Chapter 9 – Intro to Networks
Aims:
Outline the advantages of networks.
Define the usage of the OSI model.
Outline the uses of the layers of the OSI model.
Networks
• Local area networks (LANs), which connect over a relatively small
geographical area, typically connecting computers within a single office
or building. In most cases they connect to a common electronic
connection – commonly known as a network backbone. LANs can
connect to other networks either directly or through a WAN or
MAN.
• Metropolitan area networks (MANs), which normally connect
networks around a town or city. They are smaller than a WAN, but
larger than a LAN.
• Wide area networks (WANs), which connect networks over a
large geographical area, such as between different buildings, towns or
even countries.
Advantages of networks
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•
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It is easier to set up new users and equipment.
It allows the sharing of resources.
It is easier to administer users.
It is easier to administer software licenses.
It allows electronic mail to be sent between users.
It allows simple electronic access to remote computers and sites.
It allows the connection of different types of computers which can
communicate with each other.
Sharing information
• Control the users on the network, that is, who can and cannot login.
• Control which of the users are allowed to use which facilities.
• Control which of the users are allowed to run which application programs.
• Control the usage of software packages by limiting users to license
agreements.
• Standardize the set up of application programs to a single source.
• Back-up important files on a regular basis onto a mass back-up
system.
• Set up simple-to-use procedures to access programs, such as icons,
menus, and so on.
• Possibly control PC (personal computer) viruses by running automatic
scanning programs.
• Update application programs by modifying them at a single source.
Sharing disk resources
UNIX/VMS
Global file
system
File
server
PC network
Central file
server
Peer-to-peer communications
Microcomputer
(such as Microsoft
Windows)
Sun workstation
(UNIX)
HP workstation
(such as UNIX)
Other Vendors
DEC workstation
(such as VMS)
Other Vendors
Remote login
Powerful
workstation
or minicomputer
remote
login
PC
remote
login
remote
login
Workstation
X-windows
terminal
OSI model. Why?
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Allow manufacturers of different systems to interconnect their
equipment through standard interfaces.
Allow software and hardware to integrate well and be portable on
differing systems.
Create a model which all the countries of the world would use.
Virtual
data flow
Application
Presentation
P
A
S
P
A
T
S
P
A
N
T
S
P
A
N
T
S
P
A
Session
Transport
Network
Data link
Physical
Sender
D
A
Application
Presentation
Actual
data flow
Session
Transport
Network
D
Data link
Physical
Receiver
Physical layer
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Physical. TRANSMISSION OF
BINARY DATA. Defines the
electrical characteristics of the
communications channel and the
transmitted signals, such as
voltage levels, connector types,
cabling, and so on.
Application
Presentation
Session
Transport
Network
Data Link
Physical
Data link layer
•
Data link. MEDIA ACCESS. Ensures
that the transmitted bits are received
in a reliable way, such as adding extra
bits to define the start and end of the
data frame, adding extra error
detection/correction bits and ensuring
that multiple nodes do not try to
access a common communication
channel at the same time.
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network layer
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Network. ADDRESSING AND
DETERMINING THE BEST PATH.
Routes data packets through a
network. If data packets need to go
out of a network then the transport
layer routes them through
interconnected networks. Its task
may involve, for example, splitting
data for transmission and
re-assembling it upon reception. The
IP part of TCP/IP is involved with the
network layer (or IPX in Novell
NetWare).
Application
Presentation
Session
Transport
Network
Data Link
Physical
Transport layer
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Transport. END-TO-END CONNECTION RELIABILITY. Network
transparent data transfer and
transmission protocol, which supports
the transmission of multiple streams
from a single computer. The TCP part
of TCP/IP is involved with the
transport layer (or SPX in Novell
NetWare).
Application
Presentation
Session
Transport
Network
Data Link
Physical
Session layer
•
Session. INTERHOST COMMUNICATION. Provides an open
communications path with the other
system. It involves the setting up,
maintaining and closing down of a
session. The communication channel
and the internetworking of the data
should be transparent to the session
layer. A typical session protocol is
telnet, which allows for remote login
over a network.
Application
Presentation
Session
Transport
Network
Data Link
Physical
Presentation layer
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Presentation. DATA
REPRESENTATION and
INTERPRETING. Uses a set of
translations that allows the data to be
interpreted properly. For example it
may have to translate between two
systems if they use different
presentation standards, such as
different character sets or differing
character codes. The presentation
layer can also add data encryption for
security purposes.
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application layer
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Application. NETWORK SERVICES
TO APPLICATION PROGRAMS.
Provides network services to
application programs, such as file
transfer and electronic mail.
Application
Presentation
Session
Transport
Network
Data Link
Physical
OSI model
• Increased evolution. Systems are allowed to quickly change, as they
still integrate well with existing systems. This speeds evolution.
• Allows modular engineering. This allows for systems to be
designed in a modular way so that each of the components, whether
they be hardware or software, can interface well with each other.
• Guarantees interoperable technology. This allows the transfer of
data between computers of different types, either in their software,
operating system, network hardware or computer hardware.
• Reduced complexity. The task of transmitting data from one
application to another over a network is reduced in complexity as it is
reduced to seven smaller tasks.
• Simplifies teaching and learning. The OSI model has been used as
a standard method for teaching networking, and, as it is built up of
layers, allows for easier learning of networking. Students can easily
visualize the network in a given layer of abstraction.
• Standardizes interfaces. This allows for designers to design their
products so that they can be easily plugged into one or more of the
layers of the model. The actual implementation of the layer can be
invisible to other layers.
Interconnection of layers
Physical and networks addresses
Network addresses:
• 146.176.151.130 (IP)
• F5332B10:00000E645432 (IPX)
MAC address:
• 00-00-0E-64-54-32
• 146.176.151.130
Network layer functions
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Network addresses. This identifies the actual logical location of the node
(the network address), and the actual node (the node address). The form of
the network address depends on the actual protocol. IP uses a dot address,
such as 146.176.151.130 that identifies the network and the host. IPX address
(for Novell NetWare) uses an eight-digit hexadecimal address to identify the
network address and the node portion with a 12-digit MAC address, such as
F5332B10: 00000E645432.
Routing. This is passing of the data packets from one network segment to
another, and involves routers. A router reads the network address and decides
on which of its connections it should pass the data packet on to. Routing
information is not static and must change as the conditions on the network
change. Thus each route must maintain a routing table which is used to
determine the route that the data packet takes. These routing tables are
updated by each of the routers talking to each other using a routing protocol.
Transport layer functions
• Connection type. This defines the method of handshaking of data
between the source and the destination, and can be connectionoriented or connectionless.
• Name resolution. This allows for the resolution of logical names to
logical network addresses. It is often easier to access networked
devices using a logical name, rather than their logical address, as these
are easier to remember. A typical implementation on TCP/IP networks
is the Domain Name Service (DNS) which resolves domain names to
IP addresses. For example, a domain name of www.fredandco.com
could be resolved to the network address of 11.22.33.44.
Session layer
Internetworking
Bits, frames, packets and segments