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Networking - Introduction, Classification of Computer Network

Overview of Networking
The term "networking" applies to either,
(1) The exchange of information among individuals, groups, or institutions, or
(2) The process of electronic voice or data communications
This chapter introduces the basic idea of the computer networks.
In this we tell the process of connecting computers and show how data moving from one computer to another involves using a common set of rules and governs how computer communicates with each other.
In this we also covers two internationally recognized the whole data moving and communication processes.
What is Network
Put simply, a network is a connection between at least two computers usually by cable, running software which allows them to communicate with one another.
Users over a network can share computer resources such as hard drives, printers, modems, CD-ROM drives and processors.
Although most networks are more complicated than this two-computer scenario, all networks are based on the concept of sharing.
There's actually a great work of technology involved when one computer connects to and communicates with another.
In addition, there are many types of physical connections and related software to consider.
In the following sections, we discuss some fundamental concepts behind all networks, and explain what kinds of network models are appropriate for various business environments.
When the computers connected to a network are all close together, such as in the same building or campus, the network is called a Local Area Network (LAN).
A local area network (LAN) is a group of computers and associated devices that share a common communications line or wireless link.
Typically, connected devices share the resources of a single processor or server within a small geographic area (for example, within an office building).
Usually, the server has applications and data storage that are shared in common by multiple computer users.
A local area network may serve as few as two or three users (for example, in a home network) or as many as thousands of users (for example, in an FDDI network).


The metropolitan area network (MAN) is a large-scale network that connects multiple corporate LANs together.
MANs usually are not owned by a single organization; their communication devices and equipment are usually maintained by a group or single network provider that sells its networking services to corporate customers.
MANs often take the role of a high speed network that allows for the sharing of regional resources. MANs also can provide a shared connection to other networks using a WAN link
If the network uses long-range telecommunications links allowing the computers to be spread out over long distances, the network is referred to as a Wide Area Network (WAN).
A WAN spans a large geographic area, such as a state, province or country. WANs often connect multiple smaller networks, such as local area networks (LANs) or metro area networks (MANs).
Types of WANs:
A centralized WAN consists of a central computer that is connected to dumb terminals and / or other types of terminal devices.
A distributed WAN consists of two or more computers in different locations and may also include connections to dumb terminals and other types of terminal devices.
The world's most popular WAN is the Internet.
Components of Network
All networks need a base station. And the type of network you choose - wireless, wired or a combination of both - determines which type of base station you need.
A wireless access point -
enables everyone in the home or small office to effortlessly connect to the network without running wires.
An Ethernet router-
the basis of a wired network, a router lets everyone enjoy the speed of a wired connection.
A combination wireless access point/Ethernet router -
allows different users to take advantage of both technologies at the same time.
For sharing high speed Internet access, you'll also need a cable or DSL broadband modem. Connect your base station to the broadband modem and then you are ready to connect your computers (and peripherals) to the base station.
. If it's a wired network, you'll connect everything with Ethernet cables. Please make sure that the devices you want to network are Ethernet-equipped with an appropriate Ethernet adapter.
. If it's a wireless network, you'll need to add one wireless accessory to each computer.
From above figure, the component of network
1) Network Adapter Cables
Typically, network adapter cables connect networked computers. However it is possible for a wireless connection to exist between computers on a network.
2) Network Adapters-
Network Adapters must be installed in each computer on a network.
3) Workstation-
a network must have at least one computer, sometimes known as a workstation that accesses the shared resources.
4) Server-
a network must have at least one computer that can act as a server to share its resources.
5) Disk files-
Computers on a network can share resources, such as disk files. However, shared disk files are not required to build.
6) Workstation and Server-
it is possible for a computer to behave as both a workstation and a server that is a computer can both share and use resources.
Wired or Wireless or both
The three different types of networks meet different needs you may have.
Wireless: Easy and convenient
With wireless, there's no need to drill holes or run wires to join the network. Just install the software on your desktop or laptop, add a wireless accessory, and you're ready to go.
Wireless users can travel from room to room, between floors, or even outside, almost anywhere, and remain connected.
Wired: Built for Speed
Wired networks are a must when fast data transfers are your primary concern.
Wired networks really zoom - supporting data rates at up to 100 Mbps.1
1 They're easy to set up and simple to use, offering a combination of performance and consistent coverage in any area.

Wired and Wireless: The Best of Both Worlds

With both wireless and wired users on the same network, you can pick and choose which users take advantage of which technology.
Networking Models
In a network we works over a numbers of machines, adapters etc., that are manufactured by different- different vendors.
That's why we need some standards. Standards organization such as the International Standards Organization (ISO)
the Institute of Electrical and Electronics Engineers, Inc (IEEE) have developed models that have become globally recognized and accepted as standards for designing Computer Networks.
The OSI Model-
As the concept of networking became more widespread in the business world, the idea of being able to connect networks and disparate systems became a necessity.
For this type of communication to take place, however, there needed to be a standard.
The solution came in 1878 when the ISO released an architecture that would allow achieve this goal.
These specifications were revised in 1884 and became international standards for networked communication and connections.
it is important for network administrators to know the history and understand the function of this specification, which is called the OSI Reference Model.
The OSI model presents a layered approach to networking. Each layer of the model handles a different portion of the communications process.
By separating such communications into layers, the OSI model simplified how network hardware and software work together, as well as eased troubleshooting by providing a specific method for how components should function.
Now that we know why the model was implemented, let's move on to explore just how it works. Keep in mind that the OSI model is a completely conceptual reference.
The OSI model
2.Data link
Layers in the OSI model-
Physical Layer:
Layer 1:
The Physical layer converts bits into signals (like electromagnetic pulse etc.) for outgoing messages, and signals into bits for incoming ones.
This layer arranges the transmission of a data frame's bits when they are dispatched across the network.
The Physical layer manages the interface between a computer and the network medium, and instructs the driver software and the network interface as to what needs to be sent across the medium.
Physical layer is concerned with the following.
1) Physical characteristic of interfaces and media.
2) Representation of bits (encoding).
3) Data rate (Transmission rate).
4) Synchronization of bits.
5) Physical topology.
6) Transmission mode.
Data Link Layer
Layer2 is the data link layer.
This layer places data frames from the network layer onto the physical medium, the data link layer is responsible for providing the error free transfer of these frames from one computer to another through the physical layer.
Destination ID Sender ID Control Data CRC
Destination ID-
the unique identifier of the workstation to which the information is being sent.
Sender ID-
The sender ID contains the unique identifier of the workstation sending the information.
The control information is used for frame type, routing, and segmentation information.
The data is the information itself.
The cyclical redundancy check (CRC) contains error correction and verification information to ensure that the data frame is received properly.
Specific responsibilities of the data link layer include the following:
1. Framing
2. Physical addressing
3. Flow control
4. Error control
5. Access control
Network Layer-
Layer3, the network layer, is responsible for addressing messages and translating logical addresses and names into physical addresses.
This layer determines which path the data should take from the source to the destination computer based on network conditions priority of service, and other factors.
Specification responsibilities of the network layer include the following
1) Logical Addressing
2) Routing
Transport Layer
The transport layer ensures that messages are delivered error free, in sequence and with no losses or duplications.
Layer 4 repackages messages dividing long messages into several packets and collecting messages together in one package, to provide for their efficient transmission over the network.
Responsibilities of Transport layer
1) Service-point addressing.
2) Segmentation and reassembly.
3) Connection control
4) Flow control
5) Error control.
Session Layer
Layer 5, The Session layer allows two networked resources to hold ongoing communications, called a session, across a network.
In other words, applications on each end of the session are able to exchange data for the duration of the session (connection).
This layer manages session setup, information or message exchanges, and turn off when the session ends.
It is also responsible for identification so that only designated parties can participate in the session, and handles security services for controlling access to session information.
The Session layer furnishes synchronization services between tasks at each end of the session. This layer places checkpoints in the data stream so if communications fail, only data after the most recent checkpoint need be retransmitted.
The Session layer also manages issues such as who may transmit data at a certain time and for how long, and maintains a connection through transmission of messages that keep the connection active; these messages are designed to keep the connection to be closed down due to inactivity.
Works of Session layer
1) Dialog control.
2) Synchronization.
Presentation Layer
Layer 6, The Presentation layer manages data-format information for networked communications.
. Also called the network's translator, it converts outgoing messages into a generic format that can be transmitted across a network; then, it converts incoming messages from that generic format into one that makes sense to the receiving application.
This layer is also responsible for protocol conversion, data encryption and decryption, and graphics commands.
Information sent by the Presentation layer may sometimes be compressed to reduce the amount of data to be transferred (this also requires decompression on the receiving end).
It is at this layer that a special software facility known as a redirector operates.
The redirector intercepts requests for service and redirects requests that cannot be resolved locally to the networked resource that can handle them.
A utility called the redirector operates at this level. Its purpose is to redirect Input/Output operations to resources outside the local computer.
Works of presentation layer
1) Translation.
2) Encryption.
3) Compression.
Application Layer
Layer 7 of the OSI model, The Application layer is referred to as the top layer of the OSI Reference Model.
This layer allows access to network services-such as networked file transfer, message handling, and database query processing-that support applications directly.
This layer also controls general network access, the transmission of data from sender to receiver (called flow control), and error recovery for applications when appropriate.
Works of Application layer
1) Network virtual terminal.
2) File transfer, access and management.
3) Mail services.
4) Directory services.
In the OSI model the purpose of each layer is to provide services to the next higher layer and shield the upper layer from the details of how the services are actually implemented.
Relationship between OSI Model layers.
The layers are setup in such a way that each layer acts as though it were communicating with its associated layer on the other computer.
This is logical communication between peer layers.
Physical communication takes place between adjacent layers on one computer. Only the lowest layer in the networking model can pass information directly to its counterpart on one computer.
Information on the sending computer passes through all the lower layers.
The information that moves across the networking cable to the receiving computer and up that computer's networking layers until arriving at the same level that sent the information.
An exchange of data using the OSI Model:
As data passes from the application through the OSI layers, each layer wraps the data with layer-specific information.
This information, in the form of headers and/or trailers, is read later by the corresponding layer on the receiving computers.
The physical layer adds header information called the frame preamble to the outside of the frame and adds trailer information called the frame post amble to the outside of the frame, but it does not add to the data with in the frame.
Following figure gives an overall view of the OSI layer, l7 data means the data unit at layer 7, then moves from layer to layer in descending, sequential order at each layer (except layer 7 and 1), a header is added to the data unit , when the formatted data unit passes through the physical layer (layer 1),.
It is changed into an electromagnetic signal and transported along a physical link.
Upon reaching its destination, the signal passes into digital form.
The data units then move backup through the OSI layers. As each block of data reaches the next higher layer.
The headers and trailers attached to it at the corresponding sending layer are removed.
IEEE 802 Model:
Another networking model was developed by the Institute of Electrical and Electronic Engineer Inc.
IEEE's project 802 defined LAN standards for the physical and data link layers of the OSI midel.the 802 project divided the Data-Link layer into the logical link control(LLC) and the Media access control (MAC) sublayers.
The 802 specifications fall into 12 different categories, each of which has its own number, as described in the following:
802.1 Internetworking
802.2 Logical Link Control (LLC)
802.3 Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) LANs (Ethernet)
802.4 Token Bus LAN
802.5 Token Ring LAN
802.6 Metropolitan Area Network (MAN)
802.7 Broadband Technical Advisory Group
802.8 Fiber Optic Technical Advisory Group
802.8 Integrated Voice and Data Networks
802.9 Network Security
802.10 Wireless Networks
802.12 Demand Priority Access LAN, 90BaseVG-AnyLAN
The LLC sublayer manages data link communication and defines the use to transfer information from the LLC sublayer to the upper OSI layers.
The MAC sublayer provides shared access between the computers' network adapters and the physical layer.
The MAC sublayer is responsible for delivering error-free data between computers on a network.
The Internet
Networks have become a fundamental part of today's information systems.
They form the backbone for information sharing in enterprises, governmental and scientific groups.
That information can be in several forms. It can be notes and documents, data to be processed by another computer, files sent to colleagues, and even more exotic forms of data.
Most of these networks were installed in the late 60s and 70s, when network design was the "state of the art" topic of computer research and sophisticated implementers.
From the early 70s on, another aspect of networking became important: protocol layering, which allows applications to communicate with each other.
A complete range of architectural models were proposed and implemented by various research teams, organizations and computer manufacturers.
The result of all this great know-how is that today any group of users can find a physical network and an architectural model suitable for their specific needs.
This ranges from cheap asynchronous lines with no other error recovery than a bit-per-bit parity function, through full-function wide area networks (public or private) with reliable protocols such as public packet-switching networks or private SNA networks, to high-speed but limited-distance local area networks.
The down side of this exploding information sharing is the rather painful situation when one group of users wants to extend its information system to another group of users who happen to have a different network technology and different network protocols.
As a result, even if they could agree on a type of network technology to physically interconnect the two locations, their applications (such as mailing systems) still would not be able to communicate with each other because of the different protocols.
This situation was recognized rather early (beginning of the 70s) by a group of researchers in the U.S. who came up with a new principle: internetworking.
Other official organizations became involved in this area of interconnecting networks, such as ITU-T and ISO.
All were trying to define a set of protocols, layered in a well-defined suite, so that applications would be able to talk to other applications, regardless of the underlying network technology and the operating systems where those applications run.
What exactly is the Internet? First, the word internet (also internetwork) is simply a contraction of the phrase interconnected network.
However, when written with a capital "I" the Internet refers to a worldwide set of interconnected networks, so the Internet is an internet, but the reverse does not apply.
The Internet is sometimes called the connected Internet.
The Internet consists of the following groups of networks
. Backbones:
large networks that exist primarily to interconnect other networks. Currently the backbones are NSFNET in the US, EBONE in Europe.
. Regional networks
connecting, for example, universities and colleges.
. Commercial networks
providing access to the backbones to subscribers, and networks owned by commercial organizations for internal use that also have connections to the Internet.
. Local networks,
such as campus-wide university networks. In many cases, particularly for commercial, military and government networks, traffic between these networks and the rest of the Internet is restricted
The ARPANET was built by DARPA (which was called ARPA at that time) in the late 60s to accommodate research equipment on packet-switching technology and to allow resource sharing for the Department of Defense's contractors.
It soon became popular with researchers for collaboration through electronic mail and other services.
It was developed into a research utility run by the Defense Communications Agency (DCA) by the end of 1875 and split in 1883 into MILNET for interconnection of military sites and ARPANET for interconnection of research sites
This formed the beginning of the "capital I" Internet. In 1874, the ARPANET was based on 56 Kbps leased lines that interconnected packet-switching nodes (PSN) scattered across the continental U.S. and western Europe.
These were minicomputers running a protocol known as 1822 and dedicated to the packet-switching task.
Each PSN had at least two connections to other PSNs (to allow alternate routing in case of circuit failure) and up to 22 ports for user computer (host) connections.
These 1822 systems offered reliable, flow-controlled delivery of a packet to a destination node.
This is the reason why the original NCP protocol was a rather simple protocol. It was replaced by the TCP/IP protocols.
This 1822 protocol did not become an industry standard, so DARPA decided later to replace the 1822 packet switching technology with the CCITT X.25 standard.
Data traffic rapidly exceeded the capacity of the 56 Kbps lines that made up the network, which were no longer able to support the necessary throughput.
Today the ARPANET has been replaced by new technologies in its role of backbone on the research side of the connected Internet whereas MILNET continues to form the backbone of the military side.
. The ARPANET was built by DARPA (which was called ARPA at that time) in the late 60s.
. The ARPANET was based on 56 Kbps leased lines
. The ARPANET ran over a protocol known as 1822.
. These 1822 systems offered reliable, flow-controlled delivery of a packet to a destination node.
. DARPA decided later to replace the 1822 packet switching technology with the CCITT X.25 standard.
NSFNET, the National Science Foundation Network, is a three-level
. The backbone:
a network that connects separately administered and operated mid-level networks and NSF-funded supercomputer centers
The backbone also has transcontinental links to other networks such as EBONE, the European IP backbone network.
. Mid-level networks:
of three kinds (regional, discipline-based and supercomputer consortium networks).
. Campus networks:
whether academic or commercial, connected to the mid-level networks.
First Backbone , established by the National Science Foundation (NSF) as a communications network for researchers and scientists to access the NSF supercomputers, the first NSFNET backbone used five supercomputers as packet switches, interconnected by 56 Kbps leased lines.
A primary interconnection between the NSFNET backbone and the ARPANET existed at Carnegie Mellon, which allowed routing of datagrams between users connected to each of those networks.
In 1888, the NSFNET backbone circuits topology was reconfigured after traffic measurements and the speed of the leased lines increased to T1 (1.544 Mbps) using primarily fiber optics.
Due to the constantly increasing need for improved packet switching and transmission capacities, three NSSs were added to the backbone and the link speed was upgraded.
The migration of the NSFNET backbone from T1 to T3 (45Mbps) was completed in late 1882.
The subsequent migration to gigabit levels has already started and will continue through the late 1880s.
A network or internetwork that is limited in scope to a single organization or entity or, also, a network or internetwork that is limited in scope to a single organization or entity and which uses the TCP/IP protocol suite, HTTP, FTP, and other network protocols and software commonly used on the Internet.
Note: Intranets may also be categorized as a LAN, CAN, MAN, WAN, or other type of network.
A network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g., a company's customers may be provided access to some part of its intranet thusly creating an extranet while at the same time the customers may not be considered 'trusted' from a security standpoint).
Note: Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN, because an extranet must have at least one connection with an outside network.
Intranets and extranets may or may not have connections to the Internet.
If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization.
The Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet.
Commercial Use of the Internet
In recent years the Internet has grown in size and range at a greater rate than anyone could have predicted.
A number of key factors have influenced this growth.
Some of the most significant milestones have been the free distribution of Gopher in 1881, the first posting, also in 1881, of the specification for hypertext and, in 1883, the release of Mosaic, the first graphics-based browser.
Today the vast majority of the hosts now connected to the Internet are of a commercial nature.
This is an area of potential and actual conflict with the initial aims of the Internet, which were to foster open communications between academic and research institutions.
A computer network consists of two or more computers that are connected, usually by physical media, and are running software which enables them to communicate.
A LAN is a Network of computers in close proximity, while a WAN is a computer Network spread out over a wide area, minimum hardware and software components of a LAN include a server, a work station or client, a Network adapter for each computer, media to connect the computers, a network--aware operating system, appropriate network protocol drivers, and network -- aware applications.
The ISO and the IEEE have developed models that are standards for designing computer Networks. The OSI model includes the following layer; Application, Presentation, Session, Transport, Network, Data Link and Physical.
The IEEE 802 model divides the data link layer into two sub layers, the logical link control (LLC) and the Media access control (MAC) sub layer.
Classification of Computer Network
Classification by Network layer
Computer networks may be classified according to the network layer at which they operate according to some basic reference models that are considered to be standards in the industry such as the seven layer OSI reference model and the five layer TCP/IP model.
The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative.
Description of OSI layers
Layer 7: Application layer

OSI Model


Data unit






Network process to application


Data representation and encryption


Interhost communication



End-to-end connections and reliability (TCP)




Path determination and logical addressing (IP)


Data link

Physical addressing (MAC & LLC)



Media, signal and binary transmission

The Application layer provides a means for the user to access information on the network through an application.
This layer is the main interface for the user(s) to interact with the application and therefore the network.
Some examples of application layer protocols include Telnet, applications which use File Transfer Protocol (FTP), applications which use Simple Mail Transfer Protocol (SMTP) and applications which use Hypertext Transfer Protocol (HTTP).
Applications built to use a protocol, such as FTP, should not be confused with the protocols themselves, which often reside at the session layer.
Layer 6: Presentation layer
The Presentation layer transforms data to provide a standard interface for the Application layer.
MIME encoding, data compression, data encryption and similar manipulation of the presentation is done at this layer to present the data as a service or protocol developer sees fit.
Examples: converting an EBCDIC-coded text file to an ASCII-coded file, or serializing objects and other data structures into and out of, e.g., XML.
Layer 5: Session layer
The Session layer controls the dialogues/connections (sessions) between computers
It establishes, manages and terminates the connections between the local and remote application.
It provides for either full-duplex or half-duplex operation, and establishes checkpointing, adjournment, termination, and restart procedures.
The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet protocols suite.
Layer 4: Transport layer
The Transport layer provides transparent transfer of data between end users, thus relieving the upper layers from any concern while providing reliable data transfer.
The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control.
Some protocols are state and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail.
The best known example of a layer 4 protocol is the Transmission Control Protocol (TCP).
The transport layer is the layer that converts messages into TCP segments or User Datagram Protocol (UDP), Stream Control Transmission Protocol (SCTP), etc. packets.
Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatching and classification of mail and parcels sent.
Layer 3: Network layer
The Network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer.
The Network layer performs network routing functions, and might also perform segmentation/desegmentation, and report delivery errors.
Routers operate at this layer-sending data throughout the extended network and making the Internet possible (also existing at layer 3 (or IP) are routers).
This is a logical addressing scheme - values are chosen by the network engineer.
The addressing scheme is hierarchical. The best known example of a layer 3 protocol is the Internet Protocol (IP).
Perhaps it's easier to visualize this layer as the actual Air Mail or Consolidated Carrier that transfers the mail from Point A to Point B.
Layer 2: Data link layer
The Data Link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer.
. The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and Aloha for local area networks.
On IEEE 802 local area networks, and some non-IEEE 802 networks such as FDDI, this layer may be split into a Media Access Control (MAC) layer and the IEEE 802.2 Logical Link Control (LLC) layer.
It arranges bits from physical layer into logical chunks of data, known as frames.
This is the layer at which the bridges and switches operate.
Connectivity is provided only among locally attached network nodes forming layer 2 domains for unicast or broadcast forwarding.
Other protocols may be imposed on the data frames to create tunnels and logically separated layer 2 forwarding domain.
Layer 1: Physical layer
The Physical layer defines all the electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications.
Hubs, repeaters, network adapters and Host Bus Adapters (HBAs used in Storage Area Networks) are physical-layer devices.
The major functions and services performed by the physical layer are:
. Establishment and termination of a connection to a communications medium.
. Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
. Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and fiber optic) or over a radio link.
Parallel SCSI buses operate in this layer. Various physical-layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the data-link layer.
The same applies to other local-area networks, such as Token ring, FDDI, and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.
Internet protocol
The Internet protocol suite is the set of communications protocols that implements the protocol stack on which the Internet and many commercial networks run.
It is part of the TCP/IP protocol suite, which is named after the two most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were also the first two networking protocols defined.
A review of TCP/IP is given under that heading. Note that todays TCP/IP networking represents a synthesis of two developments that began in the 1970's, namely LAN's (Local Area Networks) and the Internet, that revolutionalised computing.
The Internet protocol suite - like many protocol suites - can be viewed as a set of layers.
Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers.
Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted.
The original TCP/IP reference model consisted of four layers, but has evolved into a five-layer model.
Classification by Scale
Computer networks may be classified according to the scale or extent of reach of the network, for example as a Personal area network (PAN), Local area network (LAN), Campus area network (CAN), Metropolitan area network (MAN), or Wide area network (WAN).
Personal area network
A personal area network (PAN) is a computer network used for communication among computer devices (including telephones and personal digital assistants) close to one person.
The devices may or may not belong to the person in question.
The reach of a PAN is typically a few meters.
PANs can be used for communication among the personal devices themselves (intrapersonal communication), or for connecting to a higher level network and the Internet (an uplink).
Local area network
A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings.
Current LANs are most likely to be based on switched IEEE 802.3 Ethernet technology, running at 10, 100 or 1,000 Mbit/s, or on IEEE 802.11 Wi-Fi technology.
Each node or computer in the LAN has its own computing power but it can also access other devices on the LAN subject to the permissions it has been allowed.
These could include data, processing power, and the ability to communicate or chat with other users in the network.
Campus area network
A campus area network (CAN) is a computer network made up of an interconnection of local area networks (LANs) within a limited geographical area.
It can be considered one form of a metropolitan area network, specific to an academic setting.
Metropolitan area network
Metropolitan Area Networks, or MANs, are large computer networks usually spanning a city.
They typically use wireless infrastructure or optical fiber connections to link their sites.
A MAN is optimized for a larger geographical area than is a LAN, ranging from several blocks of buildings to entire cities.
As with local networks, MANs can also depend on communications channels of moderate-to-high data rates.
A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations.
MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks.
Wide area network
Wide Area Network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries).
). Or, less formally, a network that uses routers and public communications links.
Contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively.
The largest and most well-known example of a WAN is the Internet.
Classification by connection method
Computer networks may be classified according to the technology that is used to connect the individual devices in the network such as HomePNA, Power line communication, Ethernet, or Wireless LAN.
The Home Phoneline Networking Alliance seeks to establish standards for home networking over regular phone lines within the home - for compatibility between telecom, computer and network products.
Power line communication
Power line communication (PLC), also called mains communication, power line telecoms (PLT), powerband or power line networking (PLN) or power area networking (PAN) are terms describing several different systems for using power distribution wires for simultaneous distribution of data.
The carrier can communicate voice and data by superimposing an analog signal over the standard 50 or 60 Hz alternating current (AC).
It includes Broadband over Power Lines (BPL) with data rates sometimes above 1 Mbps and Narrowband over Power Lines with much lower data rates.
Traditionally electrical utilities used low-speed power-line carrier circuits for control of substations, voice communication, and protection of high-voltage transmission lines.
High-speed data transmission has been developed using the lower voltage transmission lines used for power distribution.
A short-range form of power-line carrier is used for home automation and intercoms.
Ethernet is a large, diverse family of frame-based computer networking technologies for local area networks (LANs).
The name comes from the physical concept of the ether.
It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format.
Wireless LAN
A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers without using wires.
WLAN utilizes spread-spectrum technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set.
This gives users the mobility to move around within a broad coverage area and still be connected to the network.
Classification by Functional Relationship
Computer networks may be classified according to the functional relationships which exist between the elements of the network, for example Active Networking, Client-server and Peer-to-peer (workgroup) architectures.
Also, computer networks are used to send data from one to another by the hardrive
Active Networking
Active networking is a communication paradigm that allows packets flowing through a communication telecommunications network to dynamically modify the operation of the network.
The active network architecture is comprised of execution environments (similar to a unix shell that can execute active packets), a node operating system capable of supporting one or more execution environments, and active hardware, capable of routing or switching as well as executing code within active packets.
This differs from the traditional network architecture which seeks robustness and stability by attempting to remove complexity and the ability to change its fundamental operation from underlying network components.
Network processors are one means of implementing active networking concepts. Active networks have also been implemented as overlay networks.
Active networking allows the possibility of highly tailored and rapid "real-time" changes to the underlying network operation enabling such ideas as sending code along with packets of information allowing the data to change its form (code) to match the channel characteristics.
The smallest program that can generate a sequence of data can be found in the definition of Kolmogorov Complexity.
The use of real-time genetic algorithms within the network to compose network services is also enabled by active networking.
Client server is network architecture which separates a client (often an application that uses a graphical user interface) from a server.
Each instance of the client software can send requests to a server.
Specific Types of servers include application servers, file servers, terminal servers, and mail servers.
While their purpose varies somewhat, the basic architecture remains the same.
Although this idea is applied in a variety of ways, on many different kinds of applications, the easiest example to visualize is the current use of web pages on the internet.
Characteristics of a server:
. Passive (slave)
. Waits for requests
. Upon receipt of requests, processes them and then serves replies
Characteristics of a client:
. Active (master)
. Sends requests
. Waits for and receives server replies
Servers can be stateless or stateful. A stateless server does not keep any information between requests.
A stateful server can remember information between requests. The scope of this information can be global or session.
A HTTP server for static HTML pages is an example of a stateless server while Apache Tomcat is an example of a stateful server.
The interaction between client and server is often described using sequence diagrams. Sequence diagrams are standardized in the UML.
Another type of network architecture is known as a peer-to-peer architecture because each node or instance of the program is both a "client" and a "server" and each has equivalent responsibilities. Both architectures are in wide use.
A peer-to-peer (or P2P) computer network relies primarily on the computing power and bandwidth of the participants in the network rather than concentrating it in a relatively low number of servers.
P2P networks are typically used for connecting nodes via largely ad hoc connections. Such networks are useful for many purposes.
Sharing content files (see file sharing) containing audio, video, data or anything in digital format is very common, and realtime data, such as telephony traffic, is also passed using P2P technology.
A pure peer-to-peer network does not have the notion of clients or servers, but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network.
This model of network arrangement differs from the client-server model where communication is usually to and from a central server.
A typical example for a non peer-to-peer file transfer is an FTP server where the client and server programs are quite distinct, and the clients initiate the download/uploads and the servers react to and satisfy these requests.
Classification of peer-to-peer networks
One possible classification of peer-to-peer networks is according to their degree of centralization:
Pure peer-to-peer:
. Peers act as equals, merging the roles of clients and server
. There is no central server managing the network
. There is no central router
Hybrid peer-to-peer:
. Has a central server that keeps information on peers and responds to requests for that information.
. Peers are responsible for hosting available resources (as the central server does not have them), for letting the central server know what resources they want to share, and for making its shareable resources available to peers that request it.
. Route terminals are used addresses, which are referenced by a set of indices to obtain an absolute address.
Classification by Network Topology
Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.
Bus network
Image showing bus network layout
A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus.
There are several common instances of the bus architecture, including one in the motherboard of most computers, and those in some versions of Ethernet networks.
Bus networks are the simplest way to connect multiple clients, but often have problems when two clients want to transmit at the same time on the same bus.
Thus systems which use bus network architectures normally have some scheme of collision handling or collision avoidance for communication on the bus, quite often using Carrier Sense Multiple Access or the presence of a bus master which controls access to the shared bus resource.
Advantages and Disadvantages of a Bus Network
. Easy to implement and extend
. Requires less cable length than a star topology
. Well suited for temporary or small networks not requiring high speeds(quick setup)
. Initially less expensive than other topologies
. Cheap
. Difficult to administer/troubleshoot.
. Limited cable length and number of stations.
. If there is a problem with the cable, the entire network goes down.
. Maintenance costs may be higher in the long run.
. Performance degrades as additional computers are added or on heavy traffic.
. Low security (all computers on the bus can see all data transmissions).
. One virus in the network will affect all of them (but not as badly as a star or ring network).
. Proper termination is required.(loop must be in closed path).
. If one node fails, the whole network will shut down.
. If many computers are attached, the amount of data flowing causes the network to slow down.
Star network
Star network layout
Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer which acts as a router to transmit messages.
If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node.
An active star network has an active central node that usually has the means to prevent echo-related problems.
The star topology reduces the chance of network failure by connecting all of the systems to a central node.
When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node.
All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only.
The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.
Strictly speaking only networks that use switches have a true star topology.
If the network uses a hub, the network topology has the physical appearance of a star, but is actually a bus.
. Good performance.
. Easy to set up and to expand.
. Any non-centralised failure will have very little effect on the network, whereas on a ring network it would all fail with one fault.
. Easy to detect faults
. Data Packets are sent quickly as they do not have to travel through any unnecessary nodes.
. Expensive to install
. Extra hardware required
. If the host computer fails the entire system is affected.
Ring network
Ring network layout
A ring network is a topology of computer networks where each node is connected to two other nodes, so as to create a ring.
Ring networks tend to be inefficient when compared to Star networks because data must travel through more number of points before reaching its destination.
For example, if a given ring network has eight computers on it, to get from computer one to computer four, data must travel from computer one, through computers two and three, and to its destination at computer four.
It could also go from computer one through eight, seven, six, and five until reaching four, but this method is slower because it travels through more computers.
Ring networks also carry the disadvantage that if one of the nodes in the network breaks down then the entire network will break down with it as it requires a full circle in order to function.
The token ring network is a ring topology only at the logical level, it runs on a physical Star network, using central devices called MSAUs or MAUs.
. Data is quickly transferred without a 'bottle neck'. (very fast, all data traffic is in the same direction)
. The transmission of data is relatively simple as packets travel in one direction only.
. Adding additional nodes has very little impact on bandwidth
. It prevents network collisions because of the media access method or architecture required.
. Data packets must pass through every computer between the sender and recipient therefore this makes it slower.
. If any of the nodes fail then the ring is broken and data cannot be transmitted successfully.
. It is difficult to troubleshoot the ring.
. Because all stations are wired together, to add a station you must shut down the network temporarily.
. In order for all computers to communicate with each other, all computers must be turned on.
. Total dependence upon the one cable.
Mesh networking
Image showing mesh network layout
Mesh networking is a way to route data, voice and instructions between nodes.
It allows for continuous connections and reconfiguration around broken or blocked paths by "hopping" from node to node until the destination is reached.
A mesh network whose nodes are all connected to each other is a fully connected network.
Mobile ad-hoc networking (MANET), featured in many consumer devices, is a subsection of mesh networking.
Mesh networks are self-healing: the network can still operate even when a node breaks down or a connection goes bad. As a result, a very reliable network is formed.
This concept is applicable to wireless networks, wired networks, and software interaction.
A mesh network is a networking technique which allows inexpensive peer network nodes to supply back haul services to other nodes in the same network.
. It effectively extends a network by sharing access to higher cost network infrastructure.
Mesh networks differ from other networks in that the component parts can all connect to each other via multiple hops, and they generally are not mobile.
Tree and hypertree networks
A hypertree is an acyclic hypergraph.
Tree and hypertree networks are important special cases of star network topologies.
A Tree Network consists of star-configured nodes connected to switches/concentrators, each connected to a linear bus backbone.
Each hub/concentrator rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node.
All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only.
The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.
Classification by Services Provided
Computer networks may be classified according to the services which they provide, such as Storage area networks, Server farms, Process control networks, Value-added network, SOHO network, Wireless community network, XML appliance, Jungle Networks, khadar network, etc.
Storage area network
In computing, a storage area network (SAN) is a network (referred to as a fabric) designed to attach computer storage devices such as disk array controllers and tape libraries to servers. As of 2007, SANs are most commonly found in enterprise storage.
A SAN allows a machine to connect to remote targets such as disks and tape drives on a network for block level I/O.
From the point of view of the class drivers and application software, the devices appear as locally attached devices.
There are two variations of SANs:
1. A network whose essential purpose is the transfer of data between computer systems and storage elements.
A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust.
The term SAN is usually (but not necessarily) identified with block I/O services rather than file access services.
2. A storage system consisting of storage elements, storage devices, computer systems, and/or appliances, plus all control software, communicating over an ethernet network.
Storage networks are distinguished from other forms of network storage by the low-level access method that they use.
Data traffic on the SAN Fabric is very similar to those used for internal disk drives, like ATA and SCSI.
In a storage network, a server issues a request for specific blocks, or data segments, from specific disk drives. This method is known as block storage.
The device acts in a similar fashion to an internal drive, accessing the specified block, and sending the response across the network.
Server farm
The Server Farm.
A server farm is a collection of computer servers usually maintained by an enterprise to accomplish server needs far beyond the capability of one machine.
Often, server farms will have both a primary and a backup server allocated to a single task, so that in the event of the failure of the primary server, a backup server will take over the primary server's function.
Server farms are typically co-located with the network switches and/or routers which enable communication between the different parts of the cluster and the users of the cluster.
Server farms are commonly used for cluster computing. Many modern supercomputers consist of giant server farms of high-speed processors connected by either Gigabit Ethernet or custom interconnects such as Myrinet.
Another common use of server farms is for web hosting
Process control network
A Process Control Network (PCN) is a communications network that is used to transmit instructions and data between control and measurement units and Supervisory Control and Data Acquisition (SCADA) equipment.
These networks have, over the years, used many of the technologies and topologies utilized in other network applications.
However, Process Control Networks (PCNs) have several special requirements that must be met in order for the solution to be acceptable to the industry.
These requirements are, in no particular order: Robustness, Determinacy, Compatibility.
Robustness includes requirements such as connection redundancy, reduced sensitivity to Electromagnetic Interference (EMI), and good error checking and correction.
Determinacy involves assuring that each device is guaranteed access to the network, and in many cases mechanisms to allow priority information (such as alarms) through the system.
Compatibility allows SCADA and Distributed Control Systems (DCS) from various manufacturers to communicate with control and measurement equipment from others.
Value-added network
A value-added network (VAN) is a specialized application service provider (ASP) that acts as an intermediary between trading partners sharing data or business processes.
VANs traditionally transmitted data formatted as Electronic Data Interchange (EDI) but increasingly they also transmit data formatted as XML.
VANs usually service a given vertical or industry and provide value-added services such as data transformation between formats (EDI?XML, EDI?EDI, etc.).
At one extreme a VAN hosts only horizontal business-to-business (B2B) application integration services, hosting general-purpose integration services for any process or industry.
At the other extreme a VAN also hosts process-specific or industry-specific pre-defined integration capabilities (e.g., data synchronization services as part of the Global Data Synchronization Network (GDSN)) and applications (e.g., supply chain order visibility).
Traditionally, most VANs primarily only supported general-purpose B2B integration capabilities focused on EDI but these service providers are quickly evolving to become more process- and industry-specific over time, particularly in industries such as retail and hi-tech manufacturing.
SOHO network
"SOHO network" is occasionally used to refer to a local area network as used in a Small office/home office business.
The term is mainly useful to define a market segment which has no internal IT staff, and possibly no dedicated server, structured cabling or server room, and where very high levels of performance and robustness are not warranted.
Compared to 19-inch rack based traditional business equipment, products designed for the SOHO market tend to be simpler, quieter, cheaper and "prettier".
Wireless community network
Wireless community networks or wireless community projects are the largely hobbyist-led development of interlinked computer networks using wireless LAN technologies.
Taking advantage of the recent development of cheap, standardised 802.11b (Wi-Fi) devices to build growing clusters (group of the same or similar elements gathered) of linked, citywide networks, or in rural areas where conventional DSL services are unavailable.
Some are being used to link to the wider Internet, particularly where individuals can obtain unmetered internet connections such as ADSL and/or cable modem at fixed costs and share them with friends.
Where such access is unavailable or expensive, they can act as a low-cost partial alternative, as the only cost is the fixed cost of the equipment.



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