Physical Layer - Transmission Media

1
Physical Layer - Transmission Media

• Transmission Media
• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics
• Wireless Transmission

2
Magnetic Media (Sneaker Net)

• Floppy Disks
• Tape
• Hard Drives
• High-speed Transmission Media?
• A case of high-density tapes shipped overnight
  can transfer more bytes/second than any online
  connection.
• Never underestimate the bandwidth of a station
  wagon full of tapes hurtling down the highway
• - Tanenbaum

3
Twisted Pair

• Two insulated copper wires
• twisting the wires reduces electrical
  interference
• Used by the telephone system
• Can run several miles without amplification
• Analog or digital communication
• digital (voltage determines one of two values (0
  or 1))
• analog ( voltage determines continuous values)
• Can achieve megabits/second transmission rates

4
Analog signals can be modulated to convey digital
information.

• Three types of modulation
• Amplitude modulation
• Frequency modulation
• Phase modulation

5
Types of Twisted Pair

• UTP (Unshielded Twisted Pair)
• Category 3
• Conventional Phone-grade line
• Often used to provide 10 megabit/second Ethernet
  connections
• Category 5
• More twists/cm than category 3 line
• Teflon insulation
• Often used to provide 100 megabit/second Ethernet
  connections

6
Coaxial Cable (Coax)

• Cross section of Coaxial Cable

Plastic Covering
Braided Outer Conductor
Copper Core
Insulating Material
7
Coaxial Cable

• 1km cables provide 1-2 Gbps
• Cables longer than 1km provide slower rates and
  require amplification

8
Fiber Optic Networks

• Very fast data transmission using light traveling
  through a fiber of glass
• Pulse of light is 1
• Absence of light is 0
• May reach terabit/sec speeds in near future
• terabit is 1000 gigabits
• Components
• light source, medium, detector

9
Fiber Optic Networks (continued)

• Uses lights source to send
• Laser
• High data rate, long distance, short life,
  expensive
• Light Emitting Diode (LED)
• Low data rate, short distance, long life,
  inexpensive
• Uses Photo Diode on Receiving End
• translates light to electrical impulse
• currently the limiting factor in fiber optic
  networks
• switching time is approximately 1 nanosecond

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Fiber Optic Ring with Active Repeaters

• Two interface types
• passive interface uses 2 taps (one led o laser,
  the other photo diode).
• active interface regenerates signal at each
  point-to-point connection.

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Copper vs Fiber

• Copper
• Repeaters needed every 5KM
• Familiar Technology
• Cheaper interfaces
• Already in place

• Fiber
• MUCH higher bandwidth
• Repeaters needed every 30KM
• Not affected by power surges or electromagnetic
  interference
• Not affected by corrosive chemicals in the air
• Much lighter than copper
• by about 80 to 1 with same bandwidth
• Harder to tap

12
The Future is Fiber

• The theoretical data rates available with fiber
  are many orders of magnitude greater than those
  available from any other alternative technology.

13
Sharing the Transmission Medium

• Two Basic Methods
• Frequency Division Multiplexing (FDM)
• Different users get a portion of the available
  band
• Works like radio broadcasting
• Different channels (bands) carry simultaneous
  signals
• Time Division Multiplexing (TDM)
• Different users use the whole band in turn
• Like time-sharing computers
• The entire channel carries one signal for small
  intervals

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Time Division Multiplexing

• Pulse Code Modulation
• Sample the 4KHz telephone channel at 8000 8-bit
  cycles/second
• T1 line (1.544 Mbps)
• Can carry 24-time multiplexed voice channels
• T2 line (6.312 Mbps)
• Can carry traffic from 4 T1 lines
• T3 line (44.736 Mbps)
• Can carry traffic from 7 T2 lines
• T4 line (274.176 Mbps)
• Can carry traffic from 6 T3 lines

15
Synchronous Optical NETwork (SONET) and
Synchronous Digital Hierarchy (SDH)

• Developed by Bellcore and then by CCITT
• Goals
• Interconnet different optical (fiber) carriers
• Unify US, European, and Japanese digital systems
• Multiplex multiple digital channels together
• Support operations, administration, and
  maintenance (OAM)

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SONET

• Uses Time Division Multiplexing
• Synchronous (data sent at every clock)

17
SONET/SDF Transmissions Speeds
Synchronous Transport Signal (STS) is unscrambled
Optical Carrier (OC).
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Switching

• Three basic forms of switching
• Circuit Switching
• A physical connection is establish between source
  and destination
• Message Switching
• The message is sent in its entirety and then sent
  through a series of point-to-point transmissions
• no limit on message size
• Packet Switching
• Messages are broken up into small pieces that are
  independently routed as packets.

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Circuit Switching is used by the telephone
system.Most current computer networks use packet
switching.
20
Circuit Switching vs Packet Switching

• Circuit switching uses a dedicated path through
  the network.
• Circuit switching reserves part of the
  bandwidth.
• Circuit switching requires a call setup.
• Circuit switching can only encounter congestion
  during call setup, while packet switching can
  encounter congestion at any point in the
  transmission.
• Circuit switched networks typically charge per
  minute fee, while packet switched networks
  typically charge per packet.

21
The Telephone System
Individual phone lines consist of two copper
wires running from the home or business to an end
office. End offices connect at toll offices,
toll offices connect at primary offices, primary
offices connect at sectional offices, and
sectional offices connect at regional offices.
22
Overview of ATT Telephone Hierarchy

• Connections are made at the lowest level possible
• The first 3 digits of the number determine the
  end office
• The copper connections at the lowest level are
  the major obstacle to high-speed service

23
Modems modulate/demodulate an analog signal to
transfer data over phone lines.
Modulate
Demodulate
Computer
Computer
Modem
Modem
Digital Signal
Digital Signal
Analog Signal
24
Analog signals can be modulated to convey digital
information.
Modems typically use phase modulation.
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Modems Typically encode several bits/baud

• Baud - number of times per second that the signal
  changes its value (upper limit is 6000 Hz for
  analog phone lines).

Constellation patterns for 3 bits/baud and 4
bits/baud modulation