Development of Mobile Networks :
The generations of mobile networks are as follows.
(i) First Generation(1G) 1981- NMT launch
(ii) Second Generation(2G) 1991-GSM Launch
(iii) Second to Third Generation Bridge (2.5)2000 — GPRS launch
(iv) Third Generation( 3G) 2003- UK 3G launch
(v) Fourth Generation (4 G) 2007
(vi) Fifth Generation (5G) 2019+
Merits of Mobile Networks :
(i) It provides both voice/data services.
(ii) It connects both fixed and wireless telephone users.
(iii) It is used in areas where cables cannot be laid out due to its wireless nature.
(iv) It is easy to maintain.
(v) It is easy to upgrade the equipments.
(vi) The ‘mobile and fixed subscribers are connected immediately with cellular network as soon as mobile phones are switched on.
(vii) All the handshake signals between mobile and base station are automatically exchanged.
Demerits of Mobile Networks :
(i) Cost
(ii) Vulnerable to Security risks
(iii) Additional training is needed to use new technology.
(iv) Cyber Crime.

Open System Interconnection (OSI) model was found in the year 1934, general framework that enables network protocols along with software and systems to be developed based on general set of guidelines. It describes the standards for the inter-computer communication.
OSI Layers:
(i) Physical Layer: This is the 1^st layer, it defines the electrical and physical specifications for devices.
(ii) Data Link Layer: It is the 2^nd layer and it guarantees that the data transmitted are free of errors. This layer has simple protocols like "802.3 for Ethernet" and "802.11 for Wi-Fi".
(iii) Network Layer: It is the 3^rd layer determining the path of the data packets. This layer is responsible for routing of data packets using IP Addressing.
(iv) Transport Layer: It is the 4^th layer that guarantees the transportation/sending of data succeddfully. It includes the error checking operation.
(v) Session Layer: It is the 5^th layer, identifies the established system session between different network entities. It controls dailogues between computers. For instance, while accessing a system remotely, session is created between your computer and the remote system.
(vi) Presentation Layer: It is the 6^th layer that does the translation of data to the next layer (Prepare the data to the Application Layer). Encryption and decryption protocols occur in this layer such as, Secure Socket Layer (SSL).
(vii) Application Layer: It is the 7^th layer, which acts as the user interface platform comprising of software within the system.

INTERNET:
(i) The Internet is a worldwide system of computer networks- A network of networks where the users at any one computer can get information from any other computer.
(ii) The Internet is a network of global connections - comprising private, public, business, academic and government networks connected by guided, wireless and fiber-optic technologies.
(iii) The unique aim was to generate a network that would permit users of a research computer from one university to "talk to" research computers on other universities.
(iv) The Internet denotes to the global communication system, including infrastructure and hardware, whereas the web is one of the services interconnected over the Internet.
INTRANET:
(i) Intranet is a private network within an enterprise to share company data and computing resources between the employees.
(ii) It may consist of many interlinked local area networks.
(iii) It includes connections through one or more gateway (connects two networks using different protocols together known as protocol convertor) computers to outside Internet.
EXTRANET: Extranet is a private network that uses Internet technology and the public telecommunication system to securely share business information with suppliers, vendors, partners, customers, or other business.