FUNDAMENTALS OF NETWORKING CHAPTER 5& 6 SERIAL AND PARALLEL TRANMISSION ; TYPE OF ERROR IN DATA TRANSMISSION

TRANSMISSION MODE Data can be transmitted as entire character or as individual bits. The transmission mode determines which of these ways the network uses to send the data. Two modes of transmission Serial transmission Parallel transmission

SERIAL TRANSMISSION Involves transmitting data as individual bits, with a single bit following another on the transmission line. Serial transmission uses a single communications line to transmit the entire character 1 bit at a time. After each of the characters is broken up into individual bits and transmitted, the receiving device must reassemble the bits back into the proper characters. A modem, for example connects to a computer with a serial cable, serial port.

SERIAL TRANSMISSION The internet uses serial transmission. The protocol in this network is called TCP/IP. Two protocols commonly transmit information using this protocol for connecting to the Internet.

Serial transmission

PARALLEL TRANSMISSION Involves in transmitting an entire character (1 bit per line) at one time. The different parts of a computer normally communicate using parallel transmission because it is much faster than serial transmission.

PARALLEL TRANSMISSION The parallel connector and cable that connect these devices differ from a serial cable in that the parallel connection uses multiple lines to accommodate the number of bits required to transmit the entire character at one time. If a character is made up of eights bits, the parallel cable needs eight communication lines to transmit the character. The eight lines are put into a single cable and plugged into a single port on the computer. Because each bit in the character requires a communications line, parallel transmission is expensive for transmission of data over long distances.

Parallel transmission

SYNCHRONIZATION In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data.  Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, in order to reach an agreement or commit to a certain sequence of action.  Data synchronization refers to the idea of keeping multiple copies of a dataset in coherence with one another, or to maintain data integrity. Process synchronization primitives are commonly used to implement data synchronization

SYNCHRONOUS TRANSMISSION Data transfer method in which a continuous stream of data signals is accompanied by timing signals (generated by an electronic clock) to ensure that the transmitter and the receiver are in step (synchronized) with one another. The data is sent in blocks (called frames or packets) spaced by fixed time intervals. 

Note In synchronous transmission, we send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits. The bits are usually sent as bytes and many bytes are grouped in a frame. A frame is identified with a start and an end byte.

Synchronous transmission

SYNCHORONIZATION BETWEEN THE TRANSMITTER & RECEIVER In a serial connections, since a single wire transports the information, the problem is how to synchronise the transmitter and receiver In other words, the receiver can not necessarily distinguish the characters (or more generally the bit sequences) because the bits are sent one after the other.

ASYNCHRONOUS TRANSMISSION In contrast, asynchronous transmission works in spurts and must insert a start bit before each data character and a stop bit at its termination to inform the receiver where it begins and ends. The term asynchronous is used to describe the process where transmitted data is encoded with start and stop bits, specifying the beginning and end of each character.

Note In asynchronous transmission, we send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte.

Asynchronous transmission

SOURCE OF ERRORS Errors – Errors must be detected, prevented, and corrected Sources of Errors – Data transmission errors can come from internal as well as external sources. Some of the sources of errors in a data communications network: IMPULSE NOISE, WHITE NOISE, ATTENUATION, CROSSTALK, LINE FAILURE

IMPULSE NOISE Caused by external electrical interference, which primarily results from electrical storms. When a bolt of lightning strikes near a copper wire, a large amount of electricity travels through the wire, changing the characteristics of electrical signals travelling on that wire.  TO CONCLUDE -External electrical interference (electrical current surge).

WHITE NOISE Transmission line always include white noise, which contains component at all frequencies. In a radio signal, for example, any static , or constant hiss during transmission, indicates white noise. The movement of electrons in the wire causes white noise, and the amount of noise is directly proportional to the temperature of the medium (which is why the noise also may be called thermal noise). If the medium is very hot, the electrons move faster, and the amount of white noise present increases. To limit the amount of white noise, the medium should be kept at an acceptably cool temperature. TO CONCLUDE - Hiss or static on the line (also called thermal noise) often caused by heat.

ATTENUATION A signal travels along a transmission line, it losses some of its strength, and this loss of strength is called attenuation. To overcome attenuation, communication network include amplifiers or repeaters placed at specific intervals to boost the strength of the signal. The distance a signal can travel before it needs to be strengthened depends on the medium being used. TO CONCLUDE -Loss of the strength of the signal, which leads to errors. To overcome attenuation, amplifiers (used for analog) and repeaters (used for digital) are placed along the network to boost the signal.

CROSSTALK Occurs when the signals from two transmission line interference with one another. Broken shielding on a cable can cause the copper core to come too close to another cable and cause interference. Or from a very strong signal being sent over one two-wire pair, causing that cable to interfere with another nearby cable. TO CONCLUDE - Signals from two transmission lines interfere with each other. Broken shielding on a cable can cause this

LINE FAILURE The entire communications line may be out of service. A break in the line, a problem with telephone equipment, electrical storms that damage the line, or other electrical problems on the line can cause a line failure. In this situation, the carrier signal is lost, and any data that was transmitted is lost.

ERROR DETECTION Because so many sources of errors exist, almost any data transmission is susceptible to some type of error. Any data transmission that sustains errors must be corrected before it can be used in processing. To help with error detections and corrections, data is divided and transmitted in smaller groups called packets or frames. Some error detection and correction techniques require that extra control information be sent along with data packets and frames, while others correct the data upon receipt.

PARITY CHECKING Is designed to determine if bits have been changed as they have been sent along the transmission line. Parity checking is used with 7-bit codes or characters; an extra bit called a parity bit, is added to the character to provide the parity checking. Transmission is defined as using even parity or odd parity.

Even parity uses an even number of 1 bits as the measure of valid data; odd parity uses an odd number of 1 bits. The sender generates the parity bit, which depends on the number of 1 bits in the original character. If the character has an even number of 1 bits, the parity bit is 0. If the character has an odd number of 1 bits, the parity bit is 1. The parity bit is transmitted with each seven-bit character, making the character eights bits long.

CHARACTER TRANSMITTED   7-BIT ASCII CHARACTER PARITY BIT CHARACTER TRANSMITTED A 1000001 10000010 B 1000010 10000100 C 1000011 1 10000111 D 1000100 10001000 E 1000101 10001011 EVEN PARITY 10000011 10000101 10001001 10001010 ODD PARITY

ERROR CORRECTION When an error is detected on a data communication network, it must be corrected. Correcting errors on a data communication network can be done either by forward error correction or by error detection with retransmission. Forward error correction corrects error without retransmission of the data; with error detection with retransmission, the sender must retransmit the data if the receiver detects an error. The correction method use depends on the types of error encountered in the transmission and the effect that for error detection and correction has on the transmission line

ERROR PREVENTION Because eliminating noise entirely from the communications line is nearly impossible, steps must be taken to reduce the amount of noise and the resulting interference from the line. The error prevention technique are defines either by the user (either receiver or sender) of the line or by the telephone company that own the line. Simple way to prevent error Ex: Improve the shielding around cables The speed at which data is transmitted :faster the transmission speed, the more likely it is that the data will contain errors.