Bound or Wired Media

Bound or Wired Media

The wired media utilized within the Physical Layer of the Open Systems Interconnection (OSI) model spans various strands and gauges of copper along with plastics and glass.

Twisted Pair

The quality (and cost) of cable is determined

Pairs of insulated copper wires are twisted together to reduce of insulation, and conductive material. Two by the number of twists per inch, the type electromagnetic interference general types of twisted pair cabling are in and cross talk. The twisted

widespread use: pairs are surrounded by an

• Unshielded Twisted Pair (UTP): Does

outer jacket that physically not have external shielding surrounding protects the wires.

the twisted pair bundles and is more susceptible to electrical interference. For the same reason, UTP is also more vulnerable to surveillance with the proper equipment. Despite its drawbacks, UTP is the most common cable type. UTP is inexpensive, is more robust during installation, and the standards for its use are well defined.

• Shielded Twisted Pair (STP): Encloses its bundle of wires in a protective (metal foil jacket or shield. This electronically grounded shield better protects the signaling cables from interference and from monitoring. STP is more expensive and is bulkier and requires a larger bend radius than UTP. This makes installation more difficult.

Coaxial Cable

Coaxial cable (or simply, coax) surrounds one thick conductor with a layer of insulation wrapped with a braided grounding wire.

Protective sheath

Braided conductive shield

Dielectric Insulator Inner conductive core

The entire cable is then covered with a protective sheath. The conducting wire is much thicker than typical twisted pairs, providing greater bandwidth and longer cable lengths, as in the figure. The superior insulation protects coaxial cable from electronic interference and the shielding makes it more difficult for an intruder to monitor the signal. Coaxial cable is generally more expensive than twisted pair and requires a larger bend radius to prevent damage to the cable during installation. However, its robust construction makes it well suited for specialized applications, such as cable TV.

Instead of electrical impulses travelling on a conductor, fiber optics send light pulses to transmit information through specially constructed glass or plastic fibers (see figure).

Paired transmitters and receivers tuned to specific light frequencies use pulsecode modulation (PCM) to transform the electrical impulses from the communicating hosts into light pulses. Think of a fiber cable in terms of a long cardboard roll (from the inside roll of paper towel) that is coated with a mirror on the inside. If you shine a flashlight in one end, you can see light come out at the other end - even if it has been bent around a corner. Light pulses are reflected down the fiber-optic strand, with the distances being determined by the strength of the laser and the optical purity of the fiber path.

Sheath - External protective and strength coating

Primary coating

• Plastic sheath around a single strand Cladding "Contains" the light In the core Core - Light transmission path
• Single mode: Has a small diameter core that decreases the number of light reflections within the cable. This allows for longer transmission distances, up to 100Km.
• Multimode: Uses a larger diameter cable than single mode, allowing the use of less expensive LEDs for transmission. Multimode is typically used when the cable length is less than 2,000 meters, which makes it ideal for intra-building interconnections.
• Plastic optical fiber (POF): Replaces the glass fibers with plastic. These cables are much more robust and also use inexpensive LEDs for transmission, but the impurities in the production of the fibers limit their transmission distance to approximately 100 meters. This makes it ideal for connecting devices to wiring closets or supporting component-tocomponent connections.