Lecture Note on Transmission Basic

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Lecture Note on Transmission Basic
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Direct Connections in Communication Networks

• Building Blocks
• links coax cable, optical fiber...
• nodes general-purpose workstations...
• Connectivity Types
• point-to-point
• multiple access

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Indirect Connections in Communication Networks

• Connectivity Types
• switched networks
• gt switches
• inter-networks
• gt routers

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Connectivity Examples

• Internet
• Best-effort
• (no performance guarantees)
• Packet-by-packet
• Point-to-point link
• Always-connected
• Fixed bandwidth
• Fixed delay
• Zero-jitter

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Delays in Networks

• Propagation time or propagation delay, tprop
• Time required for a signal or waveform to
  propagate (or move) from one point to another
  point. It is defined as d/v where d is the
  distance between the two points and v is the
  velocity of the signal.
• Electromagnetic signal sent through space from
  the Sun to the Earth.
• Velocity is the speed of light, 3 108 meters
  per second.
• Say distance is 148.8 million kilometers.
• Propagation time is (148.8 /3) 109 / 108 496
  seconds.
• Transmission time, ttrans
• Links or channels for digital transmission are
  defined in bits/second or bps. This is called
  the Data Rate, R, and is the rate at which the
  transmitter can push bits onto the transmission
  line. Transmission time is the time it takes to
  send some number, say B bits, or B/R.
• What is the transmission time for a packet of
  1500 bytes on a 10 Megabit link.
• 1500 8 / 107 1.2 milliseconds.
• Queuing time or queuing delay
• Any time related to processing or waiting of
  packets in buffers.

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Representation of Information

• Block Oriented Information specific length
  blocks
• Text files
• Fax documents
• Example A4 Page
• 200 pixels per inch (horizontal) by 100 pixels
  per inch (vertical) requires 256 kilobytes prior
  to compression (a pixel is black or white).
  Compression reduces by a factor of 8 to 16.
• JPEG color images
• 8 x 10 inch photo uncompressed is about 38.4
  Mbytes before compression and about 1.2 8
  Mbytes after compression.
• Note that each of 3 colors (RGB) requires one
  byte to represent the value
• Stream Oriented Information continuous stream
  of data
• Voice PCM 4 kilohertz voice is 64 kbps, standard
  voice channel
• Voice ADPCM, compression technique reduces rate
  to 16-32 kbps
• Audio MPEG audio MP3 compression
• Video MPEG 2
• 1920 x 1080 pixel frames at 30 frames /second
  requires about 1.5 Gbps (includes 20bits/pixel
  and vertical/horizontal blanking and embedded
  audio) uncompressed is reduced to 19-38 Mbps
  compressed

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A4 Fax Document Standard 1/16 m2, 5 grams / page
typically 210 millimeters x 297 millimeters
(slightly bigger than 8 1/2 by 11)
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Data Compression Techniques

• Why data compression reduce amount of storage
  space required to represent the document
• Compression ratio size of uncompressed / size of
  compressed
• Lossless data compression
• Example run length encodings
• Sequences of likely symbols encoded into runs
• 25 0s follows by a 1 is encoded as a fixed m-bit
  code word
• Example encode 000000000000100001001 using a 3
  bit code
• Lossy data compression
• Compressed file is an approximation of the
  original data
• Better compression ratio is achieved.
• For example, MP3 can achieve a factor of 14
  reduction.
• Example unencoded MP3 requires about 1.4
  Mbits/sec (2 channels, 16 bits per sample, at a
  rate of 44100 samples per second). Thus, 1 hour
  is about 317 MB (about ½ of an audio CD and 1/14
  of common DVDs) Through MP3 compression, we can
  compress to say 128 kbits /second.

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Analog Information

• Based on a wave that varies continuously with
  time
• Example
• Sound
• Electromagnetic radiation
• Periodic signal repeats over time
• Representation of a analog signals
• Amplitude versus time
• Frequency representation
• Representation as sums of sines and cosines

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Frequency Spectrum

• Frequency cycles per second (Hertz) of a
  sinusoidal signal
• Bandwidth usable range of frequencies high
  low f2 f1
• Typical telephone voice channel is 4k Hz
• Some approximate frequencies
• 300 Hz 20,000 Hz human voice / sound
• 50 kHz navigation (ships, submarines, etc)
• 1 MHz AM radio (20 k Hz channels)
• 10 MHz CB, short wave
• 100 MHz FM radio, TV
• 1 GHz UHF TV, mobile telephony
• 10 GHz amateur satellite
• 100 GHz upper microwave many uses (up to about
  300 GHz)
• 10 T Hz 1013 Hz Infrared, 1015 Hz Visible
  light, 1018 X-rays, . . . .

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Bandwidth Versus Channel Rate

• Shannon's theorem maximum bit rate for noisy
  channel
• C Bandwidth log2 (1 S/N)
• 4 kHz voice channel equates to 56 or 64 k bits /
  second.
• This is achievable by modern modems
• Some sample bit rates
• Radio LAN in the 2.4GHz band 2 Mbps
• Fast Ethernet 100 Mbps, Gigabit Ethernet,
• Optical fiber transmission 2.4 9.6 Gbps over
  one wavelength

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Multiplexing

• Multiplexing is fundamentally sharing some amount
  of bandwidth by many different users, channels,
  etc.
• FDM Frequency Division Multiplexing dividing up
  the frequency spectrum into multiple channels,
  each channel dedicated to a particular use
• TDM Time Division Multiplexing using the entire
  frequency bandwidth allotted but uses logical
  slots to transmit different channels at specific
  points of relative time within a logical frame
  that is transmitted.
• A digital telephone speech signal (corresponding
  to a 4 kHz channel) is obtained as follows
• Sample the signal at twice the maximum frequency
  or 8000 times a second
• Quantize the sampled value ( the amplitude of the
  signal) to 7 bits or 128 different value.
• This gives a bit rate of 56 kbits/second

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T-1 Carrier System

• T-1 was developed to carry digitally multiplexed
  channels between central offices using TDM.
• T-1 multiplexed 24 voice channels in a T-1 frame
  as follows

Channel 1 Channel 2
. . . . .
. Channel 24 7 bit data, 1 bit
control
1 bit framing
Sampling rate 1 frame is sent every 125 µsec
(8000 frames /sec) 8 bits / channel x 24 channels
1 bit 193 bits per frame. Therefore, 193 bits
/ 125 µsec 1.544 Mbps This standard signaling
rate became known as DS-1 (digital signal 1)
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DS Hierarchy

• DS-1 basic block, 1.544 Mbits/sec
• DS-2 6.312 Mbits/sec (4 DS-1 or 96 voice, plus
  some control)
• DS-3 44.736 Mbits/sec
• International standards (based on the ITU
  hierarchy developed in Europe) is somewhat
  different
• E1 2.048 Mbits/sec supporting 30 voice channels
  at 64 kbits/sec each with two channels for
  control.

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SONET Digital Hierarchy

• OC-1 51.84 Mbps
• OC-3 155.52 Mbps
• OC-12 622.08 Mbps
• OC-48 2.48832 Gbps
• OC-192 9.95328 Gbps
• OC-768 ? 40 Gpbs
• Compare to the evolution of standard rates for
  Ethernet
• 10 Mbps
• 100 Mbps
• 1 Gbps
• 10 Gbps
• 40 Gbps

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Wavelength Division Multiplexing (WDM)

• Combining multiple wavelengths over the same
  optical fiber
• For example 16 wavelengths, each at 2.5 Gbps can
  provide a rate of about 40 Gbps. (OC-48 x 16)
• Using optical add drop multiplexers
• Optical switches
• A lightpath is a logical connection from a start
  node to an end node going through intermediate
  nodes.
• Same wavelength throughout
• Reallocating of a wavelength at intermediate
  nodes (switching)
• Protection paths
• Wavelength switching capabilities

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Parallel transfer of information (bits)
Parallel Wires 8 bits, 16 bits, etc.
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
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Serial Transfer (Bit Serial)
0 0 0 0 1 1 0 0 1 0 0 0 0
SOURCE
DEST
Data line
Control line / info
Transfer, for example, an octet a single bit at a
time using a fixed time interval for each bit. A
clock signal helps determine when to sample the
bit. Why use a serial line? As distances
increase, multiple wire costs increase,
complexity of line drivers and receivers
increase, etc. Thus, to send a character, first
serialize the bits, send over the line, and then
receive the bits, and convert back to character.
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Communication Modes

• Simplex Data is transmitted in one direction
  only
• Half duplex Alternating exchange of data
  between two devices. Need to switch between
  sender and receiver mode
• Full duplex Data can be exchanged in both
  directions simultaneously

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Determining Received Bit Pattern

• Determine
• Start of each bit (or center of bit). Bit or
  clock synchronization
• Start and end of the unit, a character, a byte,
  etc. Byte synchronization
• Start and end of message unit or frame. Frame
  synchronization

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Asynchronous Transmission

• Receiver and transmitter clocks are not
  synchronized. They are independent. Receiver
  resynchronizes at the start of each unit, say a
  character.
• To determine beginning of character, have a
  start bit
• To be sure about end of character, have a stop
  bit.
• Know how many bits are in a character, say 8.
• Clock frequency for sampling is usually 16 times
  bit rate. Receiver attempts to sample in the
  center of the bit.
• Start bit is usually 1 bit long and stop bits are
  often 1, 1.5, or 2 bits

(idle) start 1 0
0 1 1 1 1 0
stop (idle)
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Synchronous Transmission

• Sender and receiver clocks are synchronized
  (often through the sending of a clock signal on a
  control line)
• Start and stop bits are not used. Characters can
  be sent one after the other without the overhead
  of start and stop bits.
• Framing is still important.

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• 1850 Invention of telegraph by Morse
• 1874 - Invention of time-division multiplexing by
  Baudot
• 1876 - Invention of telephone by Bell
• 1899 - Invention of radio by Marconi
• 1928 Development of sampling theory by Nyquist
• 1936 - Invention of pulse code modulation by
  Reeves
• 1948 Development of channel capacity theory by
  Shannon
• Invention of transistor by Bell
  Laboratories
• 1962 First 1.544 Mb/s T1 cable carrier system
  by Bell System
• Telestar, first communications
  satellite be Bell System
• 1965 Early Bird, first geostationary
  communications satellite by INTELSAT
• 1966 Low-loss optical fiber proposed by Kao
• 1980 ATT introduces fiber optic transmission
  at T3 data rate
• 1984 Divestiture of the Bell System increases
  competition and accelerates introduction of
  digital
• transmission in the United States
• 1988 First transatlantic fiber optic cable
  (TAT-8) installed
• 1989 First transpacific fiber optic cable
  (HAW-4/TPC-3) installed
• 1992 Conversion of major U.S. networks to
  digital transmission completed
• 1990s- Deployment of digital transmission
  worldwide in support of the Synchronous Digital
  Hierarchy

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Performance of Transmission System
Circuit length
Type of service
Reference circuit
Analog vs.digital
Multiplex hierarchy
Performance objectives
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Voice Transmission

• For digital transmission applications, voice
  signal must first be converted to a digital
• representation by the A/D process
• PCM (Pulse Coded Modulation)
• Different PCM
• Delta modulation
• Block Diagram of Analog-to-Digital Converter

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• Standard for voice digitization
• For single channel PCM at a transmission rate of
  64 kb/s (8 kHz sampling with 8 bit quantization)
• For multi-channel
• North American standard 24 voice channels at
  a total transmission rate of 1.544 Mb/s
• European CEPT standard 30 voice channels at a
  total transmission rate of 2.048 Mb/s
• Transmission quality of digitized voice
  channels
• A/D technique
• Number of tandem A/D and D/A conversions
  (accumulation of quantization noise)
• Characteristics of transmission impairment

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Data Transmission

• Data Transmission Network Interfaces

data Interface
timing control
data Interface
timing control
DTE DCE
DCE
DTE
Data terminal equipment
Data circuit terminating equipment
Communications Network
Data circuit terminating equipment
Data terminal equipment
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• Timing Signals
• Asynchronous transmission
• Synchronous transmission
• (a) Codirectional timing
• (a) Contradirectional timing

Timing
Timing
DTE
DCE
Communication Network
DCE
DTE
Data
Data
Timing
Timing
DTE
DCE
Communication Network
DCE
DTE
Data
Data
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• (c) Buffered timing
• Synchronous Data Transmission Configuration

Timing
Timing
DTE
Buffer
DCE
Data
Data
Communication Network


Timing
Timing
DTE
Buffer
Data
Data
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• Interface standards
• End-to-end______________
• Layer7_______________
• End-to-end______________
• Layer6_______________
• End-to-end______________
• Layer5_______________
• End-to-end______________
• Layer4_______________
• ________________________
• Layer3_______________
• ________________________
• Layer2_______________
• ________________________
• Layer1_______________
• DTE DCE

Application
Presentation
Session
Transport
Network
Network
Data Link
Communications Equipment
Physical
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Video Transmission

• Compression schemes are essential because of
  very large amounts of information to be
  transmitted. (e.g.) 8-bit PCM applied to 6-MHz TV
  signal results in 96 Mb/s
• Compression schemes
• - Compression in space transform coding,
  sub-band coding
• - Compression in time motion
  estimation/compensation

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Failure Model

• Failure departure from specified performance
• Failure rate ( ) average rate at which
  failures can be expected to occur throughout
  useful life of equipment
• Reliability (R(t)) probability that equipment
  will perform without failure for a period of time
  t given by
• Note that the above equation is based on the
  Poisson assumption on the failure process.

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Performance Objectives

• Availability is defined as the probability or
  fraction of time that circuit continuity is
  maintained and usable service is provided.
• Quality is considered only when the circuit is
  available and the quality parameters commonly
  used include bit error rate, timing slips,
  jitters, and delay.
• Design objectives (DO) vs. Operation and
  Maintenance (OM) standards of performance.
• DOOM Margin
• Mean Time Between Failure (MTBF) ratio of total
  operating time divided by the number of failures
  in the same period.

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• Mean Time To Repair (MTTR) average time to
  repair the failed equipment including fault
  isolation, equipment replacement or repair, and
  test time, but excluding administrative time for
  travel and locating spare modules of equipment.
• Mean Time to Service Restoration (MSTR) average
  time to restore service including repair and
  administrative time.
• Availability (A) probability that equipment will
  be operable at any given point in time or the
  fraction of time that the equipment performs the
  required function over a stated period of time,
  which can be defined as follows
• Unavailability (U) complement of availability
• For large MTBF and small MTSR (usually the case)

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• Outage (O) condition whereby the user is
  deprived of service due to failure within the
  communication system.
• Mean Time Between Outages (MTBO) ratio of total
  operating time divided by the number of outages
  in the same period.
• Redundant equipment
• MTBO1 If the equipment consists of n identical
  units, each with the same MTBF, for which at
  least r must be operable for the system to be
  operable, the first type of outage is given by
• where the sensing of a failure and resulting
  switchover is itself to occur within failure.

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• System availability
• Series combination
• Parallel combination
• Series parallel combination
• Parallel-series combination

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• Series Combination
• Parallel Combination

N
2
1
1
2
N
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• Series-Parallel Combination

B1
A1
N1
B2
A2
N2
AM
NM
BM
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• Parallel-Series Combination

A1
AN
A2
B2
B1
BN
M1
MN
M2
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Error Objectives

• Error parameters
• Bit Error Rate (BER) ratio of erred bits to the
  total transmitted bits in a measurement interval
• Error-Free Second (EFS) or Error Seconds (ES)
  percentage or probability of one-second
  measurement intervals that are error free (EFS)
  or in error (ES)
• Error-Free Blocks (EFB) percentage or
  probability of data blocks that are error free.

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• Relationships between error parameters
• Basic assumption statistically independent bit
  errors with an average probability
• of error
• pBER, R bit rate
• EFS and BER
• P (block error)

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1
Error second probability
Probability of block error
Bit error probability
0
R(data rate)
1
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Bit Count Integrity

• Definitions
• Bit Count Integrity (BCI) preservation of
  precise number of bits (or characters or frames)
  that are originated in a message or unit of time.
• Slip loses of BCI caused by a shift of a digital
  signal.
• Performance Impact of One Slip
• Service Potential Impact
• Encrypted Text Encrypted key must be resent
• Video Freeze frame for several seconds - Loud
  pop on audio
• Digital Data Deletion or repetition of data -
  Possible reframe
• Facsimile Deletion of 4-8 scan lines Drop call
• Voice Band Data Transmission errors for 0.01 to 2
  sec. Drop call
• Voice Possible click

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• CCTTT Performance Objectives for slip rate on
  64-kb/s International Connection (Rec. G.822)(13)
• Performance Mean Slip Rate Measurement Percentage
  of
• Classification Thresholds Average Period Total
  Time
• Unacceptable gt30 slips in 1 hr 1yr lt0.1
• Degraded gt5 slips in 24 hrs and 1yr
• lt30 slips in 1 hr
• Acceptable lt5 slips in 24 hrs 1yr gt98.9

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Jitter

• Definitions
• Jitter short-term variation of sampling instant
  from its intended position in time or phase
• Wander (or drift) long-term variation of
  sampling instant fro its intended position in
  time or phrase
• Transmission impairment
• By jitter
• Degradation in system error rate
  performance
• Slips
• Distortion in the recovered analog
  signal
• By wander
• Buffer overflow (fill) or underflow
  (empty)

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• Sources of jitter
• Regenerative repeaters imperfect clock recovery
  process
• Digital multiplexers insertion/deletion of
  overhead of stuffing bits
• Sources of wander
• Oscillator instability
• Propagation delay variation