Transcript Training - Pennsylvania State University

COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
Chapter 3
Data Transmission
Review Connection/Connectionless
{
{
Connectionoriented
Connectionless
Service
Example
Reliable Message Stream
Reliable byte stream
Unreliable connection
Unreliable datagram
Acknowledged Datagram
Request-reply
Sequence of Pages
Remote logon
Digitized Voice
Electronic Junk Mail
Registered mail
Database Query
Review Connection/Connectionless
•Connection-oriented service is modeled after the Telephone Company
•Connectionless Service is modeled after the Postal System
PRIMITIVE
Request
Indication
Response
Confirm
MEANING
A Entity wants the service to do something
A Entity is informed about an event
An Entity wants to respond to an event
The response to an earlier request has come back
A Sample Connection Oriented Service
CONNECT.request
CONNECT.indication
CONNECT.response
CONNECT.confirm
DATA.request
DATA.indication
DATA.response
DATA.confirm
Layer N+1
Layer N
1
5
7
4
6
1 2 3 4 5 6 7 8 9 10
3
Layer N+1
Layer N
Request a connection
Signal the called Party
Callee accepts or rejects call
Tell Caller whether call was accepted
Request that data be sent
Signal the arrival of data
Request that connection be released
Signal peer about request
2
5
6
8
Computer 1
Time
Computer 2
LAST WEEK - OSI
We Spoke about the OSI/ISO and TCP/IP Models
•NEITHER the OSI model and its Protocols nor
the TCP/IP models and its protocols are perfect
•Bad Timing
•Bad Technology
•Bad Implementations
•Bad Politics.
•OSI Model is
•Printed Standards almost a meter thick
•The standards are difficult to implement
•The stands are inefficient in operation
LAST WEEK - TCP/IP
•The TCP/IP Model is
•The first implementation of TCP/IP was part
of Berkeley UNIX and was good
•The model does not clearly distinguish the concept of
• Service
•Interface
•Protocol
•The TCP/IP model is NOT general and is poorly
suited for describing any protocol other than TCP/IP
•The TCP/IP model does not distinguish between the
Physical and Data Link Layers, which are completely different
•While the TCP and IP stack are well thought out and
implemented, many of the other protocols were Ad Hoc, generally
produced by a couple of Grad Students hacking away until they got tired
DECIBELS
•Decibels are often used in communications when:
•Talking about signal strength
•Talking about the net gain or loss of a cascaded transmission path
•A Decibel is a measure of the ratio between two signal levels
N = 10logP2/P1
N = number of decibels
P1=input power level
P2=output power level
•dBW (decibel-watt) is the absolute power level
Power = 10log Power (watts)/1(watt)
1mW = -30dBW
1 W = 0 dBW
1000W = 30dBW
This Week: The Physical Layer
Communications and Information Theory are topics of whole courses
We’ll cover some theoretical basics regarding communications over a
physical channel
We discover that there are physical limitations to communications
over a given channel
We’ll cover some fundamental theorems
Physical Layer
Source node
Destination node
Application
Application
Presentation
Presentation
Session
Session
Intermediate node
transport
Network
Data link
Physical
Packets
Frames
Bits
Signals
transport
Network
Network
Data link
Data link
Physical
Physical
Physical / Data Link Layer Interface
Sender
Receiver
NL
DLL
PL
HDR
Frame
ACK
HDR
Transmitted Bits
Transmission Terminology (1)
Transmitter
 Receiver
 Medium

– Guided medium

e.g. twisted pair, optical fiber
– Unguided medium

e.g. air, water, vacuum
Transmission Terminology (2)

Direct link
– No intermediate devices

Point-to-point
– Direct link
– Only 2 devices share link

Multi-point
– More than two devices share the link
Transmission Terminology (3)

Simplex
– One direction (but in Europe means half
duplex)


e.g. Television
Half duplex
– Either direction, but only one way at a time


e.g. police radio
Full duplex
– Both directions at the same time

e.g. telephone
Frequency, Spectrum, and Bandwidth
•Electromagnetic signal are used to transmit data
•This transmitted signal is a function of Time
•Time-Domain
•This transmitted signal can also be a function of Frequency
•Frequency-Domain
•The Frequency domain is more important in understanding
data transmission
Electromagnetic Signals

Function of time
– Analog (varies smoothly over time)
– Digital (constant level over time, followed by a
change to another level)

Function of frequency
– Spectrum (range of frequencies)
– Bandwidth (width of the spectrum)
Time domain concepts
– A Continuous signal

Varies in a smooth way over time
– A Discrete signal

Maintains a constant level then changes to another
constant level
– A Periodic signal

Pattern repeated over time
– An Aperiodic signal

Pattern not repeated over time
Periodic Signal Characteristics
– Amplitude (A): signal value, measured in
volts
– Frequency (f ): repetition rate, cycles per
second or Hertz
– Period (T): amount of time it takes for one
repetition, T=1/f
– Phase (Φ): relative position in time, measured
in degrees or radians
amplitude (volts)
Analog Signaling

represented by sine waves
1 cycle
phase
difference
time
(sec)
frequency (hertz)
= cycles per second
amplitude (volts)
Digital Signaling

represented by square waves or pulses
1 cycle
time
(sec)
frequency (hertz)
= cycles per second
BPS vs. Baud
BPS=bits per second
 Baud=# of signal changes per second
 Each signal change can represent more
than one bit, through variations on
amplitude, frequency, and/or phase

Continuous & Discrete Signals
Periodic
Signals
Sine Wave

Peak Amplitude (A)
– maximum strength of signal
– volts

Frequency (f)
–
–
–
–

Rate of change of signal
Hertz (Hz) or cycles per second
Period = time for one repetition (T)
T = 1/f
Phase ()
– Relative position in time
Varying Sine Waves
Sin2πt
0.5Sin2πt
Sin 2
Sin (2t 

4
)
Phase Shift in
radians
Sin4πt
or
Sin2 (t  0.125)
Phase Shift in
seconds
Wavelength ()

Distance occupied by one cycle
Distance between two points of corresponding phase
in two consecutive cycles

Assuming signal velocity in space is equal to v

–
–
–
–
 = vT or
f = v
Here, v =c = 3*108 ms-1 (speed of light in free space)
Remember T=1/ f
Frequency Domain Concepts
A Signal is usually made up of many
frequencies
 Components are sine waves
 It Can be shown (Fourier analysis) that any
signal is made up of component sine waves
 One can plot frequency domain functions
instead of/in addition to time domain
functions

Addition of
Frequency
Components
(a) Sin(2πft)
(b) (1/3)Sin(2π(3f)t)
(c) (4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]
Communications Basics


Represent a signal as a single-valued function of time,
g(t), to model behavior of a signal (may be voltage,
current or other change)
Jean-Baptiste Fourier showed we can represent a
periodic signal (given some conditions) as the sum of a
possibly infinite number of sines and cosines
Period = T
g(t) = (1/2)c + n=1
S an sin(2nft) +n=1S bn cos(2nft)
f = 1/T is fundamental frequency
a & b coefficients are the amplitude of the nth harmonic
This is a Fourier Series
Original
Time ->
Harmonic spectrum
As we add
more
harmonics
the signal
reproduces
the original
more closely
Signal Transmission
 No
transmission facility can transmit signals
without losing some power
 Usually
this attenuation is frequency dependent so
the signal becomes distorted
 Generally
signal is completely attenuated above
some max frequency (due to medium
characteristics or intentional filtering)
 The
signal is bandwidth limited
Signal Transmission
 Time
T necessary to transmit a character depends
on coding method and signaling speed
 Signaling
speed = number of times per second the
signal changes value and is measured in baud
 Note
that baud rate is not necessarily the same as
the bit rate
 By
limiting the bandwidth of the signal we also
limit the data rate even if a channel is perfect
 Overcome
this by encoding schemes
Spectrum & Bandwidth




Spectrum
– range of frequencies contained in signal
Absolute bandwidth
– width of spectrum
Effective bandwidth
– Often just bandwidth
– Narrow band of frequencies containing most of
the energy
DC Component
– Component of zero frequency
Signal with DC Component
Data Rate and Bandwidth

Any transmission system has a limited
band of frequencies

This in turn limits the data rate that can be
carried
Bandwidth

Width of the spectrum of frequencies that
can be transmitted
– if spectrum=300 to 3400Hz,
bandwidth=3100Hz
Greater bandwidth leads to greater costs
 Limited bandwidth leads to distortion
 Analog measured in Hertz
 Digital measured in baud or Bps

Analog and Digital Data Transmission

Data
– Entities that convey meaning

Signals
– Electric or electromagnetic representations of
data

Transmission
– Communication of data by propagation and
processing of signals
Voice Grade Line
For a given Bit Rate of b bits/sec the time
required to send 8 bits is b/8 Hz.
 For a voice Grade Line has a cutoff
frequency near 3000Hz
 This restriction means that the number of
the highest harmonic passed through is
3000/(b/8) or 24000/b

Data

Analog
– Continuous values within some interval
– e.g. sound, video

Digital
– Discrete values
– e.g. text, integers
Acoustic Spectrum (Analog)
Signals
Means by which data are propagated
 Analog

– Continuously variable
– Various media

wire, fiber optic, space
– Speech bandwidth 100Hz to 7kHz
– Telephone bandwidth 300Hz to 3400Hz
– Video bandwidth 4MHz

Digital
– Use two DC components
Digital Text Signaling
Transmission of electronic pulses
representing the binary digits 1 and 0
 How do we represent letters, numbers,
characters in binary form?
 Earliest example: Morse code (dots and
dashes)
 Most common current form: ASCII

ASCII Character Codes
Use 8 bits of data (1 byte) to transmit one
character
 8 binary bits has 256 possible outcomes (0
to 255)
 Represents alphanumeric characters, as
well as “special” characters

Digital Image Signaling

Pixelization and binary representation
Code:
00000000
00111100
01110110
01111110
01111000
01111110
00111100
00000000
Bit rate and Baud rate

Bit rate number of bits that are transmitted in a second

Baud rate number of line signal changes (variations) per second
If a modem transmits 1 bit for every signal change
bit rate = baud rate
If a signal change represents 2 or more or n bits
bit rate = baud rate *n
Data and Signals
Usually use digital signals for digital data
and analog signals for analog data
 Can use analog signal to carry digital data

– Modem

Can use digital signal to carry analog data
– Compact Disc audio
Why Study Analog?
Telephone system is primarily analog
rather than digital (designed to carry voice
signals)
 Low-cost, transmission medium (present
almost at all places at all times
 If we can convert digital information (1s
and 0s) to analog form (audible tone), it
can be transmitted inexpensively

Voice Signals
Easily converted from sound frequencies
(measured in loudness/db) to
electromagnetic frequencies, measured in
voltage
 Human voice has frequency components
ranging from 20Hz to 20kHz
 For practical purposes, the telephone
system has a narrower bandwidth than
human voice, from 300 to 3400Hz

Analog Signals Carrying Analog
and Digital Data
QAM
•QAM - Quadrature Amplitude Modulation
•Diagrams that show legal combinations of amplitude and phase
are called CONSTELLATION PATTERNS
2 bits/Baud
8 Valid combinations
4800bps
4 bits/Baud
16 valid combinations
9600bps
ITU V.32 modem standard
•The next step after 9600bps is 14400bps and is called V.32 bis (transmits 6 bits)
•This is followed by V.34 running at 28,800bps with 128 bit constellation
Digital Signals Carrying Analog
and Digital Data
Analog Transmission
Analog signal transmitted without regard
to content
 May be analog or digital data
 Attenuated over distance
 Use amplifiers to boost signal
 Also amplifies noise

Digital Transmission
Concerned with content
 Integrity endangered by noise, attenuation
etc.
 Repeaters used
 Repeater receives signal
 Extracts bit pattern
 Retransmits
 Attenuation is overcome
 Noise is not amplified

Advantages of Digital Transmission





Digital technology
– Low cost LSI/VLSI technology
Data integrity
– Longer distances over lower quality lines
Capacity utilization
– Economical high bandwidth links
– High degree of multiplexing easier with digital techniques
Security & Privacy
– Encryption
Integration
– Can treat analog and digital data similarly
Transmission Media
The physical path between transmitter and
receiver is the Transmission Path
 Design factors

– bandwidth
– attenuation: weakening of signal over
distances
– interference
– number of receivers
Impairments and Capacity
Impairments exist in all forms of data
transmission
 Analog signal impairments result in
random modifications that impair signal
quality
 Digital signal impairments result in bit
errors (1s and 0s transposed)

Transmission Impairments
Signal received may differ from signal
transmitted
 Analog - degradation of signal quality
 Digital - bit errors
 Caused by

– Attenuation and attenuation distortion
– Delay distortion
– Noise
Transmission Impairments

Attenuation
– loss of signal strength over distance

Attenuation Distortion
– different losses at different frequencies

Delay Distortion
– different speeds for different frequencies

Noise
Attenuation
P1 watts
transmitter
P2 watts
receiver
Attenuation
10 log10 (P1/P2) dB
Amplification
10 log10 (P2/P1) dB
Attenuation
Signal strength falls off with distance
 Depends on medium
 Received signal strength:

– must be enough to be detected
– must be sufficiently higher than noise to be
received without error

Attenuation is an increasing function of
frequency
Delay Distortion





Occurs only in guided media
The velocity of propagation of a signal through a
guided medium varies with frequency.
This effect is called delay distortion
Its affect is the received signal is distorted due to
varying delays
Its more critical in digital data
– Because of delay distortion some of the signal
components in one bit position can spill into another
causing intersymbol interference which is a major
limitation to the maximum bit rate in a transmission
channel
Noise (1)

Noise is the major limiting factor in communication
system performance

Noise is the unwanted signals that inserted between
transmitter and receiver
Noise (2)
There
are 4 main types of Noise:
Thermal
–Due to thermal excitement of electrons
–Uniformly distributed, cannot be eliminated
–Noise is assumed to be independent of frequency
–White noise
Intermodulation
–Signals that are the sum and difference of original
frequencies sharing a medium
Noise (3)
 Crosstalk

– A signal from one line is picked up by another
NEXT (near-end crosstalk
)
– interference in a wire at the transmitting end of a signal
sent on a different wire

FEXT (far-end crosstalk)
– interference in a wire at the receiving end of a signal
sent on a different wire

Impulse
–
–
–
–
–
Irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude
Less predictable
Noise (4)

Effect of Noise is
– distorts a transmitted signal
– attenuates a transmitted signal

The signal-to-noise ratio to quantifies
noise by expressing in decibels the
amount by which a signal level exceeds
the noise within a specific bandwidth
S/Ndb =
10 log S
N
N= noise power
S= average signal power
Effect of noise
Signal
Noise
Logic
Threshold
Signal+Noise
Sampling times
0 1
0 1
1
0
1 1 0
1 1 0
0
0
Bit error
0
1
0 1
0 1
Data Received
Original data
Channel Capacity

Data rate
– In bits per second
– Rate at which data can be communicated

Bandwidth
– In cycles per second of Hertz
– Constrained by transmitter and medium
Maximum Data Rate
 In
1920s Nyquist (of the Nyquist Theorem)
developed an equation for the maximum data rate
of a noiseless channel
– For low pass filtered signal of bandwidth B
– Sampling at exactly 2B samples per sec allows
reconstruction of the signal
– More samples are useless since the frequencies
above B are filtered out
C=Capacity=max data rate = 2B log2 M bits/sec
for M discrete levels
Nyquist theorem
“ In a perfectly noiseless channel, if f is the
maximum frequency the medium can
transmit, the receiver can completely
reconstruct a signal by sampling it 2*f times
per second”
Nyquist, 1920
Nyquist formula
C=
2B log2 M
B = bandwidth
M = number of discrete signal levels
Theoretical capacity for Noiseless channel
Example: Channel capacity calculation for voice bandwidth (~3100 Hz):
M
2
4
8
16
Max data rate (C)
6200 bps
12400 bps
18600 bps
24800 bps
Shannon’s Law
 In
the ‘40s Shannon (of Shannon’s Law) extended
the equation to a channel subject to
thermodynamic (thermal) noise
Thermal noise measured by ratio of signal (S)
power to noise (N) power (signal-to-noise ratio - S/N)
But represented as: 10 log10 S/N
These units are called decibels (dB)
Now, for a channel with signal to noise of S/N
Capacity=C=max bits/sec = B log2 (1 + S/N)
Here, C=Theoretical Maximum capacity with noise
Note: Only much lower rates are achieved since the equation
assumes zero impulse noise and no attenuation and delay
distortion.
Maximum Data Rate of a Noisy
Channel
For a channel of 30,000Hz bandwidth and
a signal to thermal noise ratio of 30dB
The best that can be transmitted is a little over 30,000bps
No matter how many or how few signal levels are used
and no matter how often or how infrequent samples
are taken
The Telephone Company
The Telephone Network
The telephone network consists of your phone at home that
is connected (by the Local Loop) to the Central Office. The
Central Office is in turn connected to a Hierarchical Phone
Network. Worldwide, there are over 300 million
(300,000,000) telephones - 98% of them interconnected.
POTS - Plain Old Telephone Set
The POTS, or Plain Old Telephone Set, consists of these 5
sections:
i.Ringer Unit
ii.Hook Switch
iii.Dialer Unit
iv.Hybrid/Speech Network
v.Hand Set
POTS
The connection to the CO (Central Office) comprises only 2 wires:
Tip and Ring. This connection is called the "Local Loop."
The Local Loop
Tip & Ring
The Tip is +ve and colored green. The Ring is -ve and colored
Red. If you look at a phone jack in your house, you will see that it
is wired for 4 wires: Red, Green, Black and Yellow. However, black
and yellow are not normally used.
The black and yellow wires can be used for a second telephone
line or they can be used for running a Network Physical layer
protocol called Phonenet (by Farralon). Phonenet uses the black
and yellow for Network communications. It is for use with
Appletalk, and is a replacement for Localtalk. It runs at the
Localtalk speed of 230 Kbps, reasonable for small networks.
Ringer Unit
Ringer Unit
The ringer is a device that alerts you to an incoming call: it
interprets the ringing voltage from the Central Office.
Originally, the ringer was a electromagnetic bell. Today,
though, most ringers are electronic devices.
The Central Office sends the following:
•a 90 to 120 VAC ringing voltage
•Frequency of 20 Hz
•Cadence for North America is 2 sec On/ 4 sec Off
The Hook Switch
Hook Switch
The hook switch is activated by lifting the handset off of the
cradle. The position of the hook switch determines whether the
telephone is waiting for a call, or is actively using the line. The
off-hook position informs the network of a request for use. The
on-hook position releases the use of the network.
The Dialer Unit
Dialer Unit
There are two types of Dialer Units:
Rotary and Touch Tone. Rotary is the
old "put your finger in the hole and
spin" type. The rotary dial operates by
toggling the Hook Switch on and off.
Touch Tone is the modern
method where 2 frequencies
per push button are sent.
Touch Tone is a trade name;
the correct name is DTMF
(Dual Tone Multi Frequency).
Hybrid/Speech Network
Hybrid/Speech Network
The Hybrid/Speech Network performs these functions:
•It converts the Tx/Rx 4 wires from the Handset to the 2 wires
for the Local Loop.
•It interfaces the signals from the Dialer Unit to the telephone
line.
•It provides auto line compensation for line length to keep the
volume constant.
The Handset
Handset
The Handset contains transducers that convert mechanical energy into
electrical energy. The microphone converts speech into electrical energy
while the diaphragm (or speaker) converts electrical signals into audible
signals.
Functions of a Telephone Set are shown below.
i.Request use of network from the CO (Central Office).
ii.Inform you of the network status: Dial-tone, Ringing, Busy, Fast
Busy (Talk Mail)
iii.Informs CO of desired number.
iv.Informs you when a call is incoming (phone rings).
v.Releases use of network when call is complete (hang-up)
vi.Transmit speech on network & receives speech from distant caller.
vii.Adjust power levels and compensates for line length
Local Loops
Local Loops
The Local Loop is the connection between the Central
Office and the home or business. Two wires (1 pair) are run
into every home. The pair does not go directly to the Central
Office. Instead, it goes to those big green boxes--that you
see on the street corners--called "Serving Area Interfaces"
(SIA) . Large multi-conductor bundles of wires then go from
there to the Central Office.
TELCO Architecture
The Central Office
The Central Office (2)
The Central Office provides the following functions:
i.It supplies the battery voltage for the telephone system. The
On-hook voltage is 48 Vdc +/- 2V. Off-hook voltage is -6.5 Vdc.
ii.It supplies the Ringing Generator - 90 to 120 VAC, 20 Hz, 2
sec on/ 4 sec off
iii.It supplies the Busy signal (480 + 620 Hz, 0.5 sec On/ 0.5
sec Off), Dial Tone (350 + 440 Hz) and Fast Busy (480 + 620
Hz, 0.2 sec On/ 0.3 sec Off).
iv.It has the digital switching gear that determines if the
number is an Interoffice call (local) or an Intraoffice call (Toll long distance).
Central Office (3)
A Central Office can have up to 10,000 subscribers (for
example, 284-0000 to 284-9999). Most have 4,000 to 5,000
subscribers. The Central Office bases the loading
requirements on roughly 10% of the phones that will be in
use at any one time. However, the use of Internet dialup
access has drastically changed this statistic
Hierarchical Phone Networks
The PSTN (Public Switch Telephone Network) is divided into a
hierarchical network. Here are the 5 classes of switching
centers in North America:
Center Class
Description
Abbreviation
1
Regional Center
RC
2
Sectional Center
SC
3
Primary Center
PC
4
Toll Center
TC
4b
Toll Point
5
Central Office
TP
CO
Symbol
An Example
Hierarchical Structure
The Hierarchical portion is seen as follows:
Trunk
Long distance telephone cable
Toll Trunk
Connects CO (Central Office) to
TC (Toll Center)
Everything above TC (Toll
Center) and TC to TC
Between CO (Central Office)
Intertoll Trunk
Interoffice Trunk
Intraoffice Trunk
Call between 2 subscribers
within the same CO (284-7079
to 284-8181
Call Routing
Call routing:
1.Preferred route
2.Second choice
3.Third Choice
Call routing is determined by network engineering and physical location.
When all lines are idle, the call routing selects the preferred route. If the
preferred route is busy, then the call is routed to the second choice. Because
the second choice is routed through one toll center, the charge for the call is
greater than the preferred route. The third choice is used when the second
choice is busy. The third choice goes through 2 toll centers, and is the most
expensive route
END Class 3