Transcript Network Environments

Network Environments
Communications Networks
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LAN – Local Area Network
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Collection of Computers and peripherals with a
common connection in one building or site.
WAN – Wide Area Network
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Groups of networks connected together over a
larger geographical area.
Ultimate WAN ??
Network topologies
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The topology of a network is its physical layout
— the way in which the computers and other
units (commonly referred to as nodes) are
connected.
Common topologies:
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Star
Bus
Ring
LAN - Advantages
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Shared resources
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Shared Information
Efficient Storage
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Disk storage, printers, modems, scanners etc.
Programs can be stored on one computer
Local Email
Easier Set-up procedures
Different types of computers can be connected
LAN - Disadvantages
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Dependence on a single server machine.
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Hardware/software access depends on network
System Manager dependent
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System must be properly organised for efficiency
Difficult to make system secure.
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Large systems will have a back-up server.
Vulnerable to hackers – particularly via modems
Performance degrades as use increases
Types of LAN
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Server-based – Client-Server Architecture
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Devices on networks are either clients or servers.
Workstations would be clients. They request a service from
servers. For example, retrieve a file from the file server, or
print a document on a printer which is a server device
Peer-to-peer
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Alternative architecture for small businesses (3 or 4
computers)
Each workstation can communicate directly with every other
workstation with no central server.
Network Maintenance Tasks
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Any network requires the management of:
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Adequate response times
Necessary hardware upgrades
Software availability
Software upgrades
Provision of information for users
Back-up procedures
Network Security
Comparison
Server-based
Peer-to-peer
Centralised backing storage
Distributed storage
Software held centrally and shared. Server
manages distribution of data and software
to clients. Some processing may be
handled locally but most done by file
server. More speed and power but
additional cost and complication.
Copies of software held on individual
machines. Basic network services to enable
sharing of data, software and printers are
provided. Generally easier to set up and
maintain than servers. OK for small
businesses.
User Ids, passwords and access levels are
controlled centrally
No central security
Centralised back-up facilities.
Back-up responsibility is devolved to
individual users.
If server goes down, all users affected.
No central server dependence
Central node computer controls the
communication protocol. Computers may
be of a different type.
Individual computers must all have the
same software loaded to control
communication with other computers.
Wide Area Networks
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LAN is connected by cables.
WAN is spread over a wide geographical area
Mode of communication may be:
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Telephone line, microwave, satellite link
Use of global networks has increased due to:
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Telephone networks: analogue to digital
Reduced connection and network costs
Improved compression techniques
Communications links
Communications
satellite
Possible
Configuration
Mainframe
in
Norwich
modem
Mainframe
in
London
modem
Mainframe
in
USA
Communications media
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Twisted pair (copper cable)
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Coaxial cable
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sends pulses of light rather than electricity
Microwave
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high quality, well insulated cable
Fibre optic cable
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used in telephone network and LANs
almost line of sight (30 miles apart)
Communications satellite
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in geosynchronous orbit
Telephone Network Service
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BANDWIDTH determines speed of data transfer and is
defined as the range of frequencies that a line can
carry. The following high bandwidth services are
capable of sending voice, video and computer data.
ISDN Lines
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Integrated Services Digital Network
ADSL Lines
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Asymmetric Digital Subscriber Line
Other Linking Devices
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Bridge
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Gateway
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A connection between 2 LANs
Connects LANs with WAN or mainframe
Modem (Modulator/Demodulator)
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Telephones designed for speech (analogue signal)
Modems at either end convert digital-analogue and
then analogue-digital.
digital
modem
analogue
modem
digital
Transmission
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Asynchronous
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Sends 1 character at a time, with each character
preceded by a start bit and followed by a stop bit. A
parity bit is also usually included to combat incorrect
transmission. Used by PCs, fast and economical for
small amounts of data.
Synchronous
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Transmission controlled by timing signals – no start
and stop bits. Less error-prone than asynchronous.
Standardisation
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In order that equipment from different suppliers can be
linked or networked, standard sets of rules or
PROTOCOLS have been devised to ensure
compatibility. Any equipment using the same protocol
can be linked together.
Where necessary, protocol converters can be used to
overcome:
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Different types of transmission – asynchronous(PC) or
synchronous(mainframe).
Different character representations – ASCII(PC) or
EBCDIC(mainframe).
Different error checking methods.
Factors affecting rate of data
transmission
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The speed of the modem
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The nature of the transmission line
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A digital line such as an ISDN line has a much higher
transmission speed than an analogue line.
The type of cable used
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Different modems vary in data transmission rates, typically
from 9K to 56K bps (bits per second).
Twisted pair has a transfer rate of 10Mbps; fibre optic cable is
about 10 times as fast.
The type of transmission
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synchronous or asynchronous (see Ch.60)