Data Communication Data Communications System Transmitter transmit data

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.

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

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

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

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

Asynchronous Transmission Unpredictable time interval Between characters Start bit Stop bit 11 1 1 0 0 01 Start bit 00 1 0 11 0 0 8 -bit asynchronous character stream Stop bit

Asynchronous Transmission Start 0 100 200 300 400 500 600 700 800 1 0 2 3 4 5 6 7 8 Transmitter timing Stop 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 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 Transmission Synchronous frame format preamble 8 -bit Control flag fields postamble Data fields Control 8 -bit fields flag

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-232 C or an registered jack (e. g. RJ-45) connection is possible.

Diagrams on RJ-11 and RJ-45 Interfaces

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