comp 421 /cmpet 401 communications and networking chapter 3 data transmission
TRANSCRIPT
COMP 421 /CMPET 401COMP 421 /CMPET 401
COMMUNICATIONS and NETWORKING
Chapter 3
Data Transmission
Review Connection/ConnectionlessReview Connection/Connectionless
Service ExampleReliable Message Stream Sequence of PagesReliable byte stream Remote logonUnreliable connection Digitized VoiceUnreliable datagram Electronic Junk MailAcknowledged Datagram Registered mailRequest-reply Database Query
{{Connection-
oriented
Connection-less
Review Connection/ConnectionlessReview Connection/Connectionless
PRIMITIVE MEANINGRequest A Entity wants the service to do somethingIndication A Entity is informed about an eventResponse An Entity wants to respond to an eventConfirm The response to an earlier request has come back
•Connection-oriented service is modeled after the Telephone Company•Connectionless Service is modeled after the Postal System
A A Sample Connection Oriented ServiceSample Connection Oriented Service
CONNECT.request Request a connectionCONNECT.indication Signal the called PartyCONNECT.response Callee accepts or rejects callCONNECT.confirm Tell Caller whether call was acceptedDATA.request Request that data be sentDATA.indication Signal the arrival of dataDATA.response Request that connection be releasedDATA.confirm Signal peer about request
5 714 6
5 36 8 2
Computer 1
Computer 2
1 2 3 4 5 6 7 8 9 10 Time
Layer N
Layer N
Layer N+1
Layer N+1
LAST WEEK - OSILAST 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/IPLAST 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 poorlysuited 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
DECIBELSDECIBELS
•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 levelsN = 10logP2/P1 N = number of decibels
P1=input power levelP2=output power level
•dBW (decibel-watt) is the absolute power levelPower = 10log Power (watts)/1(watt)
1mW = -30dBW 1 W = 0 dBW
1000W = 30dBW
This Week: The Physical LayerThis 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 LayerPhysical Layer
Application
Presentation
Session
transport
Network
Data link
Physical
Application
Presentation
Session
transport
Network
Data link
Physical
Network
Data link
Physical
Source node Destination node
Intermediate node
Signals
Packets
Bits
Frames
Physical / Data Link Layer InterfacePhysical / Data Link Layer Interface
NL
DLL
PL
Frame
HDR
ACK
HDR
Sender Receiver
Transmitted Bits
Transmission Terminology (1)Transmission Terminology (1)
TransmitterReceiverMedium
– Guided mediume.g. twisted pair, optical fiber
– Unguided mediume.g. air, water, vacuum
Transmission Terminology (2)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)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 BandwidthFrequency, 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 SignalsElectromagnetic 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 conceptsTime domain concepts
– A Continuous signalVaries in a smooth way over time
– A Discrete signalMaintains a constant level then changes to another
constant level
– A Periodic signalPattern repeated over time
– An Aperiodic signalPattern not repeated over time
Periodic Signal CharacteristicsPeriodic 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
time(sec)
amp
litu
de
(vo
lts)
1 cycle
frequency (hertz)= cycles per second
phase difference
Analog SignalingAnalog Signaling
represented by sine waves
Digital SignalingDigital Signaling
represented by square waves or pulses
time(sec)
amp
litu
de
(vo
lts)
1 cycle
frequency (hertz)= cycles per second
BPS vs. BaudBPS vs. Baud
BPS=bits per secondBaud=# of signal changes per secondEach signal change can represent more
than one bit, through variations on amplitude, frequency, and/or phase
Continuous & Discrete SignalsContinuous & Discrete Signals
PeriodicPeriodicSignalsSignals
Sine WaveSine WavePeak 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 WavesVarying Sine Waves
Sin2πt 0.5Sin2πt
Sin4πt
2Sin )4
2( tSin
or)125.0(2 tSin
Phase Shift in seconds
Phase Shift in radians
Wavelength (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 ConceptsFrequency 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 wavesOne can plot frequency domain functions
instead of/in addition to time domain functions
Addition of Addition of FrequencyFrequencyComponentsComponents (a) Sin(2πft)
(b) (1/3)Sin(2π(3f)t)
(c) (4/π)[Sin(2πft)+(1/3)Sin(2π(3f)t)]
Communications BasicsCommunications 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 + an sin(2nft) + bn cos(2nft)n=1 n=1
f = 1/T is fundamental frequencya & b coefficients are the amplitude of the nth harmonic
This is a Fourier Series
Time -> Harmonic spectrum
Original
As we add more harmonics the signal reproduces the original more closely
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 TransmissionSignal 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
Signal TransmissionSignal Transmission
Spectrum & BandwidthSpectrum & 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 ComponentSignal with DC Component
Data Rate and BandwidthData Rate and Bandwidth
Any transmission system has a limited band of frequencies
This in turn limits the data rate that can be carried
BandwidthBandwidth
Width of the spectrum of frequencies that can be transmitted– if spectrum=300 to 3400Hz,
bandwidth=3100HzGreater bandwidth leads to greater costsLimited bandwidth leads to distortionAnalog measured in Hertz Digital measured in baud or Bps
Analog and Digital Data TransmissionAnalog and Digital Data Transmission
Data – Entities that convey meaning
Signals– Electric or electromagnetic representations of
dataTransmission
– Communication of data by propagation and processing of signals
Voice Grade LineVoice 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
DataData
Analog– Continuous values within some interval– e.g. sound, video
Digital– Discrete values– e.g. text, integers
Acoustic Spectrum (Analog)Acoustic Spectrum (Analog)
SignalsSignalsMeans by which data are propagatedAnalog
– 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 SignalingDigital 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 CodesASCII 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 SignalingDigital Image Signaling
Pixelization and binary representation
Code: 0000000000111100011101100111111001111000011111100011110000000000
Bit rate and Baud rateBit 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 SignalsData 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?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 SignalsVoice 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 Analog Signals Carrying Analog and Digital Dataand Digital Data
QAMQAM•QAM - Quadrature Amplitude Modulation
•Diagrams that show legal combinations of amplitude and phase are called CONSTELLATION PATTERNS
2 bits/Baud8 Valid combinations4800bps
4 bits/Baud16 valid combinations9600bps 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 Digital Signals Carrying Analog and Digital Dataand Digital Data
Analog TransmissionAnalog Transmission
Analog signal transmitted without regard to content
May be analog or digital dataAttenuated over distance Use amplifiers to boost signalAlso amplifies noise
Digital TransmissionDigital Transmission
Concerned with content Integrity endangered by noise, attenuation etc.Repeaters usedRepeater receives signalExtracts bit patternRetransmitsAttenuation is overcomeNoise is not amplified
Advantages of Digital TransmissionAdvantages 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 MediaTransmission 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 CapacityImpairments 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 ImpairmentsTransmission Impairments
Signal received may differ from signal transmitted
Analog - degradation of signal qualityDigital - bit errorsCaused by
– Attenuation and attenuation distortion– Delay distortion– Noise
Transmission ImpairmentsTransmission Impairments
Attenuation– loss of signal strength over distance
Attenuation Distortion– different losses at different frequencies
Delay Distortion– different speeds for different frequencies
Noise
AttenuationAttenuation
transmitter receiver
P1 watts P2 watts
Attenuation 10 log10 (P1/P2) dB
Amplification 10 log10 (P2/P1) dB
AttenuationAttenuationSignal strength falls off with distanceDepends on mediumReceived signal strength:
– must be enough to be detected– must be sufficiently higher than noise to be
received without errorAttenuation is an increasing function of
frequency
Delay DistortionDelay 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 (1)
Noise is the major limiting factor in communication system performance
Noise is the unwanted signals that inserted between transmitter and receiver
Noise (2)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)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)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= average signal power
N= noise powerSN
Effect of noiseEffect of noise
Signal
Noise
Signal+Noise
0 1 1 1 1 0 0 0 0 1 Data Received
Sampling times
Bit error
0 1 0 1 1 0 0 1 0 1 Original data
Logic Threshold
Channel CapacityChannel 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 RateMaximum 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/secfor M discrete levels
Nyquist theoremNyquist 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 formulaNyquist formula
M Max data rate (C) 2 6200 bps 4 12400 bps 8 18600 bps16 24800 bps
M Max data rate (C) 2 6200 bps 4 12400 bps 8 18600 bps16 24800 bps
C = 2B log2 MB = bandwidthM = number of discrete signal levels
Theoretical capacity for Noiseless channel
Example: Channel capacity calculation for voice bandwidth (~3100 Hz):
In the ‘40s Shannon (of Shannon’s Law) extended the equation to a channel subject to thermodynamic (thermal) noiseThermal 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)
Shannon’s LawShannon’s Law
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 Maximum Data Rate of a Noisy ChannelChannel
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 30,000bpsNo matter how many or how few signal levels are usedand no matter how often or how infrequent samplesare taken
The Telephone CompanyThe Telephone Company
The Telephone NetworkThe Telephone NetworkThe 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 Unitii.Hook Switchiii.Dialer Unitiv.Hybrid/Speech Networkv.Hand Set
POTSPOTS
The connection to the CO (Central Office) comprises only 2 wires: Tip and Ring. This connection is called the "Local Loop."
The Local LoopThe Local Loop
Tip & RingTip & 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 UnitRinger 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 SwitchThe 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 UnitThe 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 NetworkHybrid/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 HandsetThe HandsetHandset 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 LoopsLocal 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 ArchitectureTELCO Architecture
The Central OfficeThe Central Office
The Central Office (2)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)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 Symbol
1 Regional Center RC
2 Sectional Center SC
3 Primary Center PC
4 Toll Center TC
4b Toll Point TP
5 Central Office CO
An ExampleAn Example
Hierarchical Structure Hierarchical Structure
The Hierarchical portion is seen as follows:
Trunk Long distance telephone cable
Toll Trunk Connects CO (Central Office) to TC (Toll Center)
Intertoll Trunk Everything above TC (Toll Center) and TC to TC
Interoffice Trunk Between CO (Central Office)
Intraoffice Trunk Call between 2 subscribers within the same CO (284-7079 to 284-8181
Call RoutingCall Routing
Call routing: 1.Preferred route2.Second choice3.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 3END Class 3