Part II: Data Transmission

1
Part II Data Transmission
Fall 2005

• The basics of media, signals, bits, carriers, and
  modems

Qutaibah Malluhi Computer Science and
Engineering Qatar University
2
Fall 2005Transmission Media

• Overview of transmission media used in network
  systems

3
Sending Data

• At sender encode data as energy and transmit
  energy
• At receiver decode energy at destination back
  into data
• Energy can be electrical, light, radio, sound,
  etc.
• Each form of energy has different properties and
  requirements for transmission
• Transmitted energy is carried through some sort
  of medium copper, glass, air, ...

Medium
Sender (encodes data)
Receiver (decodes data)
data 10110 110
data 10110 110
4
Physical Layer and Data Transmission
5
Copper Wire

• Primary medium to connect computers
• Inexpensive
• easy to install
• low resistance to electric current gt signal
  travels further
• Interference How to minimize?
• Twisted pair
• Coaxial cable (Coax)
• Shielded twisted pair

6
Twisted Pair

• One wire for signal and other as ground reference
• Receiver uses the difference between signal
• Receiver is immune to interference if both wires
  are affected equally
• Twisting
• limits electromagnetic energy these wires emit
• protects the wires from external interference
• Two wires has same distance to noise source. One
  is closer in one twist, the other is closer in
  the second twist.

7
UTP and STP

• Unshielded Twisted Pair (UTP)
• Classified into 7 categories based on cable
  quality
• Category 1 is the lowest quality (phone) and 5 is
  the most common today (100 Mbps Ethernet)
• Shielded Twisted Pair (STP)

8
Coaxial Cable

• Can carry higher frequencies that TP
• Improved performance through shield
• Particularly useful for wires that pass next to
  equipment generating EM fields

9
Glass Fibers

• Thin glass fiber carries light with encoded data
• Plastic jacket allows fiber to bend (some!)
  without breaking
• Fiber is very clear and designed to reflect light
  internally for efficient transmission
• Light emitting diode (LED) injects light into
  fiber. Light sensitive receiver at other end
  translates light back into data
• Applications
• Backbone networks (cost-effective wide-bandwidth)
• Backbone of TV cable networks (coax for
  connection to premise)
• Some LANS (100baseFX and 1000base-X Ethernets)

10
Glass Fibers More Details
11
Glass Fiber Adv. and Disadv.

• Advantages
• Much higher bandwidth
• Signal generation and not the medium is the
  bottleneck today
• No electromagnetic interference
• Further signal distances (less signal
  attenuation)
• 50 KM for fiber vs. 5 KM for copper
• Better encoding of bits (store more bits in
  signal)
• More immune to tapping
• Disadvantages
• Special installation equipment
• difficulty of joining fibers
• Unidirectional (but requires single wire no
  ground)
• difficulty to locate fiber problems

12
Connectors
Twisted Pair Connectors
Coax Connectors
Coax Connectors
13
Radio

• Data transmitted using radio waves. Conceptually
  similar to radio, TV, pagers
• Energy travels through the air rather than copper
  or glass
• Physical connection is not required
• Antennas are needed
• Omni-directional (travels in all directions)
• Can travel through walls and through an entire
  building
• Can be long distance or short distance
• Long distance with satellite relay
• Short distance wireless computer network

14
Wireless Network

• Wireless bridge and net interface

15
Microwave

• High frequency radio waves
• 2-300 GHz
• Higher speed than lower frequency RF signals
• Unidirectional, for point-to-point communication
• Can not penetrate structures clear path is
  required
• Line of sight transmission
• Antennas mounted on towers relay transmitted data
  

16
Infrared

• Infrared light transmits data through the air
• Similar to technology used in TV remote control
• Can propagate throughout a room (bouncing off
  surfaces), but will not penetrate walls
• Becoming common in personal digital assistants

17
Laser

• Unidirectional, like microwave
• Higher speed than microwave
• Uses laser transmitter and photo-sensitive
  receiver at each end Point-to-point, typically
  between buildings
• Can be adversely affected by weather

18
Satellites
19
Summary

• Copper wire is mature technology, rugged and
  inexpensive maximum transmission speed is
  limited
• Glass fiber
• Higher speed
• More resistant to electro-magnetic interference
• Spans longer distances
• Requires only single fiber
• More expensive less rugged
• Radio and microwave don't require physical
  connection
• Radio and infrared can be used for mobile
  connections
• Laser also does not need physical connection and
  supports higher speeds