1 Physical Layer Concerned with Transmission of Unstructured Bit Stream Over Physical Medium. Data Transmission: Simplified Communication Block Diagram

1

Physical Layer

• Concerned with Transmission of Unstructured Bit Stream Over Physical Medium.

• Data Transmission:

Simplified Communication Block Diagram

Input Device

Transmitter

Medium

Receiver

Output Device

SourceSystem

DestinationSystem

m

g(t)

s(t)

r(t)

g’(t)

m’

2

Concepts & Terminology

• Medium (Simplex, Halfduplex, Fullduplex) – Hardware --- Signal is Physically Confined.

• Twisted-pair Wires,

• Coaxial Cables,

• Fiber Optics.

– Software --- Signal is Not Physically Confined.

• Propagation Through Air,

• Seawater.

3

Frequency, Spectrum, and Bandwidth:

Signal• Continuous (or Analog)• Discrete (or Digital)• Periodic -- Shape is Repeated• Aperiodic -- Shape is Not Repeated

4

Frequency, Spectrum, and Bandwidth: (cont.)

• Three Attributes:– Frequencies -- Number of Cycles per Second (Hertz

(Hz) = 1 CPS)

– Amplitude -- Instantaneous Value of The Signal During a Cycle.

– Phase -- Part of a Cycle That a Signal Has Passed When It Is Measured; Or a signal That Advanced a Certain Number of Degrees Pass The Reference Points.

5

Frequency, Spectrum, and Bandwidth: (cont.)

• All Signals Used In These Examples Will Be Sinusoidal & Can Be Described By;

V(t) = A sin(2ft + )

where A is Maximum Amplitude, f is Frequency, t is Instant of Time,

and is Phase.

6

Examples:

Sine wave representation of a signal (periodic signal)

Aperiodic Analog Signal (e.g., Human Voice)

1 Cycle

Time

0o

Amplitude

A

180o 360o

Time

Amplitude

+10V

Aperiodic Analog Signal (e.g., Human Voice)

7

Examples:(cont.)Aperiodic Discrete Signal

Continuous Signal

Time

Amplitude+5V

-5VAperiodic Discrete Signal

Period

Time0o

Amplitude+3V

180o 360o

-3V

90o

270o

0o

90o

Continuous Signal

Cycle

8

Examples:(cont.)

Note:– A period represents one full cycle– A cycle represents 360o (2 radians)– Angular velocity of the wave = number of

radians that the wave completes in a second– Total angle a sine wave completes in time t is: = t = 2ft

9

Examples:(cont.)

Discrete Signal(Digital Representation of Sine Wave)

Note: Both Examples Have Frequency of 3Hz or (3(2) = 3(360o) = 1080o

Amplitude+3V

-3VDiscrete Signal (Digital Representation of Sine Wave)

Time

10

Frequency Domain Concepts

So Far, We Have Viewed a Signal As a Function of Time. But Any Signal Can Also Be Viewed As a Function of Frequency

Example:s(t) = sin(2ft) + 1/3 sin3(2f)t + 1/5 sin5(2f)tThe Components of This Signal Are Just Sine Waves

of Frequencies f, 3f, and 5f. Using Fourier Analysis, It Can Be Shown That Any Signal Is Made Up of Components at Various Frequencies, Where Each Component is a Sinusoid.

11

Frequency Domain Concepts (cont.)

s(t) = sin(2ft) + 1/3 sin3(2f)t + 1/5 sin5(2f)tf1 3f1 5f1

f

1

s(t) Frequency Domain For Signal

12


Time12

1

f

sin(2f1)t1

1

f

Amp

-1

-0.5

0

0.5

1

0

13


Time13

1

f 1

1

f

1/3 sin 3(2f1)t

13

2

f

Amp

0

-0.5

0

0.5

1

-1

14


1/5 sin 5(2f1)t

Time15

1

f 1

1

f

Amp

0

-0.5

0

0.5

1

-1

15

2

f 15

3

f 15

4

f

15


sin(2f1)t + 1/3 sin 3(2f1)t + 1/5 sin 5(2f1)t

16


• Spectrum of Signal -- range of frequencies. From example above: spectrum extends from f1 to 5f1.

• Bandwidth -- Width of Spectrum or 4 f1

• Relationship Between Bandwidth & Data Rate. The Higher The Data Rate, The Greater The Bandwidth.

• Example: (Refer to Previous Examples)Let a Positive Pulse Represent a Binary 1 and a Negative

pulse Represent a Binary 0 Then The Signal Represents The Binary Stream 1010...

17


The Pulse Duration is 1/ (2 f1), Thus The Data Rate is 2 f1 Bits Per Second.

For f1 = 1000 Hz, The Data Rate = 2000 bps & The Bandwidth = 4000 Hz

18

Fourier Series

)2cos()2sin()(11

21 nftbnftactg

nn

nn

T

T

n

T

n

dttgT

c

dtnfttgT

b

dtnfttgT

a

0

0

0

)(2

)2cos()(2

)2sin()(2

A Way of Representing Any Periodic Function As a Sum of Harmonically Related Sinusoids.

Where f = 1/T is The Fundamental Frequency, an and bn are The Sine And Cosine Amplitudes of The nth Harmonics.

Coefficients Are:

19

Fourier Series (cont.)

dtnftkftT

)2sin()2sin(0

0 for k n

for k = nsin(2kft) for f = 1/2, or T = 2with different k

20


Multiplication of two sine waves with different k

21


)]4/6cos()4/7cos()4/cos()4/3)[cos(/(4

]|)4/cos()/(4|)4/cos()/(4[

)4/sin()4/sin(

)2sin()(

87,0

76,1

63,031,1

10,0

)(

01100010)(

76

314

1

7

6

3

141

8

0

2

nnnnn

ntnntn

dtntdtnt

dtnfttga

t

t

tt

t

tg

tg

tt

Tn

22


)]4/6sin()4/7sin()4/sin()4/3[sin(1

]|)4/sin(|)4/sin([4

1

])4/cos()4/9cos[4

1

)2cos()(2

)]4/7cos()4/6cos()4/3cos()4/)[cos(/(1

76

431

4

3

1

7

6

8

0

nnnnn

ntnt

dtndtnt

dtnfttgT

b

nnnnna

tntn

n

n

23


4

33

4

1

118

2

)(2

7

6

3

1

8

0

dtdt

dttgT

c

Note: The maximum value of sin(x) and cos(x) is 1 and the minimum value is -1. The maximum and minimum values of cos(x1) - cos(x2) + cos(x3) - cos(x4) and sin(x1) - sin(x2) + sin(x3) - sin(x4) are 4 and -4, respectively. Hence,an and bn converge to zero when n becomes infinite.

24


A binary signal and its rms Fourier amplitudes.

0

10 1 1 0 0 0 1 0

amplitude

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0.5

0.25

Successive approximations to the original signal

Time Harmonic number

0

1

1 harmonic

1

25

Fourier Series (cont.)Successive approximations to the original signal

Time Harmonic number

8 harmonies

03

1

1

2 harmonies

21

52

0

1

4 harmonies

31 42

6 84 7

0

1

26

Definition

• Digital Signal -- A Sequence of Discrete Discontinuous Voltage Pulses. Each Pulse is a Signal Element

• Baud -- Number of Signal Elements Per Second.• Note -- Baud Rate is Not Necessarily The Same As

Bit Rate.

27

Example

• Given a bit rate of b bits/sec, the time required to send 8 bits (for example) is 8/b sec, so the frequency of the first harmonic is b/8 Hz. An ordinary telephone line, often called a voice grade line, has an artificially introduced cutoff frequency near 3000 Hz. This restriction means that the number of the highest harmonic passed through is 24000/b, roughly (the cutoff is not sharp). For some commonly used data rates, the numbers work out as follows:

28

Example (cont.)

Bps T(msec) First Harmonic (Hz) Harmonic (Hz) sent

300 26.67 37.5 80600 13.33 75 40

1200 6.67 150 20 2400 3.33 300 10 4800 1.67 600 5 9600 0.83 1200 219200 0.42 2400 138400 0.21 4800 0

29

Maximum Data Rate of a Channel

• Signal-to-Noise RatioSignal Power

(S/N)dB = 10 log10

Noise Power

Expresses The Amount In Decibels(dB) That The intended signal exceeds the noise level.

A high S/N

High Quality Signal & a Low Number of Required Intermediate Repeaters.

30

Maximum Data Rate of a Channel (cont.)

• Shannon's Major Result:

Maximum Number of Bits/Sec = H log2 (1+S/N), Where H is The Bandwidth of The Channel In Hertz.

Example: Consider a Voice Channel Being Used Via Modem

to Transmit Digital Data. Assume: Bandwidth = 3100 Hz, S/N = 30dB or a Ratio of 1000:1 C = 3100 log2 (1 + 1000)

= 30,894 bps Theoretical Maximum

31

Shannon Theorem (Additional Comments)

For a Given Data Rate, We Would Expect That a Greater Signal Strength Would Improve The Ability To Correctly Receive Data In The Presence of Noise.

Key Parameter: (S/N).– Theoretical Maximum: Only Much Lower Rate is

Achievable.

– Only Assume Thermal Noise.

– Capacity --- Error Free Transmission.

32

Relation Between Data Rate, Noise, and Error.

• Noise Can Corrupt 1 or More Bits. • If The Data Rate is Increased, Then The Bits

Become “Shorter”', So More Bits Are Affected By a Given Pattern of Noise.

• Thus, At a Given Noise Level, The Higher The Data Rate, The Higher The Error Rate.

33

Nyquist's Result (Assumed Noiseless Channel)

Maximum Data Rate = 2 H log2V bits/sec.

For a System With Bandwidth H, The Maximum Data Rate Using Binary Signaling Elements (2 Voltage Levels) is 2H. So, For H = 3100 Hz, C = 6200 bps

Now, Suppose The Signal Has 8 Discrete Levels; We Have

C = 2 (3100Hz) log2(8) bits/sec

= 18,600 bps

34


Note:

1. An Increase in Data Rate Increases Bit Error Rate.

2. An Increase in S/N Decreases Bit Error Rate.

3. An Increase in Bandwidth Allows An Increase in Data Rate.

35


Noise figure

Types of Noise– Thermal Noise

– Intermodulation Noise

– Crosstalk

– Impulse Noise

36

Local Network Transmission Media

• Baseband Coaxial Cable– Digital Signaling– Entire Bandwidth Consumed By Signal– Bidirectional : Signal Inserted at Any Point Propagates in

Both Directions– Generally Uses Special-Purpose 50 Cable

• Broadband Coaxial Cable– Analog Signaling– FDM Possible– Unidirectional– Uses Standard 75 CATV Cable

37

Transmission Media

• Magnetic Media– Magnetic Tape – Floppy Disk

• Twisted Pair (Most Common)Used:– Telephone System– NetworksNote: Can Run Several Km Without Amplification

• Either Digital or Analog DataBandwidth Depends on Thickness of The Wire and The

Distance

38

Baseband Coax

• Bandwidth is a Function of The Cable Length. eg. 1km 10Mbps

Used For: – LANs– Telephone System

• Connecting to Computers:– T Junction– Vampire TAP

• Signaling:– Straight Binary– Manchester Encoding– Diff. Manchester Encoding

39

Broadband Coax (Several Channels)

Note: Can Be Used Up to 300MHz, To Support a Data Rate of 150Mbps.

+ Types of Broadband System– Dual Cable

– Midsplit Cable

– Note: Both Use a Device, Headend

Broadband Requires Skilled Radio Freq. Engineers to Plan The Cable and Amplifier Layout and Install System.

40

Which Media?

• Twisted Pair– Most Cost Effective– For Single Building, Low Traffic LAN

• Cable– Best For High Traffic, Lots of DP Devices.

• Fiber– Many Advantages, Cost-Effectiveness Improvements

Needed.• Microwave, Laser, Infrared

– Good Choices For Point-to-Point Links Between Buildings.

41

Three Different Encoding Techniques

42

Fiber Optics

• Three Components– Transmission Medium– Light Source (LED)– Detector (Photodiode)

• Unidirectional System That Accepts an Electrical Signal, Converts & Transmit It By Light Pulses, and Then Reconverts The Output to An Electrical Signal at The Receiving End.

• Multimode Fiber• Single Mode (Up to 1000 Mbps)

43

Fiber Optics (cont.)

(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.

44


A fiber optic ring with active repeaters

45


A passive star connection in a fiber optics network

46

Telephone System

(a) Fully interconnected network. (b) Centralized switch. (c) Two level hierarchy.

47

Example of Circuit Route

Typical circuit route for a medium-distance call.

48

Modems

Transforms a Digital Bit Stream Into an Analog Signal.

• Related Terms:– Modulation -- The Process of Varying Certain

Characteristics of a Signal, Called a Carrier.– Carrier -- A Continuous Frequency Capable of Being

modulated with a second signal (Information Carrying). Note: Signals Used at Local Loops Are DC, Limited by

Filters to The Frequency Range 300 Hz to 3k Hz. This is Too Slow For Digital Signaling. Therefore, AC Signaling is Used.

49

AC Signalings

A Continuous Tone in The Range of 1000Hz to 2000Hz is Introduced (Sine Wave Carrier)

Now, We Must Use An Encoding Technique, Modulation (An Operation On 1 or More of The Three Characteristics of a Carrier Signal):– Amplitude (ASK)– Frequency (FSK)– Phase (PSK)

This Produces a Signal Which Occupies a Bandwidth Centered on The Carrier Frequency.

50

AC Signalings (cont.)

Note:

ASK -- On Voice Grade, Up to 1200 bps; Used Over Fiber.

FSK -- Less Susceptible to Error, Up to 1200 bps. Can Be Used For Higher Frequencies.

51


ASK: 2 Different Binary Values Are Represented By 2 Different Amplitudes of The Carrier Frequency.

FSK: 2 Different Binary Values Are Represented By 2 Different Frequencies Near The Carrier Frequency; Offset From The Carrier By Equal But Opposite Amounts.

PSK: The Phase of The Carrier Signal is Shifted to Represent Data. A Binary 0 Sending A Signal Burst of The Same Phase as The Previous Phase. A Binary 1 Sending A Signal Burst of Opposite Phase to The Preceding One.

52


(a) A binary signal (b) Amplitude modulation(c) Frequency Modulation (d) Phase modulation

53


(a) A talking to B (b) B talking to A

54

Encoding Techniques•Amplitude-shift keying (ASK)

cos(2fct + c) binary 1

s(t) =

binary 0

Frequency-shift keying (FSK)

cos(2f1t + c) binary 1

s(t) =

cos(2f2t + c)binary 0

Phase-shift keying (PSK)

cos(2fct + ) binary 1

s(t) =

cos(2fct) binary 0

55

Encoding Techniques (cont.)

011001110001011110100(d) PCM output

(a) original signal

(b) PAM pulses

(c) PCM pulses

(d) PCM output

56

TDM

• Synchronization is Needed Over The Trunk Circuit • Example:

Bell Telephone T1 Carrier System.24TDM Channels,Sampling Rate of 8000 samples/sec.,8 Pulses/Sample (7 Standard levels Plus 1 For

Synchronization),

Frame Consists of 24 8 = 192 Bits Plus 1 Extra Bit For Framing. Yielding 193 Bits Every 125 sec., Gross Data of 1.544 Mbps (CCITT Standard).

57

TDM (cont.)

The Bell system T1 carrier (1.544 Mbps).

58

Wireless Transmission

• The Electromagnetic Spectrum-when electrons move, they create electromagnetic waves.

• By attaching an antenna to an electrical circuit, the electromagnetic waves can be broadcast efficiently & received via receiver some distance away.

• In a vacuum, all electromagnectic waves travel at the same speed: 3 (10)8 m/sec.

59

Wireless Transmission (Cont.)

• The radio, microwave, infrared, and visible portion of the spectrum can all be used for transmitting info by modulating the amplitude, frequency, or phase of the waves.

• The FCC allocates spectrum for AM and FM, TV, Cellular Phones, police, Military, Telephone Companies, Government, etc.

60

Radio Transmission

• Radio waves are omnidirectional. They are easy to generate, can travel long distances, penetrate buildings easily, & thus widely used for communication (both indoor and outdoor).

• Typically frequency ranges from 30 MHZ to 1 GHZ.

61

Radio Transmission (Cont.)

• For digital data communication, the low frequency range implies that only lower data rates are achievable (i.e., in the kilobit rather than the megabit range).

• Example: ALOHA, bandwidth 100kHz, data rate 9600 bps.

62

Microwave Transmission

• Waves travel in a straight line (above 100 MHZ), and can be narrow focused.

• Transmitting receivers and transmitters must be accurately aligned.

• Microwave (two types): Terrestrial and Satellite• Terrestrial: typical antenna is parabola “dish”,

about 10 ft in diameter, usually located at heights above the ground level.

63

Microwave Transmission (Cont.)

• Primary Uses:

long-haul telecommunication services, as an alternative to coaxial cable for transmitting TV and voice, short point-to-point link between buildings for closed-circuit TV or a data link between networks.

• Example: Microwave Communications, Inc. (MCI)• Common Frequency Range: 2 to 40 GHz.

64

Satellites

• A Communication Satellite -- A microwave Relay Station, Used to Link 2 or More Ground-Based Microwave Transmitters/Receivers.

• Satellite Receives Transmissions On One Frequency Band (Uplink), Amplifies/Re- peats It on Another Frequency (Downlink).

• Frequency Bands -- Transponder Channels or Transponders.

• Altitude: 36,000km, The Satellite Period is 24 Hours.

65

Communication Satellite (Cont.)

Uses:– TV Distribution (e.g., PBS).– Long-distance telephone transmission.– Private business networks.– Mobile Satellite Service (FCC has allocated the

L Band: 1.65 GHz-Uplink & 1.55 GHz-Downlink)

66


• Spacing Standard: > 4o Apart In The 4/6 GHz Band, & > 3o Spacing at 12/14 GHz.

• Optimum Frequency Range 1-10GHz.• Point-to-Point Bandwidth: 4/6 GHz.• Round Trip Propagation Delay: 240 - 300 ms.• TDM Used For Accessing Channel.

67


Point-to-point link via satellite microwave

68


Broadcast link via satellite microwave

69


A Two-antenna

satellite (a)

70


A Two-antenna

satellite (b)

71

Encoding for satellite

Typical Satellite Splits Its 500 MHz Bandwidth Over a Dozen Transponders, Each With a 36 MHz Bandwidth. Each Transponder can Encode a Single 50Mbps Data Stream, 800 64Kbps Digital Voice Channels, or Other Combinations.

Satellite vs Terrestrial – T1 (1.544Mbps) vs 1000 Times This Via

Rooftop-to-Rooftop Transmission.– Fiber Has More Potential Bandwidth.

72

Transmission and Multiplexing

• FDM– Effective Bandwidth of 3000 Hz (From 350 to 3350 Hz)

Can be theoretically divided into ten 300 Hz channels.– Disadvantage: Limited Number of Low-Bandwidth Can

Be Multiplexed to Share A High-Bandwidth Circuit.– Advantage: Reliability and Simplicity of Equipment.

Also, Bit-Level Synchronization is Not Needed.– Note: Filters Are Used At Both The Transmitting and

receiving station to separate one frequency from another.

• TDM– Divides The Channel Into Discrete Time Slot.

73

Multiplexing

coscos= 1/2 [cos( + ) + cos( - )]Note: As shown in the equation above, multiplying two

cosine functions yields a new signal with two new cosine components.

s(t)coscoss(t) (coss(t) cos2

s(t)/ 2 + s(t) cos2Note: By multiplying the original signal with a cosine

function (i.e. carrier signal) twice, we get the original signal plus some additional signal. Multiplication is applied once in the transmitter and once in the receiver.

74

Example

Let the original signal s(t) = 4 cos (2 10t) + 8 cos(2 50t)

Let the carrier signal be cos(2 70t)Therefore, the result of the multiplication in the

transmitter is:s(t)cos(2 70t) = 2cos(2 80t)

+ 2cos(2 60t)+ 4cos(2 120t)+ 4cos(2 20t)

75

Example (cont.)

With a filter of 70 Hz and above, the signal between 0Hz and 70Hz will be erased. The new signal 2 cos(280t) + 4cos(2 120t) , which is shifted 70Hz of original signal, will be transmitted.

Amplitude

0 20 60 80 120

4 4

22

70

Hz

76

Hardware Diagram

77

Hardware Diagram (cont.)

78

Hardware Diagram (cont.)

Note : The typical range of human voice is between 300 and 3100 Hz.However, we wish to allow for a range of 0 to 4 KHz, in order to avoid signal interference

79

Example:

Telephone line with human voice (usually in the range 300 to 3100Hz)

80

Example: (cont.)

81

Definition of Digital Signal Encoding Formats

• Nonreturn-to-Zero-Level (NRZ-L) 0 = high level 1 = low level

• Nonreturn to Zero Inverted (NRZI) 0

= no transition at beginning of interval (one bit time) 1

= transition at beginning of interval

• Bipolar-AMI 0

= no line signal 1

= positive or negative level, alternating for successive ones

82

Definition of Digital Signal Encoding Formats (cont.)

• Pseudoternary 0 = positive or negative level, alternating for successive

zeros 1 = no line signal

• Manchester 1 = transition from high to low in middle of interval 0 = transition from low to high in middle of interval

• Differential Manchester Always a transition in middle of interval 0 = transition at beginning of interval 1 = no transition at beginning of interval

83


• B8ZS Same as bipolar AMI, except that any string of eight zeros is replaced by a string with two code violations

• HDB3Same as bipolar AMI, except that any string of four zeros

is replaced by a string with one code violation

84


85

Interfacing

Most Digital Data Processing Devices Have Limited Data Transmission Capability. Typically Generate NRZ-L Digital Signals. The Distance Across Which They Can Transmit Data is Also Limited. Hence, The More Common Case is:

Digital datatransmitter/receiver

Transmissionline interfacedevice

Transmissionline interfacedevice

Digital datatransmitter/receiver

Signal andcontrol leads

Bit-serialtransmission medium

Data terminalequipment(DTE)

Data circuit-terminatingequipment (DCE)

..

..

..

..

Generic interface to transmission medium

86

Interfacing (cont.)

• DTE: Data Terminal Equipment• Examples: terminals, workstations• DTE's are rarely directly connected to transmission

media such as coaxial or fibers.• Reason?

– Signal Strength

– Bit-serial Transmission Media are widely used

• Solution: DCE(Data Circuit-terminating Equipment)

87

Characteristics of Interfacing

• Four Characteristics:– mechanical: DTE/DCE connectors– electrical: voltage, coding schemes– functional: assignment of meanings to

interchange wires– procedural: protocol (state transmissions)

• Most Popular Standards:– EIA-232-D (de facto)

– X.21 (CCITT Physical Layer under X.25)

– ISDN Physical Interface

88

EIA-232

• EIA: Electronic Industries Association• Variations: 232-C(1969), 232-D(1987)• Target Media: voice-grade telephone lines• Connector: DB25, a 25-pin connector standard• Signaling: Digital Signals are used

– Data: -3V = bit 1, > +3V = bit 0

– Control: -3V = OFF, > +3V = On

– Data Rate: 20kbps

89

EIA-232 (cont.)

• Interchange Circuits– Data(4): Support full-duplex traffic

– Control(15): transmission, testing, quality monitoring

– Timing(3):

– Ground/Shield(2):

• The procedural definition concerns: – call set-up

– data transfer

– call clearing

90

X.25 (international Standard)

• Defines the Interface Between the Host (DTE) and the Carrier's Equipment (DCE)

• X.25 Has 3 Layers: – Physical (X.21 and X.21 bis)

– Frame

– Packet

• Will Look at Digital Interface (X.21). 15 pins

91

Interfacing

• RS-232C

• RS-449/442-A/423-A

• X.21 (15 pins)

92

Interfacing (cont.)

Signal lines used in X.21

DTE DCE

T (Transport)

C (Control)

R (Receive)

I (Indication

S (Signal, i.e. bit timing)

B (Byte timing) optional

Ga (DTE common return)

G (Ground)

93

Interfacing (cont.)

An example of X.21 usage. DTE DCE

Step C I Event in telephone analog sends on T sends on R

0 Off Off No connection-line idle T = 1 R = 1 1 On Off DTE picks up phone T = 0 2 On Off DCE gives dial tone R=“+++...+” 3 On Off DTE dials phone number T = address 4 On Off Remote phone rings R=call

progress 5 On On Remote phone picked up R = 1 6 On On Conversation T = data R = data 7 Off On DTE says goodbye T = 0 8 Off Off DCE says goodbye R = 0 9 Off Off DCE hangs up R = 1 10 Off Off DTE hangs up T = 1

94

SONET / SDH

• SONET(Synchronous Optical NETwork)/SDH(Synchronous Digital Hierarchy)

– Motivated by break up of AT&T– Local telephone company had to connect to

multiple long distance carriers– Standards needed

• Started in Bell-Core

• Joined by CCITT

95

SONET Design Goals

• Enable different carriers to interwork• Unify the U.S., European, and Japanese digital

systems• Provide a way to multiplex digital channels together• Provide support for operations, administrations, and

maintenance.

Note: SONET (A Synchronous system + uses TDM)

96

A SONET path

SourceMultiplexer Repeater Multiplexer Repeater

DestinationMultiplexer

Section Section Section Section

Line Line

Path

97

Basic SONET Frame

• 810 bytes put out every 125sec.• 8000 frames/sec (matches the sampling rate of the

PCM channels used in telephone system• 8 810 = 6480 bits are transmitted, and 8000

times per sec Gross data rate 51.84Mbps, STS-1 (Synchronous Transport Signal). All SONET trunks are multiples of STS-1.

• Hence, we have OC-3, OC-12, etc.• View a sonet as a rectangle of bytes (90 9). After

factoring out overhead, 87 9 8 8000 = 50.112 Mbps user data.

98

Two back-to-back SONET frames

3 Columnsfor overhead

87 Columns

9Rows

..... Sonetframe

(125sec)

Sonetframe

(125sec)

Sectionoverhead

Lineoverhead SPE

Pathoverhead

99

Multiplexing in SONET

T1T1

T1

T3

T3

STS-1

STS-1

STS-1 STS-3

STS-3

STS-3

STS-3

STS-12 OC-12

3:1Multiplexer

4:1Multiplexer

ScramblerElectro-optical

converter

100

SONET and SDHmultiplex rates

SONET SDH Data rate(Mbps)

Electrical OpticalOptical Gross SPE User

STS-1 OC-1 51.84 50.112 49.536 STS-3 OC-3 STM-1 155.52 150.336 148.608

STS-9 OC-9 STM-3 466.56 451.008 445.824 STS-12 OC-12 STM-4 622.08 601.344 594.432

STS-18 OC-18 STM-6 933.12 902.016 891.648 STS-24 OC-24 STM-8 1244.16 1202.688 1188.864 STS-36 OC-36 STM-12 1866.24 1804.032 1783.296 STS-48 OC-48 STM-16 2488.32 2405.376 2377.728

101

Information Switching

Physicalcopperconnectionset upwhen callis made.

packetsqueued upfor subsequenttransmission

Computer

SwitchingOffice

(a) Circuit switching (b) Packet switching

102

Information Switching (cont.)(Timing of events)

(a) Circuit Sw. (b) Message Sw. (c) Packet Sw.

103

Circuit Switching

• Circuit Switching: Dedicated Path Between 2 Stations. – Circuit Establishment

– Data Transfer

– Circuit Disconnect

– Advantage: Good for Applications Which Require Continuous Data Flow (e.g. Voice)

– Disadvantage: Unused Bandwidth

104

Message Switching

• Message Switching (Store-And-Forward):

Exchange Blocks of Data Between IMPs With no Limit on Block size.

Disadvantage: Large Buffer Required and IMP-IMP Line May be Tied Up Too Long.

105

Packet Switching

• Packet Switching:

Long Message are Subdivided into ShortPackets, and Packets are Transmitted Between IMPs.

Advantage: Suited for Handling Interactive TrafficDisadvantage: Proper Routing Problem

106

ISDN Concept

• Principles of ISDN

• Support of Voice and Non-Voice Applications

• Support for Switched and Nonswitched Applications

• Reliance on 64Kbps Connections

• Intelligence in the Network

• Layered Protocol Architecture

• Variety of Configuration

107

Evolution of ISDN

• Evolution From Telephone IDN's

• Transition of One or More Decades

• Use of Existing Networks

• Interim User Network Arrangements

• Connections at Other Than 64kbps

108

Objectives of ISDN

• Standardization

• Transparency

• Separation of Competitive Function

• Leased and Switched Services

• Cost-Related Tariffs

• Smooth Migration

• Multiplexed Support

109

Comments of Service

• Videotex - Interactive Access to Remote Database. Example:

- On-line Telephone Book • Teletex -- A Form of Electronic Mail For Home

and Business Use Note: May Need Written Copies Via Fax

• Telemetry or AlarmExample:

- Electronic Meter Reading,- Smoke Detectors

110

Candidate Services for Integration

Service

Bandwidth Telephony Data Text Image

Digital Telephone Packet-switched Telexvoice Circuit-switched Teletex

(64Kbps) Leased Leased circuits Leased circuit circuits Telemetry Videotex Information Funds transfer Facsimile retrival (by voice and Information Information Information synthesis) retrieval retrieval retrieval

Mailbox Mailbox SurveillanceElectronic mail Electronic mailAlarms

111

Candidate Services for Integration

Service

Bandwidth Telephony Data Text Image

Wide Music High-speed TV band Computer

conferencing

(>64Kbps) Communication Teletex

Videophone

Cable TVdistribution

112

Comments on ISDN Architecture

• Digital Bit Pipe (64Kbps)

• Support Multiple Independent Channels by

TDMing of The Bit Stream

• Two Principal Standards

- Low Bandwidth (Home Use)

- High Bandwidth (Businesses)

113

Comments of Service

114

ISDN Architecture Continue

• NT1 -- Network Terminating Device, Connected to The ISDN Exchange.– Has Connectors For a Passive Bus Cable

– Up to 8 ISDN Devices can be Connected

– Has Electronics For Network Adm., Monitoring, Performance, Contention Resolution, \etc

115


• NT2 (PBX) -- Needed by Businesses to Handle

More Traffic Simultaneously

– Need Adapter For Non-ISDN Devices

Example: RS-232C Terminal

– CCITT Has Defined Four Reference Points:

R, S, T, and U

U is Two-wire Copper Twisted Pair, But

Will Be Replaced By Fiber.

116


(a) Example ISDN system for home use

117


(b) Example ISDN system with a PBX for use in large business

118

Block Structure of a digital PBX

Switch

Control unit

Trunkmodule

Line modulefor ISDNdevices

Line modulefor RS-232-C

terminals

Line modulefor analogtelephones

Service unit

To ISDNexchange

119


• A - 4kHz analog telephone channel• B - 64 kbps digital PCM channel for voice or data• C - 8 or 16 kbps digital channel

Interface

Interface

Customer’sequipment

Carrier’sequipment

120


• D - 16 or 64 kbps digital channel for out-of-band signaling

• E - 64 kbps digital channel for internal ISDN signaling

• H - 384, 1536, or 1920 kbps digital channel

121


• It is not CCITT's intention to allow an arbitrary combination of channels on the digital bit pipe. Three combinations have been standardized so far:

• 1. Basic rate: 2B + 1D• 2. Primary rate: 23B + 1D (U.S. and Japan) or 30B

+ 1D (Europe)• 3. Hybrid: 1A + 1C

Documents

1 Physical Layer Concerned with Transmission of Unstructured Bit Stream Over Physical Medium. Data Transmission: Simplified Communication Block Diagram