Transcript Physical Layer

Physical Layer - Transmission
Media
• Transmission Media
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Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
Wireless Transmission
Magnetic Media (Sneaker Net)
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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
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
Analog signals can be modulated to convey
digital information.
Three types of modulation:
•Amplitude modulation
•Frequency modulation
•Phase modulation
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
Coaxial Cable
•1km cables provide 1-2 Gbps
•Cables longer than 1km provide slower rates and require
amplification
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
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
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 pointto-point connection.
Copper vs Fiber
• Copper
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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
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.
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
Time Division Multiplexing
• Pulse Code Modulation
– Sample the 4KHz telephone channel at 8000 8bit 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
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)
SONET
• Uses Time Division Multiplexing
• Synchronous (data sent at every clock)
SONET/SDF Transmissions Speeds
Synchronous Transport Signal (STS) is unscrambled
Optical Carrier (OC).
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.
Circuit Switching is used by the telephone system.
Most current computer networks use packet switching.
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.
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.
Overview of AT&T 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
Modems modulate/demodulate an analog
signal to transfer data over phone lines.
Modulate
Computer
Demodulate
Modem
Digital Signal
Computer
Modem
Analog Signal
Digital Signal
Analog signals can be modulated to convey
digital information.
Modems typically use
phase modulation.
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