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Computer Networking





Networking allows various intelligent devices like computers, PDA, mobile phones to communicate with each other either through physical wires or wireless signals.


When sending emails, devices like routers, act like postmen, who look at the IP address the email address originated from and they then forward it to the destination IP address, who is the actual intended recipient of the email. Networking basically explains how these sequences of events occur in real time world and how the data packets reach their destination.


Software and Networking are inter-dependant. A Client Server application created by a software developer can run only on a network, formed by your hardware guy, whose majority of the time goes into tripping about CAT5 cables and sorting them out. The network operating system used by the system administrator is actually the successful execution of the code of a software developer, at runtime.


An internet work is a collection of individual networks, connected by intermediate networking devices, that functions as a single large network. The networking devices are the vital tools for communication. Whenever you have a set of computers or networking devices to be connected, you make the connections, depending on the physical layout and your requirements.



The most popular types of networks

LAN (Local Area Network)
A LAN covers a small region of space, typically a single networked office building school or home and enables multiple users to exchange files and messages, as well as access shared resources such as printers and file servers.


MAN (Metropolitan Area Network)
A MAN is a collection (interconnection) of various LANs within the same geographical area, for instance a city, typically owned an operated by a single entity such as a government body or large corporation.


WAN (Wide Area Network)
A WAN is a long distance network, interconnecting various systems (LAN’s) together throughout a country or even beyond the border, through telephone network, which unites geographically distributed users. The Internet is the largest WAN, spanning the Earth.



Other types of area networks

While LAN, WAN and MAN are by far the most popular network types mentioned, you may also commonly see references to these others:

Wireless Local Area Network - a LAN based on WiFi wireless network technology.


Campus Area Network - a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus.


Storage Area Network - connects servers to data storage devices through a technology like Fibre Channel.


System Area Network - links high-performance computers with high-speed connections in a cluster configuration. Also known as Cluster Area Network.


LAN, WAN and Home Networking

Residences typically employ one LAN and connect to the Internet WAN via an Internet Service Provider (ISP) using a broadband modem. The ISP provides a WAN IP address to the modem, and all of the computers on the home network use LAN (so-called private) IP addresses. All computers on the home LAN can communicate directly with each other but must go through a central gateway, typically a broadband router, to reach the ISP.



Internetworking evolved as a solution to three key problems

Isolated LANs, Duplication of resources and a Lack of network management. Isolated LANS made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did a separate support staff.

This lack of network management meant that no centralized method of managing and troubleshooting networks existed.




Communication and exchange of information between computers is made possible by using a set of rules and conventions known as protocols. The Internet runs on the TCP/IP protocol.


A wide variety of communication protocols exist, but all tend to fall into one of the following groups:

LAN protocols - define communication over the various LAN media and operate at the network and data link layers of the OSI model.


WAN protocols - define communication over the various wide-area media and operate at the lowest three layers of the OSI model.


Routing protocols - are responsible for path determination and traffic switching and are network-layer protocols.


Network protocols - are the various upper-layer protocols that exist in a given protocol suite.






The entire Net runs on a set of rules. Rules defining a technology are known as protocols. They serve as a guiding factor for the technology to build upon. The Internet runs on the TCP/IP protocol.


What is TCP/IP?

TCP/IP refers to two network protocols (or methods of data transport) used on the Internet. They are Transmission Control Protocol (TCP) and Internet Protocol (IP), respectively. These network protocols belong to a larger collection of protocols, or a protocol suite. These are collectively referred to as the TCP/IP suite. Protocols within the TCP/IP suite work together to provide data transport on the Internet. In other words, these protocols provide nearly all services available to today's Net surfer. Some of those services include :

• Transmission of electronic mail.
• File transfers.
• Usenet news delivery.
• Access to the World Wide Web. In short: TCP/IP refers to a

collection of protocols that facilitate communication between machines over the Internet (or other networks running TCP/IP).



There are two classes of protocol within the TCP/IP suite


1 - Network-Level Protocols

Network-level protocols manage the discrete mechanics of data transfer. These protocols are typically invisible to the user and operate deep beneath the surface of the system. For example, the IP protocol provides packet delivery of the information sent between the user and remote machines. It does this based on a variety of information, most notably the IP address of the two machines. Based on this and other information, IP guarantees that the information will be routed to its intended destination. Throughout this process, IP interacts with other network-level protocols engaged in data transport. Short of using network utilities (perhaps a sniffer or other device that reads IP datagrams), the user will never see IP's work on the system.

2 - Application-Level Protocols

Conversely, application-level protocols are visible to the user in some measure. For example, File Transfer Protocol (FTP) is visible to the user. The user requests a connection to another machine to transfer a file, the connection is established, and the transfer begins. During the transfer, a portion of the exchange between the user's machine and the remote machine is visible (primarily error messages and status reports on the transfer itself, for example, how many bytes of the file have been transferred at any given moment).


How does TCP/IP Work?

TCP/IP operates through the use of a protocol stack. This stack is the sum total of all protocols necessary to complete a single transfer of data between two machines. (It is also the path that data takes to get out of one machine and into another.)


To understand TCP/IP (Transmission Control Protocol/Internet Protocol), you must also consider its individual components and their functions. When combined into a suite of protocols and applications, the overall purpose of TCP/IP is to provide a structure for transmitting data across the vast majority of networks worldwide as they interconnect to compose the Internet. Here is an overview of the key components of TCP/IP and their primary purpose.



Routers act as gateways among networks and plan a path through networks so data packets reach their final destination in the most efficient manner possible.


IP (Internet Protocol)

 IP is the protocol that controls how data travels from one computer to another on separate networks. It breaks data down into packets, establishes rules for packet delivery, and directs the routing of data packets among networks until all the packets reach their destination.


IP Addresses & ARP (Address Resolution Protocol)

IP addresses identify the destination’s specific network, subnetwork, and computer. ARP helps host networks process IP addresses and use them to find the physical hardware locations that correlate to the virtual IP addresses.


TCP (Transmission Control Protocol)

TCP is needed to guarantee reliable data delivery. Once data packets reach their destination, TCP guarantees that all packets arrive safely in the proper sequence. If anything is awry, TCP requests retransmission. TCP then assembles the packets in the correct order as originally intended for the corresponding application.




OSI Model


Open Systems Interconnection (OSI) Model

The OSI model is the basic model describing the data movement through a network. This reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. It is a conceptual model composed of seven layers, each specifying particular network functions.


The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers.


When trying to device a system, it is not necessary that all these layers have to be used. A system can also make away with any of the 7 layers like say the Session Layer or the Application Layer.


The seven layers of the Open System Interconnection (OSI) reference model


Layer 7 - Application layer - Network process to application (User Interface)

Communication between programs. This is the layer that user programs talk to. This is where most Industrial Automation applications are run. Serial encapsulation of older protocols, ModbusTCP, Ethernet / IP and others operate at this level.

Layer 6 - Presentation layer - Data representation and encryption (Translation)

Data representation conversions. This layer translates data, between what the network requires and what the computers or industrial devices at each end expect. This is mostly used by the computers or equipment that present the data in a graphical way.

Layer 5 - Session layer - Interhost communication (syncs and sessions)

Establishes and maintains communications channels, so program on different devices can establish a link. Often combined with the Transport Layer and may be different for each Industrial Ethernet Protocol.

Layer 4 - Transport layer - End-to-end connections and reliability (flow control & error-handling) (TCP)

Responsible for end-to-end data transmission integrity. Makes sure that the data actually gets there, with no errors, in the right order, regardless of transmission problems.

Layer 3 - Network layer - Path determination (routing) and logical addressing (IP)

Routes data from one network node to another. This layer translates "logical" device names and addresses into their network hardware equivalents, and does Routing, if necessary, for devices that are more than one network link away. Used in Wide Area Networks WAN applications.

Layer 2 - Data Link layer - Physical addressing (data frames to bits)

Takes care of moving data from one network node to another which are all within the same Local Area Network LAN. Used with Layer 2 switches and can run other protocols such as Siemens SinecHl.

Layer 1 - Physical layer - Media, signal and binary transmission (raw bit stream).

Translates the bits generated by all the other layers into signals to send through the network, and translates them back into bits at the other end.


Basically in OSI layered protocol structure, we have two layers


1 - Upper Layer – Comprising of Sessions, Presentation and Applications Layers.

This layer basically deals with the application issues (basically software). Application Layer is the closest to the end user. For e.g. Applications like Microsoft Word, Powerpoint on which we work can be said to be closer to the end user like us.

2 - Lower Layer – Comprising of the Physical, Data Link, Network and Transport Layers.

This layer deals with the data transport issues. The Physical and Data Link layers basically deal with hardware and software. Other Lower Layers are generally implemented using software.




TCP/IP Model


The TCP/ IP model superseded the OSI model and has become firmly established as the dominant commercial architecture and as the protocol suite upon which the bulk of new protocol development is to be done. The various layers in the TCP/IP model are:

Layer 5 - Application
Layer 4 - Transport
Layer 3 - Internet work
Layer 2 - Data Link
Layer 1 - Physical

Note that the Sessions and Presentation Layer present in the OSI Model are absent in the TCP/IP Model.


There are a number of reasons for the success of the TCP/IP model over the OSI model:
Internet is built on the foundation of the TCP/IP suite. The tentacles of the Internet and the World Wide Web have spread throughout the world and that is the main reason for the success of TCP/IP model over the OSI model.


TCP/IP protocols were initially researched under a project in the Department of Defense (DOD). DOD was committed to international standards and most of its operational requirements couldn’t be met by the OSI model. So it started to develop the TCP/IP. Since the DOD is the largest consumer of software products in the world, the vendors were encouraged to develop TCP/IP based products.


The Internet Protocol (IP) Suite

The IP suite uses encapsulation to provide abstraction of protocols and services. Generally a protocol at a higher level uses a protocol at a lower level to help accomplish its aims. The Internet protocol stack can be roughly fitted to the four layers of the original TCP/IP model:




Routing protocols like BGP, which for a variety of reasons run over TCP, may also be considered part of the application or Network Layer.

3-Transport TCP, UDP, DCCP, SCTP, IL, RUDP

Routing protocols like OSPF, which run over IP, are also to be considered part of the network layer, as they provide path selection. ICMP and IGMP run over IP are considered part of the network layer, as they provide control information.

IP (IPv4, IPv6)

ARP and RARP operate underneath IP but above the link layer so they belong somewhere in between.

1-Network access Ethernet, Wi-Fi, token ring, PPP, SLIP, FDDI, ATM, Frame Relay, SMDS




Today, most commercial operating systems include and install the TCP/IP stack by default. For most users, there is no need to look for implementations. TCP/IP is included in all commercial Unix systems, MAC OS X, and all free-software Unix-like systems such as Linux distributions and BSD systems, as well as Microsoft Windows.




This material is furnished on an "as is" basis. The provider makes no warranties of any kind, either expressed or implied as to any matter including, but not limited to, warranty of fitness for a particular purpose or merchantability, exclusivity or results obtained from use of the material. The provider does not make any warranty of any kind with respect to freedom from patent, trademark, or copyright infringement.


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