Network Topology: It is graphical mapping of the configuration of physical and logical connections between nodes. LAN 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.

Classification of network topologies

There are also three basic categories of network topologies:

• physical topologies

• signal topologies

• logical topologies

Physical topologies

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.

Classification of physical topologies

Point-to-point

The simplest topology is a permanent link between two endpoints. Switched point-to-point topologies are the basic model of conventional telephony. The value of a permanent point-to-point network is the value of guaranteed, or nearly so, communications between the two endpoints.

Permanent (dedicated)

Easiest to understand, of the variations of point-to-point topology, is a point-to-point communications channel that appears, to the user, to be permanently associated with the two endpoints.

Switched:

Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically, and dropped when no longer needed.

Bus

Linear bus

The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has exactly two endpoints (this is the ‘bus’, which is also commonly referred to as the backbone or trunk)

All data that is transmitted between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network virtually simultaneously.

Distributed bus

The type of network topology in which all of the nodes of the network are connected to a common transmission medium which has more than two endpoints that are created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology.

Star

The type of network topology in which each of the nodes of the network is connected to a central node with a point-to-point link in a ‘hub’ .All data that is transmitted between nodes in the network is transmitted to this central node, which is usually some type of device that then retransmits the data to some or all of the other nodes in the network.

Extended star

A type of network topology in which a network that is based upon the physical star topology has one or more repeaters between the central node (the ‘hub’ of the star) and the other nodes, the repeaters being used to extend the maximum transmission distance of the point-to-point links between the central node and the peripheral nodes .

Ring

The type of network topology in which each of the nodes of the network is connected to two other nodes in the network and with the first and last nodes being connected to each other, forming a ring – all data that is transmitted between nodes in the network travels from one node to the next node in a circular manner and the data generally flows in a single direction only.

Mesh

Mesh topologies involve the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. In a ring, although two cable paths exist, messages can only travel in one direction. Some WANs, most notably the Internet, employ mesh routing.

Full

Fully connected

The type of network topology in which each of the nodes of the network is connected to each of the other nodes in the network with a point-to-point link – this makes it possible for data to be simultaneously transmitted from any single node to all of the other nodes.

The physical fully connected mesh topology is generally too costly and complex for practical networks, although the topology is used when there are only a small number of nodes to be interconnected.

Partial

Partially connected

The type of network topology in which some of the nodes of the network are connected to more than one other node in the network with a point-to-point link – this makes it possible to take advantage of some of the redundancy that is provided by a physical fully connected mesh topology without the expense and complexity required for a connection between every node in the network.

Tree

Also known as a hierarchical network.

The type of network topology in which a central ‘root’ node i.e. the top level of the hierarchy is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central ‘root’ node, while each of the second level nodes that are connected to the top level central ‘root’ node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central ‘root’ node being the only node that has no other node above it in the hierarchy Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the ‘branching factor’ of the hierarchical tree.

Hybrid network topologies

It is combination of different topologies such as star bus topology, star of star, star wired ring, hybrid mesh.

Star-bus

A type of network topology in which the central nodes of one or more individual networks that are based upon the physical star topology are connected together using a common ‘bus’ network whose physical topology is based upon the physical linear bus topology.

Star-of-stars

Hierarchical star

A type of network topology that is composed of an interconnection of individual networks that are based upon the physical star topology connected together in a hierarchical way to form a more complex network .

Star-wired ring

A type of hybrid physical network topology that is a combination of the physical star topology and the physical ring topology.

Hybrid mesh

A type of hybrid physical network topology that is a combination of the physical partially connected topology and one or more other physical topologies the mesh portion of the topology consisting of alternate connections between some of the nodes in the network .

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.

Logical topology

The mapping of the connections between the nodes of a network, as evidenced by the path that data appears to take when traveling between the nodes.

Classification of logical topologies

The logical classification of network topologies generally follows the same classifications as those in the physical classifications of network topologies, the path that the data takes between nodes being used to determine the topology as opposed to the actual physical connections being used to determine the topology.

1.) Logical topologies are often closely associated with media access control (MAC) methods and protocols.

2.) The logical topologies are generally determined by network protocols as opposed to being determined by the physical layout of cables, wires, and network devices or the logical flow of data.

3.) Logical topologies are able to be dynamically reconfigured by special types of equipment such as routers and switches.

Advantages and Disavantages of Topologies:

1. In Bus Topology, a device communicates with another device on the network sends a broadcast message onto the wire that all other devices , but only the intended recipient actually accepts and processes the message. A failure in network cable or wire will only take down entire LAN.Bus topology is very easy, simple and inexpensive than other topologies.

2. Star Topology is very easy to install but it is expensive due to the cost of central device. Many home networks use the star topology.A failure in any star network cable will only take down one computer’s network access.

3. Tree topology is a combination of star and bus topologies.This network is very easy to extend and facilities the organizations to meet their requirements. In tree topology if central cable breaks then all the nodes goes down.

1. In Ring Topology,All messages travel through a ring in the same direction (either “clockwise” or “counterclockwise”). A failure in any cable or device breaks the loop and can take down the entire network. Ring topologies are found in some office buildings or school campuses.

1. A mesh offers various advantages over other network topologies.Firstly, use of dedicated links guarantees that the each connection can carry its own data load, thus eliminating traffic troubles that can take place when links must be shared by multiple devices.Second is that a mesh topology is robust in nature.

Introduction

· A JSP page is a web page that contains Java code along with the HTML tags.

· When accessed by a client, the Java code within the page is executed on the server side, producing textual data.

· This data, which is surrounded by HTML tags, is sent as a normal HTML page to the client.

· JSP pages typically comprise of:

ü Static HTML/XML components.

ü Special JSP tags

ü Optionally, snippets of code written in the Java programming language called “scriptlets”.

Phase	Description
Page translation	The page is parsed and a Java file containing the corresponding servlet is created.
Page compilation	The Java file is compiled.
Load class	The compiled class is loaded.
Create instance	An instance of the servlet is created.
Call jspInit()	This method is called before any other method to allow initialization.
Call jspService()	This method is called for each request.
Call jspDestroy()	This method is called when the servlet container decides to take the servlet out of service.

Life Cycle Methods

Method	Parent Class
public void jspInit();	javax.servlet.jsp.JspPage
public void _jspService(HttpServletRequest request, HttpServletResponse response) throws javax.servlet.ServletException, java.IO.IOException;	javax.servlet.jsp.HttpJspPage
public void jspDestroy();	javax.servlet.jsp.JspPage

The jspInit(),_jspService(), and jspDestroy() methods of a JSP page are equivalent to the init(), service(), and destroy() methods of a servlet, respectively.

JSP Elements

JSP Tag Type	Description	Tag Syntax
Directive	Specifies translation time instructions to the JSP engine	<%@ Directives %>
Declaration	Declares and defines methods and variables	<%! Java Declarations %>
Scriptlet	Allows the developer to write free-form Java code in a JSP page	<% Some Java code %>
Expression	Used as a shortcut to print values in the output HTML of a JSP page	<%= An Expression %>
Action	Provides request-time instructions to the JSP engine	<jsp:actionName />
Comment	Used for documentation and for commenting out parts of JSP code	<%– Any Text –%>

Directives

— Directives provide general information about the JSP page to the JSP engine.

— page: A page directive informs the engine about the overall properties of a JSP page

<%@ page language=”java” %>

— include: An include directive tells the JSP engine to include the contents of another file in the current page.

<%@ include file=”copyright.html” %>

— taglib: A taglib directive is used for associating a prefix with a tag library.

<%@ taglib prefix=”test” uri=”taglib.tld” %>

— The tag names, their attributes, and their values are all case sensitive.

— The value must be enclosed within a pair of single or double quotes.

— A pair of single quotes is equivalent to a pair of double quotes.

— There must be no space between the equals sign (=) and the value.

Scriplets

— Scriptlets are Java code fragments that are embedded in the JSP page.

<% count++; %>

— The scriptlet is executed each time the page is accessed.

Expressions

— Expressions act as placeholders for Java language expressions.

<%= count %>

— The expression is evaluated each time the page is accessed, and its value is then embedded in the output HTML.

— We can print the value of any object or any primitive data type (int, boolean, char, etc.) to the output stream using an expression.

— We can also print the value of any arithmetic or Boolean expression or a value returned by a method call.

Actions

— Actions are commands given to the JSP engine. They direct the engine to perform certain tasks during the execution of a page.

jsp:include jsp:forward

jsp:useBean jsp:setProperty

jsp:getProperty jsp:plugin

— The first two, jsp:include and jsp:forward, enable a JSP page to reuse other web components.

— The next three, jsp:useBean, jsp:setProperty, and jsp:getProperty, are related to the use of JavaBeans in JSP pages.

— The last action, jsp:plugin, instructs the JSP engine to generate appropriate HTML code for embedding client-side components, such as applets.

Comments

— Comments are useful for documentation purposes.

— The syntax of a JSP comment is

<%– Anything you want to be commented –%>

— We can comment the Java code within scriptlets and declarations by using normal Java-style comments and the HTML portions of a page by using HTML-style comments

<html>

<body>

Welcome!

<%– JSP comment –%>

<% //Java comment %>

<!– HTML comment–>

</body>

</html>

JSP Implicit Objects

The JSP container makes available implicit objects that can be used within scriptlets and expressions, without the page author first having to create them

Identifier	Class or Interface	Description
application	interface javax.servlet.ServletContext	Refers to the web application’s Environment
session	interface javax.servlet.http.HttpSession	Refers to the user’s session
request	interface javax.servlet.http.HttpServletRequest	Refers to the current request to the page
response	interface javax.servlet.http.HttpServletResponse	Used for sending a response to the client
out	class javax.servlet.jsp.JspWriter	Refers to the output stream for the page
page	class java.lang.Object	Refers to the page’s servlet instance
pageContext	class javax.servlet.jsp.PageContext	Refers to the page’s environment
config	interface javax.servlet.ServletConfig	Refers to the servlet’s configuration
exception	class java.lang.Throwable	Used for error handling

Professor Samson Abramsky FRS is a computer scientist. Since the Year 2000, he has been a Fellow of the Royal Society of Edinburgh, a Fellow of Wolfson , Oxford and Christopher Strachey Professor of Computing at Oxford University Computing Laboratory. He has also been a Fellow of the Royal Society since 2004. His research achievements include the development of game semantics, domain theory in logical form, and categorical quantum mechanics.

He was educated at King’s College, Cambridge (BA 1975, MA Philosophy 1979, Diploma in Computer Science) and Queen Mary, University of London (PhD Computer Science 1988, supervised by Richard Bornat).

His earlier positions include:

• Programmer, GEC Computers Limited, 1976–1978
• Lecturer, Department of Computer Science and Statistics, QMUL, 1980–1983
• Lecturer, 1983–1988, Reader, 1988–1990, Professor, 1990–1995, Department of Computing,  Imperial College London
• Professor of Theoretical Computer Science, University of Edinburgh, 1996–2000

Research career

Samson Abramsky is Christopher Strachey Professor of Computing and a Fellow of Wolfson College, Oxford University. Previously he held chairs at the Imperial College of Science, Technology and Medicine, and at the University of Edinburgh.

He holds MA degrees from Cambridge and Oxford, and a PhD from the University of London.

He is a Fellow of the Royal Society (2004), a Fellow of the Royal Society of Edinburgh (2000), and a Member of Academia Europaea (1993). He is a member of the Editorial Boards of the North Holland Studies in Logic and the Foundations of Mathematics, and of the Cambridge Tracts in Theoretical Computer Science. He was General Chair of LiCS 2000-2003, and is currently a member of the LiCS Organizing Committee.

He has played a leading role in the development of game semantics, and its applications to the semantics of programming languages. Other notable contributions include his work on domain theory in logical form, the lazy lambda calculus, strictness analysis, concurrency theory, interaction categories, and geometry of interaction. He has recently been working on high-level methods for quantum computation and information.

Awards

• He was awarded an EPSRC Senior Research Fellowship in 2007
• His paper “Domain theory in Logical Form won the LiCS Test-of-Time award (a 20-year retrospective) for 1987. The award was presented at LiCS 2007.
• He was awarded an EPSRC Senior Research Fellowship on Foundational Structures and Methods for Quantum Informatics in 2007.
• Fellow of the Royal Society (2004)
• Fellow of the Royal Society of Edinburgh (2000)