February 4, 2002 Week 5: Day 1
Welcome to Lesson 5 - Making Networks Work
Lesson 5 Notes
OSI and 802 Networking Models
These two theoretical models are extremely important in understanding how networks
work.
Role of a Reference Model
The OSI reference model provides a standard framework to discuss what takes
place when two devices communicate.
“Open Systems” is the
ultimate goal of the OSI model. It has provided a basis for non-proprietary
devices and protocols.
OSI Network
Reference Model
The OSI reference model is made up of layers. These layers are centered around the idea that every function in a network can be broken up into a series of related tasks. An example which can be used to illustrate layers is vehicle production. Each team has its own function (design, engineering, production) and relies on the others to complete the task—putting a quality automobile in your garage.
OSI Reference Model Structure
The OSI reference model is divided into seven layers. Each layer performs a
specific tasks in network communication and will be discussed in detail momentarily.
Each layer interacts with the layer
above and below it. For example, the Session layer works with both the Transport
layer (immediately below), and the Presentation layer (immediately above).
The layers can be generally grouped
together according to function. Layers 1 and 2 are concerned with the physical
media, signaling, etc. Layers 3 and 4 deal with getting the data to the destination
across the network. Layers 5 through 7 handle conversations between the computers.
Each layer communicates with its
peer layer on the other computer which is often called logical or virtual communication.
This means that the Transport layer on the sending computer communicates, by
using the services of the lower layers, with the Transport layer on the receiving
computer.
As data is passed down through the protocol stack, each layer adds its own information
to be used by its peer layer on the receiving computer. When the data is received
and passed up through the layers, each layer removes the information attached
by its peer on the sending computer.
On its way through the stack, before it gets passed from one layer to the next, data is broken down into units called packets or payloads. The official OSI term here is PDU or protocol/packet/payload data unit. At each layer of the stack, the software adds its own special formatting or addressing to the PDU to allow successful delivery of its payload across the network.
OSI Model Layers
| Application | The Application layer provides the interface to the user and makes decisions on whether or not network access is needed. |
| Presentation | The Presentation layer handles data compression/decompression and encryption/decryption. |
| Session | The Session layer manages the “conversation” between applications and is responsible for synchronization |
| Transport | The Transport layer is responsible for segmenting the stream of data from the upper layers into manageable pieces. It also handles error-free delivery and thus manages flow control and error handling. |
| Network | The Network layer’s responsibility is to direct data in an internetwork. It is also responsible for logical network address to physical network address conversion. |
| Data Link | The Data Link layer is responsible for addressing the packets and defining the structure of the packets. This structure is called the frame and is discussed in more detail in Chapter 6. |
| Physical | The Physical layer converts bits into signals to be sent across the network. |
We will cover the 802 Networking Model and IEEE specifications in the next lesson.
Here is a question to ponder:
Develop an example for how layers work, then, using that example, assign names to your layers and specify their specific tasks. There are many different things you could use as examples of layers. The key is that each layer performs a specific function. Some examples might include driving a car to the store or any type of communication, phone calls, meetings, etc. Each of these has a specific set of guidelines, which govern what takes place.
See you on Friday!
mdm