on the number of symbols in the
language – English uses 26 symbols (a, b, …) and 26 for (A, B,
..) and 10 symbols for (0, 1, 2, ..)
ASCII: American Standard Code for Information Exchange:
uses 7 bit pattern (128 symbols)
Extended ASCII: 8-bit patterns – ASCII is a subset of it by
adding a 0 to the left
Unicode: 65,536 symbols because it uses a 16 bit code
Images: also represented by bit patterns, however
the mechanism is different.. In its simplest form an
image is divided into a matrix of pixels, where each
pixel is a small dot
Data Representation


Audio: is a representation of sound, it is by nature
different from text, numbers, or images. It is
continuous, not discrete.
Video: Can be produced as a continuous entity, (by
a TV camera), or it can be a combination of images,
each a discrete entity arranged to convey the idea of
motion.
Encoding Techniques




Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
Digital Data, Digital Signal

Digital signal



Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements
Terms (1)

Unipolar


Polar


One logic state represented by positive voltage the
other by negative voltage
Data rate


All signal elements have same sign
Rate of data transmission in bits per second
Duration or length of a bit

Time taken for transmitter to emit the bit
Terms (2)

Modulation rate



Rate at which the signal level changes
Measured in baud = signal elements per
second
Mark and Space

Binary 1 and Binary 0 respectively
Interpreting Signals

Need to know



Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting
of signals



Signal to noise ratio
Data rate
Bandwidth
Comparison of Encoding
Schemes (1)

Signal Spectrum




Lack of high frequencies reduces required
bandwidth
Lack of dc component allows ac coupling via
transformer, providing isolation
Concentrate power in the middle of the bandwidth
Clocking



Synchronizing transmitter and receiver
External clock
Sync mechanism based on signal
Comparison of Encoding
Schemes (2)

Error detection


Signal interference and noise immunity


Can be built in to signal encoding
Some codes are better than others
Cost and complexity


Higher signal rate (& thus data rate) lead to
higher costs
Some codes require signal rate greater than data
rate
Encoding Schemes

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Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
Nonreturn to Zero-Level (NRZL)


Two different voltages for 0 and 1 bits
Voltage constant during bit interval

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
no transition I.e. no return to zero voltage
e.g. Absence of voltage for zero,
constant positive voltage for one
More often, negative voltage for one
value and positive for the other
This is NRZ-L
Nonreturn to Zero Inverted
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
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of
signal transition at beginning of bit time
Transition (low to high or high to low)
denotes a binary 1
No transition denotes binary 0
An example of differential encoding
NRZ
Differential Encoding



Data represented by changes rather
than levels
More reliable detection of transition
rather than level
In complex transmission layouts it is
easy to lose sense of polarity
NRZ pros and cons

Pros



Cons




Easy to engineer
Make good use of bandwidth
dc component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission
Multilevel Binary


Use more than two levels
Bipolar-AMI



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


zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still
a problem)
No net dc component
Lower bandwidth
Easy error detection
Pseudoternary



One represented by absence of line
signal
Zero represented by alternating positive
and negative
No advantage or disadvantage over
bipolar-AMI
Bipolar-AMI and
Pseudoternary
Trade Off for Multilevel Binary

Not as efficient as NRZ




Each signal element only represents one bit
In a 3 level system could represent log23 = 1.58
bits
Receiver must distinguish between three levels
(+A, -A, 0)
Requires approx. 3dB more signal power for same
probability of bit error
Biphase

Manchester






Transition in middle of each bit period
Transition serves as clock and data
Low to high represents one
High to low represents zero
Used by IEEE 802.3
Differential Manchester




Midbit transition is clocking only
Transition at start of a bit period represents zero
No transition at start of a bit period represents
one
Note: this is a differential encoding scheme
Manchester Encoding
Differential Manchester
Encoding
Biphase Pros and Cons

Con




At least one transition per bit time and possibly
two
Maximum modulation rate is twice NRZ
Requires more bandwidth
Pros



Synchronization on mid bit transition (self
clocking)
No dc component
Error detection

Absence of expected transition
Modulation Rate
Scrambling


Use scrambling to replace sequences that
would produce constant voltage
Filling sequence







Must produce enough transitions to sync
Must be recognized by receiver and replace with
original
Same length as original
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS


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

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Bipolar With 8 Zeros Substitution
Based on bipolar-AMI
If octet of all zeros and last voltage pulse
preceding was positive encode as 000+-0-+
If octet of all zeros and last voltage pulse
preceding was negative encode as 000-+0+Causes two violations of AMI code
Unlikely to occur as a result of noise
Receiver detects and interprets as octet of all
zeros
HDB3



High Density Bipolar 3 Zeros
Based on bipolar-AMI
String of four zeros replaced with one
or two pulses
B8ZS and HDB3
Digital Data, Analog Signal

Public telephone system

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
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300Hz to 3400Hz
Use modem (modulator-demodulator)
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)
Modulation Techniques
Amplitude Shift Keying


Values represented by different
amplitudes of carrier
Usually, one amplitude is zero





i.e. presence and absence of carrier is used
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber
Binary Frequency Shift Keying






Most common form is binary FSK (BFSK)
Two binary values represented by two
different frequencies (near carrier)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
High frequency radio
Even higher frequency on LANs using co-ax
Multiple FSK




More than two frequencies used
More bandwidth efficient
More prone to error
Each signalling element represents
more than one bit
FSK on Voice Grade Line
Phase Shift Keying


Phase of carrier signal is shifted to
represent data
Binary PSK


Two phases represent two binary digits
Differential PSK

Phase shifted relative to previous
transmission rather than some reference
signal
Differential PSK
Quadrature PSK

More efficient use by each signal element
representing more than one bit





e.g. shifts of /2 (90o)
Each element represents two bits
Can use 8 phase angles and have more than one
amplitude
9600bps modem use 12 angles , four of which
have two amplitudes
Offset QPSK (orthogonal QPSK)

Delay in Q stream
QPSK and OQPSK Modulators
Examples of QPSF and OQPSK
Waveforms
Performance of Digital to
Analog Modulation Schemes

Bandwidth




ASK and PSK bandwidth directly related to bit rate
FSK bandwidth related to data rate for lower
frequencies, but to offset of modulated frequency
from carrier at high frequencies
(See Stallings for math)
In the presence of noise, bit error rate of PSK
and QPSK are about 3dB superior to ASK and
FSK
Quadrature Amplitude
Modulation




QAM used on asymmetric digital subscriber
line (ADSL) and some wireless
Combination of ASK and PSK
Logical extension of QPSK
Send two different signals simultaneously on
same carrier frequency




Use two copies of carrier, one shifted 90°
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary
output
QAM Modulator
QAM Levels

Two level ASK




Four level ASK



Each of two streams in one of two states
Four state system
Essentially QPSK
Combined stream in one of 16 states
64 and 256 state systems have been
implemented
Improved data rate for given bandwidth

Increased potential error rate
Analog Data, Digital Signal

Digitization







Conversion of analog data into digital data
Digital data can then be transmitted using NRZ-L
Digital data can then be transmitted using code
other than NRZ-L
Digital data can then be converted to analog
signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation
Digitizing Analog Data
Pulse Code Modulation(PCM)
(1)

If a signal is sampled at regular intervals at a
rate higher than twice the highest signal
frequency, the samples contain all the
information of the original signal





(Proof - Stallings appendix 4A)
Voice data limited to below 4000Hz
Require 8000 sample per second
Analog samples (Pulse Amplitude Modulation,
PAM)
Each sample assigned digital value
Pulse Code Modulation(PCM)
(2)


4 bit system gives 16 levels
Quantized





Quantizing error or noise
Approximations mean it is impossible to recover
original exactly
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives
64kbps
Direction of Data Flow



Simplex: the communication is Unidirectional as on
a one-way street. Only one device can transmit and
the other can receive. Keyboards and traditional
monitors are examples
Half-Duplex: Each station can transmit and receive
but not at the same time – one lane bridge with bidirectional traffic. Walkie-talkies and CB (citizen
band) radio
Full Duplex: also called duplex, both stations can
transmit and receive at the same time
Simplex
Half-duplex
Full-duplex
Networks
Distributed Processing -tasks are divide among multiple computers
Network Criteria
most important
reliability, security, and performance
Physical Structures –
devices are connected: point-to-point, and
multipoint, and network topology
how
Categories of Networks –
LAN, MAN, WAN, …
Point-to-point connection
Multipoint connection
Categories of topology
Fully connected mesh topology (for five devices)
Star topology
Bus topology
Ring topology
Categories of networks
LAN
LAN (Continued)
MAN
WAN
The Internet
A Brief History
The Internet Today
The Internet – a Brief History



ARPANET: the advance research project
agency (ARPA) in the department of defense
– DOD
1973: the idea of transmission control
protocol (TCP) came about which included
concepts as datagrame, and encapsulations.
Shortly after TCP was split into two protocols:
TCP and IP (TCP/IP)
Internet today
Terms




ISP: International Service Providers
NSPs: National Service Providers
Regional ISPs: smaller ISPs that are
connected to one or more NSP
Local Internet Service Providers:
provide direct service to the end user. It
can be connected to regional ISPs or
directly to NSPs
Protocols and Standards
Protocols
key elements are syntax semantics and
timing
Standards
Standards Organizations
T, CCITT ANSI, and IEEE, EIA
Internet Standards
ISO. ITU-
Layered Tasks
Sender, Receiver, and Carrier
Hierarchy
Services
Internet Model
Peer-to-Peer Processes
Functions of Layers
Summary of Layers
Internet layers
Peer-to-peer processes
An exchange using the Internet model
Physical layer
Note:
The physical layer is responsible for
transmitting individual bits from one
node to the next.
Data link layer
Note:
The data link layer is responsible for
transmitting frames from
one node to the next.
Node-to-node delivery
Example 1
A node with physical address 10 sends a frame to a node
with physical address 87. The two nodes are connected by
a link. At the data link level this frame contains physical
addresses in the header. These are the only addresses
needed. The rest of the header contains other information
needed at this level. The trailer usually contains extra bits
needed for error detection
Example 1
Network layer
Note:
The network layer is responsible for
the delivery of packets from the
original source to the
final destination.
Source-to-destination delivery
Example 2
We want to send data from a node with network address A
and physical address 10, located on one LAN, to a node
with a network address P and physical address 95, located
on another LAN. Because the two devices are located on
different networks, we cannot use physical addresses
only; the physical addresses only have local jurisdiction.
What we need here are universal addresses that can pass
through the LAN boundaries. The network (logical)
addresses have this characteristic.
Example 2
Transport layer
Note:
The transport layer is responsible for
delivery of a message from one process
to another.
Reliable process-to-process delivery of a message
Example 3
This is an example of transport layer communication.
Data coming from the upper layers have port addresses j
and k (j is the address of the sending process, and k is the
address of the receiving process). Since the data size is
larger than the network layer can handle, the data are split
into two packets, each packet retaining the port addresses
(j and k). Then in the network layer, network addresses (A
and P) are added to each packet.
Example 3
Application layer
Note:
The application layer is responsible for
providing services to the user.
Summary of duties
OSI Model
A comparison
OSI model
Note: