Communication systems are made up of two or more hardware devices that send and receive signals ( the source and receiver), a medium over which the signal is transmitted, and any other hardware or software needed to allow the devices to communication. We are going to talk about the hardware devices.
These devices may be mainframe computers, minicomputers, personal computers, or other types of computers that are used to input, process, and output data. A terminal is a network device that is used for input and/or output. They range from devices that perform only basic input/output functions to networked personal computers that can input, output, and process data and serve as stand-alone devices.
Two types of networks:
1. Wide Area Network (WAN) - spans a large geographical area,
such as a city, state, various cities and allows users to share hardware
devices, access data and applications, and communicate more efficiently.
2. Local Area Network (LAN) - connects devices located in a
limited geographic area such as within an office or within a building.
Computers on a network serve as a network node. A node is a device that connected to the network, has a network address, and can send and/or receive data on a network. This device can act as a host computer that is accessed by a user working at remote locations, an intermediate device that has other devices attached to it, or a termination point for a communication link.
Mainframe - is a very large computer that usually has many computers, terminals, and other devices attached to it; mainframes often serve as host computers in a network. Example - 4-2; mainframes can not be used as a terminal.
Minicomputers - smaller, slower, and has less memory than a mainframe
computers. Runs software that allows communications on the network to take
place. Can be used for local processing or serve as host computers that
allow connections to the network
4-4
Personal Computers (microcomputers) -use a NIC card to
connect to a network
Terminals - device used for input and/or output, which is connected
to a host computer or network.
1. general-purpose (cathode ray tube CRT or video display terminal
VDT) - used to communicate over a network; divided into
4 types(4-6):
a. dumb - has no
processing or storage capabilities of its own; relies entirely on the host
computer to perform such
functions; asynchronous transmission - transmitting one character, or byte
at a time
b. smart - has memory
that allows them to store data and receive information from a host computer;
stores the entire
screen of data, which consists of many characters, called a block, and
transmits data to the host computer one block
at a time. This is called synchronous transmission.
c. intelligent -
contains memory and additional functions that are programmed in its firmware,
which are permanently
programmed chips within the terminal. Use synchronous transmission
d. programmable -
has all the capabilities of an intelligent terminal and can be programmed
by the operator.
terminal emulation - software that makes it look like, or emulate,
the type of
terminal for which the host computer is looking; (Ewan, Telnet, FTP)
2. special - purpose terminals - terminals designed for specific
tasks and cannot be used for a variety of functions as can
general-purpose terminals.
a. point-of-sale terminals
- includes a monitor, a scanner, a printer, and a keyboard with special
characters commonly not
found on video display terminal keyboards. Connected to a network so it
can send and receive data from a central
pricing and inventory database stored on a host computer. Two copies of
sales receipt. Bar code reader - uses a
photocell to read the universal product code (UPC) symbol of the item to
be purchased.
b. credit card authorization
devices - connected by telephone to a credit card agency or bank appointed
as a
clearinghouse for authorizing credit card purchases. Magnetic strip encoded
with the account number along with the
type of card. Swipped through machine. Purchase is approved, the device
displays an authorization number which is
printed or written on the receipt. Can now use debit cards.
c. automated teller machines
(ATMs) - allow you to complete financial transactions and other banking-related
activities.
Host computers
contain customer files that are available at all times, so a transaction
from the ATM can be verified
against the customer account. ATM cards contain account number and personal
identification number (PIN).
d. printers - considered
terminals because they can accept output data at a remote site.
e. facsimile machine
(fax) - device that can send or receive pictures and text over a phone
line. Original document is fed
into machine,converted into signal that are transmitted over the telephone
network, and the receiving fax converts it
back into a printed document and produces a facsimile of the original.
PCs have modem that acts as a fax, called a
fax/modem.
f. remote input devices
- not connected to a computer directly, but instead transmit real-time
data over a wireless
network or are used to gather data offline to be transmitted to the host
computer later. Many remote input devices can
also output info.
g. engineering industry
devices (computer-aided design/computer-aided aided manufacturing (CAD/CAM))
- used for
developing the drawings (blueprints) for plans for buildings, heating systems,
or plant layouts. Larger monitors.
Connected to host computers or PCs that store the drawings produced on
the terminals.
h. telephone - voice
mail system that uses a computer to operate the system; home shopping network;
i. airline and travel
reservation systems - terminals connected to host computers containing
data on certain airlines and
customer information.
Network Configurations
The way computers and terminals are connected in a network is called
the line configuration. Two types with the main difference being how data
is sent between the host and other computers.
1. Point-to-Point - a direct line between a sending and receiving
device. Each terminal has its own communication line therefore the host
computer doesn't have to spend a lot of time determining what terminal
sent the data. Connections between host computers and single remote terminals
or computers.
2. Multipoint - several devices share a single communication
line. Each terminal can only transmit when no other terminal is transmitting.
Mainframe and LANs. How many terminals allowed on the communication line
depends on the channel capacity (bandwidth) and the amt. of data each terminal
will transmit. The amt. of time a terminal must wait for its data to be
sent and
to get a reply back from the host computer is called response time.
Line-access protocols are used to describe how terminals will have access
to a line. Technique depends on whether point-to-point or multipoint. Protocol
is a rule describing how devices send data on a communication network.
Two ways communication takes place on multipoint networks:
1.Polling - mainframe computers poll terminals on a multipoint
network to determine which terminal has data to transport. Mainframe is
the primary station and terminals are secondary stations. Primary station
initiates the transmission of data by sending a message to the secondary
stations, asking if each one has data to send. Secondary stations polled
in a certain order based on a predetermined list. Only one primary station.
Two types:
a. roll call polling - 21
b. hub polling - 22
2. Selection - another method by which the primary station communicates
with the secondary station. Terminal is selected if the primary station
has data to transmit to that terminal. If the primary station selects all
secondary stations on the multipoint simultaneously it is said to broadcast
a message.
interface - means a connection that allows interaction between
devices
handshaking - 4-25
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