Hardware Used For Transmitting and Receiving
Hardware for transmitting and receiving includes modems and networks as well as the internal components
of the computer.
Communication within the computer
Communication within a computer 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. A bus is a
pathway of wires and connectors that provides the link between input, storage, process and output devices.
A bus can be thought of as a highway on which data travels within the computer. The size of the bus is
called its width. The greater the width, the more data that can be transmitted at the one time. For example, a
64-bit bus has 64 lanes and can transmit data 64 bits at a time. A 32-bit bus moves 32 bits at a time. The
speed of the bus is also an important factor in the performance of the computer. A faster bus will make the
applications run faster. There are two different types of buses.
• An internal bus is located on the motherboard and links the CPU and memory. These buses are
photo chemically etched layers of metal that create electrical channels.
• An expansion 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 local bus. There are different types of
expansion buses, such as ISA, EISA, VESA and PCI.
Peripheral devices are linked to the CPU using expansion slots and ports. An expansion slot 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 expansion card or expansion board. There are many
different types of expansion cards for different purposes, such as to connect a monitor, hard disk, scanner
or other peripheral device. Expansion cards are also 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 (see
Figure 2.25). Ports have different connectors that are used to attach cables to the peripheral devices. Most
connectors are available in two types: male and female. Male connectors have one or more exposed pins
like the end of a power cord. Female connectors have matching holes like a power point. Ports are either
parallel or serial.
• Parallel ports transmit eight bits (one byte) at a time along eight parallel lines. They are used to
connect devices that send and receive large amounts of data such as printers and disk drives. There are
different types of parallel ports, such as Centronics and SCSI (small computer system interface,
pronounced ‘skuzzy’).
• Serial ports transmit data one bit at a time using only one communication line. Serial ports are
often called COM or communications ports and are assigned a number such as COM1 and COM2. Serial
ports are used to connect almost any peripheral device, such as a mouse, keyboard, modem or plotter. The
most common type of serial port is the RS-232 (recommended standard). It supports the use of a 25-pin
connector (DB-25) or a 9-pin connector (DB-9). Higher speed standards have been developed such as the
RS-423 and RS-422.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Modems
A modem is a device that enables data to be transmitted from one computer to another. The word modem
stands for modulator/demodulator. A modem converts (modulates) digital signals sent from a computer into
analog signals suitable for transmission on a telephone line. Analog signals are pulses in the form of waves.
Depending on the medium used, they can be electrical pulses or light pulses. 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. A direct connect modem is the most common type of modem as it is
connected directly to a telephone line. Most modems use a command set developed by Hayes and are
described as ‘Hayes compatible’.
Modems may be internal or external.
• Internal modems are a type of expansion card fitted into an expansion slot inside the system
unit. Internal modems have become popular because they do not occupy desk space.
• External modems are usually connected to a serial port (COM1 or COM2), require a power
supply and take up some desk space. They 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.
A common transmission speed is 56000 bps, often abbreviated to 56K. Modems operate with different
standards. The CCITT (Consultative Committee on International Telegraphy and Telephony) V series is a
set of standards that define modem operations, speed, data compression and error correction. These
standards, which are being continually updated, include V.21, V.32bis, V.34 and V.90.
Modems that have a facsimile (fax) capability are called fax modems. They can send a computergenerated document to a fax machine and receive a document from a fax machine. Some fax modems use
optical character recognition software to convert incoming fax messages to word processing files. This
allows fax messages to be edited.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Networks
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 terminals.
Terminals can be either dumb, smart or intelligent.
• Dumb terminals 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 personal computers are
classified as intelligent terminals and are called workstations on a network.
Networks are classified according to the distances they cover and include local area networks and
wide area networks.
Local area networks (LANs) connect computers (or terminals) within a building or group of
buildings on one site. LANs operate in a small geographical area and the terminals are linked together by
cables (coaxial or fibre optic). They are found in offices, hospitals, schools and tertiary institutions. LANs
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.
There are three advantages in using a LAN:
• They allow limited hardware resources such as printers, hard disks and modems to be shared.
• They allow application software (word processing, databases, spreadsheets and graphics
programs) to be shared.
• They improve communication among users on the network by allowing messages to be sent and
received.
Wide area networks (WANs) connect computers (or terminals) over hundreds or thousands of
kilometres. WANs rely on links such as coaxial cables, fibre optic cables and microwave devices. WANs
often consist of a main computer system called the host and a number of terminals (workstations or
nodes).A WAN may use a private leased line, the normal telephone network or a combination of both. A
private leased line is dedicated to the network and offers higher transmission speeds than those available
through the Public Switched Telephone Network (PSTN). PSTN links are relatively cheap if workstations
do not require constant connection but the link is not guaranteed and can be interrupted. WANs are slower
than LANs at transferring data because they are not directly connected.