Title: ICSA 411: Week 2b Data Communication
1ICSA 411 Week 2bData Communication
- Elizabeth Lane Lawley, Instructor
2Three 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
3Analog 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
4Digital 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
5Transmission 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
6Data, Signal, and Transmission Matrix
7Advantages 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 bandwidth is possible with data compression
8Why Use Analog Transmission?
- Already in place
- Significantly less expensive
- Lower attentuation rates
- Fully sufficient for transmission of voice signals
9Analog 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
10Modem
- 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
11Methods 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)
12Amplitude 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
13ASK Illustration
1
0
0
1
14Frequency 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
15FSK Illustration
1
1
0
1
16Phase 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)
17PSK Illustration
0
0
1
1
18Differential Phase Shift Keying (DPSK)
0
0
1
1
19Analog 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
20Quadrature 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
21Quadrature Amplitude Modulation Illustration
90
135
45
amplitude 1
0
180
amplitude 2
225
315
270
22QAM 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
23Trellis 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
24CCITT 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
25V.32 bis Modems
- Uses Trellis coding with QAM
- Allows transport of data up to 14400 bps
- Modulation rate is 2400 baud
26V.34 Modems
- Capable transmission up to 28.8 kbps
- Modulation rate (baud rate) and carrier frequency
can vary - Multi-dimensional Trellis-coding is employed
27V.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.
2856kbps 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)
29Digital 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
30Converting 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
31Codec
- 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
32Digital 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
33Differential 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
34Problems 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
35Biphase 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
36Manchester Code
- Transition in the middle of each bit period
- Transition provides clocking and data
- Low-to-high1 , high-to-low0
- Used in Ethernet
37Differential 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
38Digital Encoding Illustration
39Telecommunications Standards
- Where do they come from?
- Standard setting bodies
- Governments
- Two types
- Market-driven and voluntary
- Government-regulated and mandatory
40Advantages
- 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
41Disadvantages
- 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
42Institute 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
43American 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
44National 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
45Industry Associations
- Electronic Industries Association (EIA)
- Telecommunication Industry Association (TIA)
- e.g. EIA-232 (formerly RS-232-C)
46Intl 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
47ISO (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
48Internet 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
49RFC? 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