Data and Computer Comm.
Stallings – Chapters 3-5 (Quick Overview)
3: Data Transmission
4: Guided and Wireless Transmission
5: Signal Encoding Techniques
NOTE: Many figures and other materials in this presentation are borrowed from
required and reference textbooks cited on the class web page.
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QUICK OVERVIEW
Chapter 3
Data Transmission
2
Terminology (1)
Transmitter
Receiver
Medium
„
Guided medium
e.g. twisted pair, optical fiber
„
Unguided medium
e.g. air, water, vacuum
3
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
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Terminology (3)
Simplex
„
One direction
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
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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
6
Data Rate and Bandwidth
Any transmission system has a limited
band of frequencies
„
Limits the data rate that can be carried
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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 (CD) audio
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Analog Transmission
Analog signal transmitted without regard to
content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
But also amplifies noise
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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
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Advantages of Digital Transmission
Digital technology
„
Low-cost LSI/VLSI technology
Data integrity
„
Longer distances over lower quality lines
Capacity utilization
„
„
High bandwidth links can be more economical
High degree of MUXing easier with digital
Security and privacy
„
Encryption
Integration
„
Can treat analog and digital data similarly
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Channel Capacity
Data rate
„
„
In bits per second (bps)
Rate at which data can be communicated
Bandwidth
„
„
In cycles per second (Hertz)
Constrained by transmitter and medium
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QUICK OVERVIEW
Chapter 4
Transmission Media
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Overview
Guided - wired
Unguided - wireless
Characteristics and quality determined by
medium and signal
„
„
For guided, medium is more important
For unguided, bandwidth produced by
antenna is more important
Key concerns are data rate and distance
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Design Factors
Bandwidth
„
Higher bandwidth enables higher data rate
Transmission impairments
„
Attenuation
Interference
Number of receivers
„
„
In guided media
More receivers (multi-point) introduce more
attenuation
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Electromagnetic Spectrum
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Guided Transmission Media
Twisted Pair
„
„
„
Cheap, widely deployed and used
Easy to work with
Limited data rate and range
Coaxial cable
„
„
„
„
„
Highly versatile medium
Television distribution
Long distance telephone transmission
Short distance computer systems links
Local area networks
Optical fiber
„
„
„
„
„
„
„
„
Greater capacity
Smaller size & weight
Lower attenuation
EM isolation
Greater repeater spacing
Long-haul, metropolitan, and rural exchange trunks
Subscriber loops
LANs
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Example: FO Ring
with Active Repeaters
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Wireless Transmission
Unguided media
Transmission and reception via antenna
Directional
„
„
Focused beam
Careful alignment required
Omnidirectional
„
„
Signal spreads in all directions
Can be received by many antennae
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Terrestrial Microwave
Parabolic dish
„
„
Focused beam
Line of sight
Variety of applications
„
„
„
Long-haul telecommunications services
Voice and TV transmission
Point-to-point links between buildings
Higher frequencies give higher data rates
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Satellite Microwave
Point-to-point or multi-point comm.
„
„
Satellite is relay station
Satellite receives on one frequency, amplifies
or repeats signal and transmits on another
Requires geo-stationary orbit
„
Height of 35,784km
Television distribution
Long-distance telephone
Private business networks
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Example: Principal Satellite Bands
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Broadcast Radio
Omnidirectional
Variety of applications
„
„
„
FM radio
UHF and VHF television
Data (e.g. 802.11, Bluetooth)
Line of sight
Suffers from multipath interference
„
Reflections
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Infrared
Point-to-point connectivity
Transceivers modulate non-coherent
infrared light
Line of sight (or reflection)
Blocked by walls
Several areas of application
„
„
Appliance remote control
Data port
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QUICK OVERVIEW
Chapter 5
Data Encoding
25
Encoding Techniques
Four basic categories of choice
„
„
„
„
Digital data, digital signal
Analog data, digital signals
Digital data, analog signal
Analog data, analog signal
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Digital Data, Digital Signal
Digital signal
„
„
„
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements
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Terms (1)
Unipolar
„
All signal elements have same sign
Polar
„
One logic state represented by positive
voltage, the other by negative voltage
Data rate
„
Rate of data transmission in bits per sec (bps)
Duration or length of a bit
„
Time taken for transmitter to emit the bit
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Terms (2)
Modulation rate
„
„
Rate at which the signal level changes
Measured in units of baud
baud = signal elements per second
Mark and Space
„
Binary 1 and Binary 0, respectively
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Interpreting Signals
Need to know
„
„
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting of
signals
„
„
„
SNR
Data rate
Bandwidth
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Comparison of
Encoding Schemes (1)
Signal Spectrum
„
„
„
Lack of high frequencies reduces required bandwidth
Lack of DC component an advantage, as allows AC
coupling via transformer which provides isolation
Concentrate power in the middle of the bandwidth
Clocking
„
„
„
Synchronizing transmitter and receiver
External clock
Sync mechanism based on signal
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Comparison of
Encoding Schemes (2)
Error detection
„
Can be built into signal encoding
Signal interference and noise immunity
„
Some codes are better than others
Cost and complexity
„
„
Higher signal rate (and thus data rate) leads
to higher costs
Some codes require signal rate greater than
data rate
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Encoding Schemes
Simple NRZ
„
„
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Multilevel binary (>2 signal levels)
„
„
Bipolar-AMI (alternate mark inversion)
Pseudoternary
Biphase
„
„
Manchester
Differential Manchester
Scrambling techniques
„
„
Bipolar with 8-zeros substitution (B8ZS)
High-density bipolar-3 zeros (HDB3)
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NRZ-L
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
„
no transition (i.e. no return to zero voltage)
e.g. Absence of voltage for one value, constant
positive voltage for the other (e.g. EEL3701C)
Usually in data communication, negative voltage
is used for one value and positive for other value
„
For our purposes, 0 = high level, 1 = low level
This scheme is NRZ-L
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NRZI
NRZ inverted (NRZI)
„
NRZ is inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of
signal transition at beginning of bit time
„
„
Transition (either low-to-high or high-to-low)
denotes a binary 1
No transition denotes a binary 0
An example of differential encoding
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NRZ
36
Differential Encoding
Info. transmitted is represented in terms of
changes between successive signal elements
„
Rather than by signal elements themselves
More reliable detection of transition rather than
level
„
„
May be more reliable to detect transition in presence
of noise
In complex transmission layouts, it is easy to lose
sense of polarity of signal
e.g. leads from attached device to multidrop twisted-pair line
may be accidentally attached inverted
problem with NRZ-L, but not with differential encoding
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NRZ pros and cons
Pros
„
„
Easy to engineer
Makes good use of bandwidth
Cons
„
„
DC component
Lack of synchronization capability
Used for magnetic recording
Not often used for signal transmission
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Multilevel Binary (1)
Uses more than two levels
Bipolar-AMI (alternate mark inversion)
„
„
„
„
Binary 0 represented by no line signal
Binary 1 represented by positive or negative pulse
Successive Binary-1 pulses alternate in polarity
Advantages
No loss of sync if long string of ones (zeros still a problem)
No net DC component
Lower bandwidth than with NRZ (see Fig. 5.3)
Simple means of error detection
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Multilevel Binary (2)
Pseudoternary
„
„
„
„
Similar to bipolar-AMI but opposite
Binary 1 represented by absence of line
signal
Binary 0 represented by alternating positive
and negative pulses
Same advantages as bipolar-AMI
Of course, opposite regarding long strings
„
„
No loss of sync if long string of zeros
Long string of ones still a problem
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Bipolar-AMI and Pseudoternary
41
Tradeoff for Multilevel Binary
Not as efficient a code as NRZ
„
„
„
Each signal element only represents one bit
In a 3-level system, theoretically we could
represent log23 = 1.58 bits
Receiver must distinguish between three
levels (+A, -A, 0)
Analysis shows multilevel requires ~3dB more
signal power for same probability of bit error
Equivalently, at a given SNR, BER for NRZ is
significantly less than BER for multilevel binary
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Biphase
Manchester
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„
„
„
„
Transition in middle of each bit period
Transition serves as both clock and data
Low-to-high represents Binary 1
High-to-low represents Binary 0
e.g. Used by IEEE 802.3 CSMA/CD (Ethernet)
Differential Manchester
„
„
„
Mid-bit transition is for clocking only
Transition at start of bit period represents Binary 0
No transition at start of bit period represents Binary 1
Yes, it is another differential encoding scheme!
„
e.g. Used by IEEE 802.5 Token Ring
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Example
Mid-bit transition
serves as both clock
and data
0
0
1
1
0
0
0
0
1
1
1
1
0
0
0
0
0
0
1
1
1
1
Mid-bit transition for
clock, start transition
for data 44
Modulation Rate
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Biphase Tradeoffs
Cons
„
„
At least one transition per bit time and possibly two
Thus, maximum modulation rate is twice NRZ
⇒ Requires more bandwidth
Pros
„
Synchronization on mid-bit transition
These are self-clocking codes
„
„
No DC component
Error detection
Absence of expected transition
Noise would have to invert both signal before and after
expected transition to cause undetected error
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Scrambling Techniques
Biphase not well suited to long-distance applications due
to high ratio of signaling rate to data rate
Alternative is some type of scrambling scheme
„
Replace sequences that would otherwise produce constant
voltage (e.g. string of 0s in bipolar-AMI)
Filling sequence
„
„
„
Must produce enough transitions to sync
Must be recognized by receiver and replace with original data
Same length as original
Advantages?
„
„
„
„
No DC component
No long sequences of zero-level line signal
No reduction in data rate
Error detection capability
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B8ZS
Bipolar with 8-Zeros Substitution (B8ZS) code
„
Common in North America
e.g. line coding option for 64kbps channels on 1.544Mbps T1 links
Based on bipolar-AMI
Basic operation
„
„
If octet of all zeros and last voltage pulse preceding was positive,
encode as: 0 0 0 + − 0 − +
If octet of all zeros and last voltage pulse preceding was
negative, encode as: 0 0 0 − + 0 + −
Causes two violations of AMI code
„
Since + 0…0 + and − 0…0 − not permitted
These are unlikely to occur as a result of noise
Receiver detects and interprets as octet of all zeros
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B8ZS Example
000-+0+-
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HDB3
High-Density Bipolar-3 Zeros (HDB3) code
„
Common in Europe and Japan
Also based on bipolar-AMI
String of 4 zeros is replaced with sequences of 1 or 2
pulses
In each case, 4th zero replaced with code violation
For successive violations, if last violation negative then
next violation positive and vice-versa
„
so no DC component
# of Bipolar Pulses Since Last Substitution
Polarity of
Preceding Pulse
Odd
−
000−
+00+
+
000+
−00−
Even
Substitution
Rules
50
B8ZS and HDB3
Odd: 0 0 0 −
Even: + 0 0 +
(assuming odd # of
1s or pulses since
last substitution)
Even: − 0 0 −
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Other Encoding Schemes
4B/5B
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„
„
„
„
„
Used in FDDI, 100BASE-FX, 100BASE-T
Every 4 bits of data encoded in a 5-bit code word
These 5-bit codes are selected to have no more than
one leading 0 and no more than two trailing 0s
Thus, never get more than three consecutive 0s
Resulting 5-bit codes are transmitted using NRZI
Achieves 80% efficiency
8B/10B
„
„
Used in 1000BASE-T, 1000BASE-SX, 1000BASE-LX
8-bit data stream encoded as 10-bit code words
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Digital Data, Analog Signal
3 basic encoding or modulation techniques
„
„
„
Amplitude-shift keying (ASK)
Frequency-shift keying (FSK)
Phase-shift keying (PSK)
In all cases, resulting signal occupies a
bandwidth centered on a carrier frequency
53
Modulation of
Analog Signals for Digital Data
54
Example: Full-Duplex FSK
Transmission on Voice-Grade Line
fc = 1170Hz
fc = 2125Hz
From specification for Bell System 108 series modems
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Analog Data, Digital Signal
Digitization
„
„
„
„
„
„
„
Conversion of analog data into digital data
Digital data can then be transmitted using NRZ-L
Digital data can then be transmitted using code other than
NRZ-L
Digital data can then be converted to analog signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation
Example
„
„
„
Voice data limited to below 4000Hz
Require at least 8000 samples per second
If 7-bit resolution PCM (i.e. 128 levels), 8000 x 7 = 56 kb/s
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Analog Data, Analog Signals
Why modulate analog signals?
„
„
Higher frequency can give more efficient
transmission
Permits frequency-division muxing (Chp. 8)
Types of modulation
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„
„
Amplitude (AM)
Frequency (FM)
Phase (PM)
57
Example
AM
FM
PM
58
Spread Spectrum
Analog or digital data
Analog signal
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping (FHSS)
„
Signal broadcast over seemingly random series of frequencies
Direct Sequence (DSSS)
„
Each bit is represented by multiple bits in transmitted signal
Known as chipping code, spreads signal across a wider frequency
band in direct proportion to # of bits used
e.g. 10-bit chipping code spreads signal across frequency band 10x
greater than 1-bit code
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Example of DSSS
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Odds and Ends
What is “broadband” ?
„
„
Sometimes referred to as a wideband transmission, broadband
refers to telecommunications that provide a variety of channels
of data over a single communication medium (wire).
Two of the more commonly found and used broadband
technologies are cable and DSL broadband.
What is “baseband” ?
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„
„
„
Transmission of signals without modulation.
In a baseband LAN (e.g. 100BASE-T), digital signals (1s and 0s)
are inserted directly onto the cable as voltage pulses.
The entire spectrum of the cable is consumed by the signal.
This scheme does not allow FDM.
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