The actual standard defines specific electrical properties of the wire, but it is most commonly known as being rated for its Ethernet capability of 100 Mbit/s.
Its specific standard designation is EIA/TIA-568. Cat 5 cable typically has three twists per inch of each twisted pair of 24 gauge copper wires within the cable. The twisting of the cable reduces electrical interference and crosstalk.
Another important characteristic is that the wires are insulated with a plastic (FEP) that has low dispersion, that is, the dielectric constant of the plastic does not depend greatly on frequency.
Special attention also has to be paid to minimizing impedance mismatches at connection points.
It is often used in structured cabling for computer networks such as Fast Ethernet, although it is often used to carry many other signals such as basic voice services, token ring, and ATM (at up to 155 Mbit/s, over short distances).
The other well known flavor of this type of cable is the 10 Mbit/s Category 3 cable. Less well known is the 20 Mbit/s Cat 4. Cat 4 offered only a small advance in speed over Cat 3, and was generally ignored in favor of Cat 5. Cat 1 and Cat 2 are 1 Mbit/s systems for voice and low-speed data.
Patch leads created from Cat 5 are often terminated with RJ-45 electrical connectors. Normal Cat 5 cables are wired "straight through" and connect a computer to a hub or switch. In other words, pin 1 is connected to pin 1, pin 2 to pin 2, etc. The RJ-45 pin out for a Cat 5 cable can either be TIA-568A or TIA-568B.
TIA-568A is used by some phone systems and Token Ring. Most everything else, such as the Ethernet standards 10BASE-T and 100BASE-TX, use TIA-568B.
In Ethernet, "crossover" Cat 5 cables are cables in which pairs two and three are reversed. (Pairs one and four are not reversed for 100BASE-T4 but are reversed for 1000BASE-T.) These are most often used to connect two PC's NICs directly (with no intervening hub).
They can also be used to connect two hubs or switches together. However most hubs and switches either have an uplink port, a button to change a port to uplink or one or more ports with auto sense (most modern switches now have auto sense on every port).
These features eliminate the need for crossover cables when connecting them. See the Crossover cable and TIA-568B articles for pin out diagrams.
cat 5e cable is an enhanced version of Cat 5 for use with 1000BASE-T (gigabit) networks, or for long-distance 100 Base-T links (350 m, compared with 100 m for Cat 5). It must meet the EIA/TIA 568A-5 specification. Virtually all cables sold as Cat 5 are actually Cat 5e. The markings on the cable itself reveal the exact type.
