In classic Ethernet, all end devices are connected to the same physical Ethernet segment and gain access to the transmission medium via the collision procedure. Such a network segment with its own collision procedure forms a collision domain.
A collision domain is a self-contained LAN segment that operates with a self-sufficient collision procedure. The background of a self-sufficient collision domain can be seen in the increasing delay time, which increases rapidly with the number of connected stations and the increased number of accesses to the transmission medium.
The boundary conditions for the size of the collision domain are as follows: During the duration of the smallest data packet, such a data packet must run from one end of the collision domain, for example station A, to the other end, and from there a possible collision signal must run back to the output station A before station A has fed the last bit of the data packet into the transmission medium. With the classic Ethernet 10 Mbit/s, the smallest data packet is 64 bytes or 512 bits. The clock rate of 10 Mbit/s is 100 ns, the resulting slot time is 51.2 µs. This slot time corresponds to the time in which the data packet travels from station A to the end of the transmission medium and back. For the single distance to the end of the collision domain, the max. signal propagation time is 25.5 µs. The size of the collision domain can then be calculated from the signal propagation time of the cable.
Collision domains in Ethernet
In classic Ethernet according to 10Base-5, the Yellow Cable had a shortening factor, the Nominal Velocity of Propagation( NVP), of 0.77c. This means that the signal propagation speed on the coaxial cable is about 230,000 km/s and the extent of the collision domain for a 10 Mbps signal is over 5 km. With this value, however, it must be taken into account that all transceiver cables with twice the length are included in the length specification. At higher clock rates of 100 Mbit/s, as with Fast Ethernet, or at 1 Gbit/s, the collision domains are reduced to one tenth in each case. However, since a sufficient safety reserve has been planned, the extension for Fast Ethernet has been specified at 200 m.
If two LAN segments are connected via a repeater, the collision domain is not affected because the repeaters are transparent for the access procedure. In contrast to repeaters, bridges and routers separate the collision domains.