Transcript Lecture 1 - cda college

INTRODUCTION TO
COMPUTER NETWORKS, OSI MODEL
COM211 Communications and Networks
CDA College
Theodoros Christophides
Email: [email protected]
www.cdacollege.ac.cy/site/info-com-technology-ll/
Course Content
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Lecture 1: Introduction to Computer Networks, OSI Model
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Lecture 2: Hardware building blocks and encoding
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Lecture 3: Physical Media and Cabling
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Lecture 4: Protocols
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Lecture 5: LAN and WAN Technologies
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Lecture 6: Data Link Layer and Ethernet
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Lecture 7: Midterm
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Lecture 8: Network Layer of OSI
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Lecture 9: Transport Layer of OSI
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Lecture 10: Application Services
Course Content
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Lecture 11: Security in Computer Networks
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Lecture 12: Wireless Networks
•
Lecture 13: Revision for the Final examinations
References
Andrew S. Tanenbaum, Computer
Network, Prentice-Hall
 Doughlas E. Comer, Computer Networks
and Internet
 Larry L. Peterson and Bruce S. Davie,
Computer Networks: A Systems Approach

Computer Networks
A collection of autonomous computers
interconnected by a single technology.
Two computers are said to be
interconnected if they are able to
exchange information.
Connectivity
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Interconnect machines.
Maintain data confidentiality, data integrity, and
system accessibility.
Support growth by allowing more and more
computers, or nodes, to join in (scalability).
Support increases in geographical coverage.
Internet
Is not a single network but a network of
networks
Links
Each node needs one interface for each link.
point-to-point(a)
multiple-access(b)
Geographical coverage and scalability are limited.
Network Topology
The network topology
defines the way in which
computers, printers, and
other devices are
connected. A network
topology describes the
layout of the wire and
devices as well as the
paths used by data
transmissions.
Mesh Topology
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This topology connects all
devices to each other for
redundancy and fault
tolerance.
It is used in WANs to
interconnect LANs and for
mission critical networks
(banks, financial
institutions, etc.)
Implementing the mesh
topology is expensive and
difficult.
Bus Topology

All the devices on a bus topology are
connected by one single cable.
Star Topology

The star topology is
the most commonly
used architecture in
Ethernet LANs.
Tree Topology

Larger networks use the
extended star topology also
called tree topology. When
used with network devices
that filter frames or packets,
like bridges, switches, and
routers, this topology
significantly reduces the
traffic on the wires by
sending packets only to the
wires of the destination
host.
Ring Topology
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A frame travels around the ring, stopping at each
node. If a node wants to transmit data, it adds
the data as well as the destination address to
the frame.
The frame then continues around the ring until it
finds the destination node, which takes the data
out of the frame.
Types:
ring – All the devices share a single cable
 Dual ring – Allows data to be sent in both directions
and provides redundancy
 Single
Network Components
Physical Media
 Network Devices
 Computers
 Protocols
 Services

Networking Media
Twisted Pair
 Coaxial
 Fiber Optics
 Wireless Transmissions

Network Devices
Hub
 Switches ( Level 2 and 3 )
 Routers
 Wireless Access Points
 Modems
 NIC’s

Computers
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End Devices: Acts as a source/destination. For
message transmitting or receiving.
Server: In a client/server network environment,
network services are located in a dedicated
computer whose only function is to respond to
the requests of clients. The server contains file
sharing, http and other services that are
continuously available to respond to client
requests.
Client: Our computers. We request a service
from a server (ex. We log into gmail to check our
email )
Protocols
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A protocol, in contrast, is a set of rules governing
the format and meaning of the packets, or
messages that are exchanged by the peer
entities within a layer. Entities use protocols to
implement their service definitions. They are free
to change their protocols at will, provided they
do not change the service visible to their users.
TCP, UDP, IP, X.25, ICMP, IPSec
Services
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DHCP - Dynamic Host Configuration Protocol
DNS – Domain Name System
HTTP - Hypertext Transfer Protocol
SSH – Secure Shell
Telnet
SNMP - Simple Network Management Protocol
SMTP - Simple Mail Transfer Protocol
FTP – File Transfer Protocol
IRC – Internet Relay Chat
POP – Post Office Protocol
Internetworking

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To interconnect two or more
networks, one needs a
gateway or router. Host-tohost connectivity is only
possible if there’s a uniform
addressing scheme and a
routing mechanism.
Messages can be sent to a
single destination (unicast),
to multiple destinations
(multicast), or to all possible
destinations (broadcast).
Synchronous Time Division Multiplexing
(STDM)
Divide time into equal-sized quanta and
assign each them to flows on the physical
link in round-robin fashion.
Frequency-Division Multiplexing
(FDM)
Flows are transmitted simultaneously on
the link, but each one uses a different
frequency.
Code Division Multiplexing (CDM)

Flows are transmitted simultaneously on the link,
but each one uses a different coding scheme.

For a chosen group of nodes, a unique coding
scheme can be used. Each bit is encoded in
multiple pulses. Multiple senders may use the
same time slots with different coding.
Statistical Multiplexing
Flow is broken into packets and sent to a
switch, which can deal with the arriving
packets according to the switch policy
(FIFO, round-robin, etc).
Computer
A
queue
Computer
B
switch
Computer
C
Range of Coverage
Computer networks can be classified according to their geographical
coverage:
• LAN: local area network
• WLAN: wireless local area network
• MAN: metropolitan area network
• WAN: wide area network
In Interconnecting multiple networks (internetworking), we are
interested in the seamless integration of all these levels. Have in mind
that different levels use different technologies!
ISO: International Standards Organization
OSI: Open Systems Interconnection
Application
The protocol stack:
The idea behind the model: Break up the
design to make implementation simpler.
Each layer has a well-defined function.
Layers pass to one another only the
information that is relevant at each level.
Communication happens only between
adjacent layers.
Presentation
Session
Transport
Network
Data link
Physical
Layers of the OSI model
• Physical: Transmit raw bits over the medium.
• Data Link: Implements the abstraction of an error free medium (handle
losses, duplication, errors, flow control).
• Network: Routing and Addressing. IP
• Transport: Break up data into chunks, send them down the protocol
stack, receive chunks, put them in the right order, pass them up.
• Session: Establish connections between different users and different
hosts.
• Presentation: Handle syntax and semantics of the info, such as
encoding, encrypting.
• Application: Protocols commonly needed by applications (cddb, http,
ftp, telnet, etc).
Communication Between Layers
sender
receiver
data
data
AH
Application
PH
Presentation
data
Application
data
Presentation
Session
SH
data
Session
Transport
TH
data
Transport
Network
NH
data
Network
Data link
Physical
DH
data
BITS
DT
Data link
Physical
Layers in TCP
Application
Presentation
FTP
HTTP
DNS
NFS
…
Session
Transport
ICMP
Network
TCP
UDP
IGMP
IP
ARP
Data link
Physical
RARP
TCP/IP and OSI
Thank you for your time and
patience!
Questions??