A transmission medium (plural transmission media) is a material substance (solid, liquid, gas, or plasma) that can propagate energy waves. The term transmission medium also refers to a technical device that employs the material substance to transmit or guide waves. Thus, an optical fiber or a copper cable is a transmission medium.

Many transmission media are used as communications channels. For telecommunications transmission media are classified as one of the following: - guided (or bounded)waves are guided along a solid medium such as a transmission line. - wireless (or unguided)transmission and reception are achieved by means of an antenna. In both cases, communication is in the form of electromagnetic waves.

With guided transmission media, the waves are guided along a physical path. Examples of guided media: - phone lines, - twisted pair cables, - coaxial cables, - optical fibers.

Unguided transmission media are methods that allow the transmission of data without the use of physical means to define the path it takes. Examples of unguided transmission media: - microwave, - radio, - Infrared.

Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs.

Braid Shield – flexible conductive wire braided around the dielectric. Braid may be made of aluminium or bare or treated copper, Jacket – made of polyvinylchloride or polyethylene for nonplenum cable; made of Teflon or Kyanar for plenum cable. Conductor Wire – made of copper, copper treated with tin or silver, or aluminium or steel covered with copper, Dielectric – nonconductive material (such as polyethylene or Teflon) Foil Shield – made of polypropylene or polyester tape coated with aluminium of both sides (STP only),

The code before the slash designates the shielding for the cable itself, while the code after the slash determines the shielding for the individual pairs: TP = twisted pair U = unshielded F = foil shielding S = braided shielding

UTP cable is also the most common cable used in computer networking. Modern Ethernet, the most common data networking standard, utilizes UTP cables. Twisted pair cabling is often used in data networks for short and medium length connections because of its relatively lower costs compared to optical fiber and coaxial cable.

Twisted pair cables are often shielded in an attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work, and is improved by grounding the drain wire along with the shield.

150 ohm STP shielded twisted pair cable defined by the IBM Cabling System specifications and used with token ring or FDDI networks. This type of shielding protects cable from external EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

ScTP cabling offers an overall sheath shield across all of the pairs within the 100 Ohm twisted pair cable. F/TP uses foil shielding instead of a braided screen. This type of shielding protects EMI from entering or exiting the cable.

S/STP (Screened Shielded Twisted Pair) or S/FTP (Screened Foiled Twisted Pair) cabling offer shielding between the pair sets and an overall sheath shield within the 100 Ohm twisted pair cable. This type of shielding protects EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

S/STP cable is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources, and also eliminates alien crosstalk.

Old nameNew namecable screeningpair shielding UTPU/UTPnone STPU/FTPnonefoil FTPF/UTPfoilnone S-STPS/FTPbraidingfoil S-FTPSF/UTPfoil, braidingnone

CategoryType Frequency Bandwidth ApplicationsNotes Cat10.4 MHz Telephone and modem lines Not described in EIA/TIA recommmendations. Unsuitable for modern systems. Cat2? MHz Older terminal systems, e.g. IBM 3270 IBM 3270 Not described in EIA/TIA recommmendations. Unsuitable for modern systems. Cat3UTP16MHz 10BASE-T and 100BASE-T4 Ethernet Ethernet Described in EIA/TIA-568. Unsuitable for speeds above 16 Mbit/s. Now mainly for telephone cables

CategoryType Frequency Bandwidth ApplicationsNotes Cat4UTP20MHz 16 Mbit/s Token Ring Token Ring Not commonly used Cat5UTP100MHz 100BASE-TX & 1000BASE-T Ethernet Ethernet Common in most current LANs Cat5eUTP100MHz 100BASE-TX & 1000BASE-T Ethernet Ethernet Enhanced Cat5. Same construction as Cat5, but with better testing standards.

CategoryType Frequency Bandwidth ApplicationsNotes Cat6UTP250MHz 1000BASE-T Ethernet Ethernet Most commonly installed cable in Finland according to the 2002 standard. SFS-EN Cat6e 250MHz (500MHz according to some) Not a standard; a cable maker's own label. Cat6a500MHz 10GBASE-T Ethernet ISO/IEC 11801:2002 Amendment 2.

CategoryType Frequency Bandwidth ApplicationsNotes Cat7S/FTP600MHz Telephone, CCTV, 1000BASE-TX in the same cable. 10GBASE-T Ethernet. CCTV 1000BASE-TX 10GBASE-T Ethernet Four pairs, U/FTP (shielded pairs). Standard under development. Cat7a1000MHz Telephone, CATV, 1000BASE-TX in the same cable. 10GBASE-T Ethernet. CATV 1000BASE-TX 10GBASE-T Ethernet Four pairs, S/FTP (shielded pairs, braid-screened cable). Standard under development.

CategoryType Frequency Bandwidth ApplicationsNotes Cat81200MHz Under development, no applications yet. Four pairs, S/FTP (shielded pairs, braid-screened cable). Standard under development.

Coaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis.

Coaxial cable conducts electrical power using an inner conductor (usually a flexible solid or stranded copper wire) surrounded by an insulating layer and all enclosed by a shield layer, typically a woven metallic braid; the cable is often protected by an outer insulating jacket.

Short coaxial cables are commonly used to connect home video equipment, in ham radio setups, and in measurement electronics. They used to be common for implementing computer networks, in particular Ethernet, but twisted pair cables have replaced them in most applications except in the growing consumer cable modem market for broadband Internet access.

An optical fiber is a flexible, transparent fiber made of a pure glass (silica) not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber.

Diagram of a single mode (SM) optical fiber. 1.- Core 8-10 µm 2.- Cladding 125 µm 3.- Buffer 250 µm 4.- Jacket 400 µm

Optical fiber can be used as a medium for telecommunication and computer networking because it is flexible and can be bundled as cables. It is especially advantageous for long- distance communications, because light propagates through the fiber with little attenuation compared to electrical cables.