Transmission Media

Transmission Media
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Types of Transmission Media

Guided:
- The waves are guided along a physical path
Unguided:
- The medium transmits the waves but does not guide them

Some Important Terminology

Direct Link:
- The signal is propagated directly from transmitter to receiver (perhaps through amplifiers/repeaters)
Point-to-Point:
- A guided medium that provides a direct link between two and only two devices
Multipoint:
- A guided medium that is shared by more than two devices

Multiplexing

The Concept:
- Simultaneously transmit multiple channels of information on a single direct link
Common Approaches:
- Frequency Division Multiplexing (FDM) - Each signal, \(m_i\) is modulated onto a different frequency (called a subcarrier and denoted by \(f_i^c\))
- Time Division Multiplexing (TDM) - Multiple signals are interleaved (in bits or blocks) over time

Guided Media - Twisted Pair Cable

Description:
- Two insulated copper wires that are twisted together (which results in destructive interference and, hence, reduces radiation and, hence, energy loss)
- The individual pairs can be shielded (STP) or unshielded (UTP)
- The cable can also be shielded (S/STP)
Example:

Twisted Pair Cable (cont.)

Categories:
- 1 - primarily for voice
- 2 - 4Mbps
- 3 - 16MHz of bandwidth; 10Mbps
- 4 - 20MHz of bandwidth; 16Mbps
- 5/5e - 100MHz of bandwidth; 100Mbps (4 pairs/cable)
- 6 - 400MHz of bandwidth; 1Gbps (4 pairs/cable)
Common Uses:
- Voice Telephony (including private branch exchanges)
- Digital Subscriber Line (DSL) in many flavors

Twisted Pair Cable (cont.)

Advantages:
- High availability
- Low installation cost
Disadvantages:
- Usable frequency spectrum is small
- Limited data rates
- Limited range
- High susceptibility to electromagnetic and radio frequency interference

Guided Media - Coaxial Cable

Description:
- A (thick) copper wire surrounded by an insulator and an outer conductor
- The current flowing on the inner conductor is equal and opposite to the current flowing on the outer conductor so little energy is lost to radiation
Example:

Coaxial Cable (cont.)

Common Types:
- RG-6 - 1.00mm core, 75\(\Omega\)
- RG-59 - 0.81mm core, 75\(\Omega\)
- RG-58 - 0.90mm core, 50\(\Omega\)
- RG-8 - 2.17mm core, 50\(\Omega\)
Common Uses:
- Cable TV (Old: RG-59; New: RG-6)
- Ethernet (RG-58: 10Base2; RG-8: 10Base5)
- Amateur radio

Coaxial Cable (cont.)

Advantages:
- High capacity (370-1000 times more than twisted pair)
- Low susceptibility to noise and crosstalk (because of shielding)
Disadvantages:
- Noise on the return path
- High installation cost

Guided Media - Fiber Optic Cable

Description:
- A thin glass core surrounded by cladding that reflects light into the core, surrounded by a protective plastic buffer/jacket
Example:

Fiber Optic Cable (cont.)

Operation:
- Light travels through the core
- It bounces off of the cladding to "make turns"
- The signal degrades because of impurities in the glass
Illustration:
Total Internal Reflection:
- Light passing from one medium (\(m_1\)) to another (\(m_2\)) with lower index of refraction (\(r_i\)) bends away from the normal line
- At the critical angle, defined by \(\sin(\theta) = r_2 / r_2\), the refracted light will not go into the second medium but will travel along the border

Fiber Optic Cable (cont.)

Common Wavelengths (in the infrared):
- 850nm
- 1300nm
- 1550nm
Common Uses:
- Cable TV
- Voice Telephony
- Ethernet

Fiber Optic Cable (cont.)

Advantages:
- Low cost
- High capacity
- Thin
- Low interference/degradation
Disadvantages:
- Can be bent
- Each fiber is one-way
- High cost of interfaces

Guided Media - Characteristics

	Frequency	Attenuation	Delay
Twisted Pair	0-3.5kHz	0.2dB/km at 1kHz	50\(\mu\)/km
Coaxial Cable	0-500MHz	7dB/km at 10MHz	4\(\mu\)/km
Optical Fiber	186-370THz	0.2-0.5dB/km	5\(\mu\)/km

Unguided Media - Radio

Einstein on Radio:
- "You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this?
  And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."
VLF, LF and MF Bands:
- Range of about 1000km
- Penetrate buildings
- Omnidirectional
HF and VF Bands:
- Tend to be absorbed by the Earth
- Can be bounced off of the ionosphere (called sky wave propagation)

Radio - Some Examples

Pulse Modulation - WWVB in Fort Collins, Co.:
- Transmits minutes, hours, day of year, etc...
- 60,000 Hz
- 0.2 seconds of reduced power is a binary 0; 0.5 sec of reduced power is a binary 1; 0.8 sec of reduced power is a separator
- Message requires 53 bits and 7 separators

Radio - Some Examples (cont.)

Amplitude Modulation - AM Talk Radio Stations:
- The wave produced by a person's voice is overlaid onto the transmitter's wave to vary its amplitude
- AM 680 broadcasts at 680,000 Hz
- Carrier wave of frequency \(\Omega\)
  \(c(t) = C \sin(\Omega t)\)
- Signal to broadcast
  \(m(t) = M \sin(\omega t + P)\)
- AM is \(m(t)+C\)
  \(y(t) = [C + (M \sin(\omega t + P)] \sin(\Omega t)\)

Radio - Some Examples (cont.)

Frequency Modulation - Cordless Telephones:
- Vary the instantaneous frequency of a carrier wave in accordance with an input signal
- FM 101.5 broadcasts at 101,500,000 Hz

Unguided Media - Microwave

Properties:
- Travel in a nearly straight line
- Can be narrowly focused (by a parabolic antenna)
- High signal to noise ratio
- Little interference
- Low penetration
  - Do not pass through buildings
  - At 4GHz and above, the waves are very short and can be absorbed by water (e.g., rain)
  - At 60GHz the waves can be absorbed by oxygen
Range:
- 2GHz-6GHz: 70km
- 18GHz-45GHz: 1.6-8km

Unguided Media - Infrared

Mostly directional
Cheap
Do not pass through solid objects (so no license is required)

Unguided Media - Laser (i.e. Coherent Light)

Unidirectional
High bandwidth
Cannot penetrate thick fog or rain

The Electromagnetic Spectrum

International Telecommunication Union (ITU) Bands

Band	Frequency	Media
Gamma Rays	10²²
	10²¹
	10²⁰
	10¹⁹
	10¹⁸
	10¹⁷
X-Rays	10¹⁶
Ultraviolet	10¹⁵
Visible Light	10¹⁴	Fiber Optics
Infrared	10¹³	Fiber Optics
THF	10¹² (1THz)	Microwave
EHF	10¹¹	Microwave
SHF	10¹⁰	PCS, Microwave
UHF	10⁹ (1GHz)	FM Radio, TV, Cellular Radio
VHF	10⁸	TV, Coaxial Cable
	10⁷	AM Radio, Coaxial Cable
High	10⁶ (1MHz)	Coaxial Cable
Medium	10⁵	Twisted Pair Cables
Low	10⁴
Very Low	10³ (1KHz)
	10²
Extremely Low	10¹
	10⁰ (1Hz)