Description
Do some Internet research on flow control in data communications networks. Identify common examples of why flow control is needed and how it ensures reliable data transfer between senders and receivers. Provide 2-3 references. Instructions:The total word count must be 250 to 300Please also respond to at least two of your classmates with a meaningful reply of 150 words or greater Please provide references and in-text citations for your original postings in APA format.
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The way in which the electromagnetic signals are encoded to
convey data determines the efficiency and reliability of the
transmission
Data
Entities that convey
meaning or information
Transmission
The communication of
data across a computer
network by the
propagation and
processing
of signals
Signal
Electric or
electromagnetic
representation of data
Signaling
The physical propagation
of the signal along a
communication medium
Analog data
Continuous values on some interval
Voice and video
Data collected by sensors, such as temperature and
pressure
Digital data
Discrete values
Text, integers, binary data
Signals are used to encode and transmit data
Analog signal
Continuously varying electromagnetic wave that may
be transmitted over both guided and unguided media
Digital signal
Sequence of voltage pulses
Generally cheaper than analog signaling
Less susceptible to noise interference
Suffer more from attenuation than analog signals
Cannot be used on optical fiber or wireless media
Transmitter
technologies use
modulation
techniques that
enable sound and/or
video waveforms to
be conveyed as
electromagnetic
waveforms over wires
or airwaves
Modem
(modulator/demodulator)
Coverts binary voltage pulses by
modulating a carrier frequency
Demodulates the signal to recover
the original data at the other end
Codec (coder/decoder)
Takes an analog signal and
approximates that signal by a bit
stream
At the other end of a line the bit
stream is used to
reconstruct the
analog data
Digital data,
digital signal
Analog data,
digital signal
Digital data,
analog signal
Analog data,
analog signal
• The
equipment
for encoding
digital data
into a digital
signal is less
complex and
less
expensive
than digitalto-analog
equipment
• Conversion
of analog
data to
digital form
permits the
use of
modern
digital
transmission
and
switching
equipment
• Some
transmission
media, such
as optical
fiber and
satellite, only
propagate
analog
signals
• Analog data
are easily
converted to
an analog
signal
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
Cost – large scale and very large scale integration has
caused continuing drop in cost
Data Integrity – effect of noise and other impairments
is reduced
Capacity Utilization – high capacity is more easily and
cheaply achieved with time division rather than
frequency division
Security and Privacy – Encryption possible
Integration – All signals (voice, video, image, data)
treated the same
Analog or digital data must be converted into a signal for purposes of
transmission
The mapping from binary digits to signal elements is the encoding
scheme for transmission
The basis for analog encoding is a continuous constant frequency
signal known as the carrier signal
Modulation
The conversion of digital signals to analog form
Demodulation
The conversion of analog data signals back
to digital form
Digital data are
represented as variations in
the different amplitudes of
the carrier wave
Inexpensive
The amplitude (or height)
of the sine wave varies to
transmit the ones and zeros
Is susceptible to sudden
gain changes from noise,
distortions, and other
signal impairments making
it a rather inefficient
modulation technique
Also used to transmit
digital data over optical
fiber
Digital information is
transmitted through discrete
frequency changes of the
carrier wave
Simplest form is binary FSK
(BFSK) in which the two
binary values are represented
by two different frequencies
near the carrier frequency
Advantages:
• Less susceptible to noise than ASK
• Is easy to decode
• Often has a better signal-to-noise
ratio than ASK
• Is a signaling option supported by
most dial-up modems
Is commonly used for high
frequency radio transmission
The phase of the
carrier signal is shifted
to encode data
Is more noise resistant
and efficient than ASK
or FSK
Is widely used in
business networks,
especially wireless
networks
A four-phase system
(quadrature phaseshift keying, QPSK)
could encode two bits
with each signal burst
QFSK can be used to
double the data rate
while maintaining the
same bandwidth
Techniques may be combined
A common combination is PSK and ASK, where
some or all of the phase shifts may occur at one
or two amplitudes
Is commonly used in today’s networks
Examples:
56 Kbps dial-up modems
Digital subscriber line (DSL) modems
Gigabit Ethernet networks
Continue to be one of the most widely used pieces
of communications gear
Is a device that modulates an analog carrier wave
to encode digital information
Also demodulates the signals it receives to decode
transmitted information
Direct broadcast satellite, Wi-Fi, and mobile phones
use modems to communicate
Three popular types are:
Voice-grade
Cable
ADSL
Differential version is NRZI (NRZ, invert on ones)
Change=1, no change=0
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
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
Require at least one transition per bit time,
and may even have two
Modulation rate is greater, so bandwidth
requirements are higher
Maximum modulation rate is twice NRZ
Advantages
Synchronization due to predictable transitions
Error detection based on absence of a transition
Transition in the middle of each bit
period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0
Used in Ethernet and other LANs
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential encoding
Used in token-ring
Voice-generated sound wave can be represented by an
electromagnetic signal with the same frequency
components and transmitted on a voice-grade
telephone line
Modulation can produce a new analog signal that
conveys the same information but occupies a different
frequency band
A higher frequency may be needed for effective
transmission
Analog-to-analog modulation permits frequency-division
multiplexing
Clocks of transmitter and receiver
must somehow be synchronized
Block of bits transmitted in a
steady stream without start and
stop codes
• Provide a separate clock line between
transmitter and receiver – – – works well
over short distances
• Embed the clocking information in the
data signal
The data plus preamble,
postamble, and control
information are called a frame
Each block begins with a preamble
bit pattern and generally ends
with a postamble bit pattern
Involves the use of a data link
control procedure which
automatically detects
transmission error and causes a
frame in error to be retransmitted
All transmission media have potential for
introduction of errors
All data link layer protocols must provide a
method for controlling errors
Error control process has two components
Error detection
Redundancy introduced so that the occurrence of an
error will be detected
Error correction
Receiver and transmitter cooperate to retransmit frames
that were in error
Bit added to each
character to make
all bits add up to
an even number
(even parity) or
odd number
(odd parity)
Good for
detecting singlebit errors only
Noise impulses
are often long
enough to
destroy more
than one bit
The ability of
parity checking to
detect errors is
dependent on the
total number of
bits corrupted by
noise impulses
and the parity
convention that is
used
One of the
most
common
and
powerful
errordetecting
codes
Data in
frame is
treated as a
single
binary
number,
divided by a
unique
prime
binary, and
remainder is
attached to
frame
17-bit
divisor
leaves 16bit
remainder,
33-bit
divisor
leaves 32bit
remainder
For a CRC of
length N,
errors
undetected
are 2-N
Overhead is
low (1-3%)
⚫ Analog and digital data
communications
⚫ Data encoding
techniques
Analog encoding of
digital information
⚫ Digital encoding of
analog information
⚫ Digital encoding of
digital data
⚫ Analog encoding of
analog information
⚫
⚫ Asynchronous
transmission
⚫ Synchronous
transmission
⚫ Error detection
The need for error
control
⚫ Parity checks
⚫ Cyclic redundancy
check
⚫
Chapter 5: Data Communication Fundamentals
Technique for
assuring that a
transmitting entity
does not
overwhelm a
receiving entity
with data
Necessary when
data is being sent
faster than it can be
processed by
receiver
Prevents buffers
from overflowing
One of the primary
functions
performed at the
data link layer of
OSI
Two types of errors
Lost frame
A frame fails to arrive at the other side
Damaged frame
A recognizable frame does arrive, but some of the bits
are in error
Collectively referred to as automatic repeat request
(ARQ), common techniques for error control are:
Error detection
Positive acknowledgment
Retransmission after timeout
Negative acknowledgment and retransmission
The higher the data rate, the more
cost-effective the transmission
facility
• Cost per kbps declines with an increase in
Most individual data
the data rate of the transmission facility
• Cost of transmission and receiving
communicating devices require
equipment, per kbps, declines with
relatively modest data rate support
increasing data rate
Same general
architecture as
other FDM
systems
A number of
sources
generate a laser
beam at
different
wavelengths
These are sent
to a multiplexer
that
consolidates the
sources for
transmission
over a single
fiber line
Optical
amplifiers
amplify all of
the
wavelengths
simultaneously
The composite
signal arrives at
a demultiplexer
where the
component
channels are
separated and
sent to
receivers at the
destination
point
ADSL uses frequency-division
modulation (FDM) to exploit
the 1-MHz capacity of twisted
pair
Asymmetric because ADSL
provides more capacity
downstream (from the carrier’s
central office to the customer’s
site) than upstream (from
customer to carrier)
Provides a perfect fit for
Internet access
Entire frequency band for the upstream channel
overlaps the lower portion of the downstream channel
Advantages
The higher the frequency, the greater the attenuation
More flexible for changing upstream capacity
Disadvantages
Need for echo cancellation logic on both ends of line
Long-distance carrier system
designed to transmit voice
signals over high-capacity
transmission links such as
optical fiber, coaxial cable,
and microwave
Evolution of these networks
to digital involved adoption of
synchronous TDM
transmission structures
21
Transmission facilities supporting DS-1
Often used for leased dedicated transmission
between customer premises
Private voice networks
Private data network
Video teleconferencing
High-speed digital facsimile
Internet access
SONET is an optical
It has been standardized Synchronous Digital Specifications for taking
transmission interface that
by ANSI for voice, longHierarchy (SDH), a
advantage of the highwas originally designed for
haul data, and/or video compatible version, hasspeed digital transmission
the public telephone
traffic applications been published by ITU-Tcapability of optical fiber
network
Frequency-division multiple
access (FDMA)
• Radio spectrum used to connect mobile
devices and cell towers is divided into
separate frequency channels, each
capable of carrying one call
Frequency-division duplexing
(FDD)
• Two distinct frequency bands are used –
one band carries uplink channels (mobile
device to cell tower) and the second
carries downlink channels (cell tower to
mobile device)
These approaches are being
replaced by time-division
multiple access (TDMA) and codedivision multiple access (CDMA)
in cellular networks
⚫ Flow control
⚫ Error control
⚫ Motivation for
multiplexing
⚫ FDM
⚫ WDM
⚫ ADSL
⚫ Synchronous TDM
⚫ The TDM mechanism
⚫ Digital carrier systems
⚫ T-1 facilities
⚫ SONET/SDH
⚫ Cellular and cordless
phone systems
Chapter 6: Data Link Control and Multiplexing
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