The word “coaxial” completely describes this medium. Coaxial means “having a common axis.” Coaxial cable is essentially a series of layers around a central core. At the heart of the coaxial cable is a core conductor. Typically, this is a solid core of copper, but there are also hollow and stranded cores on the market. Wrapped around this core is a plastic or vinyl insulator. Around this insulator is the second conductor in the form of a braided, metal sheath. The final layer is an outer sheath, also plastic or vinyl.
For LANs, coaxial cable has historically been associated with Ethernet and came in two basic flavors, thinnet and thicknet. Thinnet (also known as cheapernet, 10BASE2, or thin coaxial cable) and thicknet (also known as 10BASE5, or thick coaxial cable) are both 50 ohm cables and were once the primary cabling plant for Ethernet LANs. Both thinnet and thicknet environments usually employ baseband (or digital) signaling schemes. ARCnet (another obsolete type of LAN) used a completely different cable, 62 ohm.
The advantages of coaxial cable are two-fold. First, the nature of the medium makes it highly immune to interference. Second, with a high signal-to-noise ratio, coaxial cable can support fairly high data rates. Over limited distances, coaxial cable can be pushed into the low gigabit per second (Gbps) range.
The disadvantages of coaxial cable include cost, size, installation, and access. Coaxial cable is slightly more expensive than twisted pair. Thicknet is a significantly thicker cable, with a solid, inflexible core. It was much more difficult to pull than thinnet cable, and significantly more difficult to pull than twisted pair.
While coaxial cable products are still available for LAN environments, it is most definitely a legacy technology that is rarely (if ever) installed today. Twisted pair and optical fiber are the media of choice. Coaxial cable remains the media of choice in cable TV installations, but even that is beginning to show signs of change.
A variant of coaxial cable, called “twinaxial cable” (or just “twinax”) is now found in some higher speed LAN environments, typically at data rates of 1 Gbps or more. As the name implies, twinax consists of two core conductors with a common outer sheath. It has the same high signal-to-noise characteristic of coaxial cable, but more carrying capacity.