Although Ethernet was originally designed as a coaxial bus system, alternate physical layers have evolved since the early 80s. The IEEE 802 committee has defined several physical layers and that is why it is important to specify the correct option when selecting Ethernet.

10BASE5

The original Ethernet was configured as a bus system with a thick coaxial cable as the medium. That is what was specified in the 1980 DIX standard. An external transceiver called a medium attachment unit (MAU) clamps at particular points on the cable marked by stripes every 2.5 meters. From the transceiver, an attachment unit interface (AUI) cable connects to an AUI port on the actual Ethernet adapter that fits into the computer. The AUI port is a DB-15 connector. A coaxial segment can be up to 500 meters long and AUI cables are each restricted to 50 meters in length. A total of 100 transceivers can occupy one trunk segment. Individual trunk segments can be cascaded using repeaters up to 2000 meters. In 1985 the IEEE standardized this configuration as 10BASE5 to signify 10 Mbps baseband signaling up to 500 meters in length.

Thick coaxial cable is indeed bulky and its topology is not always convenient to wire in a plant. Troubleshooting a 100-station segment could be a nightmare, so you do not see new 10BASE5 installations. There is no support for this cable with Fast Ethernet technology.

10BASE2

The answer to the bulkiness of 10BASE5 along with its expense was Thinnet or Cheapernet standardized in 1985 as 10BASE2. Thinnet again was a bus topology but this time with internal transceivers. A thin RG-58/u coaxial cable interconnects up to 30 stations to a maximum length of 185 meters. Segments can be repeated up to 740 meters. BNC style connectors, terminators and taps are used to cable the system. Although easier to install than 10BASE5, the focus on new installations is towards twisted-pair cabling. This cable is likewise not supported by Fast Ethernet.

10BASE-T

In 1990 the IEEE published 10BASE-T after pioneering work was done to introduce twisted-pair cabling and star topology to Ethernet installations. The 10BASE-T Ethernet adapters have internal transceivers and RJ-45 connectors. Usually two-pair unshielded cabling is attached to a hub in a point-to-point fashion. Bus connections are not allowed. The connection between an adapter and hub cannot exceed 100 meters in length. Hub-to-hub connection length can vary depending upon the medium used. If another twisted-pair connection is used, the maximum length is again 100 meters. With Thinnet it is 185 meters and with thick coaxial cable 500 meters.

The star topology is much easier to troubleshoot than a bus system; however, the reliability of the hub now must be considered in the overall reliability of the system. Another reason for the focus on twisted-pair is that development of Fast Ethernet is based on twisted-pair and not coaxial cable providing no migration path for installed coaxial cable.

10BASE-F

The 10BASE-F standard is actually a series of fiber optic standards. Fiber optics provides long distance, higher-speed migration, noise immunity and electrical isolation. There are three media standards:

10BASE-FL — This fiber link standard replaces the older FOIRL standard.
10BASE-FB — This backbone standard is not very popular.
10BASE-FP — This passive hub technology is also not popular.

The 10BASE-FL standard requires a duplex 62.5/125 µm fiber optic cable for each link. Transmission distances of up to 2 km are possible — as is full-duplex operation.