INTRODUCTION TO NETWORKS & OSI MODEL T.RAMYAAPI/EIEKLNCE.

INTRODUCTION TO NETWORKS & OSI MODEL

T.RAMYAT.RAMYA

API/EIEAPI/EIE

KLNCEKLNCE

Networking

Netprog: OSI Reference Model

Multiaccess vs. Point-to-point

• Multiaccess means shared medium.– many end-systems share the same physical

communication resources (wire, frequency, ...)– There must be some arbitration mechanism.

• Point-to-point– only 2 systems involved– no doubt about where data came from !



MultiaccessMultiaccess Point-to-pointPoint-to-point

LAN - Local Area Network

• connects computers that are physically close together ( < 1 mile).– high speed– multi-access

• Technologies:– Ethernet 10 Mbps, 100Mbps– Token Ring 16 Mbps– FDDI 100 Mbps


WAN - Wide Area Network

• connects computers that are physically far apart. “long-haul network”.– typically slower than a LAN.– typically less reliable than a LAN.– point-to-point

• Technologies:– telephone lines– Satellite communications


MAN - Metropolitan Area Network

• Larger than a LAN and smaller than a WAN- example: campus-wide network- multi-access network

• Technologies:– coaxial cable – microwave


Internetwork

• Connection of 2 or more distinct (possibly dissimilar) networks.

• Requires some kind of network device to facilitate the connection.


Net A Net B

THE NEED FOR STANDARDS• Over the past couple of decades many of the networks that were built

used different hardware and software implementations, as a result they were incompatible and it became difficult for networks using different specifications to communicate with each other.

• To address the problem of networks being incompatible and unable to communicate with each other, the International Organisation for Standardisation (ISO) researched various network schemes.

• The ISO recognised there was a need to create a NETWORK MODEL that

would help vendors create interoperable network implementations.

Open Systems Interconnection (OSI) is a set of internationally recognized, non-proprietary standards for networking and for operating system involved in networking functions.

An open system is a set of protocols that allow two different systems to communicate regardless of their underlying architecture.

Open Systems Interconnection model is fundamental to all communications between network devices

It is now the theoretical model for how communication takes place between network devices.

The OSI reference model is an arbitrary hierarchical stratification (layering) of computer networking functions. The stratification consists of seven layers.

There are various ways of implementing the protocols at any given layer. OSI is a CONCEPTUAL model.

OSI Model allows complete interoperability between incompatible systems.

The communication is governed by an agreed upon series of rules and conventions called PROTOCOLS.

The OSI Reference ModelThe OSI reference model consists of seven layers, not including layer 8, the

end user’s application, and layer 0 the physical transmission media.

7. Application Layer

6. Presentation Layer

7. Session Layer

8. Transport Layer

9. Network Layer

10. Data Link Layer

11. Physical Layer

8. O/S or User Application

0. Physical Transmission Media

Design Issues for the Layers

►AddressingAddressing►Error ControlError Control►Flow ControlFlow Control►MultiplexingMultiplexing►RoutingRouting

THE SEVEN OSI REFERENCE MODEL LAYERS

LAYER 1: PHYSICAL• The physical layer deals with the physical characteristics of the

transmission medium.

• It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

• Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other similar attributes are defined by physical layer specifications.

• Examples :- EIA/TIA-232, RJ45, NRZ.

Converts bits into electronic signals for outgoing messages

Converts electronic signals into bits for incoming messages

This layer manages the interface between the the computer and the network medium (coax, twisted pair, etc.)

This layer tells the driver software for the MAU (media attachment unit, ex. network interface cards (NICs, modems, etc.)) what needs to be sent across the medium

The bottom layer of the OSI model

Physical Layer cont…• Responsibility:

– transmission of raw bits over a communication channel.

• Issues:– mechanical and electrical interfaces– time per bit ,Distances– Transmission mode– topology

Handles special data frames (packets) between the Network layer and the Physical layer

At the receiving end, this layer packages raw data from the physical layer into data frames for delivery to the Network layer

At the sending end this layer handles conversion of data into raw formats that can be handled by the Physical Layer

Layer 2 : DATA LAYER

• The data link layer provides access to the networking media and it enables the data to locate its intended destination on a network.

• It provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses.

• This layer uses the MAC address to define a hardware or data link address in order for multiple stations to share the same medium and still uniquely identify each other.

• Concerned with network topology, network access, error notification, ordered delivery of frames, and flow control.

• Examples :- Ethernet, Frame Relay, FDDI.

Data Link Control• Responsibility:

– provide an error-free communication link– Hop-to-hop delivery

• Issues:– framing (dividing data into chunks)

• header & trailer bits– Addressing

– error notification, – ordered delivery of frames,

– and flow control

10110110101 01100010011 10110000001

LAYER 3: NETWORK• Defines end-to-end delivery of packets. • Defines logical addressing so that any endpoint can

be identified. • Defines how routing works and how routes are

learned so that the packets can be delivered. • The network layer also defines how to fragment a

packet into smaller packets to accommodate different media.

• Routers , IP address

Handles addressing messages for delivery,

Responsible for deciding how to route transmissions between computers

This layer also handles the decisions needed to get data from one point to the next point along a network path i.e source to destination delivery.

This layer also handles packet switching and network congestion control

The Network Layer• Responsibilities:

– path selection between end-systems (routing).– subnet flow control.– fragmentation & reassembly– translation between different network types.

• Issues:– packet headers– virtual circuits

LAYER 4: TRANSPORT

• The transport layer regulates information flow to ensure process – to- process delivery between host applications reliably and accurately.

• The transport layer segments data from the sending host's system and reassembles the data into a data stream on the receiving host's system.

• Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

Manages the flow of data between parties by segmenting long data streams into smaller data chunks (based on allowed “packet” size for a given transmission medium)

Reassembles chunks into their original sequence at the receiving end

Provides acknowledgements of successful transmissions and requests resends for packets which arrive with errors

The Transport Layer• Responsibilities:

– provides virtual end-to-end links between peer processes.

– end-to-end flow control

• Issues:– Segmentation and reassembly– error detection – reliable communication

LAYER 5: SESSION LAYER

• The session layer defines how to start, control and end conversations (called sessions) between applications.

• This includes the control and management of multiple bi-directional messages using dialogue control.

• It also synchronizes dialogue between two hosts' presentation layers and manages their data exchange.

• Examples :- SQL, ASP(AppleTalk Session Protocol).

Enables two networked resources to hold ongoing communications (called to exchange data for the duration of the session responsible for initiating, maintaining and terminating sessions a session) across a network

Responsible for security and access control to session information (via session participant identification)

Responsible for synchronization services

(checkpoints)

The Session Layer

• Responsibilities:– establishes, manages, and terminates sessions

between applications.– service location lookup

LAYER 6: PRESENTATION

• The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system.

• Concerned with Semantics and syntax of the information

• Provides encryption and compression of data.• Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.

Manages data-format information for networked communications (the network’s translator)

For outgoing messages, it converts data into a generic format for network transmission; for incoming messages, it converts data from the generic network format to a format that the receiving application can understand. If necessary, the presentation If necessary, the presentation layer translates between multiple data formats by using a layer translates between multiple data formats by using a common format. common format.

This layer is also responsible for certain protocol conversions, data encryption/decryption, or data compression/decompression.

The Presentation Layer

• Responsibilities:– data encryption– data compression– data conversion

The top layer of the OSI model

Provides a set of interfaces for sending and receiving applications to gain access to and use network services, such as: networked file transfer, message handling and database query processing

LAYER 7: APPLICATIONLAYER 7: APPLICATION

• The application layer is the OSI layer that is closest to the user.

• It provides network access to the user. • It differs from the other layers in that it does not

provide services to any other OSI layer, but rather, only to applications outside the OSI model.

• Examples of such applications are spreadsheet programs, word processing programs, and bank terminal programs.

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

7 Application

6 Presentation

5 Session

4 Transport

3 Network

2 Data Link

1 Physical

Tran

smission M

edia

UserApplication

UserApplication

Sending Computer Receiving Computer

Layering & Headers • Each layer needs to add some control information

to the data in order to do it’s job. • This information is typically appended to the data

before being given to the lower layer.• Once the lower layers deliver the the data and

control information - the peer layer uses the control information.

Headers

Process

Transport

Network

Data Link

Process

Transport

Network

Data Link

DATA

DATA

DATA

DATA

H

H

H

H

HH

What are the headers?

Physical: no header - just a bunch of bits.

Data Link: – address of the receiving endpoints– address of the sending endpoint– length of the data– checksum.

Network layer header - examples

• type of service • length of the data• packet identifier• fragment number• time to live

protocolprotocol header checksumheader checksum source network source network

addressaddress destination network destination network

addressaddress

Network SoftwareProtocol Hierarchies

• Layers, protocols, and interfaces.

Summary

• Data-Link: communication between machines on the same network.

• Network: communication between machines on possibly different networks.

• Transport: communication between processes (running on machines on possibly different networks).

Protocol Hierarchies

Services to Protocols Relationship

• The relationship between a service and a protocol.

Reference Models

The OSI reference model.

Reference Models

Hybrid Model

• The hybrid reference

A Critique of the OSI Model and Protocols

• Why OSI did not take over the world

• Bad timing• Bad technology• Bad implementations• Bad politics

Bad Timing

• The apocalypse of the two elephants.

INTRODUCTION TO NETWORKS & OSI MODEL T.RAMYAAPI/EIEKLNCE.

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