ICSA 411: Week 2 Data Transmission

1
ICSA 411 Week 2Data Transmission

• Elizabeth Lane Lawley, Instructor

2
Electromagnetic Signals

• Function of time
• Analog (varies smoothly over time)
• Digital (constant level over time, followed by a
  change to another level)
• Function of frequency (more important)
• Spectrum (range of frequencies)
• Bandwidth (width of the spectrum)

3
Periodic Signal Characteristics

• Amplitude (A) signal value, measured in volts
• Frequency (f) repetition rate, cycles per second
  or Hertz
• Period (T) amount of time it takes for one
  repetition, T1/f
• Phase (F) relative position in time, measured in
  degrees

4
Bandwidth

• Width of the spectrum of frequencies that can be
  transmitted
• if spectrum300 to 3400Hz, bandwidth3100Hz
• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion

5
Why Study Analog in a Data Comm Class?

• Much of our data begins in analog form must
  understand it in order to properly convert it
• Telephone system is primarily analog rather than
  digital (designed to carry voice signals)
• Low-cost, ubiquitous transmission medium
• If we can convert digital information (1s and 0s)
  to analog form (audible tone), it can be
  transmitted inexpensively

6
Data vs Signals

• Analog data
• Voice
• Images
• Digital data
• Text
• Digitized voice or images

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Analog Signaling

• represented by sine waves

phase difference
1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
8
Voice/Audio Analog Signals

• Easily converted from sound frequencies (measured
  in loudness/db) to electromagnetic frequencies,
  measured in voltage
• Human voice has frequency components ranging from
  20Hz to 20kHz
• For practical purposes, the telephone system has
  a narrower bandwidth than human voice, from 300
  to 3400Hz

9
Image/Video Analog Data to Analog Signals

• Image is scanned in lines each line is displayed
  with varying levels of intensity
• Requires approximately 4Mhz of analog bandwidth
• Since multiple signals can be sent via the same
  channel, guardbands are necessary, raising
  bandwidth requirements to 6Mhz per signal

10
Data Analog Signals

• Requires conversion from digital data to analog
  signal
• Discussed in more detail later tonight

11
Digital Signaling

• represented by square waves or pulses

1 cycle
amplitude (volts)
time
(sec)
frequency (hertz)
cycles per second
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Digital Text Signals

• Transmission of electronic pulses representing
  the binary digits 1 and 0
• How do we represent letters, numbers, characters
  in binary form?
• Earliest example Morse code (dots and dashes)
• Most common current form ASCII

13
ASCII Character Codes

• Used to represent alphanumeric characters
• Use 7 bits of data to transmit one character
• 7 bits have 128 possible outcomes (0 to 127)
• Most characters are stored and transmitted using
  8 bits
• In communication, the eighth bit is used as a
  parity bit for error correction
• In applications, use of an eighth bit allows
  high-order bit characters, such as diacritics
  and symbols

14
Digital Image Signals

• Analog facsimile
• similar to video scanning
• Digital facsimile, bitmapped graphics
• uses pixelization
• Object-oriented graphics
• image represented using library of objects
• e.g. Postscript, TIFF

15
Pixelization and Binary Representation

• Used in digital fax, bitmapped graphics

1-bit code 00000000 00111100 01110110 0111
1110 01111000 01111110 00111100 00000000
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Transmission Media

• the physical path between transmitter and
  receiver (channel)
• design factors affecting data rate
• bandwidth
• physical environment
• number of receivers
• impairments

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Electromagnetic Spectrum for Transmission Media
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Impairments and Capacity

• Impairments exist in all forms of data
  transmission
• Analog signal impairments result in random
  modifications that impair signal quality
• Digital signal impairments result in bit errors
  (1s and 0s transposed)

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Transmission Impairments

• Attenuation
• loss of signal strength over distance
• Attenuation Distortion
• different losses at different frequencies
• Delay Distortion
• different speeds for different frequencies
• occurs only in guided media
• Noise
• distortions of signal caused by interference

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Types of Noise

• Thermal (aka white noise)
• Uniformly distributed, cannot be eliminated
• Intermodulation
• When different frequencies collide (creating
  harmonics)
• Crosstalk
• Overlap of signals
• Impulse noise
• Irregular spikes, less predictable

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Classes of Transmission Media

• Conducted or guided media
• use a conductor such as a wire or a fiber optic
  cable to move the signal from sender to receiver
• Wireless or unguided media
• use radio waves of different frequencies and do
  not need a wire or cable conductor to transmit
  signals

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Guided Transmission Media

• Transmission capacity depends on the distance and
  on whether the medium is point-to-point or
  multipoint
• Examples
• twisted pair wires
• coaxial cables
• optical fiber

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Twisted Pair Wires

• Consists of two insulated copper wires arranged
  in a regular spiral pattern to minimize the
  electromagnetic interference between adjacent
  pairs
• Often used at customer facilities and also over
  distances to carry voice as well as data
  communications
• Low frequency transmission medium

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Types of Twisted Pair

• STP (shielded twisted pair)
• the pair is wrapped with metallic foil or braid
  to insulate the pair from electromagnetic
  interference
• UTP (unshielded twisted pair)
• each wire is insulated with plastic wrap, but the
  pair is encased in an outer covering

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Ratings of Twisted Pair

• Category 3 UTP
• data rates of up to 16mbps are achievable
• Category 5 UTP
• data rates of up to 100mbps are achievable
• more tightly twisted than Category 3 cables
• more expensive, but better performance
• STP
• More expensive, harder to work with

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Twisted Pair Advantages

• Inexpensive and readily available
• Flexible and light weight
• Easy to work with and install

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Twisted Pair Disadvantages

• Susceptibility to interference and noise
• Attenuation problem
• For analog, repeaters needed every 5-6km
• For digital, repeaters needed every 2-3km
• Relatively low bandwidth (3000Hz)

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Coaxial Cable (or Coax)

• Used for cable television, LANs, telephony
• Has an inner conductor surrounded by a braided
  mesh
• Both conductors share a common center axial,
  hence the term co-axial

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Coax Layers
outer jacket (polyethylene)
shield(braided wire)
insulating material
copper or aluminum conductor
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Coax Advantages

• Higher bandwidth
• 400 to 600Mhz
• up to 10,800 voice conversations
• Can be tapped easily (pros and cons)
• Much less susceptible to interference than
  twisted pair

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Coax Disadvantages

• High attenuation rate makes it expensive over
  long distance
• Bulky

32
Fiber Optic Cable

• Relatively new transmission medium used by
  telephone companies in place of long-distance
  trunk lines
• Also used by private companies in implementing
  local data communications networks
• Require a light source with injection laser diode
  (ILD) or light-emitting diodes (LED)

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Fiber Optic Layers

• consists of three concentric sections

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Fiber Optic Types

• multimode step-index fiber
• the reflective walls of the fiber move the light
  pulses to the receiver
• multimode graded-index fiber
• acts to refract the light toward the center of
  the fiber by variations in the density
• single mode fiber
• the light is guided down the center of an
  extremely narrow core

35
Fiber Optic Signals
fiber optic multimode step-index
fiber optic multimode graded-index
fiber optic single mode
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Fiber Optic Advantages

• greater capacity (bandwidth of up to 2 Gbps)
• smaller size and lighter weight
• lower attenuation
• immunity to environmental interference
• highly secure due to tap difficulty and lack of
  signal radiation

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Fiber Optic Disadvantages

• expensive over short distance
• requires highly skilled installers
• adding additional nodes is difficult

38
Wireless (Unguided Media) Transmission

• transmission and reception are achieved by means
  of an antenna
• directional
• transmitting antenna puts out focused beam
• transmitter and receiver must be aligned
• omnidirectional
• signal spreads out in all directions
• can be received by many antennas

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Wireless Examples

• terrestrial microwave transmission
• satellite transmission
• broadcast radio
• infrared

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Terrestrial Microwave Transmission

• uses the radio frequency spectrum, commonly from
  2 to 40 Ghz
• transmitter is a parabolic dish, mounted as high
  as possible
• used by common carriers as well as by private
  networks
• requires unobstructed line of sight between
  source and receiver
• curvature of the earth requires stations (called
  repeaters) to be 30 miles apart

41
Microwave Transmission Applications

• long-haul telecommunications service for both
  voice and television transmission
• short point-to-point links between buildings for
  closed-circuit TV or a data link between LANs
• bypass application

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Microwave Transmission Advantages

• no cabling needed between sites
• wide bandwidth
• multichannel transmissions

43
Microwave Transmission Disadvantages

• line of sight requirement
• expensive towers and repeaters
• subject to interference such as passing airplanes
  and rain

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Satellite Microwave Transmission

• a microwave relay station in space
• can relay signals over long distances
• geostationary satellites
• remain above the equator at a height of 22,300
  miles (geosynchronous orbit)
• travel around the earth in exactly the time the
  earth takes to rotate

45
Satellite Transmission Links

• earth stations communicate by sending signals to
  the satellite on an uplink
• the satellite then repeats those signals on a
  downlink
• the broadcast nature of the downlink makes it
  attractive for services such as the distribution
  of television programming

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Satellite Transmission Process
satellite transponder
dish
dish
22,300 miles
uplink station
downlink station
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Satellite Transmission Applications

• television distribution
• a network provides programming from a central
  location
• direct broadcast satellite (DBS)
• long-distance telephone transmission
• high-usage international trunks
• private business networks

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Principal Satellite Transmission Bands

• C band 4(downlink) - 6(uplink) GHz
• the first to be designated
• Ku band 12(downlink) -14(uplink) GHz
• rain interference is the major problem
• Ka band 19(downlink) - 29(uplink) GHz
• equipment needed to use the band is still very
  expensive

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Satellite Advantages

• can reach a large geographical area
• high bandwidth
• cheaper over long distances

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Satellite Disadvantages

• high initial cost
• susceptible to noise and interference
• propagation delay

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Three Components of Data Communication

• Data
• Analog Continuous value data (sound, light,
  temperature)
• Digital Discrete value (text, integers, symbols)
• Signal
• Analog Continuously varying electromagnetic wave
• Digital Series of voltage pulses (square wave)
• Transmission
• Analog Works the same for analog or digital
  signals
• Digital Used only with digital signals

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Analog Data?Signal Options

• Analog data to analog signal
• Inexpensive, easy conversion (eg telephone)
• Data may be shifted to a different part of the
  available spectrum (multiplexing)
• Used in traditional analog telephony
• Analog data to digital signal
• Requires a codec (encoder/decoder)
• Allows use of digital telephony, voice mail

53
Digital Data?Signal Options

• Digital data to analog signal
• Requires modem (modulator/demodulator)
• Allows use of PSTN to send data
• Necessary when analog transmission is used
• Digital data to digital signal
• Requires CSU/DSU (channel service unit/data
  service unit)
• Less expensive when large amounts of data are
  involved
• More reliable because no conversion is involved

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Transmission Choices

• Analog transmission
• only transmits analog signals, without regard for
  data content
• attenuation overcome with amplifiers
• signal is not evaluated or regenerated
• Digital transmission
• transmits analog or digital signals
• uses repeaters rather than amplifiers
• switching equipment evaluates and regenerates
  signal

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Data, Signal, and Transmission Matrix
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Advantages of Digital Transmission

• The signal is exact
• Signals can be checked for errors
• Noise/interference are easily filtered out
• A variety of services can be offered over one
  line
• Higher utilization of bandwidth is possible with
  data compression

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Why Use Analog Transmission?

• Already in place
• Significantly less expensive
• Lower attentuation rates
• Fully sufficient for transmission of voice signals

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Analog Encoding of Digital Data

• Data encoding and decoding technique to represent
  data using the properties of analog waves
• Modulation the conversion of digital signals to
  analog form
• Demodulation the conversion of analog data
  signals back to digital form

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Modem

• An acronym for modulator-demodulator
• Uses a constant-frequency signal known as a
  carrier signal
• Converts a series of binary voltage pulses into
  an analog signal by modulating the carrier signal
• The receiving modem translates the analog signal
  back into digital data

60
Methods of Modulation

• Amplitude modulation (AM) or amplitude shift
  keying (ASK)
• Frequency modulation (FM) or frequency shift
  keying (FSK)
• Phase modulation or phase shift keying (PSK)

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Amplitude Shift Keying (ASK)

• In radio transmission, known as amplitude
  modulation (AM)
• The amplitude (or height) of the sine wave varies
  to transmit the ones and zeros
• Major disadvantage is that telephone lines are
  very susceptible to variations in transmission
  quality that can affect amplitude

62
ASK Illustration
1
0
0
1
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Frequency Shift Keying (FSK)

• In radio transmission, known as frequency
  modulation (FM)
• Frequency of the carrier wave varies in
  accordance with the signal to be sent
• Signal transmitted at constant amplitude
• More resistant to noise than ASK
• Less attractive because it requires more analog
  bandwidth than ASK

64
FSK Illustration
1
1
0
1
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Phase Shift Keying (PSK)

• Also known as phase modulation (PM)
• Frequency and amplitude of the carrier signal are
  kept constant
• The carrier signal is shifted in phase according
  to the input data stream
• Each phase can have a constant value, or value
  can be based on whether or not phase changes
  (differential keying)

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PSK Illustration
0
0
1
1
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Differential Phase Shift Keying (DPSK)
0
0
1
1
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Analog Channel Capacity BPS vs. Baud

• Baud of signal changes per second
• BPSbits per second
• In early modems only, baudBPS
• Each signal change can represent more than one
  bit, through complex modulation of amplitude,
  frequency, and/or phase
• Increases information-carrying capacity of a
  channel without increasing bandwidth
• Increased combinations also leads to increased
  likelihood of errors

69
Quadrature Amplitude Modulation (QAM)

• Commonly used method for quadbit transfer
• Combination of 8 different angles in phase
  modulation and two amplitudes of signal
• Provides 16 different signals, each of which can
  represent 4 bits

70
Quadrature Amplitude Modulation Illustration
90
135
45
amplitude 1
0
180
amplitude 2
225
315
270
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QAM Example CCITT V.22bis Modem

• Uses QAM
• "bis" qualifier is a French term for "duo" or
  "twice"
• Supports transmission of full-duplex 2400 bps
  synchronous or asynchronous data over a switched,
  2-Wire, voice circuit
• Modulation rate is 600 baud, with each baud
  representing four data bits

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Trellis Coded Modulation (TCM)

• Sophisticated mathematics are used to predict the
  best fit between the incoming signal and a large
  set of possible combinations of amplitude and
  phase changes
• Forward Error Correcting (FEC)
• Used in V.32 (9600 bps) and higher speed modems

73
CCITT V-Series Modem Recommendations

• V.22 1200 bps duplex modem standardized for use
  in the PSTN and on leased circuits
• V.29 9600 bps modem standardized for use on
  point-to-point 4-wire leased telephone circuits
• V. 32 2-wire, duplex modems operating at data
  rate of up to 9600 bps for use on the PSTN and on
  leased circuits

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V.32 bis Modems

• Uses Trellis coding with QAM
• Allows transport of data up to 14400 bps
• Modulation rate is 2400 baud

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V.34 Modems

• Capable transmission up to 28.8 kbps
• Modulation rate (baud rate) and carrier frequency
  can vary
• Multi-dimensional Trellis-coding is employed

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V.34 Modems

• Data rate up to 33.6 kbps over dial-up circuits
• Can achieve the above data rate only over
  extremely clean lines (see class handout from
  dbTechnology site)
• Use a range of adaptive techniques that enable a
  modem to learn and adjust to line conditions.

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56kbps Modems

• Asymmetrical can download at 56kbps but upload
  at 33.6kbps only
• Requires digital T-1 or ISDN PRI connection at
  central site or ISP, single hop between sender
  and receiver
• Two incompatible systems, no official standard
• U.S. Robotics (56K x2)
• Rockwell (56K flex)

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Digital Encoding of Analog Data

• Primarily used in retransmission devices
• The sampling theorem If a signal is sampled at
  regular intervals of time and at a rate higher
  than twice the significant signal frequency, the
  samples contain all the information of the
  original signal.
• 8000 samples/sec sufficient for 4000hz

79
Converting Samples to Bits

• Quantizing
• Similar concept to pixelization
• Breaks wave into pieces, assigns a value in a
  particular range
• 8-bit range allows for 256 possible sample levels
• More bits means greater detail, fewer bits means
  less detail

80
Codec

• Coder/Decoder
• Converts analog signals into a digital form and
  converts it back to analog signals
• Where do we find codecs?
• Sound cards
• Scanners
• Voice mail
• Video capture/conferencing

81
Digital Encodingof Digital Data

• Most common, easiest method is different voltage
  levels for the two binary digits
• Typically, negative1 and positive0
• Known as NRZ-L, or nonreturn-to-zero level,
  because signal never returns to zero, and the
  voltage during a bit transmission is level

82
Differential NRZ

• Differential version is NRZI (NRZ, invert on
  ones)
• Change1, no change0
• Advantage of differential encoding is that it is
  more reliable to detect a change in polarity than
  it is to accurately detect a specific level

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Problems With NRZ

• Difficult to determine where one bit ends and the
  next begins
• In NRZ-L, long strings of ones and zeroes would
  appear as constant voltage pulses
• Timing is critical, because any drift results in
  lack of synchronization and incorrect bit values
  being transmitted

84
Biphase Alternatives to NRZ

• Require at least one transition per bit time, and
  may even have two
• Modulation rate is greater, so bandwidth
  requirements are higher
• Advantages
• Synchronization due to predictable transitions
• Error detection based on absence of a transition

85
Manchester Code

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high1 , high-to-low0
• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• Has added advantage of differential encoding
• Used in token-ring

87
Digital Encoding Illustration
88
Telecommunications Standards

• Where do they come from?
• Standard setting bodies
• Governments
• Two types
• Market-driven and voluntary
• Government-regulated and mandatory

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Advantages

• Assures a large market, which encourages mass
  production and often lowers costs
• Encourages vendors to enter market because
  investment is protected
• Allows products from multiple vendors to
  communicate, providing consumers with wider
  selection

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Disadvantages

• Standards process can freeze technology too
  early, due to the length of the standards-setting
  process and the speed with which technology
  changes
• Current process allows for multiple standards for
  the same thing

91
Institute of Electrical and Electronics Engineers
(IEEE)

• The largest professional society in the world
• Develops standards in the area of electrical
  engineering and computing
• Publishes scores of journals and runs numerous
  conferences each year
• e.g. IEEE 802.x network standards

92
American National Standards Institute (ANSI)

• Non-governmental and nonprofit organization
• Members are U.S. manufacturers and other interest
  groups
• Sets a variety of a standards, not just
  computer-related
• ANSI proposals are usually approved by ISO as
  international standards
• e.g. 802.x, created by IEEE, approved by ANSI,
  passed on and approved by ISO

93
National Institute of Standards and Technology
(NIST)

• Formerly known as the National Bureau of
  Standards (NBS)
• Agency of the U.S. Dept.. of Commerce
• Issues standards that are mandatory for purchases
  made by the U.S. Government except the Department
  of Defense

94
Industry Associations

• Electronic Industries Association (EIA)
• Telecommunication Industry Association (TIA)
• e.g. EIA-232 (formerly RS-232-C)

95
Intl Telecommunications Union (ITU)

• Formerly known as Consultative Committee on
  International Telegraph and Telephone (CCITT)
• Standardize techniques and operations in the
  telecommunications field
• e.g.
• CCITT Group 3 Fax
• CCITT V.x modem standards

96
ISO (International Standards Organization)

• Founded in 1946
• Issues standards on a vast number of subjects,
  ranging from nuts and bolts to telephone pole
  coatings
• Has almost 200 Technical Committees
• A member of ITU-T

97
Internet Engineering Task Force (IETF)

• Part of the Internet Architecture Board (IAB)
• IETF proposes and published Internet RFCs
• IAB determines which RFCs become standards, based
  on IETF recommendations

98
RFC? Internet Standard

• Stable and well-understood
• Technically competent
• Numerous independent and interoperable
  implementations in operation
• Significant public support
• Recognizably useful
• Differs from other standards processes because of
  the emphasis on operational experience