Understanding Transmission Media Types

What is Transmission Media?

• A way of carrying information from a source to a destination.
• Transmission media in data communications is usually free space, metallic cable, or fiber-optic cable.
• Transmission media are located below the physical layer and are directly controlled by it.

Transmission Media Types

• Guided (Wired)
  • Twisted-pair cable
  • Coaxial cable
  • Fiber-optic cable
• Unguided (Wireless)
  • Free space

Guided Media

• Provide a channel from one device to another.
• Twisted-pair and coaxial cables use metallic (copper) conductors, carrying signals via electric current.
• Optical fiber cables transmit signals in the form of light.

Twisted Pair Cable

• Consists of two conductors, typically copper, each insulated in plastic and twisted together.
• There are two main types:
  • Unshielded Twisted-Pair (UTP)
  • Shielded Twisted-Pair (STP)
• Metal casing improves cable quality by preventing noise or crosstalk penetration, but it is bulkier and more expensive.

UTP Categories

• Category 1: used in telephone, with a data rate of less than 0.1 Mbps.
• Category 2: originally used in T-1 lines, with a data rate of 2 Mbps.
• Category 3: used in LANs, with a data rate of 10 Mbps.
• Category 4: used in Token Ring networks, with a data rate of 20 Mbps.
• Category 5: cable wire is normally 24 AWG cable with a jacket and outside sheath, at 100 Mbps
• Category 5E: is an extension to cat 5, data rate is 125 Mbps.
• Category 6: requires matched components from the same manufacturer, tested at 200 Mbps.
• Category 7: shielded screen twisted-pair (SSTP), wrapped in helical metallic foil & shield to decrease crosstalk, data rate 600 Mbps.

Twisted Pair Performance

• One way to measure twisted-pair cable performance is to compare attenuation versus frequency and distance.
• They can pass a wide range of frequencies.
• Attenuation, measured in decibels per kilometer (dB/km), increases with frequency.
• Gauge measures wire thickness.

Coaxial Cable

• Carries signals of higher frequency ranges than twisted-pair cables.
• Components include:
  • An insulator.
  • An inner conductor.
  • A plastic coat
  • An outer conductor (shield)

Coaxial Cable Standards

• Cables are categorized by Radio Government (RG) ratings.
• Each RG number denotes unique specifications for:
  • Wire gauge of the inner conductor
  • Thickness/type of inner insulator
  • Construction of the shield
  • Size/type of outer casing
• RG ratings are adapted for specialized functions.
  • RG-59 is used for Cable TV with 75 impedance.
  • RG-58 is used for Thin Ethernet with 50 impedance.
  • RG-11 is used for Thick Ethernet with 50 impedance.
• Attenuation has a much higher rate in coaxial cables than in twisted-pair cable.

Fiber Optic Cable

• Made of glass or plastic, transmitting signals via light.
• The critical angle affects how light travels between substances of differing densities.

Angle of Incidence

• The angle a ray makes with the line perpendicular to the interface between two substances.
• When I < critical angle, refraction occurs, and the ray moves closer to the surface.
• When I = critical angle, the light bends along the interface.
• When I > critical angle, reflection occurs, and the ray turns back into the denser substance.

Optical Fiber

• Utilizes reflection to guide light through a channel.
• The core's density must be higher than the cladding's to cause reflection instead of refraction.
• Cladding is the glass or plastic surrounding the core.
• Core is the glass or plastic center.

Propagation Mode

• Current technology supports multimode and single mode propagation, each requiring different fiber characteristics.
• Multimode: multiple beams from a single light source move through the core in different paths, dependent on core structure.
  • Step-index fiber: the core's density remains constant from the center to the edges.
  • Graded-index fiber: density is highest at the core center and gradually decreases to its lowest at the edge.
• Single-Mode: uses step-index fiber (lower constant density) and limits beams to a small range of angles close to horizontal.

Fiber Sizes

• Optical fibres defined by the ratio of core diameter to cladding diameter, in micrometers (μm).
  • 50/125: Core 50.0 μm, Cladding 125 μm - Multimode, graded index
  • 62.5/125: Core 62.5 μm, Cladding 125 μm - Multimode, graded index
  • 100/125: Core 100.0 μm, Cladding 125 μm - Multimode, graded index
  • 7/125: Core 7.0 μm, Cladding 125 μm - Single mode

Cable Composition

• Outer jacket of PVC or Teflon.
• Kevlar strands strengthen the cable.
• Plastic coating cushions the fiber.
• The fiber at the cable's center consists of cladding and core.

Fiber Optic Cable Advantages

• Higher bandwidth
• Less signal attenuation
• Immunity to electromagnetic interference
• Resistance to corrosive materials
• Light weight
• Greater immunity to tapping

Fiber Optic Cable Disadvantages

• Tricky Installation and maintenance
• Unidirectional light propagation
• Increased cost