The TCP / IP layer set corresponds to the OSI reference model levels as follows:

This correspondence is theoretical because, as the TCP / IP protocols developed before the OSI reference model, there are substantial differences, such as:

Random Access, which is to issue a completely random time, relies on the Aloha method. The latter takes its name from an experiment performed on a network connecting the various islands of the Hawaiian Archipelago early 1970. In this method, when a coupler has information to transmit, it sends it without worry about other users. If there is a collision, that is to say superposition of two signals or more users, the signals become indecipherable and are lost. They are subsequently transmitted, as shown in Figure, in which the couplers 1, 2 and 3 collide. The coupler 1 transmits its field first because he shot the smallest timer. Then, the module 2 emits, and its signals collide with the coupler 1. Both derive a random time of retransmission. The coupler 3 is listening while the couplers 1 and 2 are silent, so that the frame of the coupler 3 passes successfully. Technical aloha is the origin of all the random access methods.

SDH recommendation was standardized by ITU-T (G.707 and G.708):

• G.707: Synchronous Digital Bit Rate;

• G.708: Network Node Interface for the Synchronous Digital Hierarchy.

Found in SDH flows at 155, 622 and 2488 Mbit/s SONET.

In the 1970s, the Department of Defense, or DOD (Department Of Defense), decided before the proliferation of machines using different, incompatible communication protocols, to define its own architecture. This architecture, called TCP / IP, is the source of the Internet. It is also adopted by many private networks, called intranets.

In 1969, the Advanced Research Projects Agency (ARPA), belonging to the United States Department of Defense, funded a project. ARPA established a network of switching packets of computers connected through rented point-to-point lines called the Network of the Agency for Advanced Research Projects (ARPANET), which provided the basis for the first investigations in network interconnection. The conventions developed by ARPA to specify how individual computers could communicate over the network became TCP / IP.

The TCP / IP protocol, acronym for Transmission Control Protocol / Internet Protocol, protocols used to control Internet transmission. It allows different types of computers or computers to communicate through heterogeneous networks.

Data encapsulation simply means adding the IP header to the data. The IP header consists of five or six 32-bit words; the sixth word is attributed to the IP options field.

For digital sources, two alternative technologies have evolved for multiplexing. These are Time Division Multiplexing (TDM) and Code division Multiplexing (CDM). TDM provides a way to merge data from several sources into a single channel for communication over telephone lines, a microwave system or a satellite system. TDM can be implemented in two ways. These are synchronous TDM and asynchronous TDM. Asynchronous TDM is popularly known as Statistical TDM.

CDMA (Code Division Multiple Access) also called spread-spectrum and code division multiplexing, one of the competing transmission technologies for digital MOBILE PHONES. The transmitter mixes the packets constituting a message into the digital signal stream in an order determined by a PSEUDO-RANDOM NUMBER sequence that is also known to the intended receiver, which uses. it to extract those parts of the signal intended for itself. Hence each different random sequence corresponds to a separate communication channel. CDMA is most used in the USA.

• In FDMA, the available bandwidth is divided into various frequency bands.

• Each station is allocated a band to send its data. This band is reserved for that station for all the time.

To reduce the impact of collisions on the network performance, Ethernet uses an algorithm called CSMA with Collision Detection (CSMA / CD): CSMA/CD is a protocol in which the station senses the carrier or channel before transmitting frame just as in persistent and non-persistent CSMA. If the channel is busy, the station waits. it listens at the same time on communication media to ensure that there is no collision with a packet sent by another station. In a collision, the issuer immediately cancel the sending of the package. This allows to limit the duration of collisions: we do not waste time to send a packet complete if it detects a collision. After a collision, the transmitter waits again silence and again, he continued his hold for a random number; but this time the random number is nearly double the previous one: it is this called back-off (that is to say, the "decline") exponential. In fact, the window collision is simply doubled (unless it has already reached a maximum). From a packet is transmitted successfully, the window will return to its original size.