OSI Open System Interconnection (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design.In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

A class is the code for an object and an object is the instance of a class in memory. When you create an object from a class it is called instantiating the object.

A definition by its official site:-

"A class is a blueprint or prototype from which objects are created. This section defines a class that models the state and behavior of a real-world object. It intentionally focuses on the basics, showing how even a simple class can cleanly model state and behavior."

There are 7 Layers of the OSI model which are:-

1. . Application Layer (Top Layer)

2.  Presentation Layer

3.  Session Layer

4.  Transport Layer

5.  Network Layer

6.  Data Link Layer

7.  Physical Layer (Bottom Layer)

An object-relational database (ORD) or object-relational database management system (ORDBMS) is a database management system (DBMS) similar to a relational database, but with an object-oriented database model: objects, classes and inheritance are directly supported in database schemas and in the query language. In addition, it supports extension of the data model with custom data-types and methods.
Example of a Object-Oriented Database Model.

In such a database the data and relations between them are organised in tables. A table is a collection of records and each record in a table contains the same fields.

A relational database contains multiple tables, each similar to the one in the "flat" database model. One of the strengths of the relational model is that, in principle, any value occurring in two different records (belonging to the same table or to different tables), implies a relationship among those two records.

Network model can be seen as refinement of hierarchical model.Hierarchical model provide only 1:M relationship(Means one parent and multiple childs) but now in network model it provide M:M relationship(Many To Many relationship means each child can have multiple parents and parents can have multiple childs as well).The network model is a database model conceived as a flexible way of representing objects and their relationships.

• The network model allows each record to have multiple parent and child records, forming a lattice structure.

In a hierarchical model, data is organized into a tree-like structure, implying a single upward link in each record to describe the nesting, and a sort field to keep the records in a particular order in each same-level list. Hierarchical structures were widely used in the early mainframe database management systems, such as the Information Management System (IMS) by IBM, and now describe the structure of XML documents. This structure allows one 1:N relationship between two types of data.

The table model consists of a single, two-dimensional array of data elements, where all members of a given column are assumed to be similar values, and all members of a row are assumed to be related to one another. For instance, columns for name and password that might be used as a part of a system security database. Each row would have the specific password associated with an individual user. Columns of the table often have a type associated with them, defining them as character data, date or time information, integers, or floating point numbers.

A database model or database schema is the structure or format of a database, described in a formal language supported by the database management system. Schemas are generally stored in a data dictionary.A data model is a collection of concepts for describing data.

A schema is a description of a particular collection of data, using the a given data model.

There are some disadvantages of the Waterfall Model.
1) As it is very important to gather all possible requirements during the Requirement Gathering and Analysis phase in order to properly design the system, not all requirements are received at once, the requirements from customer goes on getting added to the list even after the end of "Requirement Gathering and Analysis" phase, this affects the system development process and its success in negative aspects.