Selective Repeat Protocol Video

      A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP).

          Conceptually, each portion of the transmission (packets in most data link layers, but bytes in TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The problem with this is that there is no limit of the size of the sequence numbers that can be required.

By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers.

 Working :

           When used as the protocol for the delivery of messages, the sending process continues to send a number of frames specified by a window size even after a frame loss. Unlike Go-Back-N ARQ, the receiving process will continue to accept and acknowledge frames sent after an initial error; this is the general case of the sliding window protocol with both transmit and receive window sizes greater than 1.

The receiver process keeps track of the sequence number of the earliest frame it has not received, and sends that number with every acknowledgement (ACK) it sends. If a frame from the sender does not reach the receiver, the sender continues to send subsequent frames until it has emptied its window. The receiver continues to fill its receiving window with the subsequent frames, replying each time with an ACK containing the sequence number of the earliest missing frame. Once the sender has sent all the frames in its window, it re-sends the frame number given by the ACKs, and then continues where it left off.

              The size of the sending and receiving windows must be equal, and half the maximum sequence number (assuming that sequence numbers are numbered from 0 to n−1) to avoid mis-communication in all cases of packets being dropped. To understand this, consider the case when all ACKs are destroyed. If the receiving window is larger than half the maximum sequence number, some, possibly even all, of the packages that are resent after timeouts are duplicates that are not recognized as such. The sender moves its window for every packet that is acknowledged

 Video explanation :

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OSI Model - How an email passes through each layer

The Open Systems Interconnection model (OSI model) is a product of the Open Systems Interconnection effort at the International Organization for Standardization.

 

It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. An instance of a layer provides services to its upper layer instances while receiving services from the layer below.

This video explains the process of how an email passes from one machine to another while traversing through the 7 layers of the OSI  model. Very Interesting video and clearly mentions about what happens to the email at each layer. Next time someone asks you "How OSI model plays an important role in our day to day life" , share this video or explain it yourself.


A complete self-explanatory video on the 7 layers of OSI model and its real-time application. It's an interactive video , so make sure that you click on continue buttons once understand the description at each step.

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Go Back N Protocol (With Animation) - Computer Networks

Go-Back-N ARQ is a specific instance of the Automatic Repeat-reQuest (ARQ) Protocol, in which the sending process continues to send a number of frames specified by a window size even without receiving an ACK packet from the receiver. Learn this from this perfect animation.

The receiver process keeps track of sequence number of the next frame it expects to receive, and sends that number with every ACK it sends. The receiver will ignore any frame that does not have the exact sequence number it expects -- whether that frame is a "past" duplicate of a frame it has already ACK'ed, or whether that frame is a "future" frame past the lost packet it is waiting for. Once the sender has sent all of the frames in its window, it will detect that all of the frames since the first lost frame are outstanding, and will go back to sequence number of the last ACK it received from the receiver process and fill its window starting with that frame and continue the process over again.

Three cases are covered: 
1. All Packets reach the receiver and send acknowledgements back to the sender.
2. Some Packets a lost on their way to destination. 
3. Some Acknowledgements are lost on their way back to sender.

                View the animation to understand The Protocol

(Click the Play Button to start)

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Data Communication [Comp - Sem 5 ] [IT - Sem 4]

Introduction

         The subject Data Communication introduced in 4th semester of IT Engineering and 5th semester of Computer Engineering is a very important subject in the curriculum of Engineering. This subject lays the foundation of Networking in Engineering. This subject needs to be understood thoroughly as you will be learning Computer Networks in the next semester. The Engineering syllabus includes in all, 3 subjects in networking : viz Data Communication, Computer Networks and Advanced Computer Networks (Elective). All these are very important for an engineer who wants to establish a career in Networking. 'Networking' is a very vast field for education and has immense scope and applications in the  industry. The demand for Network Engineers keeps on increasing day by day.

       Now , one point I would precisely state is, syllabus of Data Communication is a bit different for both Computer and IT. Its not totally different, but definitely there is  a considerable amount of difference. Beginning a word with the OSI model, I now assume that you are aware of the OSI model. If not please refer this link.

     In the OSI model you have been introduced that there are seven layers in the model : i.e.  Physical layer, data link layer ..... Presentation Layer. Now from these seven layers : the bottom most 2 layers viz : physical layer and data link layer will be introduced in the subject Data Communication.

   Now the rest 5 layers namely.,Network layer on wards will  be covered in the subject Computer Networks along with a few other important networking topics.

Analysis

Unit  1 :  Layer models and Signals

The first unit gives you introduction and a brief idea about the different terminologies and concepts which are pre-requisites for any networking concepts. This unit is extremely lengthy and a bit boring. The concepts introduced here do not allow you to enjoy unlike other networking topics. This unit is relatively easy to learn even very vast. Normally, most of the questions asked in this subject have appeared in the previous university papers.

Unit  2 : Modulation and Multiplexing

Relatively short unit , when compared to the first unit but a little bit confusing. A few concepts introduced here have already been introduced to you in the subject 'Basic Electronics Engineering' [FE - Sem 2 ]. These concepts again appear as they are. people who find Electronics tough will have to invest relatively little more efforts in this unit.

Unit 3 : Transmission media and Switching

Introduces the basic Networking topics to you. A very simple and easy to learn unit. Concepts are really interesting and can be easily understood with a simple glance. Diagrams are an important part of this unit. Scoring unit and very interesting.

Unit 4 : Error control and Data link Control

A lengthy  unit , relatively tedious and very a  bit boring. You will understand many concepts of networking in this unit ....which you will be required to understand in the other unit. Not difficult , but a few sums make learning this unit a bit complicated, relatively time consuming. This unit will  definitely need multiple glances to understand the concepts well.

Unit 5 : Multiple Access and Ethernet

Very easy unit. I enjoyed learning the concepts introduced in this unit. Many new concepts a glance will give you a brief idea about the concepts. Really interesting, most of the questions from this unit can be easily predicted. Scoring, simple and very easy unit.

Unit 6 :  Devices , Backbone networks and SONETS

Lengthy unit,mainly covers 3 main topics.Devices, Backbone networks and SONETS , which are completely different from each other. Neat diagrams allow you to score in this unit. Simple but not as simple as 5th unit.

Books to Refer

     Among the local author's I would suggest only Techmax. The book is written by the well known author and publisher J. S. Katre. The book covers all the concepts mentioned in the syllabus. The book is complete in itself. Students, sometimes tend to avoid techmax and prefer Technical , since the Techmax books are huge in  size when compared to no of pages. Please, do not prefer Technical for D.C as many concepts are incomplete.

Among the foreign authors, I would suggest , the best one : Data Communication and Networking -Behrouz Forouzan. This book will help you to learn the concepts with simple diagrams and a lucid language.  

Verdict

    Data Communications a.k.a  D.C is a very lengthy  and a bit difficult for the beginners. The subject is very interesting as well as  a very important subject in the curriculum. Scoring is not that difficult...neat diagrams and quality answers will easily fetch you 65+ marks in this subject. Scoring 70+ will need good efforts and very precise answers . Totally theoretical  but very easy to learn and not that difficult.

Books for Download

Data Communication & Networking
Behrouz Forouzan


(Most Recommended)

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Understanding Data Communications

Gilbert Held

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Data and Computer Communications 

William Stallings

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Learn the OSI Model - [Data Communications, CNT]

Introduction

Data Communications and Computer Networks are two very important subjects in the curriculum of IT and Computer Engineering. The OSI model is very important concept in both of these subjects. The entire subject is layed down on the foundation of the OSI model.

What is  the OSI Model?

The Open Systems Interconnection model (OSI model) was a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers. Similar communication functions are grouped into logical layers. 

 There are 7 layers in the OSI model :

Description of all Layer 

Application Layer

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation Layer

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session Layer

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport Layer

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network Layer

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling,congestion control and packet sequencing.

Data-link Layer

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers:

·         The Media Access Control (MAC) layer

The MAC sub layer controls how a computer on the network gains access  to the data and permission to  transmit it.

       ·         The Logical Link Control (LLC) layer.

 The LLC layer controls framesynchronization, flow control and error checking.

Physical Layer

This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Video Demonstration

 Learn how an message passes through the OSI model using this video:

A complete self-explanatory video on the 7 layers of OSI model and its real-time application. It's an interactive video , so make sure that you click on continue buttons once understand the description at each step.

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