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Data Communication
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Data Communications System
Transmitter – transmit data to another medium. Receiver – receive data from a transmitter. Medium of transfer – the medium for transfer of data.
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Communications Model Source Receiver Transmitter Destination
Transmission system Transmitter Receiver Destination Source System Destination System Workstation Modem Public Telephone Network Server
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Simplex Transmission Simplex communication means that communication can only flow in one direction and never flow back the other way. Data
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Half-duplex Transmission
Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time. Data
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Full-duplex Transmission
Full-duplex data transmission means that data can be transmitted in both directions on a signal carrier at the same time. Data
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Real life examples Simplex transmission Half-duplex transmission
Pager Half-duplex transmission Telephone, facsimile Full-duplex transmission Dual Carriageway
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Data Transmission Rate
Data transmission rate: bps, Kbps, Mbps bps – bits per second Kbps – kilo-bits per second Mbps – mega-bits per second Bps – bytes per second 1 Byte = 8 bits
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Serial Transmission The transfer of discrete signals one after another. Bits travel sequentially along the same wire. Send information over a single line one bit at a time, as in modem-to-modem connections.
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Parallel Transmission
The simultaneous transmission of a group of bits over separate wires. The transmission of 1 byte (8-bits) with computers.
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Parallel Transmission
Relatively fast Limited distance before data is lost As short as possible (no longer than 15 feet) As the length of cable increases so does the danger of cross-talk.
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Serial Transmission Not as fast as parallel transmission
Can transmit data for longer distances
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Asynchronous Transmission
In modem communication, a form of data transmission in which data is sent one character at a time. In addition, a parity bit is usually used for error checking. Avoid timing problem by not sending long, uninterrupted streams of bits. Start bit Data bits Parity bit Stop bit The coding of a typical character sent in asynchronous transmission
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Asynchronous Transmission
1 Idle state of line Start bit Stop 5 to 8 data bits Odd, even, or unused P 1 – 2 bit times Remain idle or next start bit Character format
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Asynchronous Transmission
1 Start bit Stop Unpredictable time interval Between characters 8-bit asynchronous character stream
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Asynchronous Transmission
Transmitter timing Start Stop 1 2 3 4 5 6 7 8 100 200 300 400 500 600 700 800 93 186 279 372 465 558 651 744 Receiver timing Effect of timing error Assumptions: data rate of 10 kbps 0.1 ms each bit. The receiver is off by 7% or ms per bit-time The receiver samples the incoming character every ms (based on the transmitter’s clock).
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Asynchronous Transmission
Advantages: simple cheap Disadvantages: requires an overhead of 2 – 3 bits per character (start and stop bits) (>=20%) cannot send large blocks or bits between start and stop bits with great cumulative timing error
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Synchronous Transmission
Data transfer in which information transmitted in block (frames) of bits separated by equal time intervals A block of bits is transmitted in a steady stream without star and stop codes
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Synchronous Transmission
Method 1: Provide a separate clock line between transmitter and receiver The other side uses these regular pulses as a clock This technique works well over short distances Method 2: Embed the clocking information in the data signal
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Synchronous Transmission
8-bit flag Control fields Data fields preamble postamble Synchronous frame format
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Synchronous Transmission
Advantage: For sizable/large blocks of data, synchronous transmission is far more efficient that asynchronous. The control information, preamble, and postamble are typically less than 100 bits. E.g. 48 bits of control, preamble, and postamble with 1000-character block of data, each frame consists of 48 bits of overhead and 8000 bits of data, so % overhead = 48/8048 x 100% = 0.6%
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Data Transfer Directed from PC to PC
Direct Cable Connection A null modem cable allows you to connect your PC to another nearby PC or serial device using its modem protocol. A null modem cable is limited to 30 feet in length. A null modem cable is sometimes called crossover cable.
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Crossover Cable A crossover cable is a cable that is used to interconnect two computers by "crossing over" (reversing) their respective pin contacts. Either an RS-232C or an registered jack (e.g. RJ-45) connection is possible.
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Diagrams on RJ-11 and RJ-45 Interfaces
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Data Modem Modem – Modulator and Demodulator
Modulator – convert digital signal (data in PC) to analogue signal (data via telephone line) Demodulator – convert analogue signal to digital signal
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Data Modem Modulation Demodulation Digital signal Analogue signal PC
Public Telephone Network PC Modem Digital signal Analogue signal Demodulation
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Data Modem Baud Rate Bit Rate
This refers to the number of signals per one second transmitted Bit Rate The bit rate is multiplied by the bits per signal
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Sources of errors during data transmission
Attenuation Signal grows weak over distance White noise Caused by molecular movement Impulse noise Caused by electrical interference Cross-talk Caused by interference from adjacent lines
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DCE and DTE DTE Data Terminal Equipment which is the ultimate source or final destination of data messages DCE Data Circuit-Terminating Equipment which connects the DTE to the communication circuits
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Bandwidth The capacity at which you can transfer data is called bandwidth Typical telephone line: 33,600 kilobits per second (33.6 Kbps) Cable TV: 10 megabits per second (10 Mbps) – almost 300 times the capacity of the normal phone connection
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