computer networks

1. Reference models

2. Objectives
• Understand the architecture of Internet with
an analogy.
• ISO/OSI Reference model
• TCP/IP Reference model
• Compare ISO/OSI and TCP/IP

3. Analogy: Airline System

4. Horizontal Layering

5. Protocol Layers
• To reduce the design complexity, most
networks are organized as a stack of layers or
levels, each one built upon the one below it.
• The purpose of each layer is to offer certain
services to the higher layers, shielding those
layers from the details of how the offered
services are actually implemented.

6. The OSI Reference Model
• The model is called the ISO OSI (Open Systems
Interconnection) Reference Model because it
deals with connecting open systems—that
is, systems that are open for communication
with other systems.

7. The OSI Reference Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data link layer
Physical Layer
• The seven layers of ISO/OSI model

8. The OSI Reference Model…

9. • The physical layer is concerned with
transmitting raw bits over a communication
channel.
• The design issues here largely deal with
– mechanical, electrical, and timing interfaces, and
– the physical transmission medium, which lies
below the physical layer
The Physical Layer

10. • The main task of the data link layer is to take a
raw transmission facility and transform it into
a line that appears free of undetected
transmission errors to the network layer.
• The sender break the input data up into data
frames and transmit the frames sequentially.
The Data Link Layer
Data
Frame1
Frame2
Data Link Layer

11. • The network layer is concerned with
controlling the operation of the subnet.
• A key design issue is determining how packets
are routed from source to destination.
The Network Layer

12. • The basic function of the transport layer is to
accept data from above, split it up into smaller
units if need be,
• Pass the DU to the network layer, and ensure
that the pieces all arrive correctly at the other
end.
The Transport Layer
DU1
DU2
Data Transport Layer

13. • The session layer allows users on different
machines to establish sessions between them.
• Dialog control
– (keepingtrack of whose turn it is to transmit)
• Token management
– (preventing two parties from attempting the same
critical operation at the same time) and
• Synchronization
– (checkpointing long transmissions to allow they to
continue from where they were after a crash).
The Session Layer

14. • Concerned with the syntax and semantics of
the information transmitted.
The Presentation Layer

15. • The application layer contains a variety of
protocols that are commonly needed.
• One widely-used appplication protocol is HTTP
(HyperText Transfer Protocol), which is the basis
for the World Wide Web.
• When a browser wants a Web page, it sends the
name of the page it wants to the server using
HTTP.
• The server then sends the page back.
• Other application protocols are used for file
transfer, electronic mail, and network news.
The Application Layer

16. The TCP/IP Reference Model
• ISO OSI Model TCP/IP Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data link layer
Physical Layer
Application Layer
Transport Layer
Internet Layer
Link Layer
Physical Layer

19. • Permit hosts to inject packets into any
network and have them travel independently
to the destination.
• The internet layer defines an official packet
format and protocol called IP (Internet
Protocol).
• The job of the internet layer is to deliver IP
packets where they are supposed to go.
The Internet Layer

20. • The first protocol defined is TCP(Transmission
Control Protocol)
• TCP is a reliable connection-oriented protocol
that allows a byte stream originating on one
machine to be delivered without error on any
other machine in the internet.
• It fragments the incoming byte stream into
discrete messages and passes each one onto
the internet layer.
The Transport Layer

21. • The second protocol in this layer, UDP (User
Datagram Protocol)
• UDP is an unreliable, connectionless protocol for
applications that do not want TCP’s sequencing
or flow control and wish to provide their own.
• It is also widely used for one-shot, client-server
type request-reply queries and applications in
which prompt delivery is more important than
accurate delivery, such as transmitting speech or
video.
The Transport Layer…

22. • It contains all the higher-level protocols.
• The early ones included virtual terminal
(TELNET), file transfer (FTP), and electronic mail
(SMTP)
• The TELNET allows a user on one machine to log
into a distant machine and work there.
• The FTP provides a way to move data efficiently
from one machine to another.
• SMTP was originally just a kind of file transfer,
but later a specialized protocol was developed
for it.
The Application Layer

23. • Domain Name Service (DNS) for mapping host
names onto their network addresses
• HTTP, the protocol used for fetching pages on the
World Wide Web
The Application Layer…

24. • Below the internet layer is a great void.
• The host has to connect to the network using
some protocol so it can send IP packets over
it.
• This protocol is not defined and varies from
host to host and network to network.
The Host-to-Network Layer

25. OSI Vs TCP/IP
•Did not originally clearly
distinguish between
service, interface, and
protocol
• The protocols came first,
and the model was really
just a description of the
existing protocols.
•Four layers
•connectionless in the
network layer but
supports both modes in
the transport layer
• Three concepts are
central:
Services, Interfaces, Proto
cols
• The OSI reference model
was devised before the
protocols were invented
• Seven layers
• Connectionless and
connection-oriented
communication in the
network layer, but only
connection-oriented
communication in the
transport layer