Transcript Client-server - Department of Computer Science and Information

Computer networks
Fundamentals of Information Technology
Session 6
Computer networks
• A computer network is a group of interconnected
computers
• This can involve as few as two computers. At the other
extreme it can involve millions of computers (e.g. the
Internet)
• Computers in a network are generally connected for a
purpose
– Share data (e.g. files, multimedia)
– Provide access to resources (e.g. web pages, applications)
– Provide services (e.g. processing, printing, communications)
Network types
• There are two main types of computer network in use :
– Peer-to-peer (P2P)
– Client-server
Client server
Peer-to-peer
Peer-to-peer
• In a P2P network, the
relationship between computers
is one of equality
• All machines are essentially the
same in terms of what they do
• The point is simply to provide
access to files
• P2P networks are commonly
used for file-sharing and
internet telephony
• Examples of P2P networks are
Napster, Gnutella, etc.
Files
Applications
files
Files
Applications
data
Peer 2
Peer 1
Files
Applications
data
Files
Applications
files
Peer 3
Peer4
Client-server
• In client-server networks, one type of machine (the
server) is used by another (the client) to provide data
or services. This is done through a series of requests
and responses sent over a network
Files
Applications
Server
response
request
Client
Client-server
• In client-server networks, different types of server are
used to provide different types of services to users:
– File server
– Application server
– Web server
– Print server
– Mail server
File Server
• File servers provide a location for the shared storage of files so
that they can be accessed by multiple workstations in the network
response
request
Network
Files
Client
File server
Client
Files
Back-up
Client
• To the user the network appears transparent. Files appear to be
stored on his her computer
• File servers usually have a back-up server in case of problems with
the main server
Application server
• Application servers store whole applications or major components
of applications and make these available to client machines
Network
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File server
Client
Applications
Application server
Client
• Again, to the user the network appears transparent. The
applications he/she uses appear to be installed on his/her computer
• Use of an application server makes updating software easier, as it is
done centrally on the server, not on individual clients
Web server
• Web servers provide a location for online resources
(e.g. web pages, multimedia)
• They control access to these resources from users on the
Web or company intranet
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Internet
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Company
intranet
Web server
Print server
• Print servers provide access to networked printers,
manage the print queue and direct print jobs
print job (2)
print job (4) - queued
print job (3)
Print server
print job (1)
print job (1)
Network
Client
File server
print job (2)
print job (4)
print job (3)
Client
Application server
Client
Mail server
• Mail servers control the flow of email in and out of an
organisation and to and from internal client computers
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Internet
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Mail server
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Advantages of client-server
networks
• Access, resources and security are all controlled centrally
at server level
• All data is stored in a central repository and usually
backed-up
• File-management and version control is easier, as files
are stored in one central location
• Utilities (e.g. anti-virus) are installed at server level
• Data is workstation independent, which means if one
workstation goes down, data can be accessed through a
different workstation
• Applications (where there is an application server) can
be updated centrally, at server level
Network scale
• Local Area Network (LAN)
Covers a small geographical area. Used for networks in small and
medium companies, schools, colleges, hospitals, etc.
• Wide Area Network (WAN)
Covers a wider geographical area. Used to connect LANs together
LAN
LAN
WAN
LAN
Network software
• Clients must run an operating system that allows
networking (e.g. Windows XP)
• Servers require specialist server software (e.g. Microsoft
Server 2003, Apache, Novell Netware). This software
controls and monitors:
–
–
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–
Access
Security
Resources
Performance
Network hardware – client and
server
• Any typical desktop or notebook computer can act as a
client in a network
• Any computer can act as a server, but it must have the
processing power and storage capacity needed for the
task
• Typical servers have multiple processors (up to 32 in
some cases), multiple hard-drives and extensive RAM
• Server computers must be robust and reliable because of
the central role they play in maintaining a network
• Thus, failsafe strategies, such as multiple power supply
options are often built in
Data transmission
• So that data can be moved between networks, universal
standards (protocols) for data transmission have been
implemented
• The de-facto standard is Transmission Control
Protocol/Internet Protocol (TCP/IP)
• The TCP part of TCP/IP relates to the rules of how data
should be sent
• The IP part relates to where it should be sent. Each
computer has its own unique IP address (e.g.
193.61.29.155 ). This enables data to be routed to its
correct destination
• You can find your own unique IP address at:
• http://www.ip-adress.com/
Data transmission
•
Sending a message across a network involves several
steps:
1. The message is decomposed into a number of smaller packets
using the TCP protocol
2. Each packet is addressed with the recipient IP address
3. The packets are sent to the client’s network/mail server
4. The client’s network/mail server sends the packets onto the
recipient’s PC
5. The packets are reassembled into the original message
Virtual Private Network (VPN)
• A VPN is an extension of a private network across a
public network (usually the Internet)
• VPNs use "virtual" connections routed through the
Internet from the company's private network to the
remote site or employee
• VPNs use encryption and tunnelling to ensure that
only authorized users can access the network and that
the data cannot be intercepted
• VPNs require special VPN software to be installed on the
remote computer to deal with authorisation encryption
and decryption
Virtual Private Network (VPN)
LAN
Internet
encrypted data
User
Computer
VPN Server
Users area on
file server
FIT Session 6 – Activities
• Now do
– Activity 6 – Computer Networks