The OSI Reference Model

The model is called the ISO OSI (Open systems  Interconnection) model because it deals with connecting open systems i.e, systems that are open for Communication with other systems, The OSI model is not a network architecture because it does not specify. the exact services and protocols to be used in each layer, It just tells what each layer should do.

The OSI model has seven layers  - (following is an overview)

1. The Physical layer

The physical layer is concerned with transmitting raw bits over a communication channel.
This layer deals with mechanical and electrical specifications of  primary connections such as cables, connectors and signaling options that physically link 2 nodes on a network

2. The Data Link Layer (DLL)

The Data Link Layer is responsible for delivering data units from one station to the next without errors.

A data unit with header and trailer is called a frame.

Responsibilities: 

a. Node-to-node delivery.
b. Addressing: Physical addressing is done at data link layer.
c. Access Control: When two or more devices are connected to the same link, which device has control over the link is decided by DLL.
d. Flow Control : Flow control avoids the fast transmitter from drowning the slower receiver.
e. Error control: Error control means error correction. It allows the receiver to inform the sender of any frame lost or damaged in transmission and co-ordinates the re-transmission of those frames by the sender.
f. Synchronization:

DLL is divided into: 

1.  LLC 
2.  MAC 

3. The Network Layer: 

The network layer is responsible for source to destination delivery of packet across multiple network links.

Responsibilities:

a. Source to destination delivery.
b. Logical addressing.
c. Routing
d. Multiplexing

4. The Transport Layer: 

The transport layer is responsible for source to destination delivery of entire message.

Responsibilities : 

a. End-to-end message delivery
b. Service-point(port or sockect) addressing Guaranteeing delivery of message to appropriate application of computer.
c. Segmentation and reassembly.
d. Connection control deciding whether or not to send all packets by a single path.

5. The Session Layer:

One of the services of the session layer is to manage dialogue control.
It establishes, maintains and synchronizes. the interaction between communicating devices.

Responsibilities :

a. Session management : 
Introduction of checkpoints and dialog units appropriate for transmission.

b. Synchronization: 
Deciding in what order to pass the dialog units to the transport layer and where in the transmission to require confirmation from the receiver.

c. Dialog control: 
Deciding who sends and when.

d. Graceful close: 
Ensuring that exchange has been appropriately completely before the session closes.

6. Presentation Layer: 

The Presentation layer ensures interoperability among communicating devices.
It provides the necessary translation of different control codes, character sets, graphics characters.

Responsibilities:

a. Translation
b. Encryption
c. Compression
d. Security validating passwords and login codes.

7. Application layer: 

The Application 'layer provides user interfaces and suppprt for services such as e-mail, remote file access and transfer, shared database management and others.

Applications:

a. Network virtual terminal
b. Mail services
c. File access, transfer and management
d. Directory services.

Some Definitions

1. Protocol

A protocol is a set of rules that govern data communication.
A protocol defines what is communicated and when it is communicated.

2. Internet

A collection of interconnected networks is called an Inter network or just Internet. A common form of internet is a collection of LAN's connected by a WAN.

3. Intranet

• An intranet is a private network that is contained with an enterprise. It may consist of many interlinked local area networks and also use leased lines in the wide area network. 
• An intranet includes connections through one or more gateway computers to the outside internet. 
• The main purpose of intranet is to share company information and computing resources among employees. 
• An intranet can also be used to facilitate working in groups and for teleconferences. 

Topology

The term topology refers to the way a network is laid out, either physically or logically.
Five basic technologies are :

MESH

Every device has dedicated point-to-point link to every other device.
(n(n - 1))/2 physical channels are required to link n devices.

Advantages

1. No Traffic 
The use of dedicated links guarantees that each connection can carry its data load, thus eliminate traffic problems.

2. Robust
If one link becomes unusable, it does not affect the entire problem. .

3. Privacy or Security
Every message travels along a dedicated link, hence only the intended recipient sees it.

4. Fault identification' and fault isolation is easy.

Disadvantages

1. Installation and reconfiguration is difficult.
2. The sheer bulk of wiring can be greater than the available space can accommodate.
3. The hardware required to. connect each link (I/O port and cable) are expensive.

STAR 

Each device has a dedicated point-to-point link only to a central controller, usually called a hub.

Advantages 

1. Less expensive than mesh topology. 

2. Easy to install and configure 

3. Less cabling required compared to mesh 

4. Robust

One link is damaged then the complete system is not affected. This also leads to easy fault identification and isolation.

Disadvantages

1. More cabling compared to bus or ring topology. 

2. Central controller is required. 

TREE: (Variation of STAR)

In TREE majority devices are connected to a secondary hub that in turn is connected to the 

central hub.

Advantages

1. Less expensive than mesh topology.
2. Easy to install and configure
3. Less cabling required as compared to mesh
4. Robust
5. More devices can be attached to a single hub.
6. Allows network to isolate and prioritize communication from different Computers.

Disadvantage

1. More cabling compared to bus or ring topology.
2. Central controller is required.

BUS

It uses multi point configuration.
One long cable acts as a backbone to link all devices in the network.

Advantages

1. Ease of installation
2. Backbone cable can be laid along the most efficient path.
3. Less cabling required.

Disadvantages

1. Difficult reconfiguration and fault isolation.
2. Signal reflection at the taps can cause degradation in quality,
3. A fault or break in the bus cable stops transmission.

RING

Each device has a dedicated point-to-point line configuration only with the two devices on other side of it.

Advantages

1. Relatively easy to install and configure.

Disadvantages

1. Unidirectional traffic can be a disadvantage.
2. A break in the ring can disable the entire network.

Transmission Mode

Transmission mode term is used to define the direction of signal flow between two linked devices.
There are three types of transmission modes:

1. Simplex: 

In simplex mode, the communication is unidirectional. Only one of the two stations on a link can transmit, the other can only receive.

e.g. keyboard, T.V., pager.

2. Half duplex: 

In half duplex mode, each station can both transmit and receive, but not at the same time. e.g. Walky talky.

3. Full duplex: 

In full duplex mode, both stations can transmit and receive simultaneously. e.g. Telephone.

Network

A network is a set of devices (often referred to as nodes) connected by a media links. The link connecting the devices are called communication channels.

Network Goals

1. Resource sharing
2. High Reliability
3. Saving money
4. Powerful communication medium

Transmission technology

Broadcast networks : Broadcast network have a single communication channel that is shared by all the machine on the network. Packets sent by any machine are received by all the others. An address field within the packets specifies for whom it is intended. Upon receiving the packets, the machine checks the address field. If the packets is intended for some other machine, it is just ignored.

Point-to-Point Networks : Point-to-point network consist of many connections between individual pairs of machine. To go from the source to destination, a packet on this type of network may have to first visit one or more intermediate machines.

As a general rule, (although there are exceptions), smaller geographically localized network tends to use broadcasting, whereas larger network usually are point-to-point. 

Network Topologies

Local Area Network (LAN) : LAN's are privately-owned networks within a single building or campus of upto few kilometers in size.

They are widely used to connect personal computers and workstations in a company offices and factories to share resources and exchange information.

Traditional LAN's runs at speeds of 10 to 100Mbps.

Metropolitan Area Network (MAN) : MAN is basically a bigger version of LAN and normally similar technology. It might cover a group of nearly corporate offices or acity and might be either private or public.

A MAN can support both data and voice, and might even be related to local cable television network.

Wide Area Network (WAN) :  A W AN spans a large geographical area, often a country Or continent. It contains a collection of machines intended for running user programs. 

In most wide area networks, the subnet consists of two distinct components : transmission lines and switching elements. 

Transmission lines move bits between machines. 

The switching elements are specialized computers used to connect two or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line to forward them on. 

Line Configuration

Line configuration refers to the way two or more communication devices attach to link. There are two possible line configurations - 

Point-to-point 

A point-to-point line configuration provides a dedicated link. between two devices. 

Multipoint

A multipoint (also called multidrop) line configuration is one in which more than two specific devices share a single link.