Transmission Media

1
Transmission Media

• EECE 542

2
Classes of Transmission Media

• Guided (uses a physical conduit)
• Twisted-pair cable
• Unshielded
• Shielded
• Coaxial cable
• Fiber-optic cable
• Unguided (no conduit)

3
Propagation Speed

• Speed of light in a vacuum 3x108 m/s c
• Other media
• ? dielectric constant of the medium, gt 1
• Copper v 2.3x108 m/s
• Fiber-optic cable v 2x108 m/s

4
Unshielded Twisted-Pair (UTP) Cable

• A pair of coated wires is twisted together
• Why twist the wires together?
• Reduces the undesirable effects of noise and
  crosstalk (watch out for NEXT)
• Inexpensive and easy to use/install
• EIA Categories
• Category 1 telephone/low data
• Category 2 voice/data up to 4 Mbps
• Category 3 3 twists/ft 10 Mbps new phone std.
• Category 4 3 twists/ft 16 Mbps
• Category 5 100 Mbps

5
Shielded Twisted-Pair (STP) Cable

• Each cable is encased in a metal foil or mesh.
• Less noise susceptibility
• Less susceptibility to crosstalk from adjacent
  cables
• More expensive than UTP
• More difficult to install than UTP

6
Coaxial Cable
7
Coax cont.

• Very low susceptibility to noise
• Cable dimensions and dielectric material can be
  configured for different applications
• Radio Government (RG) ratings
• RG-8, RG-9, and RG-11 Thick Ethernet
• RG-58 Thin Ethernet and RF Comm. (50 ohm)
• RG-59 CATV (75 ohms)

8
Optical Fiber

• Cable made of glass or plastic to transmit light
• Transmission depends on refraction and reflection

9
Propagation Modes of Fiber

• The physical construction of the fiber can lead
  to three modes or types of propagation
• Different material densities can be used to
  affect the reflective and refractive properties
  of the cable

10
Multimode, step-index

• Multiple beams take different paths
• Core density remains uniform
• Reflections occur at the core/cladding interface
  due to a step change in density
• Considerable distortion and loss

11
Multimode, graded-index

• Multiple beams, multiple paths
• Density of the core decreases as it approaches
  the cladding interface
• No reflections
• Distortion occurs, but less signal loss

12
Single Mode

• Light source is highly focused
• Core density uniform and much lower
• Critical angle is close to 90 degrees (no
  reflections)
• Smaller diameter than multimode
• Low distortion and low loss

13
Unguided Media

• Uses free space as the medium
• Always modulated signals (sinusoidal carriers)
• Ex WLANs, Bluetooth, cell phones, satellite
  phones, broadcast TV, wireless PDAs

14
Propagation Time

• Propagation time Distance / Propagation Speed
• Determines the amount of data in the channel
  (pipe) during a transmission
• A major factor in network performance!!!

15
An Example

• A SONET fiber-optic system using STS-48 signaling
  carries data at 2488 Mbps over a distance of 2000
  km. How much data is in the cable at any given
  time?
• Prop. Speed 2x108 m/s
• so the propagation time
• 2000x103/2x108 .01 sec.
• of bits in the fiber at any time
• 2488x106 bps x .01 sec. 24.88 x 106 bits