osi model

Prof. Mukesh N Tekwani [Mobile: 9869 488 356]

OSI & TCP/IP Reference Models – of 9

OSI Reference Model

Mukesh N. Tekwani

[email protected]

Mumbai

OSI Reference Model is a standard approach to network design. It is developed by

the International Standards Organization (ISO). The word “Open” signifies that the

model deals with connecting open systems – that is, systems that are open for

communication with other systems.

The OSI model has 7 layers. The following principles were applied to arrive at the

seven layers:

1. The complex task of network architecture is divided into a set of functions.

2. A layer should be created where a different level of abstraction is needed.

3. Each layer should perform a well defined function.

4. The layer boundaries should be chosen to minimize the information flow across

the interfaces.

5. The number of layers should be large enough that distinct functions need not be

thrown together in the same layer out of necessity, and small enough that the

architecture does not become unwieldy. Functions which are similar are grouped

together within layers.

6. The internal design of a layer is independent of the function it provides.

7. Each layer knows only about its immediately adjacent layers. A layer utilizes the

services of the layer below and provides services to the layer above.

8. Each layer can be regarded as a black box

Network Model: � What is a model? – A model is a hypothetical description of a complex entity

or process. The process of data communication along a network is very

complex involving hardware and software.

� Network model - A method of describing and analyzing data communications

networks by breaking the entire set of communications process into a number

of layers.

Network Architecture: � A set of layers and protocols is called a network architecture

� It refers to the physical and logical design of a network

Protocol: It is a formal description of message formats and the rules that two computers must

follow in order to exchange messages. This set of rules describes how data is

transmitted over a network. OSI is not a protocol. Protocols are needed for the

following reasons:



� In what format will the messages be transmitted?

� At what speed should messages be transmitted?

� What to do if errors take place?

� What to do if parts of a message are lost?

Layered Approach To Networking: � Divide the complex task into a number of simpler sub-tasks. A layer performs

that simple sub-task.

� Each layer builds upon the layer below it, adding new functionality.

� The entities comprising the corresponding layers on different machines are

called peers.

� It is the peers that communicate by using the protocols.

� Actually, data is not transferred from layer n on one machine to layer n on

another machine.

� Each layer passes data and control information to the layer immediately below

it, until the lowest layer is reached.

� Actual data communication takes place through the lowest layer – the physical

layer.

Design Issues for Layers: � Addressing - some mechanism is needed for specifying the source and

destination systems.

� Error control – error detection and correction

� Order of messages must be preserved.

� Flow control – fast sender and slow receiver – prevent the receiver from being

flooded with data

� Disassembling, transmitting, and reassembling large messages – individual

parts of messages must be numbered serially so that reassembly is possible,

and lost / damaged messages can be retransmitted.

� Multiplexing / de-multiplexing – Multiplexing is a technique that allows

several signals to be transmitted over a single communications channel.

Multiplexing permits efficient utilization of channel resources. De-

multiplexing is carried out at the receiving end to separate the signals.

� Routing - it is the act of moving information across an internetwork from a

source to a destination

Concept of Services and Protocols: � A service is a set of operations that a layer provides to the layer above it.

� Service defines what operations the layer is prepared to perform.

� A service relates to the interface between two layers – the lower layer is

service provider and the upper layer is service user.

� A protocol is a set of rules governing the format and meaning of the packets.

� Protocols relate to packets sent between peer entities on different machines.

� Entities use protocols.

� Protocols can be changed provided the services visible to the user do not

change. Thus services and protocols are completely decoupled.



� Analogy with programming languages:

• A service is like an object in an object-oriented language

• What operations can be performed on this object is defined

• How these operations are to be performed is not defined

� Protocol relates to the implementation of the service – how it is done

Layers of the OSI Model:

Application

Presentation

Session

Transport

Network

Data Link

Physical

Useful mnemonics to remember the order of these layers:

All People Seem To Need Data Processing

Please Do Not Tell Secret Passwords Anytime

Application Layer:

• This is the top layer of the reference model.

• Examples: email client, Internet browser, file transfer

• How can two applications communicate in a meaningful way? - The particular

needs of an application specify the communication characteristics at the

application layer.

• Contains protocols that allow the users to access the network (FTP, HTTP,

SMTP, etc

• Does not include application programs such as email, browsers, word processing

applications, etc

• Protocols contain utilities and network-based services that support email via

SMTP, Internet access via HTTP, file transfer via FTP, etc

Presentation Layer:

• Examples: compression, encryption

• What does the data look like? - The Presentation layer provides services that

affect how data passed between applications will look, such as character set

conversion, or encryption/decryption.

• Translation

o Different computers use different encoding systems (bit order

translation)

o Convert data into a common format before transmitting.



o Syntax represents info such as character codes, data types, and file

formats (how many bits to represent data – 8 or 7 bits

• Compression – reduce number of bits to be transmitted

• Encryption – transform data into an unintelligible format at the sending end for

data security

• Decryption – at the receiving end.

Session Layer

• Main functions of this layer are:

o Dialog control – allows two systems to enter into a dialog, keep a track

of whose turn it is to transmit

o Synchronization – adds check points (synchronization points) into

stream of data.

• How can more than one resource talk to only one of me? – The session layer

answers the need for individual hosts to support more than one active

connection at a time.

Transport Layer


o Responsible for source-to-destination delivery of the entire message

o Segmentation and reassembly – divide message into smaller segments,

number them and transmit. Reassemble these messages at the receiving

end.

o Error control – make sure that the entire message arrives without errors –

else retransmit.

• What if data arrives garbled or out of order or too fast? – The Transport Layer

provides end-to-end communication integrity. If a packet is missing or garbled,

or arrives out-of-order, or too soon, the protocol at this layer will attempt to

correct.

Network Layer Main functions of this layer are:

o Responsible for delivery of packets across multiple networks

o Routing – Provide mechanisms to transmit data over independent

networks that are linked together.

o Network layer is responsible only for delivery of individual packets and

it does not recognize any relationship between those packets

Data Link Layer

• Responsible for delivery of data between two systems on the same network


o Framing – divides the stream of bits received into manageable data units

called frames.

o Physical Addressing – Add a header to the frame to define the physical

address of the source and the destination machines.



o Flow control – Impose a flow control – control rate at which data is

transmitted so as not to flood the receiver

o Error Control – Adds mechanisms to detect and retransmit damaged or

lost frames; also prevents duplication of frames. This is achieved by

adding a trailer to the end of a frame.

Physical layer

• Functions of Physical Layer:

o Bit representation – encode bits into electrical or optical signals

o Transmission rate – The number of bits sent each second

o Physical characteristics of transmission media

o Synchronizing the sender and receiver clocks

o Transmission mode – simplex, half-duplex, full duplex

o Physical Topology – how devices are connected – ring, star, mesh, bus

topology

• Transmit raw bit-stream over physical cable.

• Define physical aspects of media, network cards, etc.

Connection-oriented and Connectionless Services: Layers can offer two types of services t5o the layers above them: connection-oriented

and connectionless.

� Connection-oriented service is modelled after the telephone system. To talk to

someone, pick up a phone, dial the number, talk and disconnect. Similarly, in a

network, the service user will

1. Establish a connection

2. Use the connection

3. Release the connection

4. The sender, receiver and the network may conduct a negotiation about data

transfer speed, maximum message size, etc.

When a connection is established, the sender and receiver may conduct a negotiation

about parameters to be used such as maximum message size, data transfer speed, etc.

Connection-oriented service is used when reliability is important. Only a single path is

established for the call, and all the data follows that path.

� Connectionless service is modelled after the post office system. Each message

(letter) carries the full address of the destination system. Each message is routed

through the network system independent of the other messages. If two messages

are sent to the same destination, normally the first one to be sent should arrive

first. But it is possible that the second message arrives first. Since each message

(packet) is routed independently, the network cannot guarantee that all the

messages will arrive at the destination in the transmitting order. Different parts of

a message may travel along different routes.



TCP / IP Protocol

What is TCP / IP? TCP – Transmission Control Protocol

IP – Internetwork Protocol TCP/IP refers to a collection of protocols

The name TCP/IP is misleading because TCP and IP are only two of the many

protocols in this collection of protocols.

TCP is a reliable connection-oriented protocol with the following features:

1. Allows error-free transmission.

2. Incoming byte stream is fragmented into a number of shorter messages and these

are passed on to the next layer.

3. At the receiving end the TCP reassembles the messages into an output stream

4. TCP also handles flow control – to control data transfer rate so that a slow

receiver is not flooded with data from a fast sender.

5. A connection must be established between the sender and the receiver before

transmission begins.

6. TCP creates a virtual circuit between sender and receiver for the duration of the

transmission.

7. TCP begins each transmission by alerting the receiver that segments are on their

way (connection establishment).

8. Each transmission is ended with connection termination.

Layers of TCP/IP Model:

Application

Transport

Internet

Data Link

Physical

The TCP / IP suite does not define any specific protocols at the data link and physical

layers.

The Application layer: This is equivalent to the combined OSI Session, Presentation,

and Application layers. All the functions handled by these 3 layers in the OSI model

is handled by the Application layer. This layer contains the higher level protocols:

a. FTP – File Transfer Protocol – basic file transfer between hosts (computers)

b. SMTP – Simple Mail Transfer Protocol (for email)

c. HTTP – Hyper Text Transfer Protocol (for web browsing)

A data unit created at the application layer is called a message.



Encapsulation of Data: TCP/IP protocol suite encapsulates data units at various

layers of the model

a. At the Application layer, the data unit created is called a message.

b. The Transport layer adds a header to form either a segment with TCP or a user

datagram with UDP.

c. Each segment created by TCP includes:

i. A sequencing number for re-ordering after receipt.

ii. An acknowledgement ID number

iii. Source address and destination address

iv. Checksum – for error detection

v. Data

vi. And other fields

d. The Network (or Internet) layer adds another header to form a datagram. A

datagram is a self-contained message unit which contains sufficient information to

allow it to be routed from the source to the destination. The protocol used at the

data link layer encapsulates the datagram into a frame and this is transmitted

across the transmission medium.

Transport Layer:

• This layer is represented by two protocols – TCP (Transmission Control

Protocol ) and UDP (User Datagram Protocol).

• UDP is simpler but is used when reliability and security are less important

than size and speed – such as speech and video where a loss of few data is

not significant, but speed is important.

Network Layer or Internetwork Layer

Internetwork Protocol (IP) is an unreliable and connectionless datagram protocol.

It offers a best–effort delivery service. There is no error checking. IP does its best to

get a transmission through to its destination but with no guarantees. Noise can cause

bit errors during transmission, and datagrams maybe discarded due to timeout errors.

IP transports data in packets called datagrams:

• Each datagram is transported separately.

• Datagrams can be of variable lengths (upto 64 KB).

• Datagrams may travel along different routes and may arrive out of sequence.

• IP does not keep track of the routes.

• IP does not have the facility to reorder datagrams once they arrive.

• A datagram contains a header and data.

• The header contains a number of fields including source and destination address.

Comparison of OSI and TCP/IP Models: o The OSI model makes a clear distinction between services, interfaces and

protocols

o Each layer performs some service for the layer above it



o A layer’s interface tells the processes above it how to access it. It specifies what

the parameters are and what results to expect (somewhat like a function

declaration)

o The protocols used in a layer are used to get the job done.

o The OSI model has 7 layers while the TCP/ IP model has 5 layers

o Both have network, transport, and application layers, but the other layers are

different

o OSI model supports both connectionless and connection-oriented communication

o TCP/IP supports only connectionless communication

How TCP/IP works? User application programs communicate with the top layer in the protocol stack. This

layer passes information to the next subsequent lower layer of the stack, and so on till

the information is passed to the lowest layer, the physical layer, which transfers the

information to the destination network.

At the destination, the lower layer passes the received information to its higher levels,

which in turn passes the data to the destination application. Each protocol layer

performs various functions which are independent of the other layers. Each layer

communicates with equivalent layers on another computer; this is called peer

interaction

File Transfer using TCP / IP: An application program, transferring files using TCP/IP, performs the following:

At the sending end:

• the application layer passes the data to the transport layer of the source

computer

• the transport layer

- divides the data into TCP segments

- adds a header with a sequence number to each TCP segment

- passes the TCP segments to the IP layer

• the IP layer

- creates a packet with a data portion containing the TCP segment

- adds a packet header containing the source and destination IP

addresses

- determines the physical address of the destination computer

- passes the packet and destination physical address to the datalink

layer

• the datalink layer transmits the IP packet in the data portion of a frame

At the receiving end:

• the destination computers datalink layer

- discards the datalink header and passes the IP packet to the IP layer



• the destination’s IP layer

- checks the IP packet header and checksum

- if okay, discards IP header & passes the TCP segment to the TCP

layer

• the destination’s TCP layer

- computes a checksum for the TCP segment data and header

- if okay, sends acknowledge to the source computer

- discards the TCP header and passes the data to the application

Questions

1. What is a peer-to-peer process? Explain with examples.

2. State the layers of the OSI mode and state the functions of each layer.

3. What is the difference between connectionless communication and connection-

oriented communication?

4. What are the reasons for using layered protocols?

5. What does “negotiation” mean when discussing network protocols?

6. If the unit exchanged at the data link layer is called frame and the unit exchanged

at the network layer is called a packet, do frames encapsulate packets or do

packets encapsulate frames? Explain your answer.

7. List two ways in which OSI and TCP/IP reference models are same and two ways

in which they are different.

8. How do the layers of the TCP/IP model correlate to the layers of the OSI model?

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