Modem: Modem combined with two words Modulator and Demodulator. A device that can convert a digital bit stream into an analog signal for transmission over some communication channel is called Modulation and can convert incoming analog signals back into digital signals known as Demodulation. Modems are used to connect digital devices across analog transmission lines .
Protocol: An entity is capable of sending or receiving information and a system is a physically distinct object that contains one or more entities. The key elements of Protocol are:
ii. Semantics: Includes control information for co-ordination and error handling.
iii. Timing: Includes speed matching and sequencing.
HDLC is an example of Protocol.The data to be exchanged must be sent in frames of a specific format.
Ethernet: The most commonly used medium access control techniques for bus/tree and star topologies is multiple access with collision detection or CSMA/CD. CSMA/CD can be termed random access or contention techniques.They are random access that there is no predictable or scheduled time for any station to transmit; station transmissions are ordered randomly. An important rule followed in CSMA/CD systems, Including the IEEE standard, is that frames should be long enough to allow collision detection prior to the end of transmission.
Broadband: Communication uses a modulation to apply a data signal to a transmission medium in analog form. Multiple signal can be present simultaneously using Frequency division multiplexing. Ethernet are inherently broadband in nature. Communication systems that use Broadband networking techniques on Coaxial cable. There are two classifications of Broadband Coaxial system. In one way system signal travel in only one direction in the cable. This kind of system is Cable TV systems. In two-way system signals can travel in both directions on the cable.Two-way systems fall into mid-split or sub-split categories .
ATM: ATM stands for Asynchronous Transfer Mode. ATM is widespread application its use as part of broadband. ATM does not provide error control and flow control mechanisms. ATM is the data transfer interface for B-ISDN. ATM is not based on HDLC,but it is based on completely new frame format known as cell, that provides minimum processing overhead. The cell has a fixed length of 53 octets or 424 bits.
Frame Relay: Frame Relay is a data link control facility designed to provide a capability for use over high-speed packet-switched networks. Frame relay provides a more efficient packet switching than X.25 and widespread use, not only in ISDN but in other networking context. A critical component of Frame relay is congestion control.
ISDN: The ISDN is intended to be a worldwide public telecommunications network to replace existing public telecommunications network. ISDN support of a wide range of voice and non-voice applications in same network. ISDN support a variety of applications including both switched and non-switched connection.
An ISDN will contain intelligence for the purpose of providing service features , maintenance and network management functions. ISDN will be based on the concepts for telephone IDN. ISDN support both circuit switching and packet switching.
The components of DBMS are the following :
1. DDL Compiler : DDL compiler processes schema definitions, specified in the DDL, and, stores description of the schemas in the DBMS catalogue. The catalogue includes information as the name of file, data items, mapping information among schemas and constraints.
2. Run-time database processor : It handles database accesses at run time; it receives retrieval or update operations on the database. The query compiler handles high-level queries that are entered interactively.
3. Pre-compiler : The pre compiler extracts DML commands from an application program written in a host programming language. These commands are sent to the DML compiler for compilation into object code for database access.
Restricting Unauthorized Access
Providing Multiple user interfacing
Providing Backup and Recovery
Representing complex relationship among data
Enforcing integrity constraints
1. A foreign key can refer to
its own relation .
2. One or more Foreign key
can be treated in Parent
3. Foreign key constraints
can be defined at the
column or table constraint
1. The Primary key clause speci-
-fies one or more attributes that
make up primary key of a
2. One Primary key can be treated
for each table .
3. Primary key constraint is a
column or set of columns that
uniquely identifies each row in
in a table .
Entity integrity : The entity integrity constraint state that no Primary key value can be NULL, because the primary key value is used to identify individual tuple in a relation having NULL values. Key constraints and entity integrity constraints are specified on individual relations.
Relation : A relation is an association among several entities. A relation is a set of relationship of the same type. A relation may also have descriptive attributes. The number of entity set that participate in a relationship is also the degree of the relationship set.
To create a database it is not necessary to adopt a database management system we can create its own database system with the help of some conventional languages like C , C++ ,VB etc. But this type of database system require complex programming, complex retrieval, complex query management.
Due to this reason,we adopt the database management environment. So that, we retrieve our data easily and with consistency.
The DBA ( Database Administrator ) creates the database schema by a set of definitions, that is, translated by the DDL compiler, i.e. stored permanently in the Data dictionary.
The description of a database is called the database Schema, which is specified during database design.
The three-schema architecture is to separate the user applications and the physical database :
- The Internal level has an Internal schema , which describes of the database .The internal schema uses a Physical data model.
- The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of a user. A high-level data model or an implementation data model can be used at this level .
- The External or View level includes a number of External schemas or User views . Each External schema describes the part of database that a particular user group is interested in and hides the rest of the database from the user group .
For example : A schema for a Student and a Course are
A collection of concepts that can be used to the structure of a database. By structure of a database, the data types, relationships, and constraints that should hold on data.
Categories of data model are :
1. High-level or Conceptual data model provides concepts that are close to many users perceive data.
2. Low-level or Physical data model provide concepts that describe the details of how data is stored in the computer
3. Representational or Implementation data models ,which provide concepts that may be understood by end users but that are not too far removed from the way data is organized within the computer.
4. Conceptual data models use concepts as –entities , attributes and relationship . Entity-relationship model a popular high-level conceptual data model .
WWW : WWW stands for World Wide Web or Web is A system that has made to access any document on the net , including sound , pictures , video. On the WWW , information is displayed in the form of Web pages . The Web pages can hold :
Web pages also contain Hyperlink. Hyperlink are text or other objects that are linked to other pages.
TFTP : TFTP stands for Trivial File Transfer Protocol , supports the transfer of small Files without user authentication . It transfers 512-byte packets using unacknowledged UDP services . RFC 1350 defines TFTP . The syntax of TFTP is that :
tftp [ -i ] host [ get / put ] source [ destination ]-i
This option is used to transfer the file in Binary mode , byte-by-byte .