First what is LAN means Local Area Network?

This type of network is mostly used in one building and in which the connected computers are not more than 256.
A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs, and occasionally a LAN will span a group of nearby buildings. In IP networking, one can conceive of a LAN as a single IP subnet (though this is not necessarily true in practice).
Besides operating in a limited space, LANs include several other distinctive features. LANs are typically owned, controlled, and managed by a single person or organization. They also use certain specific connectivity technologies, primarily Ethernet and Token Ring.
The distinction between a LAN and a WAN involves the physical distance that the network spans. A third category, the MAN, also fit into this scheme as it too is centered on a distance-based concept.

What the topology is:- the way in which connections are made and arrangement of computer is referred as topology.
Two kinds of Topologies are: Physical and Logical.
Physical topologies are bus, star , ring and physical mesh topology.
Bus:- in this type of topology the computers are connected by a single cable and we use terminator at the end of the cable that is used to bounce back the signal. In which we have the logical topology Ethernet 10 Base 2. Repeaters are used in the place of Hubs.

Star:- the computers are arranged in the form of star and in which we have a central hub . the logical topology that works here is Ethernet 10 Base 5. trouble shooting is easy in star just b’coz of Hub.

Ring:- when we need high performance network we use ring topology in which computers are arranged in form of ring . it is an expensive one . we use token passing method in this topology . logical topology is FDDI(Fiber Distributed Data Interface).


Physical Mesh Topology: combination of different topologies. Most expensive topology provides


MAN: A Metropolitan Area Network connects an area larger than a LAN but smaller than a WAN, such as a city, with dedicated or high-performance hardware.
For example: if we have network b/w Rawalpindi and Islamabad or Karachi and Hyderabad then it is dubbed as MAN.

WAN(Wide Area Network): A wide-area network spans a large physical distance. A WAN like the Internet spans most of the world!
A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address.
WANs differ from LANs in several important ways. Like the Internet, most WANs are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs use technology like ATM, Frame Relay and X.25 for connectivity. WAN requires special media , which are provided by telephone companies. WAN also requires special hardware.
Physical media for WAN: WANs use special purpose telephone wires, fiber optic cables, microwaves or satellites for communication. The simplest WANs use dedicated lines, which are special conditioned telephone lines that directly and permanently connects two computers.
Microwaves are radio waves that have a very high frequency .the disadvantage of using them is that they depend on line of sight transmission. No obstruction can get in the way, and they can travel only about 50 miles.
Communication satellites are placed in geosynchronous orbit thousands of miles above the earth. In this orbit, the satellite rotates with the earth so that it is always above a given spot. The latest WANs use long distance fiber optic cables.
WAN uses some special hardware items and these are multiplexor(combines input signals from several computers and sends the combines signal along the communication channel) routers(they receive packets of data and examine their address, they decide where to send each packet) , front-end processors(handle all the communications tasks for large computers, and also provide security to prevent unauthorized access).
This is what the WAN , I discussed WAN in detail as compare to MAN, b’coz if you have good understanding of LAN and WAN , you can easily understand MAN as it is stuck between LAN and WAN.. the only difference is of distance.


Entity Relationship model is a logical representation of data of an organization or a business area.As visible from its name, it's about the relationship between certain entities which are logically related.What is an entity?Entity is about(the description of) a person, an object, place or an event or even concept in the user environment about which an organization may wish to organize data.Where there is a name of "Relationship", it is obvious that this is a term belongs to the RDBMS(Relational Database Management System).And as we know that in this kind of DBMS we develop relations between two similar kind of entitiy instances which may posses (conditionally)different attributes('Charecteristics of an entity type which may be an interest of an organization') but must represent a common objective.Attributes have three primary types;Simple,Multivalued and Derived.Another term used quite fluently in E-R model is Instance or Entity Instance.
Instance is the occurance of an entity type.And is necessary to bring into notice before creating any sort of E-relationship is made.That was a brief introduction to Entity and terms linked to it.

Now what is a relationship?It is a binding factor which associates two or more (instances of)entity types together to form a smart Database which will be helpful for user(easily modified or updated) as well as for the MIS and DSS (to get information whenever needed in a detailed fashion).What happens when a relationship is created?Relationship acts like a glue to the entities to join them together.Or should I illustrate it more meaningfully as:"Relationship is an association among the instances of one or more than one entity types".
Here Part-I "introduction to Entity, Relationship and E-R Model" finishes here.In the next part I will describe how an E-R model is developed and what are different kinds of E-R models.

Part-II "Constructs of an E-R model and Types of relationships "
While constructing an E-R Model one should have good acquaintance with the cardinal constraints.
A constraint which specifies the number of the instances of an entity type for example B which may associate with another called A.It specifies minimum and maximum no of instances of an entity type.Possible constraints are Mandatory one,Mandatory many,optional one ,optional many and a specific no.A minimum cardinality of 0 specifies optional participation while 1 represents mandatory.
Relationships are of three types in general.Unary(relationship between the single entity type's instances);Binary(relationship between the instances of two entity types);Ternary(relationship between the instances of three entity types).
Degree is the number of entity types which participate in a relationship.(Unary;1,Binary;2 and Ternary;3).
In unary type of relationship there may be one-to-one or one-to-many relations.
In Binary type there may all the three major kinds involved;one-to-one,one-to-many and many-to-many(instances of entity type).
In ternary a simultaneous relationship between three different entity types('s instances)takes place.
Another thing which I want to mention here is the two types of entities.1.Strong Entity,2.Weak Entity.The independent entity which posseses its own identifier and doesn't need another entity to work is a strong entity.On contrary, the entity whose function depends upon the other entities and it may only posses a partial identifier is known as weak entity.
Part-III is another kind of E-R which is known as E-E-R(Enhanced Entity Relationship).

(Thanx yushrah and ABK )