OSI Reference Model

The Open System Interconnect (OSI)

The OSI reference model was developed by the International Standard Organization (ISO) to establish a framework of standards for computer-to-computer communication. The OSI model allows hardware and software companies to develop their products to work within certain parameters and guidelines of the model. This allows products to work with other products that operate within these guidelines.

The “Open” in OSI means that the model deals with systems that are open for communication with other systems. The OSI model is also called a layered protocol because of the seven (7) layers that its comprised of.

Benefits of the OSI Model

  • A network layer can be replaced by a layer from another network vendor.
  • Networks can be upgraded easily by replacing individual layers
  • The user and network designer are not restricted to using the product (hardware/ software) of a specific vendor.

Layer Function

Layer 1: Physical

This layer defines the the standards that provide guidelines on how to move data bits between modems. This layer also specifies the electrical connections between the transmission medium and the computer.

Layer 2: Data Link

This layer is responsible for ensuring that the data transmitted between two locations on the network is accurate. This layer also controls access to the network and breaks up data to be sent into frames. The data link layer solves competition problems using Media Access Control (MAC) and Carrier Sense Multiple Access/Collision Detection (CSMA/CD). This layer also solves lost, duplicated or destroyed frames.

Layer 3: Network

The network layer is concerned with determining how packets are routed from source to destination. Routes (paths) can be hard-wired into the LAN or they can be determined at the time a packet is sent. This layer solves congestion problems and ensures that the line is not overloaded with packets while other lines are under-utilized.

Layer 4: Transport

The basic function of the transport layer is to accept data from the session layer, break them into smaller pieces and pass them on to the network layer. This layer also ensures that pieces of data arrive at the other end correctly.

Layer 5: Session

The session layer allows users on different machines to establish and terminate a session between them. A session is used to allow a user to log into a remote time-sharing system or to transfer a file between two machines. The session layer manages communication and keeps track of whose turn it is to talk.

Layer 6: Presentation

This layer ensures that the receiving computer understands the information sent to it. Different computers represent data differently and it is the responsibility of this layer to provide a standard encoding (data representation) to be used by computers in the network. Data is formatted using this standard encoding before it is presented to destination machines. This layer is also responsible for data compression and encryption.

Layer 7: Application

This layer facilitates user functions (e.g. File Transfer, Email) and provides guidelines for network services.

Network Topologies

Computers can be connected together in many different ways. The layout/topology of the network will influence how reliable the network is and how easy it is to access. There are two types of topologies:Wired and Wireless. The main physical wired topologies are: Star, Ring, Bus and Mesh.

STAR TOPOLOGY

In a Star configured network, all devices are connected to a central device called a hub. Nodes communicate across the network by passing data through the hub.

Advantages:

  1. New stations can be added easily
  2. A single cable failure won’t bring down the network
  3. Relatively easy to troubleshoot

Disadvantages:

  1. Single point of failure – if hub goes down, the network goes down
  2. Total installation cost may be high due to the number of cables needed for each node

RING TOPOLOGY

All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. When a computer sends data, the data travels to each computer on the ring until it reaches it destination.

Advantages:

  • Growth of ring has minimal effect on performance
  • Each node is able to filter and amplify the data before sending it to the next node.
  • Can cover a larger area than star.

Disadvantages:

  • A break in the cable will shut down the network
  • Most expensive topology
  • One computer failure affects others
  • Difficult to add or remove stations

BUS TOPOLOGY

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Devices are connected to a single central cable called the bus or backbone. Using the BUS and destination address, data is transmitted from source to destination along the backbone.

Advantages:

  1. – Inexpensive and easy to install
  2. – You can add and subtract devices without affecting the network
  3. – Failure of one device does not affect the network
  4. – Requires less cabling than star

Disadvantages:

  1. Network shuts down if there is a break in the cable
  2. Terminators are needed at both ends of the cable
  3. Difficult to troubleshoot

MESH TOPOLOGY

Devices are connected with many redundant interconnections between nodes.

Advantages:

  1. Failure of one node doesn’t affect the network
  2. Easy to expand
  3. More secure

Disadvantages:

  1. Very expensive due to the number of cable that would be required
  2. Difficult to implement