Lession 4

INTRODUCTION…

WHY PROTOCOLS? A

network protocol or computer communication protocol is a set of rules that specify the format and meaning of messages exchanged between computers across a network Protocols are implemented by protocol software 2

ONE OR MANY PROTOCOLS?  Computer

communication across a network is a very hard problem

 Complexity

requires multiple protocols, each of which manages a part of the problem

 May

be simple or complex; must work well together

INTRODUCTION…

PROTOCOL SUITES A

set of related protocols that are designed for compatibility is called a protocol suite



Protocol suite designers: 

Analyze communication problem



Divide problems into sub-problems



Design a protocol for each subproblem

INTRODUCTION…

PROTOCOL SUITES… A

well-designed protocol suite

Is

efficient and effective - solves the problem without redundancy and makes best use of network capacity Allows replacement of individual protocols without changes to other protocols



INTRODUCTION…

LAYERED PROTOCOL DESIGN 

Layering model is a solution to the problem of complexity in network protocols



Model suggests dividing the network protocol into layers, each of which solves part of the network communication problem



These layers have several constraints, which ease the design problem



Network protocol designed to have a protocol or protocols for each layer

INTRODUCTION…

A PLAN PROTOCOL DESIGN 

International Organization for Standards (ISO) defined a 7-layer reference model as a guide to the design of a network protocol suite known as Open Systems Interconnection (OSI)



Layers are named and numbered; reference to “layer n” often means the nth layer of the ISO 7-layer reference model

7

7 LAYERS 7

Application Layer

6.

Presentation Layer

5.

Session Layer

4.

Transport Layer

3.

Network Layer

2.

Data Link Layer

1.

Physical Layer

All People Seem To Need Data Processing

The OSI 7-layer Model OSI - Open Systems Interconnection Defined in 1984 and become an international standard All

Away

People

Pizza

Seem

Sausage

To

Throw

Need

Not

Data

Do

Processing

Please

10

Relationship of OSI layers

Virtual Communication

Physical Communication

11

TASKS INVOLVED IN SENDING LETTER

7: THE APPLICATION LAYER 

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

APPLICATION LAYER (CONT’D)  Specific

services

Network

virtual terminal

File

access, transfer, and management

Mail

services

Directory

services

Example: File Transfer • The most traditional network task • Implemented by a simple Application Layer protocol called FTP FTP Client

FTP Server

PictureTel

PictureTel

PictureTel

Network 15

The application layer is responsible for providing services to the user.

6: PRESENTATION LAYER  Ensures

interoperability among communicating devices.  Is responsible for the encryption and decryption of data for security purpose and for the compression and expansion of data when necessary for transmission efficiency.

PRESENTATION LAYER (CONT’D)  Specific

responsibility Translation Encryption Compression

PRESENTATION LAYER (CONT’D)

5: THE SESSION LAYER 

Enables two networked resources to hold ongoing communications (called a session) across a network



Applications on either end of the session are able to exchange data for the duration of the session

This layer is: 

Responsible for initiating, maintaining and terminating sessions



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



Responsible for synchronization services, and for checkpoint services

SESSION LAYER Specific Session

responsibility management

Synchronization Dialog

control : Deciding who sends, and when

5. Session Layer • Establishment, maintaining and release of session • Provide dialog management • Regulate which side transmit, when, for how long (Sync.) • Provide synchronization between user tasks • Example : Winsock, UNIX

Appl.

Appl.

Pres.

Pres.

Sess.

Sess.

Tran.

Tran.

Netw.

Netw.

Data.

Data.

Phys.

Phys.

23

LAYER 4 – THE TRANSPORT LAYER 

Manages the transmission of data across a network



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

TRANSPORT LAYER (CONT’D) 

Reliable End-to-end delivery of a message

LAYER 3 – THE NETWORK LAYER 

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



This layer also handles packet switching and network congestion control

NETWORK LAYER  Is

responsible for the source-todestination delivery of a across multiple network link  provides two related services. Switching Routing

NETWORK LAYER (CONT’D) Switching ~ refer to temporary connections between physical links, resulting in longer links for network transmission.(ex: telephone conversation)  routing ~ means selecting the best path for sending a packet from one point to another when more than one path is available 

NETWORK LAYER (CONT’D)

NETWORK LAYER (CONT’D)  Specific

responsibilities

Source-to-destination

delivery(packet) Logical

addressing to physical addressing

Routing

Network Layer • Logical address to physical address translation For TCP/IP running on Ethernet Logical address: IP address 158.132.148.99 Physical address: Ethernet address 00 00 E2 15 1A CA

• Determine the route from source to destination computer • Example protocols: IP, IPX

LAYER 2 – THE DATA LINK LAYER 

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

DATA LINK LAYER  Is

responsible for delivering data units(group of bits) from one station to the next without errors.

 It

accepts a data unit from the third layer and adds meaningful bits to the beginning(header) and end(trailer) that contain addresses and other control information: Frame

NODE-TO-NODE DELIVERY

LAYER 1 – THE PHYSICAL LAYER 

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’D) 

Physical Layer

SUMMARY

Q&A