Computer is an information tool. Networks enhance the computer's ability
to exchange, preserve and protect information. Network makes it easier to
share expensive hardware and software.
THE NETWORK
Computers connected over a network can make the information exchange easier and faster. The information moves directly from computer to computer rather than through a human intermediary. A group of computer and other devices connected together is called a NETWORK and the concept of connected computers sharing resources is called NETWORKING.
LAN Local Area Network
LANs are perfect fro sharing resources within a small geographical area
Approxiamtaly 500 M.
WAN Wide Area Network
The links WAN can made are as follows:
Over the telephone lines
Using satellite links
Packet Radio Networks
Microwave transceivers
Most WANs are private and are owned by the business that operates with them.
Forming private WANs are usually called VPN i.e. Virtual Private Networks.
As WAN suffers from limited bandwidth, a typical T1 line is considered as fast WAN link.
NETWORK STRUCTURES
A collection of machines running for the users is called HOST.
The hosts are interconnected through the communication subnets or just a subnets.
WAN consists of two distinct components:
Transmission line:
Are also commonly known as Circuits, Channels and trunks which moves bits between the machines / hosts.
Switching elements:
They are specialized for connection between two or more transmission lines.
It is commonly known as the Interface Message Processors i.e. IMPs.
Designs implemented through WAN
- point to point channels
when a message in the form of a packet is send from one IMP to another IMP via one or more intermediate IMPs, the packet is received at each intermediate IMP is in entirely, stored there until the required output line is free, and then forwarded.
A subnet using this principle is called point to point, store and forward or packet-switched subnet.
- broadcast channels
Broadcast Systems have a single communication channel shared by all other machines on the network. Packets sent by any machine are received by all the others.
NETWORK ARCHITECTURE
Protocol Hierarchies is design to reduce complexity.
The set of layers and protocol is called the Network Architecture.
Layer n on one Machine carries on a conversation with layer n on another machine. The rules and conventions used in this conversation are collectively known as Layer n Protocol.
Design Issues for Layers
- Every layer must have a mechanism for connection establishment.
- mechanisms for terminating when no longer connection required.
- Data Transfer Rules:
- Simplex Communication
- Half-duplex Communication
- Full duplex Communication
- Error control mechanisms
- Message Ordering
OSI REFERNCE MODEL An Overview
Open Systems Interconnection OSI, the layered model that dominated data communications and networking literature before 1990.
It was established in 1947, ISO International Standards Organization
Open Systems:
A set of protocols that allows any two different systems to communicate regardless of their underlying architecture.
Purpose of OSI model:
To facilitate communication between different systems without requiring changes to the logic of underlying hardware and software.
As the OSI model is not a protocol. It is model for understanding and designing a network architecture that is flexible, robust and interoperable.
Layered Architecture:
The OSI Model is composed of seven ordered Layers:
Layer 1 The Physical Layer
Layer 2 The Data Link Layer
Layer 3 The Network Layer
Layer 4 The Transport Layer
Layer 5 The Session Layer
Layer 6 The Presentation Layer
Layer 7 The Application Layer
