Download Physical Layer

Physical Layer
• Concerned with Transmission of Unstructured Bit
Stream Over Physical Medium.
m
• Data Transmission:
Input Device
g(t)
Transmitter
Source
System
s(t)
Medium
r(t)
Simplified
Communication
Block Diagram
Receiver
g’(t)
Output Device
Destination
System
m’
1
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.
2
Frequency, Spectrum, and
Bandwidth:
Signal
•
•
•
•
Continuous (or Analog)
Discrete (or Digital)
Periodic -- Shape is Repeated
Aperiodic -- Shape is Not Repeated
3
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.
4
Frequency, Spectrum, and
Bandwidth: (cont.)
• All Signals Used In These Examples Will Be
Sinusoidal & Can Be Described By;
V(t) = A sin(2pft + q)
where A is Maximum Amplitude, f is Frequency, t
is Instant of Time,
and q is Phase.
5
Examples:
Sine wave representation of a signal (periodic signal)
Amplitude
A
Time
1 Cycle
0o
180o
360o
Aperiodic Analog Signal (e.g., Human Voice)
Amplitude
+10V
Aperiodic Analog Signal (e.g., Human Voice)
Time
6
Examples:(cont.)
Aperiodic Discrete Signal
Amplitude
+5V
Time
-5V
Aperiodic Discrete Signal
Continuous Signal
Amplitude
+3V
90o
180o 360o
0o
90o
Cycle
0o
-3V
Time
270o
Period
Continuous Signal
7
Examples:(cont.)
Note:
– A period represents one full cycle
– A cycle represents 360o (2p 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: q
= wt = 2pft
8
Examples:(cont.)
Discrete Signal(Digital Representation of Sine Wave)
Amplitude
+3V
Time
-3V
Discrete Signal (Digital Representation of Sine Wave)
Note: Both Examples Have Frequency of 3Hz or (3(2p)
= 3(360o) = 1080o
9
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(2pft) + 1/3 sin3(2pf)t + 1/5 sin5(2pf)t
The 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.
10
Frequency Domain Concepts
(cont.)
s(t) Frequency Domain For Signal
1
f
f1
3f1
5f1
s(t) = sin(2pft) + 1/3 sin3(2pf)t + 1/5 sin5(2pf)t
11
Frequency Domain Concepts
(cont.)
1
Amp
0.5
0
-0.5
-1
0
1
2 f1
1 Time
f1
sin(2pf1)t
12
Frequency Domain Concepts
(cont.)
Amp
1
0.5
0
-0.5
-1
0
1
3 f1
2
3 f1
1
f1
Time
1/3 sin 3(2pf1)t
13
Frequency Domain Concepts
(cont.)
Amp
1
0.5
0
-0.5
-1
0
1
5 f1
2
5 f1
3
5 f1
4
5 f1
1
f1
Time
1/5 sin 5(2pf1)t
14
Frequency Domain Concepts
(cont.)
sin(2pf1)t + 1/3 sin 3(2pf1)t + 1/5 sin 5(2pf1)t
15
Frequency Domain Concepts
(cont.)
• 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...
16
Frequency Domain Concepts
(cont.)
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
17
Fourier Series
A Way of Representing Any Periodic Function As a
Sum of Harmonically Related Sinusoids.


n 1
n 1
g (t )  12 c   an sin( 2pnft )   bn cos( 2pnft )
Where f = 1/T is The Fundamental Frequency, an and
bn are The Sine And Cosine Amplitudes of The nth
Harmonics.
2 T
an 
Coefficients Are:
T
0
g (t ) sin( 2pnft )dt
2 T
bn   g (t ) cos( 2pnft )dt
T 0
2 T
c   g (t )dt
T 0
18
Fourier Series (cont.)
 0 for k n
0 sin( 2pkft) sin( 2pnft)dt   T/2for k = n
sin(2pkft) for f = 1/2p, or T = 2pwith different k
T
19
Fourier Series (cont.)
Multiplication of two sine waves with different k
20
Fourier Series (cont.)
g (t )  01100010
 0,0  t  1
 1,1  t  3

g (t )  0,3  t  6
1,6  t  7

0,7  t  8
an 
8
2
T 0

g (t ) sin( 2pnft )dt
3
7

  sin( pnt / 4)dt   sin( pnt / 4)dt 
 1

6
 14 [4 /(pn) cos(pnt / 4) |3t 1   4 /(pn) cos(pnt / 4) |t76 ]
1
4
 4 /(pn)[cos( 3pn / 4)  cos(pn / 4)  cos(7pn / 4)  cos(6pn / 4)]
21
Fourier Series (cont.)
an  1 /(pn)[cos(pn / 4)  cos(3pn / 4)  cos(6pn / 4)  cos(7pn / 4)]
2 8
bn   g (t ) cos( 2pnft )dt
T 0
7
1 3
 [  cos 9pnt / 4)dt   cos(pn / 4)dt ]
6
4 1
1 4
 [ pn sin( pnt / 4) |3t 1  p4n sin( pnt / 4) |t76 ]
4
1
 [sin( 3pn / 4)  sin( pn / 4)  sin( 7pn / 4)  sin( 6pn / 4)]
pn
22
Fourier Series (cont.)
2 8
c   g (t )dt
T 0
7
2 3
  1dt   1dt 

6
8  1
1
3
 3 
4
4
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.
23
Fourier Series (cont.)
A binary signal and its rms Fourier amplitudes.
amplitude
1
0
1
1
0
0
0
1
0
0.5
0.25
0
1 2
3
4
5
6
7
8
9 10 11 12 13 14 15
Successive approximations to the original signal
1
1 harmonic
0
1
Time
Harmonic number
24
Fourier Series (cont.)
Successive approximations to the original signal
1
2 harmonies
0
1
2
1
4 harmonies
0
1
2
3
4
1
8 harmonies
0
1
Time
2
3
4 5 6
7
8
Harmonic number
25
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.
26
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:
27
Example (cont.)
Bps T(msec)
300
600
1200
2400
4800
9600
19200
38400
26.67
13.33
6.67
3.33
1.67
0.83
0.42
0.21
First
Harmonic (Hz)
Harmonic (Hz)
sent
37.5
80
75
40
150
20
300
10
600
5
1200
2
2400
1
4800
0
28
Maximum Data Rate of a
Channel
• Signal-to-Noise Ratio
(S/N)dB = 10 log10 Signal Power
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.
29
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
30
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.
31
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.
32
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
33
Nyquist's Result (Assumed
Noiseless Channel)
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.
34
Nyquist's Result (Assumed
Noiseless Channel)
Noise figure
Types of Noise
–
–
–
–
Thermal Noise
Intermodulation Noise
Crosstalk
Impulse Noise
35
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 50W Cable
• Broadband Coaxial Cable
– Analog Signaling
– FDM Possible
– Unidirectional
– Uses Standard 75 W CATV Cable
36
Transmission Media
• Magnetic Media
– Magnetic Tape
– Floppy Disk
• Twisted Pair (Most Common)
Used:
– Telephone System
– Networks
Note: Can Run Several Km Without Amplification
• Either Digital or Analog Data
Bandwidth Depends on Thickness of The Wire and The
Distance
37
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
38
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.
39
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.
40
Three Different
Encoding Techniques
41
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)
42
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.
43
Fiber Optics (cont.)
A fiber optic ring with active repeaters
44
Fiber Optics (cont.)
A passive star connection in a fiber optics network
45
Telephone System
(a) Fully interconnected network. (b) Centralized
switch. (c) Two level hierarchy.
46
Example of Circuit Route
Typical circuit route for a medium-distance call.
47
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.
48
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.
49
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.
50
AC Signalings (cont.)
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.
51
AC Signalings (cont.)
(a) A binary signal
(b) Amplitude modulation
(c) Frequency Modulation (d) Phase modulation
52
AC Signalings (cont.)
(a) A talking to B (b) B talking to A
53
Encoding Techniques
•Amplitude-shift keying (ASK)
 Acos(2pfct + qc) binary 1
s(t) = 

 0binary 0
Frequency-shift keying (FSK)
 Acos(2pf1t + qc) binary 1

s(t) = 

 Acos(2pf t + q ) binary 0
2
c
Phase-shift keying (PSK)
 Acos(2pfct + p) binary 1

s(t) = 

 Acos(2pf t)
binary 0
c
54
Encoding Techniques (cont.)
(a) original
signal
(b) PAM
pulses
(c) PCM
pulses
011001110001011110100
(d) PCM output
(d) PCM
output
55
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 msec., Gross Data
of 1.544 Mbps (CCITT Standard).
56
TDM (cont.)
The Bell system T1 carrier (1.544 Mbps).
57
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.
58
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.
59
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.
60
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.
61
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.
62
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.
63
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.
64
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 GHzDownlink)
65
Communication Satellite (Cont.)
• 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.
66
Communication Satellite (Cont.)
Point-to-point link via satellite microwave
67
Communication Satellite (Cont.)
Broadcast link via satellite microwave
68
Communication Satellite (Cont.)
A Two-antenna
satellite (a)
69
Communication Satellite (Cont.)
A Two-antenna
satellite (b)
70
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.
71
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.
72
Multiplexing
cosa cosb= 1/2 [cos(a + b) + cos(a - b)]
Note: As shown in the equation above, multiplying
two cosine functions yields a new signal with two
new cosine components.
s(t)cosa cosas(t)  (cosa)2 s(t)  [(1cos2a)/2]
s(t)/ 2 + s(t) cos2a/2
Note: 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.
73
Example
Let the original signal
s(t) = 4 cos (2p 10t) + 8 cos(2p 50t)
Let the carrier signal be cos(2p 70t)
Therefore, the result of the multiplication in the
transmitter is:
s(t)cos(2p 70t) = 2cos(2p 80t)
+ 2cos(2p 60t)
+ 4cos(2p 120t)
+ 4cos(2p 20t)
74
Example (cont.)
Amplitude
70
4
4
2
2
Hz
0
20
60
80
120
With a filter of 70 Hz and above, the signal between
0Hz and 70Hz will be erased. The new signal
2 cos(2p80t) + 4cos(2p 120t) , which is shifted
70Hz of original signal, will be transmitted.
75
Hardware Diagram

76
Hardware Diagram (cont.)

77
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
78
Example:
Telephone line with human voice (usually in the
range 300 to 3100Hz)

79
Example: (cont.)

80
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
81
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
82
Definition of Digital Signal
Encoding Formats (cont.)
• B8ZS
Same as bipolar AMI, except that any string of eight
zeros is replaced by a string with two code violations
• HDB3
Same as bipolar AMI, except that any string of four
zeros is replaced by a string with one code violation
83
Definition of Digital Signal
Encoding Formats (cont.)
84
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:
Signal and
control leads
Digital data
transmitter/
receiver
..
..
Data terminal
equipment(DTE)
Bit-serial
transmission
medium
Transmission
line interface
device
Transmission
line interface
device
..
..
Digital data
transmitter/
receiver
Data circuit-terminating
equipment (DCE)
Generic interface to transmission medium
85
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)
86
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
87
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
88
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
89
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
90
Interfacing
• RS-232C
• RS-449/442-A/423-A
• X.21 (15 pins)
91
Interfacing (cont.)
Signal lines used in X.21
T (Transport)
C (Control)
R (Receive)
I (Indication
DTE
DCE
S (Signal, i.e. bit timing)
B (Byte timing) optional
Ga (DTE common return)
G (Ground)
92
Interfacing (cont.)
An example of X.21 usage.
Step C
0
1
2
3
4
5
6
7
8
9
10
Off
On
On
On
On
On
On
Off
Off
Off
Off
I
Event in telephone analog
Off
Off
Off
Off
Off
On
On
On
Off
Off
Off
No connection-line idle
DTE picks up phone
DCE gives dial tone
DTE dials phone number
Remote phone rings
Remote phone picked up
Conversation
DTE says goodbye
DCE says goodbye
DCE hangs up
DTE hangs up
DTE
sends on T
T=1
T=0
DCE
sends on R
R=1
R=“+++...+”
T = address
T = data
T=0
R=call progress
R=1
R = data
R=0
R=1
T=1
93
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
94
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)
95
A SONET path
Source
Multiplexer
Repeater
Section
Multiplexer
Section
Repeater
Section
Line
Destination
Multiplexer
Section
Line
Path
96
Basic SONET Frame
• 810 bytes put out every 125msec.
• 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.
97
Two back-to-back
SONET frames
3 Columns
for overhead
87 Columns
Sonet
frame
(125msec)
.....
9
Rows
Sonet
frame
(125msec)
Section
overhead
Line
overhead
Path
overhead
SPE
98
Multiplexing in SONET
T1
T1
T1
T3


STS-1
STS-1
Electro-optical
Scrambler converter
STS-3
STS-3
STS-12
OC-12
STS-3
T3
STS-1
STS-3
3:1
Multiplexer
4:1
Multiplexer
99
SONET and SDH
multiplex rates
SONET
SDH
Electrical Optical
Optical
STS-1
STS-3
STS-9
STS-12
STS-18
STS-24
STS-36
STS-48
OC-1
OC-3
OC-9
OC-12
OC-18
OC-24
OC-36
OC-48
STM-1
STM-3
STM-4
STM-6
STM-8
STM-12
STM-16
Data rate(Mbps)
Gross
SPE
User
51.84
50.112
49.536
155.52 150.336 148.608
466.56 451.008 445.824
622.08 601.344 594.432
933.12 902.016 891.648
1244.16 1202.688 1188.864
1866.24 1804.032 1783.296
2488.32 2405.376 2377.728
100
Information Switching
Switching
Office
Computer
Physical
copper
connection
set up
when call
is made.
packets
queued up
for
subsequent
transmission
(a) Circuit switching (b) Packet switching
101
Information Switching (cont.)
(Timing of events)
(a) Circuit Sw.
(b) Message Sw.
(c) Packet Sw.
102
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
103
Message Switching
• Message Switching (Store-And-Forward):
Exchange Blocks of Data Between IMPs With no
Limit on Block size.
Disadvantage: Large Buffer Required and IMPIMP Line May be Tied Up Too Long.
104
Packet Switching
• Packet Switching:
Long Message are Subdivided into ShortPackets,
and Packets are Transmitted Between IMPs.
Advantage: Suited for Handling Interactive Traffic
Disadvantage: Proper Routing Problem
105
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
106
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
107
Objectives of ISDN
• Standardization
• Transparency
• Separation of Competitive Function
• Leased and Switched Services
• Cost-Related Tariffs
• Smooth Migration
• Multiplexed Support
108
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 Alarm
Example:
- Electronic Meter Reading,
- Smoke Detectors
109
Candidate Services for
Integration
Service
Bandwidth Telephony
Data
Text
Digital
voice
Telephone
Packet-switched Telex
Circuit-switched Teletex
(64Kbps)
Leased
circuits
Information
retrival (by
voice and
synthesis)
Leased circuits
Telemetry
Funds transfer
Leased circuit
Videotex
Information
retrieval
Mailbox
Electronic mail
Alarms
Information
retrieval
Mailbox
Electronic mail
Image
Facsimile
Information
retrieval
Surveillance
110
Candidate Services for
Integration
Service
Bandwidth Telephony
Data
Wide
band
High-speed
Computer
TV
conferencing
Communication
Teletex
(>64Kbps)
Music
Text
Image
Videophone
Cable TV
distribution
111
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)
112
Comments of Service
113
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
114
ISDN Architecture Continue
• 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.
115
ISDN Architecture Continue
(a) Example ISDN system for home use
116
ISDN Architecture Continue
(b) Example ISDN system with a PBX for use in large business
117
Block Structure of a
digital PBX
Line module
for ISDN
devices
Line module
for RS-232-C
terminals
Control unit
Trunk
module
Switch
To ISDN
exchange
Line module
for analog
telephones
Service unit
118
ISDN Architecture Continue
Interface
Interface
Customer’s
equipment
Carrier’s
equipment
• A - 4kHz analog telephone channel
• B - 64 kbps digital PCM channel for voice or data
• C - 8 or 16 kbps digital channel
119
ISDN Architecture Continue
• 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
120
ISDN Architecture Continue
• 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
121