acn 4

 Switching 

acn 3

 

LAYERED TASKS : Open System Interconnection & TCP/IP Model

LAYERED TASKS

We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail the process of sending a letter to a friend would be complex if there were no services available from the post office.

Sender, Receiver, and Carrier

At the Sender Site

Let us first describe, in order, the activities that take place at the sender site.

• Higher layer. The sender writes the letter, inserts the letter in an envelope, writes the sender and receiver addresses, and drops the letter in a mailbox.

• Middle layer. The letter is picked up by a letter carrier and delivered to the post office.

• Lower layer. The letter is sorted at the post office; a carrier transports the letter.

0n the Way: The letter is then on its way to the recipient. On the way to the recipient's local post office, the letter may actually go through a central office. In addition, it may be transported by truck, train, airplane, boat, or a combination of these.

At the Receiver Site

• Lower layer. The carrier transports the letter to the post office.

• Middle layer. The letter is sorted and delivered to the recipient's mailbox.

• Higher layer. The receiver picks up the letter, opens the envelope, and reads it. 

Reference Models:

In computer networks, reference models give a conceptual framework that standardizes communication between heterogeneous networks.

The two popular reference models are −
•OSI Model
•TCP/IP Protocol Suite

OSI(Open System Interconnection) Model:

• OSI stands for Open Systems Interconnection
• Created by International Standards Organization (ISO)
• Was created as a framework and reference model to explain how different networking technologies work together and interact
• It is not a standard that networking protocols must follow
• Each layer has specific functions it is responsible for
• All layers work together in the correct order to move data around a network

Application Layer: Contains all services or protocols needed by application software or operating system to communicate on the network

Examples
–Firefox web browser uses HTTP (Hyper-Text Transport Protocol)
–E-mail program may use POP3 (Post Office Protocol version 3) to read e-mails and SMTP (Simple Mail Transport Protocol) to send e-mails

Presentation Layer: Concerned with how data is presented to the network
•Handles three primary tasks: 
 Translation , Compression , Encryption

Session Layer

• Responsible for managing the dialog between networked devices
• Establishes, manages, and terminates connections
• Provides duplex, half-duplex, or simplex communications between devices
• Provides procedures for establishing checkpoints,  adjournment,  termination, and restart or recovery procedures

Transport Layer

• Takes data from higher levels of OSI Model and breaks it into segments that can be sent to lower-level layers for data transmission
• Conversely, reassembles data segments into data that higher-level protocols and applications can use
  
• Also puts segments in correct order (called sequencing ) so they can be reassembled in correct order at destination
• Concerned with the reliability of the transport of sent data
• May use a connection-oriented protocol such as TCP to ensure destination received segments
• May use a connectionless protocol such as UDP to send segments without assurance of delivery
• Uses port addressing

Network Layer 

• Responsible for moving packets (data) from one end of the network to the other, called end-to-end communications
• Requires logical addresses such as IP addresses
• Device example: Router
• Routing is the ability of various network devices and their related software to move data packets from source to destination

Data Link Layer

• Is responsible for moving frames from node to node or computer to computer
• Can move frames from one adjacent computer to another, cannot move frames across routers
• Encapsulation = frame
• Requires MAC address or physical address
• Protocols defined include Ethernet Protocol and Point-to-Point Protocol (PPP)
• Device example: Switch,Bridge
• Two sublayers: Logical Link Control (LLC) and the Media Access Control (MAC)

Logical Link Control (LLC)

• Data Link layer addressing, flow control, address notification, error control

Media Access Control (MAC)

• Determines which computer has access to the network media at any given time
• Determines where one frame ends and the next one starts, called frame
• synchronization

Physical Layer

• Deals with all aspects of physically moving data from one computer to the next
• Converts data from the upper layers into 1s and 0s for transmission over media
• Defines how data is encoded onto the media to transmit the data
• Defined on this layer: Cable standards, wireless standards, and fiber optic standards.
• Copper wiring, fiber optic cable, radio frequencies, anything that can be used to transmit data is defined on the Physical layer of the OSI Model
• Device example: Hub, Repeater
• Used to transmit data

An exchange using the OSI model

TCP/IP Model (Transmission Control Protocol/Internet Protocol):
A protocol suite is a large number of related protocols that work together to allow networked computers to communicate

Application Layer

Application layer protocols define the rules when implementing specific network applications Rely on the underlying layers to provide accurate and efficient data delivery
Typical protocols:

• FTP – File Transfer Protocol
• For file transfer
• Telnet – Remote terminal protocol
• For remote login on any other computer on the network
• SMTP – Simple Mail Transfer Protocol
• For mail transfer
• HTTP – Hypertext Transfer Protocol
• For Web browsing

Transport Layer 
•TCP is a connection-oriented protocol

• Does not mean it has a physical connection between sender and receiver
• TCP provides the function to allow a connection virtually exists – also called virtual circuit

•UDP provides the functions:

•   Dividing a chunk of data into segments
•   Reassembly segments into the original chunk
•   Provide further the functions such as reordering and data resend
•   Offering a reliable byte-stream delivery service
•   Functions the same as the Transport layer in OSI
•   Synchronize source and destination computers to set up the session between the respective computers

Internet Layer

• The network layer, also called the internet layer, deals with packets and connects independent networks to transport the packets across network boundaries. The network layer protocols are the IP and the Internet Control Message Protocol (ICMP), which is used for error reporting.

Host-to-network layer
The Host-to-network layer is the lowest layer of the TCP/IP reference model. It  combines  the  link layer and  the  physical layer of  the  ISO/OSI  model.  At this layer, data is transferred between adjacent network nodes in a WAN or between nodes on the same LAN.

OSI Model Vs TCP/IP Model:

Introduction to Computer Networks

Syllabus of unit 1:

DATA COMMUNICATION: Characteristics, Components, Data flow, Network criteria, Topologies, Network model, Layered tasks, ARPANET, OSI model, TCP/IP protocol suite, Addressing 

PHYSICAL LAYER: Transmission Media: Guided and unguided, Connecting devices: Hub, switch, bridge, router, Gateway.

A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.

“Computer network’’ means a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information.

The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used, with  the Internet being the most well-known example of a network of networks.

There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, a collection of independent computers appears to its users as a single coherent system. Usually, it has a single model or paradigm that it presents to the users. Often a layer of software on top of the operating system, called middleware, is responsible for implementing  this  model.  A  well-known example of a distributed system is the World Wide Web. It runs on top of the Internet and presents a model in which everything looks like a document (Web page).

A network must be able to meet certain criterias, these are mentioned below:

Performance
Reliability
Scalability 

Performance: It can be measured in the following ways:
Transit time : It is the time taken to travel a message from one device to another.
Response time : It is defined as the time elapsed between enquiry and response.
Other ways to measure performance are :
·       Efficiency of software
·       Number of users        
·       Capability of connected hardware
 

Reliability: It decides the frequency at which network failure take place. More the failures are, less is the network's reliability.

Security: It refers to the protection of data from any unauthorized user or access. While travelling through network, data passes many layers of network, and data can be traced if attempted. Hence security is also a very important characteristic for Networks.

USES OF COMPUTER NETWORKS

1. Business Applications

• To distribute information throughout the company (resource sharing). sharing physical resources such as printers, and tape backup systems, is sharing information

• client-server model. It is widely used and forms the basis of much network usage.

• communication medium among employees. email (electronic mail), which employees generally use for a great deal of daily communication.

• Telephone calls between employees may be carried by the computer network instead of by the phone company. This technology is called IP telephony or Voice over IP (VoIP) when Internet technology is used.

• Desktop sharing lets remote workers see and interact with a graphical computer screen

• Doing business electronically, especially with customers and suppliers. This new model is called e-commerce (electronic commerce) and it has grown rapidly in recent years.

2.Home Applications

• peer-to-peer communication

• person-to-person communication

• electronic commerce

• entertainment.(game playing,)

3.Mobile Users

• Text messaging or texting

• Smart phones,

• GPS (Global Positioning System)

• m-commerce

• NFC (Near Field Communication)

4.Social Issues

With the good comes the bad, as this new-found freedom brings with it many unsolved social, political, and ethical issues. 

Social networks, message boards, content sharing sites, and a  host  of other applications allow people to share their views with  like-minded individuals. As long as the subjects are restricted to technical topics or hobbies like gardening, not too many problems will arise.

The trouble comes with topics that people actually care about, like politics, religion, or sex. Views that are publicly posted may be deeply offensive to some people. Worse yet, they may not be politically correct. Furthermore, opinions need not be limited to text; high-resolution color photographs and video clips are easily shared over computer networks. Some people take a live-and-let-live view, but others feel that posting certain material (e.g., verbal attacks on particular countries or religions, pornography, etc.) is simply unacceptable and that such content must be censored. Different countries have different and conflicting laws in this area. Thus, the debate rages.

Computer networks make it very easy to communicate. They also make it easy for the people who run the network to snoop on the traffic. This sets up conflicts  over  issues  such  as  employee  rights  versus  employer   rights. Many people read and write email at work. Many employers have claimed the right to read and possibly censor employee messages, including messages sent from a home computer outside working hours. Not all employees agree with this, especially the latter part.

Another conflict is centered around government versus citizen’s rights.A new twist with mobile devices is location privacy. As part of the process of providing service to your mobile device the network operators learn where you are at different times of day. This allows them to track your movements. They may know which nightclub you frequent and which medical center you visit.

Phishing ATTACK: Phishing is a type of social engineering attack often used to steal user data, including login credentials and credit card numbers. It occurs when an attacker, masquerading as a trusted entity, dupes a victim into opening an email, instant message, or text message.

BOTNET ATTACK: Botnets can be used to perform distributed denial-of-service attack (DDoS attack), steal data, send spam, and allows the attacker to access the device and its connection.

Data Communications System :

The effectiveness of a data communications system depends on  four fundamental characteristics: delivery, accuracy, timeliness, and jitter.

I. Delivery. The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.

2 Accuracy. The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable.

3.Timeliness. The system must deliver data in a timely  manner.  Data delivered late are useless. In the case of video and audio, timely delivery means delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real-time transmission.

4.Jitter. Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. For example, let us assume that video packets are sent every 30 ms. If some of the packets arrive with 30-ms delay and others with 40-ms delay, an uneven quality in the video is the result.

A data communications system has five components

1.Message. The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 

2 Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on.

3.Receiver. The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on.

4.Transmission medium. The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.

5.Protocol. A protocol is a set of rules that govern data communications. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating, just as a person speaking French cannot be understood by a person who speaks only Japanese.

Data Flow

Communication between two devices can be simplex, half-duplex, or full-duplex as shown in Figure.

Simplex 

In simplex mode, the communication is unidirectional, as on a one- way street. Only one of the two devices on a link can transmit; the other can only receive (Figure a). Keyboards and traditional monitors are examples of simplex devices.

Half-Duplex

In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa (Figure b). Walkie-talkies and CB (citizens band) radios are both half- duplex systems.

Full-Duplex

In full-duplex, both stations can transmit and receive simultaneously (Figure c). One common example of full-duplex communication is the telephone network. When two people are communicating by a telephone line, both can talk and listen at the same time. The full-duplex mode is used when communication in both directions is required all the time.