Physical Layer - TransmissionMedia
•Transmission Media
–Magnetic Media
–Twisted Pair
–Coaxial Cable
–Fiber Optics
–Wireless Transmission
Magnetic Media (Sneaker Net)
•Floppy Disks
•Tape
•Hard Drives
•High-speed Transmission Media?
–A case of high-density tapes shipped overnightcan transfer more bytes/second than any onlineconnection.
–Never underestimate the bandwidth of a stationwagon 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 conveydigital 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/secondEthernet connections
•Category 5
–More twists/cm than category 3 line
–Teflon insulation
–Often used to provide 100 megabit/secondEthernet connections
Coaxial Cable (Coax)
•Cross section of Coaxial Cable
Plastic Covering
Braided OuterConductor
InsulatingMaterial
Copper Core
Coaxial Cable
•1km cables provide 1-2 Gbps
•Cables longer than 1km provide slower rates and requireamplification
Fiber Optic Networks
•Very fast data transmission using light traveling through a fiber ofglass
–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 opticnetworks
•switching time is approximately 1 nanosecond
Fiber Optic Ring with Active Repeaters
•Two interface types
–passive interface uses 2 taps (one led o laser, theother photo diode).
–active interface regenerates signal at each point-to-point connection.
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 orelectromagnetic interference
–Not affected by corrosive chemicalsin the air
–Much lighter than copper
•by about 80 to 1 with samebandwidth
–Harder to tap
The Future is Fiber
•The theoretical data rates available withfiber are many orders of magnitude greaterthan those available from any otheralternative 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 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
Synchronous Optical NETwork (SONET) andSynchronous Digital Hierarchy (SDH)
•Developed by Bellcore and then by CCITT
•Goals
–Interconnet different optical (fiber) carriers
–Unify US, European, and Japanese digitalsystems
–Multiplex multiple digital channels together
–Support operations, administration, andmaintenance (OAM)
SONET
•Uses Time Division Multiplexing
•Synchronous (data sent at every clock)
SONET/SDF Transmissions Speeds
Synchronous Transport Signal (STS) is unscrambledOptical Carrier (OC).
Switching
•Three basic forms of switching
–Circuit Switching
•A physical connection is establish between sourceand destination
–Message Switching
•The message is sent in its entirety and then sentthrough a series of point-to-point transmissions
–no limit on message size
–Packet Switching
•Messages are broken up into small pieces that areindependently 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 thetransmission.
• Circuit switched networks typically charge per minute fee, whilepacket 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 madeat the lowest levelpossible
•The first 3 digits of thenumber determine theend office
•The copper connectionsat the lowest level arethe major obstacle tohigh-speed service
Modems modulate/demodulate an analogsignal to transfer data over phone lines.
Computer
Computer
Modem
Modem
Digital Signal
Digital Signal
Analog Signal
Modulate
Demodulate
Analog signals can be modulated to conveydigital information.
Modems typically use
phase modulation.
Modems Typically encode several bits/baud
•Baud - number of times per second that thesignal changes its value (upper limit is 6000Hz for analog phone lines).
Constellationpatterns for 3bits/baud and 4bits/baudmodulation