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Chapter 1
Data Communications
Introduction
• Look at:
– Multiplexing (1.1)
– Signaling (1.2)
– Encoding and Decoding (1.3)
– Error Detection and Recovery (1.4)
– Flow Control (1.5)
– Sliding Window (1.6)
– Congestion Management (1.7)
Introduction
• Networking involves much more than
managing computers; it is
understanding what makes them work
• We can now send multiple signals
simultaneously over the same cable
segment
• Multiplexing combines several
connections into one larger channel
Multiplexing
• There are many different types of
multiplexing:
– Frequency division multiplexing (FDM) is a
method of transmission in which numerous
signals are combined on a single
communications line or channel
Multiplexing
• There are many different types of
multiplexing:
– Wave division multiplexing (WDM) is a
form of frequency division multiplexing
specifically for combining many optical
carrier signals into a single optical fiber
Multiplexing
• There are many different types of
multiplexing:
– Time division multiplexing (TDM) is multiple
data streams that are combined in a single
signal and transmitted over the same link
by allocating a different time slot for the
transmission of each channel
Multiplexing
• There are many different types of
multiplexing:
– Pulse code modulation (PCM) converts the
human signal into a digital representation
– In PCM a digital signal is obtained by
sampling the analog signal at various
points in time
Signaling
• Signaling is communication of
information between network nodes by
using telecommunications signals
• The carrier signal has three major
characteristics:
– amplitude
– frequency
– phase
Signaling
• Analog signaling adds information or
encodes information to an alternating
current (AC) base signal by modifying
the frequency or signal strength
• Digital signaling uses discrete steps to
represent information in binary format
as zeros (0s) or ones (1s)
Signaling
• Each binary digit or bit represents one
piece of information, where a bit being a
1 means one thing, and a 0 means
another
• Within the circuitry of the PC, ones and
zeros are represented by voltage levels
Signaling
• A signal can be any type of information
• The transition from a 0 to a 1 is called
the rising edge
• The transition from 1 to 0 is called the
falling edge
• Signals change as time progresses, and
this is what enables the flow of data
Signaling
• Some common terms used in signaling:
– Clock - sets the tone for everything that
happens within a particular electronic
circuit
– Cycle -This is a single pass through of the
signal, from the rising edge through the
falling edge, until the start of the next rising
edge
Signaling
• Some common terms used in signaling:
– Cycle length - The amount of time required
for the signal to complete one full cycle.
– Rise time and fall time - This measures
how long it takes for the level to change
from 0 to 1, or 1 to 0.
Encoding and Decoding
• Encoding is putting electronic data into
a standard format
• Data is encoded into signals to send
them from one place to another
• Once the data has been encoded and
sent along a physical medium, it must
be decoded on the other end
Encoding and Decoding
• Because information can be either
digital or analog, and signals can be
digital or analog there are several types
of encoding
– Analog-to-analog
– Analog-to-digital
– Digital-to-analog
– Digital-to-digital
Encoding and Decoding
• The various types of encoding include:
– Amplitude modulation (AM) is the encoding
of a carrier wave by the changes of its
amplitude along with the changes in input
signal
– Frequency modulation (FM) ) is the method
of encoding data onto an AC wave by
changing the instantaneous frequency of
the wave
Encoding and Decoding
• The various types of encoding include:
– Phase shift modulation (PSM) is when
digital signals are conveyed by shifting
phases
– PSM is typically used for digital signaling,
or satellite communication
– Multilevel signaling is the use of light or
sound to encode and transmit information
Encoding and Decoding
• When converting digital data to digital
signals common techniques are used :
– Non-Return to Zero (NRZ) uses two levels
of signaling or is bipolar
– The two levels or states can be expressed
as either on or off or high or low
– This is the most basic and simplistic
method of encoding
Encoding and Decoding
• When converting digital data to digital
signals common techniques are used :
– Non-Return to Zero Inverted (NRZ-I), is an
inverted signal level
– If the bit is a 1, the signal transitions
– If the bit is a 0, the level stays current (no
transition)
Encoding and Decoding
• When converting digital data to digital
signals common techniques are used :
– Manchester is a synchronous clock
encoding technique used to encode the
clock and data of a synchronous bit stream
– It uses the rising or falling edge in the
middle of each bit time to indicate a zero or
one
Encoding and Decoding
• When converting digital data to digital
signals common techniques are used :
– Differential Manchester is similar to
Manchester encoding
– A 1 bit is indicated by making the first half
of the signal equal to the last half of the
previous bit's signal
– There is no transition at the start of the bit
time
Encoding and Decoding
• Additional methods of encoding include:
– 4B/5B this takes data in four-bit codes and
maps it to corresponding five-bit codes
– This is done to guarantee no more than
three 0s in a row so that synchronization is
more adequate is TCP/IP
Encoding and Decoding
• Additional methods of encoding include:
– MLT-3 works much in the same in way that
NRZ-I works
– It encodes based on presence of a
transition or lack of a transition
– This is a three-level form of data encoding
used to concentrate the signal power
below 30 MHz
Encoding and Decoding
• Additional methods of encoding include:
– 8B/6T the value of the data byte is
compared to the values in the 8B/6T table
– The remapping table has nine symbols
used for starting and ending delimiters and
control characters
Encoding and Decoding
• Additional methods of encoding include:
– 8B/10B is an encoding scheme in which 8bit binary data values are represented by
10-bit symbols
– The data octet is split up into the three
most significant bits and the five least
significant bits
Encoding and Decoding
• Additional methods of encoding include:
– 4D-PAM5 is a four-dimensional, five-level
pulse amplitude modulation.
– This is a way of encoding bits on copper
wires to get a 1 GB per second transfer
rate when the maximum rate of a single
wire is 125 MHz
– A five-level signal called pulse amplitude
modulation 5 is used
Error Recovery
• Error detection and recovery is
accomplished using:
– Parity checking ensures when data is
transmitted from one device to another
there is a way to recover lost transactions
– Parity checks are used during transmission
of data to detect errors that are caused by
interference or noise
Error Recovery
• Error detection and recovery is
accomplished using :
– Checksums are a simple error-detection
scheme
– Each message has a value based on the
number of bits in the message
– If the value matches, it's assumed that the
complete transmission was received
Error Recovery
• Error detection and recovery is
accomplished using :
– Cyclic redundancy checking is a technique
for checking errors in data that has been
transmitted on a communications link
– It is substantially reliable in detecting
transmission errors and is commonly used
in modems
Error Recovery
• Error detection and recovery is
accomplished using :
– Error correction is a sophisticated form of
checking where errors are corrected when
they are detected
– Error-correcting code is an algorithm for
expressing a sequence of numbers
Flow Control
• Flow control is a method by which the
data flow between devices is managed
so that the data can be handled at an
efficient pace
• Flow control can take place at various
levels such as user process to user
process, host to host, and router to
router
Flow Control
• Flow control occurs when the receiving
system tells the sending system to stop
because it has nowhere to put the data
• Two types of flow control are generally
supported:
– Hardware uses two of the wires that are
connected between the machine and the
modem
– Software uses control bytes
Sliding Window
• Windowing is a flow-control method whereby
the source requires an acknowledgement
from the destination after a certain amount of
data has been transmitted
• It requires the receiving device to
communicate with the sending device by
sending back an acknowledgement when it
receives data
Sliding Window
• Sliding Window Protocols use two
different methods:
– In data acknowledgement, both sender and
receiver keep a window of
acknowledgment
– In the Stop-and-Wait or One Bit, the sender
transmits one data unit then waits for an
acknowledgment before sending the next
data unit
Congestion Management
• Congestion can be managed in several
ways:
– Preallocation schemes try to prevent
congestion from happening by requiring
that resources be preallocated before any
data can be sent
– This guarantees that the resources will be
available to process the data when it is
received
Congestion Management
• Congestion can be managed in several
ways:
– Data discard is a simple but inefficient way
to handle congestion
– If insufficient resources are present to
process existing data, the queued data is
simply discarded
Congestion Management
• Congestion can be managed in several
ways:
– Isarithmic congestion control is an
approach to congestion avoidance
– It sets an upper limit on the number of data
packets allowed to be present on the
network at any one time
Congestion Management
• Congestion can be managed in several
ways:
– Choke packets ask the sender to cut back
traffic voluntary.
– Traffic shaping is a range of techniques
used to prioritize the transmission of data
over a network link.
– The idea behind shaping is to change
bursts of traffic to uniform, regular traffic.
Congestion Management
• Congestion can be managed in several
ways:
– Load shedding is a process of
systematically reducing the demand by
temporarily decreasing the load in
response to shortages
– Jitter control a method to make sure traffic
gets through the network smoothly