NETWORK AND ITS COMPNENTS

Dr. Usha Munshi
Chief Librarian
Indian Statistical Institute
Calcutta
E-mail: umunshi@isical.ac.in, umunshi@gmail.com

NETWORK AND ITS COMPNENTS


Introduction

We are breathing in the computer age and gradually computer has become an integral part of our life. Computer is an indispensable and multipurpose tool performing multifaceted functions in multivariate environments. To make things more simpler, computer is touching every sphere of our life and has become a dire necessity be it in business, education, medicine and health care, science, engineering, manufacturing, legal practices, law enforcement, defense, government , at home, etc. Communication is another area which computers have made much easier and cheaper too. Internet – the mother of all networks which is a world wide network and connects people all over the globe and facilitates the global access to information that is available in the cyberspace and also that one wishes to send and share with other on the network.

The field of computer networking and today's Internet trace their beginnings back to the early 1960s, a time at which, the telephone network was the world's dominant communication network. The initial ARPAnet was a single, closed network. In order to communicate with an ARPAnet host, one had to actually be attached to another ARPAnet IMP (Internet Messaging Proggram). In the early to mid 1970's, additional packet-switching networks besides ARPAnet came into being; ALOHAnet, a satellite network linking together various universities. The number of networks was beginning to grow. In 1973, Robert Metcalfe's PhD thesis laid out the principle of Ethernet, which would later lead to a huge growth in so-called Local Area Networks (LANs) that operated over a small distance based on the Ethernet protocol.

3.0. Definition of A Network

Making devices to talk to each other for communication purposes is nothing new Early forays into telephony such as telegraph & telephone has since evolved into more complicated devices and now a computer can be networked to the Internet; a PC or to a home stereo for same purposes. Taking the case of early 1960s, individual computers – were physically shared and hence sharing of data & information was not easy.

Due to this impractical approach the seed of the need germinated by way of connecting computers & computer network was born. These Networks provided the basis for early ARPANET, the forerunner of the modern Internet. On June 3, 1968, a project proposal christened “ Resource Sharing & Computer Networks” submitted by the Advanced Research Projects Agency (ARPA), with an aim of not only to allow boosting sharing of their data, but also further their research activities in wide variety of military & scientific fields After being tested in 4 locations; the network spread. New protocols have since been created that evolved in today’s World Wide Network. The then new communication protocol –Packet Switching facilitated several applications, such as secure voice messaging in military channels. These new circuits provided basis for the communication technologies of the rest of 20th century. Their further development and fine –tuning were applied to Computer Networks. During 1977, early PC based LANs were spreading, initially restricted to academics and hobbyists, but eventually, found their way into the workplace & homes. LAN variants developed (MANs, WANs, etc) in academics sector. The corporate world responded quickly and the landscape of how people conduct business altered This was primarily due to widespread use of computers, and speed and ease of using them for communications and data transfer. The past few years have seen a remarkable growth in the global networking infrastructure. The Internet has grown from a research curiosity to something we all take for granted, beginning to become as essential as the ubiquitous telephone and utility networks. It has been able to withstand rapid growth fairly well and its core protocols have been robust enough to accommodate applications that were unforeseen by the original Internet designers, such as the amazing success story that is the World Wide Web.
3.1. What is a Network?
Computer networking is the scientific and engineering discipline concerned with communication between computer systems. Such communicating computer systems constitute a computer network and these networks generally involve at least two devices capable of being networked with at least one usually being a computer. The devices can be separated by a few meters (e.g. via Bluetooth) or thousands of kilometers (e.g. via the Internet). Experts in the field of networking debate whether two computers that are connected together using some form of communications medium constitute a network. Therefore, some sources will state that a network requires three connected computers.,

A network or communications networks, is a system of interconnected computers, telephones, or other communications devices that can communicate with one another and share applications and data. It is the tying together of so many communications devices in so many ways, that is changing the world we live in.







Fig1: A View of Computers Connected in a
Network Environment


Alternatively you could also say that - A computer network is an interconnection of several computers which communicate with one another to share records, files and / or peripheral devices. The users can select services required and communicate with computer as a local user


In more simpler terms, one could say in three distinct parts: that A Computer Network is:
(i) A group of two or more computers connected together
(ii) With the ability to communicate with each other
(iii) For sharing information, applications or resources
Groth[1] states that "the term network describes two or more connected computers"

According to Federal Standard 1037C[2] a computer network is "A network of data processing nodes that are interconnected for the purpose of data communication", the term "network" being defined in the same document as "An interconnection of three or more communicating entities" While a communications network in which the end instruments are computers.

Here, let us also understand the two main gadgets associated with it that is:
1. Computer System?
2. Networking

Computer System: A complete, working computer includes not only the computer, but also any software and peripheral devices that are necessary to make the computer function. Every computer system, for example, requires an operating system. Computers on a network are sometimes called nodes, while Computers and devices that allocate resources for a network are called servers. Networking : It refers to: hardware and software, used to connect computers, allowing them to communicate with one another. For instance cabling, Network Operating systems, and a variety of network components flesh out a network that can include a wide variety of computers and devices

In general we can say that a computer network is any set of computers or devices connected to each other. Examples of networks are the Internet, a wide area network that is the largest to ever exist, or a small home local area network (LAN) with two computers connected with standard networking cables connecting to a network interface card in each computer.

Thus a network is an interconnected system of nodes, or objects. In computer terminology, networks usually refer to either the electric distribution systems some large IT providers manage, or systems of interconnected computers. The largest of these computer networks is the Internet. Other computer networks may be as small as a single personal computer connected to the Internet via a router, or as large as a university campus with several thousand machines communicating via a central server. Other common networks include small home LAN's, company intranets, and VPN (Virtual Private Network )virtual networks.
































Fig 2: A Sample Network Diagram

(Source :http://upload.wikimedia.org/wikipedia/en/1/12/Sample-network-diagram.png)

4.0.Importance of Networking

Why is networking important? How is networking facilitating us and what are we able to do with this facility, these are some issues that we will grapple with in this course, as they form the basis of the networking in global context.

The early goal was to make computers talk to one another. But today millions of computers are connected with some means of communication. Sharing of resources and communication are two main reasons (amongst many) that makes a computer network very important in this Internet Society. There are numerous applications (email, entertainment, commerce…) that are becoming increasingly dependent. Networks have proliferated into many walks of life. The ubiquitous computing and Internet keeps people connected (no matter what and where they are), such as with capable cellular phones. Every day functions are getting mutated and improved (especially in the corporate world) , thereby allowing data to flow at (almost) just-instant speed .

Computer networks, and the technologies needed to connect and communicate through and between them, continue to drive computer hardware, software, and peripherals industries. This expansion is mirrored by growth in the numbers and types of users of networks from the researcher to the home user.
Today, computer networks are a requirement of modern communication. The scope of communication has increased significantly in the past decade and this boom in communications might not have been possible without the advancements in computer networks.
A glimpse in to the future may predict a way change in the way we conceived the thoughts of these networks would aim at. :While initial aim was to see everyone on the planet – “Wired” It seems to be happening other way round now: Technological advances are advancing towards “wireless Technology. Coming up are cable free alternative to Wired Networks. Not long ago from now, it is predicted that
by 2009 wearable computers which will replace the Personal Digital Assistant or PDA will be fully integrated in the workplace with ability to connect to both wired and wireless networks. Other emerging technologies include - Smart appliances that are products which have enhanced the capabilities and facilitated further access to internet. Secondly, the fully automated home that have appliances for heating, cooling, entertainment, etc connected via a LAN.
The following advantages are particularly true for LANs, although, they apply to MANs and WANs as well.

 Sharing of peripheral devices
 Sharing of programs and data
 Better communications
 Security of information
 Access to databasesThey act as Discourse Facilitators for carrying out conversation/discussion
 Facilitate Resource Location for Research and other academic activities
 Support Resource Creation that is they help us in e-Publishing (self publishing, web publishing : scholarly contents, business stuff, etc.
 Computer Network also Facilitates
 Cooperation
 Coordination
 Collaboration
 Promotes Concept of : Global Reach
 No Geographical Barriers
 Access to All You can improve efficiency by sharing information such as common files, databases and business application software over a computer network.You can improve communication by connecting your computers and working on standardised systems.Network facilitates among other things:
 Directory service that provides map of the network so that resources can be accessed without knowing their exact physical location
 Convergence – The focus here is on technological convergence where multiple products come together to form one product, with the advantages of each initial component.
 Convergence in the media has led to the removal of entry barriers across the IT, telecoms, media and consumer electronics industries, creating one large "converged" industry”. For example Computer Telephony Integration (CTI) is the merging of traditional telephony with computer systems to provide such services as call routing, reporting, and integration with business applications
 Call Center
 IP-PBX IP-based or IP-enabled
 IVR Integrated Voice Response
 VoIP Voice over Internet Protocol (Voice Over IP or VoIP) refers to technologies that have been developed to transfer voice in digital format using the Internet Protocol.
 Messaging – these are processes associated with any electronic data that is created on, stored on or transferred over a network
Networks and their Classification

Networks can be classified on the basis of various features and functionalities and goodies. Broadly speaking we can consider their classification pattern on the following features and functionalities:

5.1. Classification Based on Features & Functionalities

5.1.1. 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.

5.1.2. 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).


5.1.3. 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 Power line communication, Ethernet, or Wireless LAN.

5.1.4. 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

5.1.5. 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. Topology can be arranged in a Geometric Arragement.

5.1.6. 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 ("occasionally used to refer to a local area network as used in a Small office/home office business), Wireless community network, XML appliance, , etc.

5.1.7.By Protocol
Computer networks may be classified according to the communications protocol that is being used on the network.

6.0 Types of Networks

There are many types of computer networks, which differentiate one type from another based on primarily on geographic area they cover. Types can also be differentiated on the basis of the characteristics they depict.

As your activities develops, you might wish to consider other networks such as wide area networks (WANs), virtual private networks (VPNs), intranets and extranets. These networks each offer different business benefits, such as linking together systems in different offices, allowing remote workers to access your office systems securely, or providing up-to-date information for your staff and business partners . WANs are used by companies with offices at different locations. The WAN connects different local area networks together, into a more complex network. You will need to use client/server networks, which are based around central server computers, so that you can connect the various servers over a telecommunications network. WANs use cables/lines that are leased from a telecoms company

A VPN allows the user to connect across the Internet to the business' private network. It creates a secure link between the remote worker's computer and the central system. A VPN can be cheaper to use than leased lines or domestic-type broadband connections
Many businesses now build internal networks. These intranets store information on a central system at a private Internet address. Employees can access business information by connecting to the Internet, making it particularly useful for employees who travel or work from home.

Businesses can also open up their intranets to partners, such as suppliers and customers, typically with a password. This is an extranet. For example, a business can let clients track the progress of their orders, and the payment of suppliers online can be linked directly to the business accounts system.

Many types of computer networks which differentiate from one type to another [based on primarily on geographic area] For instance:
 LAN
 WAN
 CAN
 MAN
 HAN
 Others

The most common types of computer networks. are described below.

6.1.Local Area Network (LAN):
A network that is limited to a relatively small spatial area such as a room, a single building, a ship, or an aircraft. Local area networks are sometimes called a single location network.

For administrative purposes, large LANs are generally divided into smaller logical segments called workgroups. A workgroup is a group of computers that share a common set of resources within a LAN.

In general we can say that LANs are •computer networks that spans a relatively small area. The computers are geographically closely located in relation to each other. Most LANs are confined to a single building or group of buildings and • connect workstations and personal computers Each node or individual computer in a LAN has its own CPU with which it executes programs, but it also is able to access data and devices anywhere on the LAN This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions
•A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings.[3] 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.[2]
Based on the above definition, we can infer there are many different types of LANs, Ethernets being the most common for PCs.. Most Apple Macintosh networks are based on Apple's - AppleTalk network system, which is built into Macintosh computers
LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited, and there is also a limit on the number of computers that can be attached to a single LAN
The defining characteristics of LANs, in contrast to WANs (wide area networks), are: their much higher data rates, smaller geographic range, and that they do not require leased telecommunication lines.

Since LANs have become an important component of Libraries and information Centres, due to the astonishing developments in the Information Technology. Therefore it may be relevant to focus on some more details about LAN.

The following characteristics differentiate one LAN from another
Topology : The geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line.

Protocols : The rules and encoding specifications for sending data. The protocols also determine whether the network uses a peer-to-peer or client/server architecture.
Media : Devices can be connected by twisted-pair wire, coaxial cables, or fiber optic cables.
However, some networks do without connecting media altogether, communicating instead via radio waves
Local area networks are made up of several standard components that are discussed in another unit.







Fig 3: Local Area Network Scheme

(Source: http://upload.wikimedia.org/wikipedia/commons/7/7e/Schema_di_una_LAN.png)

6.2.Wide Area Networks (WANs)
One LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN) The computers are farther apart and are connected by telephone lines or radio waves.
Therefore, a computer network that spans a relatively large geographical area. Can be termed as WAN. Typically, a WAN consists of two or more local-area networks (LANs)Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites

Alternatively, a WAN is a data communications network that covers a relatively broad geographic area and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer. The largest WAN in existence is the InternetThere are broadly two types of WANs:
Centralized: A centralized WAN consists of a central computer that is connected to dumb terminals and / or other types of terminal devices.

Distributed: 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.
6.3 Campus Area Networks (CANs)
The computers are within a limited geographic area, such as a campus or military base. An interconnection of local-area networks within a limited geographical space.
a network can be treated as a CAN that connects two or more LANs but that is limited to a specific (possibly private) geographical area such as a college campus, industrial complex. A CAN is thus generally limited to an area that is smaller than a Metropolitan Area Network
Controller Area Network.
This type of CAN is basically a serial bus network of micro controllers that connects devices, sensors and actuators in a system or sub-system for real-time control applicationsfirst developed for use in automobiles. However their applications have moved beyond automobiles, and now controller area networks can be used as an embedded communication system for micro controllers as well as an open communication system for intelligent devices.
•6.4.Metropolitan Area Network (MAN)
A metropolitan area network (MAN) is a communications network covering a geographic area, the size of a city or suburb. A network that connects two or more LANs or CANs together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area. Multiple routers, switches & hubs are connected to create a MAN.
Broadly we can say that MAN is a data network designed for a town or city. In terms of geographic breadth, MANs are larger than local-area networks (LANs), but smaller than wide-area networks (WANs)MANs are usually characterized by very high-speed connections using fiber optical cable or other digital media. The purpose of a MAN is often to avoid long-distance telephone charges. Cellular phone systems are often MANs.


6.5. Home Area Networks (HAN)
HAN is a network contained within a user's home that connects a person's digital devices, from multiple computers and their peripheral devices to telephones, VCRs, televisions, video games, home security systems, "smart" appliances, fax machines and other digital devices that are wired into the network
In simple terms - A network contained within a user's home that connects a person's digital devices is a HAN.
•6.Internetwork

Internetwork refers to a scenario when two or more networks or network segments are connected using devices that operate at layer 3 that is the 'network' layer of the Open Systems Interconnection Basic Reference Model (OSI Basic Reference Model for short), such as a router.

Again Internetwork may also be defined as : Any interconnection among or between public, private, commercial, industrial, or governmental networks .

6.Intranet:
A network or internetwork that is limited in scope to a single organization or entity and which uses the TCP/IP protocol suite. . Intranets may also be categorized as a LAN, CAN, MAN, WAN, or other type of network.
68. Extranet:
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

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.

7.0. Network Topology

The classification of computers in an network environment can be based on Ways of networking. Computers can be connected to a network in a number of ways. The most commonly used are standard wireless (wi-fi) networks using something like a router that has wireless functions built in, normally in 802.11b/g or using cables such as CAT5E or fiber optic. This will create a LAN on all computer system using the network.

Computer Network types Can also be differentiated on the basis of the characteristics.
Broadly these could be classified under this segment on the following basis:

 Topology
 Protocol
 Architecture

7.1. Topology
The Topology refers to the shape of a network, or the network's layout. How different nodes in a network are connected to each other & how they communicate are determined by the network's topology.

Therefore, Network topology is the study of the arrangement or mapping of the elements such as links, nodes, etc. of a network, especially the physical that is real and logical that is virtual interconnections between nodes














:
Fig 4: Schematic Representation of Different Network Topologies.
Source : http://en.wikipedia.org/wiki/Network_topology

Any particular network topology is determined only by the graphical mapping of the configuration of physical and/or logical connections between nodes - Network Topology is, therefore, technically a part of graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical.

7.1.1. Types of Topologies
The arrangement or mapping of the elements of a network gives rise to certain basic topologies which may then be combined to form more complex topologies commonly refered to as hybrid topologies. The most common of these basic types of topologies are Bus ,star , Ring , Mesh , Tree .
7.1.2. Common Network Topologies
The Common topologies include :
 Mesh Topology
 Star Topology
 Bus Topology
 Ring Topology
 Tree Topology
Here shall be describing these common network topologies briefly.

7.1.2..1.Mesh Topology
•Devices are connected with many redundant interconnections between network nodesIn a true mesh topology every node has a connection to every other node in the network
Therefore a 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.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.


Fig 5: Images Showing Mesh Network Layout




7.1.2.2. Star Topology

Here all devices are connected to a central hubnodes communicate across the network by passing data through the hub.
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. 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.
The advantages of this type of network lies in its good performance., easy to set up and to expand, and easy to detect faults. Besides any non-centralised failure will have very little effect on the network, whereas on a ring network it would all fail with one fault. While its drawbacks are being expensive to install, and in its extra hardware requirement. Moreover, if the host computer fails the entire system is affected.
7.1.2.3. Bus Topology

In this all devices are connected to a central cable, called the bus or backbone .
A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. 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. A true bus network is passive – the computers on the bus simply listen for a signal; they are not responsible for moving the signal along. However, many active architectures can also be described as a "bus", as they provide the same logical functions as a passive bus; for example, switched Ethernet can still be regarded as a logical bus network, if not a physical one. With the dominance of switched Ethernet over passive Ethernet, passive bus networks are uncommon in wired networks. However, almost all current wireless networks can be viewed as examples of passive bus networks, with radio propagation serving as the shared passive medium..
easy to implement and extend Easy to implement and extend They requires less cable length than a star topology and are well suited for temporary or small networks not requiring high speeds
However, they have several drawbacks as well. For instance , difficulty in administering the /troubleshoot. If there is a problem with the cable, the entire network goes down. Hence maintenance costs may be higher in the long run. In terms of performance , as additional computers are added or on heavy traffic, it degrades. Proportionally. Besides low security, one virus threats for affecting the entire network and the like.

7.1.2.4. Ring Topology

Here all devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it.
a ring network is a topology of computer networks where each node is connected to two other nodes, so as to create a ring.
The importance of this type of network lies in the fact that the data transmission is quick and relatively simple as packets travel in one direction only. It prevents network collisions because of the media access method or architecture required
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. The other drawbacks of this type networks are that If any of the nodes fail then the ring is broken and data cannot be transmitted successfully..There is total dependence upon the one cable. As such in order for all computers to communicate with each other, all computers must be turned on. Besides to add a station you must shut down the network temporarily, . because all stations are wired together. 7.1.2.5. Tree Topology
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.
In simple terms we may say that it is a hybrid topology. Groups of star-configured networks are connected to a linear bus backbone.


Fig 6: Image Showing Tree Network Layout

7.2. Classification of Network Topologies
There are three basic categories of network topologies: This means that network topologies could fall under one of these categories.
 Physical topologies
 Signal topologies
 Logical topologies
It may be indicated here that the terms signal topology and logical topology are often used interchangeably even though there is a subtle difference between the two and the distinction is not often made between the two.7.2.1.Physical Topology
•Every LAN has a topology, or the way that the devices on a network are arranged and how they communicate with each other. The way that the workstations are connected to the network through the actual cables that transmit data that is the physical structure of the network -- is called the physical topology. Alternatively we can say that Physical Topology refers to the mapping of the nodes of a network and the physical connections between them – i.e., the layout of wiring, cables, the locations of nodes, and the interconnections between the nodes and the cabling or wiring system

7.2.2.Signal Topology
The mapping of the actual connections between the nodes of a network, as evidenced by the path that the signals take when propagating between the nodes.
The term 'signal topology' is often used synonymously with the term 'logical topology', however, , by definition, the term 'logical topology' refers to the apparent path that the data takes between nodes in a network while the term 'signal topology' generally refers to the actual path that the signals e.g., optical, electrical, electromagnetic, etc. take when propagating between nodes.


7.2.3.Logical Topology

Logical topology is the way that the signals act on the network media,.

Alternatively, the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices
Logical topologies are bound to the network protocols that direct how the data moves across a network. For instance: Ethernet protocol is a common logical bus topology protocol.; LocalTalk is a common logical bus or star topology protocol.. or IBM's Token Ring is a common logical ring topology protocol. A network's logical topology is not necessarily the same as its physical topology. For example, twisted pair Ethernet is a logical bus topology in a physical star topology layout, while IBM's Token Ring is a logical ring topology. It is physically set up in a star topology.

However there could be another group of topologies that is formed by combination of different type of topologies, hence form hybrid topologies.

7.2.4. Hybrid Network Topologies
The hybrid topology is a type of network topology that is composed of one or more interconnections of two or more networks that are based upon different physical topologies or a type of network topology that is composed of one or more interconnections of two or more networks that are based upon the same physical topology, but where the physical topology of the network resulting from such an interconnection does not meet the definition of the original physical topology of the interconnected networks. For example, the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical star topology might create a hybrid topology which resembles a mixture of the physical star and physical bus topologies or a mixture of the physical star and the physical tree topologies, depending upon how the individual networks are interconnected, while the physical topology of a network that would result from an interconnection of two or more networks that are based upon the physical distributed bus network retains the topology of a physical distributed bus network.
Examples could be : Star-Bus; Star-Wired Ring, etc.

7.3 Network Architecture and Services
Architecture basically is a design. The term architecture can refer to either hardware or software, or to a combination of hardware and software. The architecture of a system always defines its broad outlines, and may define precise mechanisms as well
•
There could be two types of mechanisms for network architecture :
(i) Open Architecture
(ii) Closed Architecture
Open Architecture : An open architecture allows the system to be connected easily to devices and programs made by other manufacturers Open architectures use off-the-shelf components and conform to approved standards
Closed ArchitectureA system with a closed architecture, on the other hand, is one whose design is proprietary, making it difficult to connect the system to other systems
The architectural principles that Robert Kahn articulated for creating a so-called "open network architecture" are the foundation, on which today's Internet is built (In electronic format: http://gaia.cs.uman.edu/kurose/introduction/history.htm) Broadly these principles are:
 Minimalism, Autonomy: This implies that a network should be able to operate on its own, with no internal changes required for it to be internetworked with other networks;
 Best Effort Service: Internetworked networks would provide best effort, end-to-end service. If, reliable communication was required, this could be accomplished by re-transmitting lost messages from the sending host;
 Stateless Routers: The routers in the internetworked networks would not maintain any per-flow state about any ongoing connection
 Decentralized Control: There would be no global control over the internet-worked networks.

7.3.1 Network Architecture

Networks can be broadly classified as using either a -
 Peer-to-peer or
 Client/server architecture

Both client/server and peer-to-peer architectures are widely used, and each has unique advantages and disadvantages.




7.3.1.1. Peer-to-Peer Architecture

Often referred to as peer-to-peer or P2P, it is the type of network in which each workstation has equivalent capabilities and responsibilities. Peer-to-peer networks are generally simpler, but usually do not offer the same performance under heavy loads.In simpler terms, Network architecture known as a peer-to-peer architecture because each node has equivalent responsibilitiesdiffers from client/server architectures, in which some computers are dedicated to serving the others
An important goal in peer-to-peer networks is that all clients provide resources, including bandwidth, storage space, and computing power. Thus, as nodes arrive and demand on the system increases, the total capacity of the system also increases.
The distributed nature of peer-to-peer networks also increases robustness in case of failures by replicating data over multiple peers, and -- in pure P2P systems -- by enabling peers to find the data without relying on a centralized index server. In the latter case, there is no single point of failure in the system.
7.3.1.2.Client/Server Architecture Network architecture in which each computer or process on the network is either a client or a server is called client/server architecture.
Servers are powerful computers or processes dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers )
While clients are PCs or workstations on which users run applicationsClients rely on servers for resources, such as files, devices, & even processing power. Client-server architectures are sometimes called two-tier architectures
The plus points of this type of architecture are: that all the data are stored at the servers, so it has better security control ability. It is more flexible than P2P paradigm in the sense that if a server in client/server paradigm wants to update the data or other resources, it can do so without any hassel. . There are already a lot of matured technologies designed for client/server paradigm which ensures the security, user-friendly interface and ease of use. Besides any element of a C/S network can be easily upgraded.
The disadvantage in a client-server architecture is that, with a fixed set of servers, in which adding more clients could mean slower data transfer for all users. In client server paradigm , traffic congestion has always been a problem. And also it does not have as good robustness as P2P network has.



7.3.1.3.Other Network Architectures
It may be appropriate to throw light on some other emerging forms of network architectures. Here mention is being briefly made of Grid Computing and Internet2.

Grid Computing
An emerging computing model that distributes processing across a parallel infrastructure. Grid computing combines the processing power of multiple computers on a network to tackle a single problem at the same time.

Grid computing, in simple terms can be stated, to be a distributed computing taken to the next evolutionary level.

The goal of Grid Computing being to create the illusion of a simple yet large and powerful self-managing virtual computer out of a large collection of connected heterogeneous systems sharing various combinations of resources.
Standardization of communications between heterogeneous systems created the Internet explosion!. The emerging standardization for sharing resources, along with the availability of higher bandwidth, are driving a possibly equally large evolutionary step in grid computing
•Internet2 –
A consortium being led by 206 universities working in partnership with industry and government to develop and deploy advanced network applications and technologies, accelerating the creation of the next Internet.
•7.4. Network Protocol
Internetworking requires the joining of many communication facilities, which need to efficiently communicate. These communications are prepared for transmission by using standard conventions called communication protocols.
A protocol is a standard method for transmitting data through a network or an agreed-upon format for transmitting data between two devices
The protocol determines the following:
 the type of error checking to be used
 data compression method, if any
 how the sending device will indicate that it has finished sending a message
 how the receiving device will indicate that it has received a message
In simpler terms: the protocol defines a common set of rules and signals that computers on the network use to communicate
Protocols are arranged in layers; each layer contributes something in preparing the data for its journey. Each protocol layer involved in the encoding process provides a header which contains information which can be used in un-packaging the message when it is received.
There are many different specialized protocols to accommodate the many kinds of data that might be transmitted. Each of these protocols perform specific roles/functions. Some of the protocols are listed below and only a few important ones are being described briefly here.

DHCP, DNS , HTTP , IMAP , IP, IPv6 , MGCP/Megaco , MPLS / Frame Relay , POP3, PPP/SLIP, PPTP , RTP/RTSP, S/MIME, SCCP (Skinny), SIP, SMTP, SNMP, SSL/TLS, TCP/IP, VoIP, X.25.•DHCP : Dynamic Host Configuration Protocol (DHCP) is a TCP/IP protocol used to assign (IP) numbers to users on a network.
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DNS : Domain Name System (DNS) translates domain names into Internet Protocol (IP) addresses. An IP address may have multiple domain names, which are intended to be text-based names that users will remember
•HTTP: (Hypertext Transfer Protocol) is a standard transmission method for text, graphic images, sound, video, and other multimedia files over the Internet
•IMAP: Internet Message Access Protocol (IMAP) is used to retrieve and read messages on a mail server. It has additional functionality that lets you manage messages while they are still located on the mail server•MGCP/Megaco : MEGACO/H.248 is the latest industry standard protocol for interfacing between hosts and call agents called Media Gateway Controllers (MGC's) and Media Gateways (MG's) – eg. an IP Telephone and the PSTN. Derived from MGCP•MGCP (VOiCOM)- As IP based telephony services began to gain some realism in the late 1990's, there was a clear need for 'gateway' control to interface to the many non-IP based networks & technologies (such as Residential Gateways, ATM, Access Gateways and the PSTN. MPLS / Frame Relay
•POP3 POP (Post Office Protocol), DMSP (Distributed Mail System Protocol), and IMAP (Internet Message Access Protocol). Of the three, POP is the oldest and consequently the best known •Local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. Nearly all subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3.
••PPP/SLIP : SLIP and PPP are two communication protocols which allow a computer connected to a server via a serial line (such as a modem) to become a actual node on the internet. This allows you to run network applications on your home computer directly. With a SLIP/PPP feed and the right software on your computer, you can not only run graphical web browsing software, you can also FTP files directly to your computer, store your mail on your computer, and many other functions.
•
While SLIP and PPP are largely similar, there are some key differences. PPP is a newer protocol, better designed, and more acceptable to the sort of people who like to standardize protocol specifications.
•
The most significant advantage PPP can offer is the automatic login and configuration negotiation. With these features, your PPP software only needs to know your login userid/password and the telephone number of your service provider. The software can then dial-up into your service provider and figure out everything else on its own. Once connected, PPP does NOT give you more of the internet than SLIP can.
•••PPTP Point-to-Point Tunneling Protocol (PPTP) provides corporations with the ability to conduct secure communications on Virtual Private Networks (VPNs) across dial-up, local area network, wide area network, or Internet connections•SMTP Simple Mail Transfer Protocol (SMTP) is the most widely used protocol for sending electronic mail (email) on the Internet. SMTP sends and receives email messages through the use of a Sender-SMTP process and a Receiver-SMTP process that perform email transmission and receipt services.
••SNMP The Simple Network Management Protocol (SNMP) serves as a network management tool that reports on the status of devices attached to a network. Usually these reports are limited to stating whether or not the devices are functioning properly•SSL/TLS The Secure Socket Layer, developed by Netscape, is a common protocol for providing secure transmission of messages on the Internet. It uses the program level between the HTTP and TCP layers. It is being replaced by the Transport Layer Security protocol, which ensures that a third party does not monitor communications between two sources
•TCP/IP The Transmission Control Protocol (TCP) and the Internet Protocol (IP) manages the flow of information over the Internet. TCP manages the assembly (and reassembly) of data into packets that can be easily transmitted over the Internet, while IP directs the packets to the their correct destination
VoIP Voice over Internet Protocol (Voice Over IP or VoIP) refers to the technologies that have been developed to transfer voice in digital format using the Internet Protocol•X.25 is an ITU-T standard protocol suite for wide area networks using the phone or ISDN system as the networking hardware. It defines standard physical layer, data link layer and network layers (layers 1 through 3) of the OSI model. Packet switched network was the common name given to the international collection of X.25 providers, typically the various national telephone companies
From a user's point of view, the important aspect about protocols is that your computer or device must support the right ones if you want to communicate with other computers
The protocol can be implemented either in hardware or in software7.5. Network Standards There is another set of parameters that adds to the characteristics of a robust, interoperable network - that is standard. The heterogeneity behviour of the systems over the network would lead to chaos, if there are no standards or standard practices followed by everyone participating in various activities over the network (particularly on the Internet) It is essential that some standard practices are followed in order to let things happen the way we want them to, thereby facilitating effective communication across the systems participating or required to participate for a particular transaction.
a definition or format that has been approved by a recognized standards organization or is accepted as a de facto standard by the industry

Standards exist for -
 Programming languagesOperating systemsData formats
 Communications protocols, and
 Electrical interfaces

Examples for different standards for Hardware, software and data transmission Include:
Hardware: IEEE is generally concerned with hardware, sets standards for most types of electrical interfaces. Most famous standard is probably RS-232C, which defines an interface for serial communication. This is the interface used by most modems, and a number of other devices, including display screens and mice.

Software: ANSI is primarily concerned with software. ANSI has defined standards for a number of programming languages
Data Transmission : ITU defines international standards, particularly communications protocols. It has defined a number of standards, that specify protocols for transmitting data over telephone lines. (including V.22, V.32, V.34 and V.42, )
From a user's standpoint, standards are extremely important in the computer industry because they allow the combination of products from different manufacturers to create a customized systemWithout standards, only hardware and software from the same company could be used together. In addition, standard user interfaces can make it much easier to learn how to use new applications
Most official computer standards are set by one of the following organizations:
 ANSI (American National Standards Institute)
 ITU (International Telecommunication Union)
 IEEE (Institute of Electrical and Electronic Engineers)
 ISO (International Standards Organization)
 VESA (Video Electronics Standards Association)
de facto standards
In addition to standards approved by organizations, there are also de facto standards. These are formats that have become standard simply because a large number of companies have agreed to use them. They have not been formally approved as standards, but they are standards nonetheless. PostScript is a good example of a de facto standard
7.5.1Standards & Protocols
There are a variety of standard protocols from which programmers can choose. Each has particular advantages and disadvantages. For example, some are simpler than others,
some are more reliable, and some are faster
One of the most popular protocols for LANs is called Ethernet, while another popular LAN protocol for PCs is the IBM token-ring network

7.5.1.1.Modern Network Hardware & Infrastructure Standards

Since the OSI Model is a reference rather than a standard, it leaves room for standards of many different designs and applications to be made in its image. The detailed description about this reference model has been given in another module.

Based upon OSI Layers one and two, these standards fill the gaps that exist in both technology and geographical distance allowing digital communication to : spread farther, carry information faster, and cost less.
7.5.1.1.1.Ethernet
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.
Ethernet (standardized as IEEE 802.3) is used to link computers in both small residential and large commercial situations and is the most widely used Network Hardware Standard today.

Often delivers internet access from other longer range hardware standards to multiple computers within a home or workplace. Ethernet equipment is relatively small, affordable, and can carry data at high speeds. The original specification called for speeds of 10mbps. A newer version of Ethernet, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second is one of the most widely implemented LAN standards, though has range limitations. Ethernet -most used standard for the transmission of digital information over short distances. A LAN may then be linked to other LANs via network infrastructure standards with long distance capabilities
7.5.1.1.2.Bluetooth

Bluetooth is an industrial specification for wireless personal area networks (PANs). Bluetooth provides a way to connect and exchange information between devices such as mobile phones, laptops, PCs, printers, digital cameras and video game consoles via a secure, globally unlicensed short-range radio frequency.
Bluetooth, (standardized as IEEE 802.15.1), is a short range wireless network standard originally developed by Ericsson Corporation. Designed to allow nodes to participate in a network using the lowest possible amount of power. Bluetooth's current maximum transmission rate is 2.1mbps. While this seems very low compared to much older Wi-Fi standards (such as 802.11b) Bluetooth is designed to fit a special section of the market, rather than to be a widespread, high-performance technology.

It is almost always used in a paired or "ad-hoc" type network. In an ad-hoc network, no router exists, but the nodes are simply responsible for negotiating communication among themselves automatically. A paired network is simply an ad-hoc network with only two nodes













It has several uses in devices using or requiring less bandwidth or are low power applications. Common Bluetooth devices and applications include mobile phone headsets, PC-to-organizer/PDA synchronization, and other situations in which small devices need low power, short range communication capability. Bluetooth is a staple feature on most of today's newest and smallest portable information and communication devices
As is evident from the above statements, Bluetooth is a radio standard and communications protocol primarily designed for low power consumption, with a short range based around low-cost transceiver microchips in each device.

Bluetooth lets these devices communicate with each other when they are in range. The devices use a radio communications system, so they do not have to be in line of sight of each other, and can even be in other rooms, so long as the received transmission is powerful enough.

Bluetooth technology will continue catering to the needs of very low power applications such as mice, keyboards and mono headsets, enabling devices to select the most appropriate physical radio for the application requirements, thereby offering the best of both worlds.7.5.1.1.3.WI-FI
Wi-Fi (IEEE 802.11x) is a standard developed to perform nearly the same role as Ethernet does in consumer settings, but without the wires. Taking to the air, Wi-Fi allows a node to lie anywhere within a 100 to 1000 ft range of a Wi-Fi enabled router and have a constant, secure connection to the Local Area Network.

Wi-Fi originated with speeds of just 11mbps in the form of IEEE 802.11b, but today can achieve speeds between 54mbps and 108mbps. Initially (Early 90s ) designed to provide wireless communication for cashier systems in retail locations and operated at speeds of 1 or 2mbps. Late 90s, the IEEE ratified the 802.11b specification, providing wireless Ethernet-like connectivity for nodes at speed steps between 1 and 11mbps. In 2003 and 2004 IEEE 802.11g, a newer standard based on the Wi-Fi specification, emerged. In 2005, an even newer standard began to emerge known as 802.11n
Let us see what are the uses of Wi Fi . It allows LANs to be deployed without cabling, typically reducing the costs of network deployment and expansion. Wi-Fi chipset pricing continues to come down, making Wi-Fi a very economical networking option and driving inclusion of Wi-Fi in an ever-widening array of devices. Wi-Fi is a global set of standards. Hence the same Wi-Fi client works in different countries around the world.
However there are some drawbacks also like power consumption is fairly high compared to some other standards, making battery life and heat a concern. Wi-Fi networks have limited range. Wi-Fi networks can be monitored and used to read and copy data transmitted over the network when no encryption such as VPN is used