The OSI Reference Model
The OSI bla bla bla bla
This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing. The Physical layer is primarily concerned with the transmission of bits.
Defines physical means of sending data over network devices.
Interfaces between network medium and devices.
Defines optical, electrical and mechanical characteristics. UTP, coaxial cable, FDDI (fiber), RS232, or even RF.
The purpose of this layer is to make sure that the bits transmitted over the physical layer are transmitted free of errors. Bridges operate here.
The level 3 deals primarily with determination of route path. Each host receiving the message "looks inside" the data i.e. analyzing frames and packets to determine what is the next destination for the received packet.
This layer provides to the upper layers a means of transmitting "data-grams" over the network to a specified host. This datagram service provides no confirmation of safe delivery of the information. The transmissions are "connectionless" meaning that there is no "continuing conversation" set up between the two hosts. One datagram may have nothing to do with the next one, and indeed, may arrive out of order if they are related. This layer is also responsible for assigning addresses to the hosts and routing packets between interconnected networks.Routes packets according to unique network device addresses.
Provides flow and congestion control to prevent network resource depletion.
Routers operate on this Layer.
The main function of this host-host layer is to accept data from the layer above, session layer, split it into smaller parts, if necessary, pass it to the network layer (below) and make sure that the data is successfully received by the other end. All this must be done in such a way so as to the session layer is completely shielded from the changes at lower layers. This layer provides methods of flow control, ordering of received data, and acknowledgement of correctly received data. It duplicates, senses errors and discards bad packets.
Reports upper
layer errors. This layer establishes connections or 'conversations' between
processes over the network.
Masks the differences of data formats between dissimilar systems.
Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data.
This is the layer where network applications that facilitate network connectivity reside (file transfer/FTP, virtual terminal/Telnet, and electronic mail, for example), and where service advertisement and service availability are managed. Service Advertisement is the process of letting systems know which services are available on the network. Once it has been made advertised, it must be made available to the computers local operating system. Responsible for final set-up and transfer of message to application requesting it.
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