WHAT IS INTERNET INFRASTRUCTURE

I believe, now you know how packets travel from one computer to another over the Internet. But what's in-between? What actually makes up the Internet infrastructure or backbone?

 The Internet backbone is made up of many large networks, which interconnect with each other. These large networks are known as Network Service Providers or NSPs. These networks peer with each other to exchange packet traffic. Each NSP is required to connect to Network Access Points or NAPs. At the NAPs, packet traffic may jump from one NSP's backbone to another NSP's backbone.

Internet Backbone

NSPs also interconnect at Metropolitan Area Exchanges or MAEs. MAEs serve the same purpose as the NAPs but are privately owned. NAPs were the original Internet interconnects points. Both NAPs and MAEs are referred to as Internet Exchange Points or IXs. NSPs also sell bandwidth to smaller networks, such as ISPs and smaller bandwidth providers. Below is a picture showing this hierarchical infrastructure.

This is not a true representation of an actual piece of the Internet. The above figure is only meant to demonstrate how the NSPs could interconnect with each other and smaller ISPs. None of the physical network components are shown in this figure. This is because a single NSP's backbone infrastructure is a complex drawing by itself. Most NSPs publish maps of their network infrastructure on their web sites and can be found easily. To draw an actual map of the Internet would be nearly impossible due to it's size, complexity, and ever changing structure.

The Internet Routing Hierarchy

 So how do packets find their way across the Internet? Does every computer connected to the Internet know where the other computers are? Do packets simply get 'broadcast' to every computer on the Internet? The answer to both the preceding questions is 'no'. No computer knows where any of the other computers are, and packets do not get sent to every computer. The information used to get packets to their destinations is contained in routing tables kept by each router connected to the Internet. 

 

Routers Connecting in Network

Routers are packet switches. A router is usually connected between networks to route packets between them. Each router knows about it's sub-networks and which IP addresses they use. The router usually doesn't know what IP addresses are 'above' it. Examine the figure below. The black boxes connecting the backbones are routers. The larger NSP backbones at the top are connected at a NAP. Under them are several sub-networks, and under them, more sub-networks. At the bottom are two local area networks with computers attached.

When a packet arrives at a router, the router examines the IP address put there by the IP protocol layer on the originating computer. The router checks it's routing table. If the network containing the IP address is found, the packet is sent to that network. If the network containing the IP address is not found, then the router sends the packet on a default route, usually up the backbone hierarchy to the next router. Hopefully the next router will know where to send the packet. If it does not, again the packet is routed upwards until it reaches a NSP backbone. The routers connected to the NSP backbones hold the largest routing tables and here the packet will be routed to the correct backbone, where it will begin its journey 'downward' through smaller and smaller networks until it finds it's destination.

Domain Names and Address Resolution

But what if you don't know the IP address of the computer you want to connect to? What if you need to access a web server referred to as www.anothercomputer.com? How does your web browser know where on the Internet this computer lives? The answer to all these questions is the Domain Name Service or DNS. The DNS is a distributed database, which keeps track of computer's names and their corresponding IP addresses on the Internet.

Many computers connected to the Internet host part of the DNS database and the software that allows others to access it. These computers are known as DNS servers. No DNS server contains the entire database; they only contain a subset of it. If a DNS server does not contain the domain name requested by another computer, the DNS server re-directs the requesting computer to another DNS server.

DNS Hierarchy

The Domain Name Service is structured as a hierarchy similar to the IP routing hierarchy. The computer requesting a name resolution will be re-directed 'up' the hierarchy until a DNS server is found that can resolve the domain name in the request. Figure 6 illustrates a portion of the hierarchy. At the top of the tree are the domain roots. Some of the older, more common domains are seen near the top. What is not shown are the multitude of DNS servers around the world which form the rest of the hierarchy. When an Internet connection is setup (e.g. for a LAN or Dial-Up Networking in Windows), one primary and one or more secondary DNS servers are usually specified as part of the installation. This way, any Internet applications that need domain name resolution will be able to function correctly. For example, when you enter a web address into your web browser, the browser first connects to your primary DNS server. After obtaining the IP address for the domain name you entered, the browser then connects to the target computer and requests the web page you wanted.

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WHAT ARE INTRANET AND EXTRANET

Intranets are basically 'small' Internets. They use the same network facilities that the Internet does, but access is restricted to a limited sphere. For instance, a company can set up an intranet within the confines of the company itself. Access can be tightly controlled and limited to authorized employees and staff. There is no connection to the Internet or any other outside network. Functions like web sites, file uploads and downloads, and e-mail is available on intranets within the confines of the network. Since frivolous sites are no longer available, there is no employee time lost due to accessing them. There is, of course, the limitation of the networking area. The very benefit of restricting access to all of the facilities available on the Internet also restricts communication to other desirable locations. This is where the extranet steps in.


An Extranet is network that connects a number of intranets into a truly mini-Internet Access is extended to all the intranets connected through the extranet, but, again, not to the Internet.  Extranets requires a constant Internet connection and a hypertext transfer protocol (http) server. Extranets can also be used to connect an intranet to the Internet so that remote offsite access can be made into a company’s intranet by an authorized individual. This can facilitate through an extranet.

Basically, it uses passwords and smart cards to log in to a gateway server that checks the requester’s security credentials. If the user checks out, he or she is allowed access into the company’s intranet structure.

A number of URL address are set aside for intranet and extranet use. Essentially because intranets are self-contained networks, the same set of addresses can be used by all intranets without conflict. Extranet addresses are designed to recognize the intranets they connect and correctly preface each intranet address with an identifier. This allows two interconnected intranets to retain the same set of address values and keep them from being mistaken. One class A address, ranging from 10.0.0.0 to 10.255.255.255 is reserved for intranet usage. Again, since an intranet is a self-contained system, it only needs one class A network to designate the main network. Sub networks use reserved class B and class C addresses. There are 16 class B addresses, from 172.16.0.0 ti 172.31.255.255 and 256 class C addresses, which range from 192.168.0.0 to 192.168.255.255.

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HOW DOES THE INTERNET WORK?

As discussed in the previous articles, every computer connected to the Internet has a unique address. Let's say your IP address is 1.2.3.4 and you want to send a message to the computer with the IP address 5.6.7.8. The message you want to send is "Hello computer 5.6.7.8!” Let's say you've dialed into your ISP from home and the message must be transmitted over the phone line. Therefore the message must be translated from alphabetic text into electronic signals, transmitted over the Internet, and then translated back into alphabetic text.


How is this accomplished? Through the use of a protocol stack. Every computer needs one to communicate on the Internet and it is usually built into the computer's operating system (i.e. Windows, UNIX, etc.). The protocol stack used on the Internet is referred to as the TCP/IP protocol stack

If we were to follow the path that the message "Hello computer 5.6.7.8!" took from our computer to the computer with IP address 5.6.7.8, it would happen something like this:

• The message would start at the top of the protocol stack on your computer and work it's way downward.
• If the message to be sent is long, each stack layer that the message passes through may break the message up into smaller chunks of data. This is because data sent over the Internet (and most computer networks) are sent in manageable chunks. On the Internet, these chunks of data are known as packets.
• The packets would go through the Application Layer and continue to the TCP layer. Each packet is assigned a port number, which is used by program on the destination computer to receive the message because it will be listening on a specific port.
• After going through the TCP layer, the packets proceed to the IP layer. This is where each packet receives its destination address, 5.6.7.8.
• Now that our message packets have a port number and an IP address, they are ready to be sent over the Internet. The hardware layer takes care of turning our packets containing the alphabetic text of our message into electronic signals and transmitting them over the phone line.
• On the other end of the phone line your ISP has a direct connection to the Internet. The ISPs router examines the destination address in each packet and determines where to send it. Often, the packet's next stop is another router. More on routers and Internet infrastructure later.
• Eventually, the packets reach computer 5.6.7.8. Here, the packets start at the bottom of the destination computer's TCP/IP stack and work upwards.
• As the packets go upwards through the stack, all routing data that the sending computer's stack added (such as IP address and port number) is stripped from the packets.
• When the data reaches the top of the stack, the packets have been re-assembled into their original form, "Hello computer 5.6.7.8!"

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WHAT IS INTERNET ADDRESS?

Addresses are essential for virtually everything we do on the Internet. The IP in TCP/IP is a mechanism for providing addresses for computers on the Internet. Internet addresses have two forms:

• Person understandable which is expressed as words.
• Machine understandable which is expressed as numbers.

The following can be a typical person understandable address on Internet:

VWW @ ****.com

VWW is an username which in general is the name of the Internet account. This name is same as the one, which you may use when logging into the computer on which you have your Internet account. Logging in is the process of gaining access to your account on a computer, which is shared by several users. Your Internet account is created on it.

@ Connect “who” with where:

“ **** ” is a sub domain (could be several in each could be separated by (dot). Last one is referred to a domain).

.com is a domain top or what part in – It refers to “where” part which is a country code.

Structure of Internet Servers Address

The structure of an Internet server’s address keyed into a client’s software is as follows:

http://www.microsoft.com

Where,

• http is the communication protocol to be used
• www is the notation for World Wide Web
• .Microsoft is the registered domain Name associated with the IP address of an Internet Server.
• .com the server provides commercial services to clients who connect to it.

To help to speed up access, its IP address can be directly represented in form of numbers. 107.56.23.1 instead of the domain name, microsoft.com. In this case no name resolution needs to take place. An Internet address is a unique 32-bit number that is typically expressed as four 8-bit octets, with each octet separated by a period. Each of the octets can take on any number from 0 through 255.

Hosts, Domains and Sub domains

Hosts are in general, individual machines at a particular location. Resources of a host machine are normally shared and can be utilized by any user on Internet. Hosts and local networks are grouped together into domains, which then are grouped together into one larger domain. For an analogy a host computer is considered as an apartment building in a housing complex and your account is just an apartment in it.

Domain may be an apartment complex, a town or even a country. Sub-domains may correspond to organizations such as *****. Computers termed as name servers contain database of Internets host addresses. They translate word addresses or persons understandable into numeric equivalents.

Let us see another example of Internet address,

http://www.*******.ac.in

What does it all mean? Actually to the ISP server, very little. The server wants to see something quite different. It wants to see a 32-bit number as an Internet address. Something like this equivalent decimal grouping:

198.168.45.249

The Internet addresses, known as universal resources locators (URL), are translated from one form to the other using an address resolution protocol. The first address is in the form we are most used to and that user use to access an Internet site. In this example, the address is for a website, identified by the hypertext transfer protocol (http), which controls access to web pages. Following http is a delimiter sequence,://, and identification for the World Wide Web (www).

The domain name, *******.ac follows www and identifies the general site for the web. (dot) edu is one example of a domain top, which is a broad classification of web users. Other common domain tops are:

.com for commerce and businesses .gov for government agencies .mil for military sites .org for all kinds of organizations.

Lastly, in this example is a country code, again preceded by a dot. Here we are using in for the India, which is the default country.

Addresses may be followed by sub domains separated by dots or slashes (/) as needed. These addresses are translated into a 32-bit (4 decimal numeric groups) address shown as for http:// www.*******.ac.in we will further discuss this topic in the next section.

What does it all mean? Actually to the ISP server, very little. The server wants to see something quite different. It wants to see a 32-bit number as an Internet address. Something like this equivalent decimal grouping:

198.168.45.249

The Internet addresses, known as universal resources locators (URL), are translated from one form to the other using an address resolution protocol. The first address is in the form we are most used to and that user use to access an Internet site. In this example, the address is for a website, identified by the hypertext transfer protocol (http), which controls access to web pages. Following http is a delimiter sequence,://, and identification for the World Wide Web (www).

The domain name, ****.ac follows www and identifies the general site for the web.(dot) com is one example of a domain top, which is a broad classification of web users. Other common domain tops are:

.com for commerce and businesses .gov for government agencies .mil for military sites .org for all kinds of organizations.

Lastly, in this example is a country code, again preceded by a dot. Here we are using us for the U.S, which is the default country.

Addresses may be followed by sub domains separated by dots or slashes (/) as needed. These addresses are translated into a 32-bit (4 decimal numeric groups) address shown as for http:// www.google.in we will further discuss this topic in the next ARTICLES.

Address Space

Internet addresses are divided into five different types of classes. The classes were designated A through E. class A address space allows a small number of networks but a large number of machines, while class C allows for a large number of networks but a relatively small number of machines per network. The following figure lists five address classes used in classical network addresses. Regardless of the class of address space assigned, organizations assigned a particular class of address will not utilize the entire address space provided. This is especially in the case of class A and Class B addresses allocation schemes.

Ports

A port is an additional 16-bit number that uniquely identifies the particular service on any given machine on the Internet. Port numbers are 16 bit wide, therefore each computer on the Internet has a maximum number of 216 or 65,536 ports. The particular application is identified by its unique port number in the same way that a specific television station has a unique channel number.

Port numbers are divided into three ranges:

Well-known ports are those from 0 through 1,023.

Registered ports are those from 1,024 through 49,151.

Dynamic and private ports are those from 49,152 through 65,535.

Well-known ports, those ranging from 0 through 1,023 are where most common services on the Internet are residing. These ports are controlled and assigned by the Internet Assigned Number Authority (IANA) and on most systems can be used only by system (root) processes or by programs executed by privileged users.

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WHAT ARE INTERNET PROTOCOLS?

A communication protocol is an agreement that specifies a common language two computers use to exchange messages. For example, a protocol specifies the exact format and meaning of each message that a computer can send. It also specifies the conditions under which a computer should send a given message and how a computer should respond when a message arrives. Different types of protocols are used in Internet such as IP and TCP. 
 
A computer connected to the Internet needs both TCP and IP software. IP provides a way of transferring a packet from its source to destination and TCP handles the lost datagram’s and delivery of datagram’s. Together, they provide a reliable way to send data across the Internet. We discuss about these protocols in brief in the following section.

Internet Protocol (IP)

The Internet protocol specifies the rules that define the details of how computers communicate. It specifies exactly how a packet must be formed and how a router must forward each packet on toward its destination. Internet Protocol (IP) is the protocol by which data is sent from one computer to another on the Internet. Each computer (known as a host) on the Internet has at least one IP address that uniquely identifies it from all other computers on the Internet. When sending or receiving data, the message gets divided into little chunks called packet. Each of these packets contains both the senders Internet address and the receiver’s address. The packet that follows the IP specification is called an IP datagram. The Internet sends an IP datagram across a single network by placing it inside a network packet. For network the entire IP datagram is data. When the network packet arrives at the next computer, the computer opens the packet and extracts the datagram. The receiver examines the destination address on the datagram to determine how to process it. When a router, determines that the datagram must be sent across another network, the router creates a new network packet, encloses the datagram inside the packet and sends the packet across another network toward its destination. When a packet carrying a datagram arrives at its final destination, local software on the machine opens the packet and processes the datagram. Because a message is divided into a number of packets a different route can send each packet across the Internet. Packets can arrive in a different order than the order they were sent in. The Internet Protocol just delivers them. It's up to another protocol, the Transmission Control Protocol to put them back in the right order. IP is a connectionless protocol, which means that there is no established connection between the end points that are communicating. Each packet that travels through the Internet is treated as an independent unit of data without any relation to any other unit of data. In the Open Systems Interconnection (OSI) communication model, IP is in layer 3, the Networking Layer.

Transmission Control Protocol (TCP)

TCP makes the Internet reliable. TCP solves many problems that can occur in a packet switching system. TCP provide the following facilities:

• TCP eliminates duplicate data.
• TCP ensures that the data is reassembled in exactly the order it was sent.
• TCP resends data when a datagram is lost.
• TCP uses acknowledgements and timeouts to handle problem of loss.

The main features of TCP are:

Reliability: TCP ensures that any data sent by a sender arrives at the destination as it was sent. There cannot be any data loss or change in the order of the data. 

Reliability at the TCP has four important aspects:

• Error Control
• Loss control
• Sequence control
• Duplication control 

Connection-oriented: TCP is connection-oriented. Connection-oriented means a connection is established between the source and destination machines before any data is sent i.e. a connection is established and maintained until such time as the message or messages to be exchanged by the application programs at each end have been exchanged. The connections provided by TCP are called Virtual Connections. It means that there is no physical direct connection between the computers.

TCP is used along with the Internet Protocol to send data in the form of message units between computers over the Internet. While IP takes care of handling the actual delivery of the data, TCP takes care of keeping track of the individual units of data (called Packet) that a message is divided into for efficient routing through the Internet. TCP provides for a reliable, connection-oriented data transmission channel between two programs. Reliable means that data sent is guaranteed to reach its destination in the order sent or an error will be returned to the sender.

For example, when an HTML file is sent to someone from a Web server, the Transmission Control Protocol (TCP) program layer in that server divides the file into one or more packets, numbers the packets, and then forwards them individually. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end (the client program in our computer), TCP reassembles the individual packets and waits until they have arrived to forward them as a single file.

TCP is responsible for ensuring that a message is divided into the packets that IP manages and for reassembling the packets back into the complete message at the other end. In the Open Systems Interconnection (OSI) communication model, TCP is in layer 4, the Transport Layer.

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HOW TO ACCESS THE INTERNET?

Before we can use the Internet, we have to gain access to it.  This access is achieved in one of several ways, which we will discuss in this How to Access the Internet article. Above all, the Internet is a collection of networks that are connected together through various protocols and hardware.

Dial-up Connection:

one of the commonest ways of connection to Internet is through dial up connection using a modem and a telephone line. Using these you can connect to a host machine on the Internet. Once connected the telecommunications software allows you to communicate with the Internet host. When the software runs it uses the modem to place a telephone call to a modem that connects to a computer attached to the Internet.

The SLIP (Serial Line Internet Protocol) or PPP (Point to Point Protocol):

Two protocols; serial line interface protocol (SLIP) and the point-to-point protocol (PPP), allow a user to dial into the Internet. They convert the normal telephone data stream into TCP/IP packets and send them to the network. With these, the user becomes a peer station on the Internet and has access to all of the Internet’s facilities.

Internet Service Providers

As mentioned earlier, nobody truly owns the Internet, but it is maintained by a group of volunteers interested in supporting this mode of information interchange.  Central to this control is the Internet service provider (ISP) which is an important component in the Internet system. Each ISP is a network of routers and communication links.  The different ISP's provide a variety of different types of network access to the end systems, including 56 Kbps dial-up modem access, residential broadband access such as cable modem or DSL, high-speed LAN access, and wireless access.  ISP's also provide Internet access to content providers, connecting Web sites directly to the Internet. To allow communication among Internet users and to allow users to access worldwide Internet content, these lower-tier ISP's are interconnected through national and international upper-tier ISP's, such as Sprint.  An upper-tier ISP consists of high-speed routers interconnected with high-speed fiber-optic links.  Each ISP network, whether upper-tier or lower-tier, is managed independently, runs the IP protocol (see below), and conforms to certain naming and address conventions.

ISDN (Integrated Services Digital Network) Service

The whole idea of ISDN is to digitize the telephone network to permit the transmission of audio, video and text over existing telephone lines.  The purpose of the ISDN is to provide fully integrated digital services to users.

The use of ISDN for accessing the Internet has breathed new life into the ISDN service. ISDN’s slow acceptance was due mostly to a lack of a need for its capabilities. Being a digital interface, ISDN has provided a means for accessing web sites quickly and efficiently.  In response to this new demand, telephone companies are rapidly adding ISDN services.

The ISDN standard defines three channels types, each with the different transmission rate: bearer channel (B), data channel (D) and hybrid channel (H). The B channel is defined at a rate of 64 Kbps.  It is the basic user channel and can carry any type of digital information in full duplex mode as long as the required transmission does not exceed 64 kbps.  A data channel can be either 16 or 64 kbps depending on the needs of the user used to carry control signals for B channels. Of the two basic rate B channels, one is used to upload data to the Internet and one to download from the Internet.
 
The D Channel assists in setting up connection and maintaining flow control. There are three ways ISDN can be used to interface to the Internet, by using a modem, adapter, or bridge/router.  ISDN modems and adapters limit access to a single user.  Both terminate the line into an ISDN service.  The difference between them is that the ISDN modem takes the Internet traffic and pushes it through the computer serial port, while, the faster ISDN adapter connects directly to the computer’s buses. 

ISDN bridge/routers allow for local network connections to be made through ISDN to the Internet.  The ISDN termination is made into an Ethernet-type LAN so that multiple users can achieve access to the Net through a single access address.  Transfer rates between user and the Internet are between 56 and 128 Kbps.

Direct ISP Service through Leased Line

The most costly method of accessing the Internet is to use leased lines that connect directly to the ISP.  This will increase access rate to anywhere between 64 K and 1.5 Mbps, depending on the system in use.  Equipment called data service units (DSU) and channel service units (CSU) are set up in pairs, one pair at the customer site and the other at the ISP site. There is no phone dialing required since the connection is direct. Also the only protocol needed to complete the access is TCP/IP, for much the same reason. Depending on the transfer rate required and the distance between the sites, cabling between them can be made with fiber optic cables or unshielded twisted-pair (UTP) copper wire.

Cable Modem 

One more way of accessing the Internet currently being developed is the use of cable modems.  These require that you subscribe to a cable service and allow you two-way communication with the Internet at rates between 100K and 30 Mbps.  The cable modem performs modulation and demodulation like any other modem, but it also has a tuner and filters to isolate the Internet signal from other cable signals.  Part of the concern for use of the cable modem is to formulate LAN adapters to allow multiple users to access the Internet.  A medium access control (MAC) standard for sending data over cable is being formulated by the IEEE 802.14 committee.

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What is Packet Switching?

End systems are connected together by communication links.  There are many types of communication links, which are made of different types of physical media, including fiber optics, twisted pair, coaxial cable and radio links. Different links can transmit data at different rates.  The link transmission rate is often called the bandwidth of the link, which is typically measured in bits/second.  The highest the bandwidth, the more is the capacity of the channel.  End systems are not usually directly attached to each other via a single communication link.  Instead, they are indirectly connected to each other through intermediate switching devices known as routers.


A router takes a chunk of information arriving on one of its incoming communication links and forwards that chunk of information on one of its outgoing communication links.  In the jargon of computer networking, the chunk of information is called a packet.  The path that the packet takes from the sending end system, through a series of communication links and routers, to the receiving end system is known as a route or path through the network.  Rather than providing a dedicated path between communicating end systems, the Internet uses a technique known as packet switching that allows multiple communicating end systems to share a path, or parts of a path, at the same time.  Similar to a router, there is another special machine called gateways used in the network that allows different networks to talk to the Internet, which uses TCP/IP.

Packet switching is used to avoid long delays in transmitting data over the network. Packet switching is a technique, which limits the amount of data that a computer can transfer on each turn. Packet switching allows many communications to proceed simultaneously. Each packet contains a header that specifies the computer to which the packet should be delivered and the destination is specified using computer’s address. Computers that share access to a network take turns in sending packets. On each turn, a given computer sends one packet.
 
IP uses this packet switching concept to deliver messages on the Internet If the destination address does not exist on the local network, it is the responsibility of that network’s router to route the message one step closer to its destination. This process continues until the destination machine claims the message packet.

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What are Network Models?

There are two types of networking models available: OSI reference Model and the TCP/IP Network Model for the design of computer network system. In this post we shall look at these models.

OSI (Open System Interconnection) Networking Model

An open system is a model that allows any two different systems to communicate regardless of their underlying architecture.  The purpose of the OSI model is to open communication between different devices without requiring changes to the logic of the underlying hardware and software.

The OSI model is not a protocol, it is a model for understanding and designing a network architecture that is inter- operable, flexible and robust. The OSI model has a seven-layered architecture.  These are:

• Application layer
• Presentation layer
• Session layer
• Transport Layer
• Network layer
• Data link layer
• Physical Layer

Physical layer: 

The physical layer is concerned with sending raw bits between the source and destination nodes over a physical medium. The source and destination nodes have to agree on a number of factors. 

• Signal encoding: how are the bits 0 and 1 to be represented?
• Medium: what is the medium used and its properties?
• Bit synchronization: is the transmission synchronous or asynchronous?
• Transmission type: whether the transmission is serial or parallel?
• Transmission mode: is the transmission simplex, half-duplex or full duplex?
• Topology: what is the network topology i.e. star, mesh, ring or bus?

Data link layer:

The data link layer is responsible for transmitting a group of bits between the adjacent nodes. The group of bits is known as frame. The network layer passes a data unit to the data link layer and data link layer adds the header information to this data unit. The data link layer performs the following functions:   

Addressing:

Headers and trailers are added containing the physical addresses of the adjacent nodes and removed on a successful delivery.

Framing:

Grouping of/bits received from the network layer into manageable units called frame

Flow control:

To regulate the amount of data that can be sent to the receiver.

Media access control (MAC):

Who decide who can send data, when and how much.

Synchronization:

This layer also contains bits to synchronize the timing to know the bit interval to recognize the bit correctly.

Error control:

It incorporates the CRC to ensure the correctness of the frame. Node to node 

delivery:

It is also responsible for error-free delivery of the entire frame/packet to the next adjacent node. 

Network layer:

The network layer is responsible for routing a packet within the subnet that is,  from source to destination nodes across multiple nodes in the same network or across multiple networks. This layer also ensures the successful delivery of a packet to the destination node. The network layer performs the following functions:

Routing:

To find the optimal route 

Congestion control:

Which is based on two approaches (i) Increase on the resources (ii) Decrease the word.

Transport layer:

This layer is the first end-to-end layer. Header of the transport layer contains information that helps send the message to the corresponding layer at the

destination node. The message is broken into packets and may travel through a number of intermediate nodes. This layer takes care of error control and flow control both at the source and destination for the entire message. The responsibilities of the transport layer are: 

• Host-to-host message delivery 
• Flow Control 
• Segmentation and reassembly 

Session layer:

The main functions of this layer are to establish, maintain and synchronize the interaction between two communication hosts. It makes sure that once a session is established it must be closed gracefully. It also checks and establishes connections between the hosts of two different users. The session layer also decides whether both users can send as well as receive data at the same time or whether only one host can send and the other can receive. The responsibilities of session layer are:
    Sessions and sub sessions: this layer divides a session into sub session for avoiding retransmission of entire message by adding the checkpoint feature.
    Synchronization: this layer decides the order in which data needs to be passed to the transport layer.
     Dialog control: this layer also decides which user application sends data and at what point of time and whether the communication is simplex, half duplex or full duplex.
     Session closure: this layer ensures that the session between the hosts is closed gracefully. 

Presentation layer: 

When two hosts are communicating with each other they might use different coding standards and character sets for representing data internally. This layer is responsible for taking care of such differences. This layer is responsible for:

• Data encryption and decryption for security.
• Compression.
•  Translation.

Application layer: 

It is the topmost layer in the OSI model, which enables the user to access the network. This layer provides user interface for network applications such as remote login, World Wide Web and FTP. The responsibilities of the application layer are:

• File access and transfer 
• Mail services 
• Remote lo-gin 
• World Wide Web

TCP/IP Networking Model: 

TCP/IP is an acronym for Transmission Control Protocol / Internet Protocol. TCP/ IP is a collection of protocols, applications and services. TCP/IP protocol were developed prior to the OSI model therefore its layers do not match with the OSI model.

The TCP/IP protocol suit is made of the five layers: Physical, data link, network, transport & application. The first four layers provide physical standards network interface, inter-networking and transport mechanism whereas the last layer comprises of the functionalities of the three topmost layers in the OSI model.

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WHAT IS INTERNET?

INTRODUCTION TO INTERNET

The Internet is worldwide computer network that interconnects, million of computing devices throughout the world.  Most of these devices are PC’s, and servers that store and transmit information such as web pages and e-mail messages.  Internet is revolutionizing and enhancing the way we as humans communicate, both locally and around the globe. Everyone wants to be a part of it because the Internet literally puts a world of information and a potential worldwide audience at your fingertips.

The Internet evolved from the ARPANET (Advanced Research Projects Agency) to which other networks were added to form an inter network. The present Internet is a collection of several hundred thousand of networks rather than a single network. From there evolved a high-speed backbone of Internet access for sharing these of networks. The end of the decade saw the emergence of the World Wide Web, which heralded a platform-independent means of communication enhanced with a pleasant and relatively easy-to-use graphical interface.

World Wide Web is an example of an information protocol/service that can be used to send and receive information over the Internet. It supports:

• Multimedia Information (text, movies, pictures, sound, programs . . . ).
• Hyper Text Information (information that contains links to other information resources).
• Graphic User Interface (so users can point and click to request information instead of typing in text commands).

 The World Wide Web model follows Client/Server software design. A service that uses client/server design requires two pieces of software to work: Client Software, which you use to request information, and Server Software, which is an Information Provider.The server software for the World Wide Web is called an HTTP server (or informally a Web server). Examples are Mac HTTP, CERN HTTP, and NCSA HTTP. The client software for World Wide Web is called a Web browser. Examples are: Netscape, and Internet Explorer.

OBJECTIVES

After reading this article, you will be able to understand:

• Make classification of networks;
• understand two types of networking models;
• understand the concept of packet switching;
• understand how to access to the internet;
• list the services available on Internet; and
• understand how does the Internet works. 

CLASSIFICATION OF NETWORKS:

There are different approaches to the classification of compute Networks.  One such classification is based on the distance approach.  In this section we will discuss such networks.

The networks can be classified into LAN, MAN and WAN networks. Here, we describe them into brief to understand the difference between the types of network.

Local Area Network (LAN)

LAN is a privately - owned computer networks confined to small geographical area, such as an office or a factory widely used to connect office PCs to share information and resources. In a Local area network two or more computers are connected by same physical medium, such as a transmission cable. An important characteristic of Local Area networks is speed. i.e. they deliver the data very fast compared to other types of networks with typical data transmission speed are 10-100 Mbps.

A wide variety of LANs have been built and installed, but a few types have more recently become dominant.  The most widely used LAN system is the Ethernet system. Intermediate nodes (i.e. repeaters, bridges and switches) allow LANs to be connected together to form larger LANs.  A LAN may also be connected to another LAN or to WANs and MANs using a “router”.

In summary, a LAN is a communications network, which is:

• local (i.e. one building or group of buildings)
• controlled by one administrative authority
• usually high speed and is always shared

LAN allows users to share resources on computers within an organization.

Metropolitan Area Network (MAN)

A MAN, basically a bigger versions of a LAN is designed to extend over an entire city.  It may be single network such as a cable television network, or it may be a means of connecting a number of LANs into a large network so that resources may be shared for example, a company can use a MAN to connect the LANs in all of its offices throughout a city.

A MAN typically covers an area of between 5 and 50 km diameter. Many MANs cover an area the size of a city, although in some cases MANs may be as small as a group of buildings

The MAN, its communications links and equipment are generally owned by either a consortium of users or by a single network provider who sells the service to the users.  This level of service provided to each user must therefore be negotiated with the MAN operator, and some performance guarantees are normally specified.

A MAN often acts as a high-speed network to allow sharing of regional resources (similar to a large LAN).  It is also frequently used to provide a shared connection to other networks using a link to a WAN.

Wide Area Network (WAN)

The term Wide Area Network (WAN) usually refers to a network, which covers a large geographical area, and use communications subnets (circuits) to connect the intermediate nodes.  A major factor impacting WAN design and performance is a requirement that they lease communication subsets from telephone companies or other communications carriers.  Transmission rates are typically 2 Mbps, 34 Mbps, 155 Mbps, 625 Mbps (or sometimes considerably more). The basic purpose of the subnet is to transmit message from one end to another end through intermediate nodes.

In most WAN a subnet consists of two types of elements:

1. Transmission lines.
2. Switching element.

Transmission lines also called channels move about from one machine to another machine. The basic purpose of the switching element is to select the outgoing path for forwarding the message.

Numerous WANs have been constructed, including public switched networks, large corporate networks, military networks, banking networks, stock brokerage networks, and airline reservation networks. A WAN is wholly owned and used by a single company is often referred to as an enterprises network.

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Every new blogger or some other thinks a lot to get more traffic or visitors for their blogs or websites. In order to get more traffic or visitors to your blog or website, there are lot of services or traffic exchange sites which gives or sends visitors to your sites. There may be some paid traffic exchange sites. I think many of you have already gone through all these tips:

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Choosing the wrong niche

Just like entering a crowded marketplace can spell doom for a blog, blogging about a topic that there is little interest in won’t let you go either.I’m able to say that you simply won't extremely apprehend till you are attempting, however if your topic isn't mentioned anyplace else on-line – no blogs, no websites, no forums, nothing – don’t assume it is an unused niche. It could be that there just is not an audience.

Influenced by money

If you just blog for the money, as you head into month three of your blog’s life and you have yet to earn a penny, chances are you won’t be there in month six when you are still not making anything.Focus your motivation on enjoying the process of writing about a topic you care about and interacting with others who share your interest. You will have more sticking power and you will enjoy the journey. You will keep at it long enough to be financially rewarded as well. 

Do not to start a blog about making money from blogging or making money online, unless you already make very good money from blogging or Internet business and can present this information in a way that is not already being done by someone else(unique writing style, unique life experiences, etc). The same rule applies to any topic that is already saturated with established experts. It is okay to blog about a topic you enjoy and may not be an expert in, but if there are a thousand similar blogs to yours the only ones that will do well are those with credibility. Credibility can be established with proof or through the quality of the information provided, which generally comes only from people with experience. If this isn’t you, don’t make your life hard. Move on and find a topic you can dominate.

Jumping from blog to blog

I am amazed how many good bloggers out there have a blog that is doing well and then suddenly decide to start another one and divide their output across two blogs. Or even worse, they shut down their good blog right when it’s just gathering momentum to go start another. If you want to be in the blogging game long-term you have to focus long enough on one blog to give it a chance to succeed.

Thinking blogging is easy money

Many of the people have entered the blogs-world thinking that with an hour or two a day they can become famous and earn several thousand dollars a month or more. I support part-time blogging for a full-time income, but that is not an overnight end result. If you think it will happen to you within a week, a month or even six months, you are sure going to be disappointed with it. It takes work, particularly at the beginning to make a winning blog, however the rewards create it worthy. If your timetable is presently full and you're not ready to create space for normal and that I mean regular work on your blog, you almost certainly shouldn’t jump into blogging simply however.

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Just try giving any keyword in Google Search Engine and you will acquisition file-names accent with the keyword you accept given. So it artlessly proves that your File-name should accept adapted keyword.

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• Who would be your ambition audience?
• What you intend to advertise to them. Is it a actual account or just argument content?
• What will accomplish your business abstraction altered or altered than aggregate abroad that is already on the market?

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Why am I doing this? What square measure my goals? What do i would like to achieve?

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