Jkkd Mti-ui Ch02
Transcript of Jkkd Mti-ui Ch02
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MTI - UI Ganjil 2009/2010 1
Chapter 2
Data Communications Concepts
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Data Digitization The process of process of transforming
humanly readable characters into
machine readable code is character encoding.
Characters are turned into a series of
ones and zeroes.
The most commonly used standards are ASCII, EBCDIC, and UNICODE
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7-Bit ASCII Code table
�Ascii encoding is used on most computers today
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EBCDIC Code Table
� EBCDIC is used on IBM Mainframes
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Serial vs. Parallel Data Transmission
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Serial vs. parallel Data Transmission
Transmission
Characteristic
Serial Parallel
Transmission Description One bit after another, one at a
time
All bits in a single character
transmitted simultaneously
Comparative Speed Slower Faster
Distance Limitation Farther Shorter
Application Between two computers, fromcomputers to external devices,local and wide area networks
Within a computer along thecomputer¶s busses, between adrive controller and a hard drive
Cable Description All bits travel down a single wire,one bit at a time
Each bit travels down its ownwire simultaneously with other bits.
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Synchronous vs. Asynchronous
transmission
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Modulation vs. demodulation
This process is done by a mo(dulator)dem(odulator)
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Modem based communication channels
The dial-up modem allows connections throughthe phone network
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Half vs. full duplex
Data communications sessions are bi-directional in nature.
There are two environments available for handling this bi-directional traffic: full and half duplex.
In a full duplex communications environment
both devices can transmit at the same time. In a half duplex environment you can only
hear or talk at any given point in time.
Given the choice of full or half duplex it is
usually better to choose full duplex.
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Carrier Wave
There are three properties of a wave that canbe modulated or altered:
Amplitude
Frequency
Phase
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Amplitude Modulation
Each vertical lines separates opportunities to identifya 1 or 0 from another .
These timed opportunities are known as signalingevents.
The proper name for one signaling event is a baud
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Frequency Modulation
f requency shif t keying or FSK
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Phase Modulation
phase shif t keying or PSK
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Detecting Phase Shifting
Quadrature Phase Shift Keying
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Increasing transmission efficiency
There are two ways in which a given
modem can transmit data faster:
increase the signaling events per second,
or baud rate.
find a way for the modem to interpret more
than one bit per baud.
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Differential Quadrature Phase Shift keying
This technique improves transmission rate byincreasing the number of events per baud
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Data Compression
Data compression techniques improvethroughput.
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Data compression The sending device replaces strings of
repeating character patterns with a
special code that represents the pattern. The code is significantly smaller than
the pattern it represents.
This results in the amount of data sentbetween the sending device and the
receiving device to increase.
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Packetization The process of dividing the data steam
flowing between devices into structured
blocks known as packets.
A packet is a group of bits organized in
a pre-determined, structured manner
consisting of a piece of the data streamto which management information is
added.
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Packetization
This data stream is divided into 3 packets
Note the addition of header information to the dataportion
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Packetization The predetermined structure of a
packet is critical.
Through the use of standards, devicesknow the number of bits in eachsection; the header, data portion and
trailer.
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Encapsulation / De-encapsulation
In a layered protocol, each layer adds aheader according to the layers syntax.
The sending device adds thisinformation in a process of encapsulation
The receiving device reverses the thisprocess (de-encapsulation)
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Encapsulation / De-encapsulation in the OSI model
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Multiplexing
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Frequency Division Multiplexing
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Time Division Multiplexing
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Statistical Time Division
Multiplexing
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Switching
Switching allows temporary connections to beestablished, maintained and terminatedbetween sources and destinations
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Circuit Switching The work to create a signal path is done up
front; a switch fabric creates a direct path
between the source and the destination. Communication takes place just as if the
temporary circuit were a permanent direct connection:
The switched dedicated circuit makes it appear to the user of the circuit as if a wirehas been run directly between thecommunicating devices.
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Packet switching In a packet switched network, packets of
data travel one at a time from the message
source to the message destination. The physical path taken by one packet may
be different than that taken by other packetsin the data stream.
The path is unknown to the end user. A series of packet switches pass packets
among themselves as they travel from sourceto destination
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Datagram Delivery
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Connectionless vs. Connection-Oriented
Networks
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The error detection process
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Error detection process ·The transmitting and receiving devices agree on how
the error check is to be calculated
· The transmitting device calculates and transmits theerror check along with the transmitted data
· The receiving device re-calculates the error check
based on the received data and compares its newly
calculated error check to the error check received
with the data
·If the two error checks match, everything is fine. If
they do not match, an error has occurred
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Parity Checking
Simple parity checking
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Parity Checking
Parity checks can miss multiple bit errors
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Alternatives to parity
LRC improves parity checking at the cost of extra data transmitted
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Error Control Techniques Error Prevention
Error Detection
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Error detection techniques yParity (VRC)
yLongitudinal Redundancy Checks (LRC)
yChecksums
yCyclic Redundancy Checks (CRC)
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Parity
Parity, also known as a (VerticalRedundancy Check or VRC), is thesimplest error detection technique.
Parity works by adding an error check
bit to each character.
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Longitudinal Redundancy
Checks
Longitudinal Redundancy Checks(LRC) seek to overcome the weaknessof simple, bit-oriented one directionalparity checking..
LRC adds a second dimension to parity.
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Checksums Checksums are also block-oriented error detection
characters added to a block of data characters.
a checksum is calculated by adding the decimal facevalues of all of the characters sent in a given data
block and sending only the least significant byte of
that sum.
The receiving modem generates its own checksum
and compares the locally calculated checksum withthe transmitted checksum
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Error Correction The receiving device detects the error
and requests a re-transmission
The sending device then retransmits the
the portion that contained the error .
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Flow Control To prevent buffer overflows the receiving
device sends a signal to the sending device
The flow control software constantly monitorsthe amount of free space available in buffer
memory and tells the sending device to stop
sending data when there is insufficient
storage space.
When the buffer once again has room the
sending device is told to resume transmitting
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