Network Protocols and the OSI Model Explained
Network Protocols
A protocol is a set of rules that enable the transfer of data between two or more computers.
Level Architecture
The purpose of the level architecture is to reduce the complexity of logically grouping data communication functions in certain areas of responsibility (levels).
Features
Each level provides services to the upper level and receives services from the lower level.
A message from a top level contains a header with information to be used in the receiving node.
The set of services that a level provides is called an entity, and each entity consists of a driver (manager) and an element (worker).
Standards
- OSI (International Organization for Standardization)
- IEEE (Institute of Electrical and Electronic Engineers)
OSI Model
In this model, the purpose of each level is to provide services to the higher level, freeing it from the implementation details of each service. The information that is sent from one computer to another must pass from the upper level to the lower level across all other levels in descending order within the computer that originates the data.
On their way through each level, data has information added that will be removed when it reaches its destination. The added information is classified as:
- Information Control: Aimed at its level in the target computer. Each level behaves as if it were communicating with its counterpart on the other computer.
- Interface Information: Addressed to the adjacent level with which it is interacting. The purpose of this information is to define the services provided by the lower level and how these services should be accessed. This information, after being employed by the adjacent level, is removed.
The OSI model is structured in seven levels:
Physical Level
This level addresses the transmission of bit streams without apparent structure on a means of connection. It is related to electro-optical, mechanical, and functional interface to the transmission medium. It is also responsible for providing the signal used to transmit data generated by the higher levels.
At this level, it defines how to connect the cable to the card of network, which pin each connector must have, and the functional use of each of them. It also defines the transmission technique to be used for sending data on the medium used. It is responsible for activating, maintaining, and deactivating a physical circuit. This level is the consolidation and synchronization of bits and is responsible for getting the bits from one computer to another.
Data Link Layer
This level is responsible, in the computer of origin, for staying in a logical grouping of bits, called a frame, with data from higher levels. In the target computer, it is responsible for grouping the bits from the physical level into data frames to be delivered to the network level. This level is responsible for ensuring the transfer of error-free frames from one computer to another through the physical level.
Network Layer
This layer is responsible for routing messages and converting logical addresses and names into physical addresses. It is also responsible for determining the appropriate path for the data based on network conditions, priority service, etc. The network layer comprises small fragments of messages to be sent together through the network.
Transport Layer
This layer is responsible for recovery and error detection. It also ensures the delivery of computer messages originating from the application level. The level is responsible for reporting to senior levels of the network status.
Session Layer
This layer allows two applications residing in different computers to set, use, and end a connection called a session. This level makes recognition of names and functions necessary for two applications to communicate across the network, as in the case of functions of security.
Presentation Layer
This layer determines the format to use for exchanging data over the network. It can be called the translator of the network. This level also handles security for broadcast; the network provides services such as data encryption.
Application Layer
This layer serves as a window for processes that require access to network services.