Data Communication

Data Communication

Data Communications System • Transmitter – transmit data to another medium. • Receiver – receive data from a transmitter. • Medium of transfer – the medium for transfer of data.

Source System Destination System Source Transmitter Transmission system Receiver Destination Workstation Modem Public Telephone Network Modem Server Communications Model

Simplex Transmission • Simplex communication means that communication can only flow in one direction and never flow back the other way. Data

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

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

Real life examples • Simplex transmission • Pager • Half-duplex transmission • Telephone, facsimile • Full-duplex transmission • Dual Carriageway

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

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.

Parallel Transmission • The simultaneous transmission of a group of bits over separate wires. • The transmission of 1 byte (8-bits) with computers.

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.

Serial Transmission • Not as fast as parallel transmission • Can transmit data for longer distances

Data bits Parity bit Start bit Stop bit The coding of a typical character sent in asynchronous transmission 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.

Idle state of line Odd, even, or unused Start bit 5 to 8 data bits 1 – 2 bit times 0 Stop 1 Remain idle or next start bit P bit Asynchronous Transmission Character format

Unpredictable time interval Between characters Start bit Stop bit Start bit Stop bit 1 1 1 1 0 0 0 1 0 0 1 0 1 1 0 0 Asynchronous Transmission 8-bit asynchronous character stream

Transmitter timing Start 0 100 200 300 400 500 600 700 800 1 2 3 4 5 6 7 8 Stop Receiver timing 0 93 186 279 372 465 558 651 744 Asynchronous Transmission Effect of timing error Assumptions: data rate of 10 kbps  0.1 ms each bit. The receiver is off by 7% or 0.007 ms per bit-time The receiver samples the incoming character every 0.093 ms (based on the transmitter’s clock).

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

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

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

Synchronous frame format preamble 8-bit flag Control fields Control fields postamble 8-bit flag Data fields Synchronous Transmission

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%

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.

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.

Diagrams on RJ-11 and RJ-45 Interfaces

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

Data Modem Modulation Digital signal Analogue signal PC Modem Public Telephone Network PC Modem Digital signal Analogue signal Demodulation

Data Modem • Baud Rate • This refers to the number of signals per one second transmitted • Bit Rate • The bit rate is multiplied by the bits per signal

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

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

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

Data Communication