C01- Network Models

 A protocol can be defined as a set of rules that governs the communications between computers on network.

These rules include guidelines that regulates the following characteristics of a network: access method, allowed physical topologies, types of cabling, and speed of data transfer.

Both OSI reference and TCP/IP models are based on concepts of layering.

What is layering?

  • Dividing a task into sub-tasks and then solving each sub-task independently.
  • Establishing a well defined interface between layers make porting easier.
  • Some major advantages are:
  1. Code reusability.
  2. Extensibility
  3. Accelerates evolution.
  4. Simplifies teaching and learning.

The OSI (Open System Interconnection) Reference Model is basically a description for layered communications and computer network protocol and transmission design.

It divides data movement into seven layers from top to bottom and is often called the OSI Seven Layer Model.

The seven models includes:

  • Application Layer
  • Presentation Layer
  • Session Layer
  • Transport Layer
  • Network Layer
  • Data Link Layer
  • Physical Layer.
The first three layers are the Software Layers.
The Transport Layer is the Heart of the OSI model.
The last three layers are the Hardware layers.

Again, the OSI models are divided into two layers:
  1. The Host Layers
  2. Media Layers.
Again they are divided into 5 more layers based on data units:
  1. Data
  2. Segments
  3. Packets
  4. Frame
  5. Bit
Host Layers.

Data:
Application: It processes Network to Application.
Presentation: Helps in data representation, encryption and decryption.
Session: Interhost communication.

Segments:
Transport: End-to-end connections and reliability, Flow control.

Packet:
Network: Path determination and logical addressing.

Frame:
Data link: Physical addressing

Bit:
Physical: Media, signal and binary transmission.

Layer 1: Physical Layer:
  • It defines the relationship between a device and a physical medium.
  • This includes the layout of pin, voltages, cable specification, hubs, repeaters, network adapters, host bus adapters, and more.
  • The major functions and services performed by the Physical layer are: 
  •   Signals being sent and received in 0s and 1s.
  • some of its functions includes:
  1. Converts bits to signals.
  2. Bit synchronization
  3. bit rate control
  4. physical topology
  5. switching
  6. transmission mode.
Layer 2: Data Link Layer:
  • It provides the functional and procedural means to transfer data.
  • MAC address is used at this layer.
  • switches function at this layer.
Layer 3: Network Layer:
  • Performs routing functions.
  • Routers operate at this layer-sending data throughout the extended network and making the internet possible.
Layer 4: Transport Layer:
  • Provides transparent transfer of data between end users and providing reliable data transfer services to the upper layers.
  • Controls the reliability of a given link through flow control, segmentation/de-segmentation, and error flow control.
Layer 5: Session Layer:
  • Controls connections between computers.
  • Establishes, manages and terminates the connection between the local and remote application.
Layer 6: Presentation Layer:
  • This layer provides independence from differences in data representation.
  • It formats and encrypts data to be sent across a network, providing freedom from compatibility problems.
  • Sometimes called the syntax layer.
Layer 7: Application layer:
  • This layer is the closest to the end user.
  • Its functions typically include identifying communication partners and determining resource availability.
  • Some examples of its implementations include:
  1. Hypertext Transfer Protocol (HTTP).
  2. File Transfer Protocol (FTP).
  3. Simple Mail Transfer Protocol (SMTP).
Computers transfer information across networks in short bursts of about 1500 bytes of data (bursty- unlike video streaming).

TCP/IP Model

  • It forms the base of present-day internet.
  • TCP and IP are two protocols of this model.
  • It was initially used by APARNET.
  • It has four layers:
  1. Host-to-network.
  2. Internet.
  3. Transport.
  4. Application.
Host-to-network Layer:
  • The lowest layer of the TCP/IP model
  • Its main purpose is to connect the host to the network.
Internet Layer:
  • Responsible for delivering IP packets to their destinations.
  • An important protocol of this layer is the Internet protocol (IP).
Internet Protocol:
  • It is an unreliable and connectionless protocol.
  • Transports data in packets called datagrams.
  • Does not keep track of the routes.
TCP:
  • Transport layer uses TCP for reliable connection-oriented services.
  • Some various functions of TCP are error control, flow control, and sequencing.
UDP:
  • Transport layer uses this for unreliable connectionless services.
  • No assurance that packet reached.
  • No sequencing nor error checking.
  • Useful in real time data transfer and quick transfer of large data.
  • It follows deliver us more important than accurate delivery.
Some similarities between OSI and TCP/IP models.
  • Both are based on the concepts of a stack of independent protocols.
  • functionality is similar for both.
  • Up to transport is network oriented while above it is user oriented.

Some differences are:
  • OSI model has seven layers while TCP/IP model has only 4 layers.
  • OSI model provides clear distinction between services, interfaces and protocols while the TCP/IP model does not.
  • In OSI model, transport layer is connection oriented while in TCP/IP model it is both connection oriented and connectionless.




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