COMP 421 /CMPET 401

COMMUNICATIONS and NETWORKING Chapter 3 Data Transmission

Review Connection/Connectionless

Connection oriented

{

Connection-

{

less Service

Reliable Message Stream Reliable byte stream Unreliable connection Unreliable datagram Acknowledged Datagram Request-reply

Example

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

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 Layer N+1 Layer N 1 5 7 4 6 1 2 3 4 5 6 7 8 9 10 Computer 1 Time Layer N+1 Layer N 2 3 6 5 8 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 Application Presentation Session transport Network Data link Physical Packets Intermediate node Network Frames Data link Bits Physical

Signals

Destination node Application Presentation Session transport Network Data link 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

Analog Signaling

 represented by sine waves

1 cycle frequency (hertz) = cycles per second phase difference time (sec)

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

Sin2πt

Varying Sine Waves

Sin

2 Sin4πt 0.5Sin2πt

Sin

( 2 

t

  4 ) Phase Shift in radians or

Sin

2  (

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*10 8 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 +

S

n=1

a

n sin(2



nft) + n=1

S

b

n cos(2



nft) f = 1/T is fundamental frequency a & b coefficients are the amplitude of the n th harmonic This is a Fourier Series

Time -> Harmonic spectrum Original 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

the

bit rate

is not necessarily the same as  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

transmitter P 1 watts Attenuation Amplification P 2 watts receiver

10 log 10 (P 1 /P 2 ) dB 10 log 10 (P 2 /P 1 ) 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/N db = 10 log S N

N= noise power S= average signal power

Effect of noise

Logic Threshold

0 1 1 1 1 0 0 0 0 1 0 1 0 1 1 0 0 1 0 1 Bit error

Signal Noise Signal+Noise Sampling times

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 log 2 for M discrete levels M bits/sec

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 log 2 M

B = bandwidth M = number of discrete signal levels Theoretical capacity for

Noiseless channel Example: Channel capacity calculation for voice bandwidth (~3100 Hz): M Max data rate (C)

2 6200 bps 4 12400 bps 8 18600 bps 16 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 log 10 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 log 2 (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 1 Description Abbreviation Symbol Regional Center RC 2 Sectional Center SC 3 4 4b 5 Primary Center Toll Center Toll Point TP Central Office PC TC CO

An Example

Hierarchical Structure

The Hierarchical portion is seen as follows: Trunk Long distance telephone cable Toll Trunk Intertoll Trunk Interoffice Trunk Intraoffice Trunk Connects CO (Central Office) to TC (Toll Center) Everything above TC (Toll Center) and TC to TC Between CO (Central Office) 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