HPFOCUS

Understanding the Host-to-Host Communications Model
Older model

Proprietary
Application and combinations software controlled by one vendor

Standards-based model

Multivendor software
Layered approach

Why a Layered Network Model?

Reduces complexity
Standardizes interfaces
Facilitates modular engineering
Ensures interoperable technology
Accelerates evolution
Simplifies teaching and learning

The Seven Layers of the OSI Model

The OSI reference model defines the network functions that occur at each layer.

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.
The data link layer defines how data is formatted for transmission and how access to the physical media is controlled.
The network layer provides connectivity and path selection between two host systems that may be located on geographically separated networks.
The transport layer segments data from the system of the sending host and reassembles the data into a data stream on the system of the receiving host.
The session layer establishes, manages, and terminates sessions between two communicating hosts.
The presentation layer ensures that the information sent at the application layer of one system is readable by the application layer of another system.
The application layer provides network services to the applications of the user, such as e-mail, file transfer, and terminal emulation.

Data Encapsulation
Data De-Encapsulation
Peer-to-Peer Communication

TCP/IP Stack
Defines four layers
Uses different names for Layers 1 through 3
Combines Layers 5 through 7 into single application layer


TCP/IP Stack vs. the OSI Model

The OSI reference model defines the network functions that occur at each layer.

TCP/IP is now the most widely used protocol for a number of reasons, including its flexible addressing scheme, its usability by most operating systems and platforms, its many tools and utilities, and the need to use it to connect to the Internet.

The components of the TCP/IP stack are the network access, Internet, transport, and application layers.

The OSI model and the TCP/IP stack are similar in structure and function, with correlation at the physical, data link, network, and transport layers. The OSI model divides the application layer of the TCP/IP stack into three separate layers.