Transmitting And Receiving

Tran smi tting and R eceiv ing Trans mit ting and R ecei ving is the transfer of data within and between information systems.

Co mm unic ation Co nce pt s There are two main types of digital data transmission: serial and parallel. Type

Descri pti on

Seria l Tran smissio n

The bits in a message are sent in single file, one byte behind the other

Par all el Tran smissio n

-0 0 1 1 0 1 -> 8 bits in a byte are sent along 8 separate paths (or multiples of 8 bits along multiple paths) at the same time. 0 0 1 1 0 1 1 0

Adv antages/Disadv s

antage

Examples of use

Slower than parallel but used over longer distances. Signal fades over distance so the signal must be amplified using hardware eg. Repeaters.

Modems Telephone lines Satellite systems

Quicker but expensive. Errors occur because of data skew if the distance is more than a few metres.

Disk drives Printers

Serial transmission can be either asynchronous or synchronous. As ynchr onous tr ansm iss ion: each byte is identified with special start and stop bits. It has become the standard for PCs. Synchr onous t ransmiss ion: requires all the data to be sent at the same rate. It is faster and more efficient than asynchronous transmission, and is used on larger computer systems.

The direction of data flow is either simplex, half duplex or full duplex. Type

Simple x tr ans mission

Description

Data travels along the medium in one direction only. Sender

Half Du ple x tr ans mission

Data travels along the medium in both directions, but not simultaneously. Sender

Full Dup lex tr ans mission

Receiver

Examples of Use

Slower than other methods. The same channels cannot be used to send data back to the transmission station

Keyboard Printer Teletext

Taking turns to send and receive slows down the transfer of data but reduces errors.

Disk drive CB radio Internet chat sessions

Fastest of the three methods. Allows true interactive transmission and reception.

CPU Newer modems

Receiver

Data travels along the medium in both directions simultaneously Sender

Advantages & Disadvantages

Receiver

The speed of data transfer is measured either by its baud rate or by the number of bits per second (bps). Bau d r at e is a measure of the maximum number of data symbols or electrical signals that can be sent per second over the communication link.

Har dwar e Hardware for transmitting and receiving includes modems and networks as well as the internal components of the computer. Communicati on Within the

Co mputer

Communication within a computer usually involves transferring data between peripheral devices, the hard disk and the CPU. Every part of the computer is linked to the CPU either directly or indirectly by a bus. Bus : a pathway of wires and connectors that provides the link between input, storage, process and output devices. Can be thought of as a highway on which data travels within the computer. The size of a bus is called its wi dth . The greater the width, the more data can be transmitted at the one time. Faster bus = faster running of applications. There are two types of buses; Internal bus: located on the motherboard and links the CPU and memory. These buses are photochemically etched layers of metal that create electrical channels. Expansi on bus : connects peripheral devices to the CPU and memory. If the expansion bus connects directly to the CPU and bypasses RAM it is called a loca l bus . Different types of expansion buses: ISA, EISA, VESA & PCI. Peripheral devices are linked to the CPU using expansion slots and ports. An expansion sl ot is an opening where a circuit board can be inserted into the motherboard to extend the capabilities of the computer. The circuit board that is inserted is called an expansi on car d/b oar d. There are many types of expansion cards for different purposes ie. To connect a monitor, hard disk, scanner, etc. Expansion cards are used to extend memory. A port is a socket used to connect peripheral devices. It is usually located at the rear of the system unit. Ports have different connectors that are used to attach cables to the peripheral devices. They come in male and female. Male connectors have one or more exposed pins, female connectors have the hole the pins insert into. Parallel po rts: transmit eight bits (one byte) at a time along eight parallel lines. Used to connect devices that send and receive large amounts of data ie. Printers and disk drives. There are different types like Centronics and SCSI (Small Computer System Interface). Ser ial po rts: transmit data one bit at a time using only one communication line. Also known as COM or communications ports and are assigned a number like COM1

or COM2. Used connect almost any peripheral device eg. Mouse, keyboard, modem. Most common type of serial port is the RS-232 (Recommended Standard). Mo dems A modem is a device that enables data to be transmitted from one computer to another. Modem stands for mo dulator/dem odulator. Modems convert(modulate) digital signals sent from a computer into analog signals suitable for transmission on a telephone line. When the signal is received by another modem, it reverses the process by converting(demodulating) the analog signal into a digital signal suitable for the receiving computer. Direct connect modem is the most common type of modem, connected directly to telephone line. Internal m odems: a type of expansion card fitted into an expansion slot inside the system unit. Popular because they do not occupy desk space. External m odems: usually connected to a serial port, require a power supply and occupy desk space. Have a set of status lights to indicate what the modem is doing, and are generally more expensive than internal modems. The speed of a modem is measured by the number of bits transmitted per second or the baud rate. Common transmission speed is 56 000 bps or 56K. Modems that have a fax capability are called fax mo dems . They can send a computer-generated document to a fax machine and receive a document from a fax machine. Networ ks When a number of computers (or terminals) and their peripheral devices are connected, it is called a network. The simplest form of a network is when one computer is connected directly to another computer using a cable. However, a network can also consist of hundreds of computers connected together. The devices used to send data to a computer system or receive data from a processor are called termina ls . Dumb termina ls: do not contain a processor and usually consist of a keyboard and monitor. Smart terminals : can do some low-level processing such as text editing. Intelligent terminals : have both memory and processing capabilities. Most PCs are intelligent terminals. Called workstations on a network.

Networks are classified according to the distances they cover and include local area networks and wide area networks. Lo cal Ar ea Netwo rks (L ANs ): connect computers/terminals within a building or group of buildings on one site. Operate in a small geographical area, the terminals are linked together by cables (coaxial or fibre optic). Found in offices, hospitals, schools, tertiary institutions. Often contain a central computer called the network server. File servers are computers that perform a function for other computers on the network such as sharing hardware resources. Advantages: - Allow limited hardware resources eg. Printers, hard disks and modems to be shared. - Allow application software (word processing, databases, spreadsheets, graphics programs) to be shared - Improve communication among users on the network by allowing messages to be sent and received. Wide Ar ea Netwo rks (W ANs): connect computers/terminals over hundreds or thousands of kilometres. WANs rely on links such as coaxial cables, fibre optic cables and microwave devices. Often consist of a main computer system called the host and a number of terminals (workstations or nodes). May use a private leased line, the normal telephone network or a combination of both. Private lines are dedicated to the network and has higher transmission speeds than a Public Switched Telephone Network (PSTN). WANs are slower than LANs at transferring data because they are not directly connected.

Softw are All application software transmits and receives data within an individual computer. Communicati ons s oftw ar e: manages transfer of data, files and commands between computers. Uses protocols (set of rules that govern the transfer of data between computers) to establish connection. Two computers must use the same protocols, otherwise the data transfer may fail. Most popular protocols for networks: Ethernet and Token Ring. Com pr essi on: reduces the number of bits required to represent data. Compacts the data so that it takes up less space on a disk and requires less time to download. Compressed data must be decompressed to extract the original information. Electr onic ma il: email. Allows communication with other users by sending and receiving electronic messages using a computer.

Data encrypt ion: used to prevent data from being intercepted and read during transmission. The data is ‘scrambled’ so that it is meaningless to anyone other than the recipient, who uses the same encryption software to convert it back.

No n- com pu ter Tool s Non-computer tools used to transmit and receive include conventional mail, telephone, fax, radio and television. Mail sy stem: makes it possible to send a letter or package to the address of any person, anywhere. Delivery of mail can be on road, rail or air transportation systems. Tele phone system: transmits sounds or speech between distant places along telephone lines. Fax mach in es: transmit and reproduce documents by means of signals sent over telephone lines. Fax machine scans the document and converts it into a bit-mapped image. This image is compressed and transmitted along the telephone network to a destination fax machine. This machine decompresses the image and reconstructs the bit-mapped image. A printer then prints the image. Radi o: transmission and detection of sound using electromagnetic waves that travel through the air. When a radio receiver is tuned to the frequency of a radio station, a signal is received. Tele vis ion: transmission and detection of images and sounds using electromagnetic waves that travel through the air. A scene to be televised is translated into a sequence of electrical pulses that are transmitted via an antenna. The television translated the electrical pulses into a corresponding sequence of images and sounds that are displayed by the screen and speakers.