Ethernet (also known as IEEE 802.3 standard) is a data transmission standard for local area networks based on the following principle:
All machines on an Ethernet network are connected to the same communication line, made up of cylindrical cables
Different variants of Ethernet technologies are distinguished according to the type and diameter of the cables used:
|10Base2||Thin Ethernet||Coaxial cable (50 Ohms) with a thin diameter||BNC||10 Mb/s||185m|
|10Base5||Thick Ethernet||Coaxial cable with a thick diameter (0.4 inch)||BNC||10Mb/s||500m|
|10Base-T||Standard Ethernet||Twisted pair (category 3)||RJ-45||10 Mb/s||100m|
|100Base-TX||Fast Ethernet||Double twisted pair (category 5)||RJ-45||100 Mb/s||100m|
|100Base-FX||Fast Ethernet||Multimode fibre optic (type 62.5/125)||100 Mb/s||2 km|
|1000Base-T||Gigabit Ethernet||Double twisted pair (category 5)||RJ-45||1000 Mb/s||100m|
|1000Base-LX||Gigabit Ethernet||Monomode or multimode fibre optic||1000 Mb/s||550m|
|1000Base-SX||Gigabit Ethernet||Multimode fibre optic||1000 Mbit/s||550m|
|10GBase-SR||10Gigabit Ethernet||Multimode fibre optic||10 Gbit/s||500m|
|10GBase-LX4||10Gigabit Ethernet||Multimode fibre optic||10 Gbit/s||500m|
All the computers on an Ethernet network are connected to the same transmission line, and communication is carried out using a protocol called CSMA/CD (Carrier Sense Multiple Access with Collision Detect which means that it is a multiple access protocol with carrier monitoring (Carrier Sense) and collision detection).
With this protocol any machine is authorized to transmit over the line at any time and with no concept of priority between machines. This communication is conducted simply:
This principle is based on several limitations:
The waiting time varies depending on the frequency of collisions:
Until now, the Ethernet topology described has been that of the shared Ethernet (any message transmitted is heard by all connected machines, available bandwidth is shared by all machines).
Over several years a significant development has taken place: that of the Switched Ethernet.
The physical topology remains a star, but is organized around a switch. The switch uses a filtering and switching mechanism very similar to that used by gateways where these techniques have been used for a long time.
It inspects the source and destination addresses of messages, draws up a table which then allows it to know which machine is connected on which port of the switch (in general this process is done by self-learning, i.e. automatically, but the switch manager can carry out additional adjustments).
Knowing the recipient port, the switch will only transmit the message on the appropriate port, the other ports remaining free for other transmissions which can be carried out simultaneously.
The result is that each exchange can be carried out at nominal speed (greater division of the bandwidth), without collisions, with a noticeable increase in the network bandwidth as a result (also at nominal speed).
As for knowing whether all the ports on a switch can communicate at the same time without loosing messages, that depends on the quality of the switch (non blocking switch).
Since switching makes it possible to avoid collisions and the 10/100/1000 base T(X) technologies have separate circuits for transmission and reception (a twisted pair per transmission direction), the majority of modern switches make it possible to deactivate collision detection and move into full-duplex mode on the ports. In this way, the machines can transmit and receive at the same time (which again contributes to network performance).
Full-duplex mode is particularly interesting for servers which must serve several clients.
Modern Ethernet switches also detect the transmission speed used by each machine (autosensing) and if the machine supports several speeds (10 or 100 or 1000 megabits/sec) begin by negotiating with it to choose a speed as well as the semi-duplex or full-duplex mode of the transmission. This makes it possible to have a store of machines with different performances (for example a set of computers with various hardware configurations).
As the traffic transmitted and received is no longer transmitted over all ports, it becomes much more difficult to sniff what is happening. This contributes to the general security of the network, which is a highly sensitive subject today.
To finish, the use of switches makes it possible to build geographically larger networks. On shared Ethernet, a message must be able to wait for any other machine on the network for a specified time period (slot time) without which the collision detection mechanism (CSMA/CD) does not work correctly.
This no longer applies with Ethernet switches. The distance is no longer limited except by the technical limits of the medium used (fibre optic or twisted pair, strength of signal transmitted and sensitivity of the receiver,...).