Physical Layer Issues Theoretical Underpinning –or, Bandwidth 101 Media Characteristics –Optical...
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Transcript of Physical Layer Issues Theoretical Underpinning –or, Bandwidth 101 Media Characteristics –Optical...
Physical Layer Issues
• Theoretical Underpinning– or, Bandwidth 101
• Media Characteristics– Optical Fiber– Coax– Copper Wire (Twisted Pair)– Wireless
• Other Useful Ideas
Signals
• Propagation - {how fast does the signal travel in that media, esp. compared to light?}
• Frequency - {number of oscillations per second of the electromagnetic field of the signal}
• Bandwidth - {the width/size, in Hz, of the signal -- usually defined by where most of the energy is}
• Data Rate - {the number of bits per second. Distinct from, but related to, frequency and bandwidth}
• Baud - {Changes per second in the signal. Limited by bandwidth.}
Freq/BW/DR
Power
Frequency
BW
FREQ
{see Fig 2-1}
1
0
0 1 1 0 0 0 1 0
TTime
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic number
0.50
0.25
rms
ampl
itude
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic number
0.50
0.25
rms
ampl
itude
1 harmonic
1
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic number
0.50
0.25
rms
ampl
itude
2 harmonics
1
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic number
0.50
0.25
rms
ampl
itude
4 harmonics
1
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Harmonic number
0.50
0.25
rms
ampl
itude
8 harmonics
1
0
Maximum Data Rates• Nyquist:
– DataRate <= 2*BandWidth * log2 V
where ‘V’ is the number of values which are encoded into
the signal. In the On/Off, 0/1 world, V = 2. Your highspeed
modem has V = 16.
• Shannon:
– The real world is noisy, so Nyquist was an optimist.
• Marti:
– Complexity costs money and adds fragility.
So be choosy.
DR ~ 2 * BW {Max by Theory}
DR ~ 1/2 * BW {Practical}
<- In an On/Off world (V = 2)
Physical Effects
Bandwidth Limits - {Signals consist of many (infinite) different sine waves, not all of which can be carried by the media}
Dispersion - {Particularly for multimode fiber, different parts of thesignal may move at different speeds, thus changing the shape ofthe signal at the receiver}
Jitter - {Imperfect clock synchronization along the transmission path}
Noise - {Unwanted, external energy that may corrupt the signal}
Media
• Optical Fiber {pg 87} Multimode Single Mode
• Coax {pg 84,85} Broadband Baseband
• Twisted Pair {pg 83} Shielded Unshielded
Cost and PerformanceMedia Types:
UTP
Coax Baseband Broadband
Fiber Multimode Single Mode
IncreasingBandwidth
IncreasingCost
But remember, cost includes --material--LABOR--electronics
Biggest part of installation cost
DistancesMedia Types:
UTP
Coax Baseband Broadband
Fiber Multimode Single Mode
Typically 100m +/-
200m-500mup to 40km
depends on power budget;can be 100s of km*
* Most LANs use 2km between devices
Fiber Facts
CoreCladding
Protective Coating
Core Cladding50 125 Multimode (microns)62.5 125 " "8 to 10 n/a Singlemode
Fiber Facts, cont.
"photons"
Modes == Different paths thru core
Since the photons travel at the same speed, but for different distances,the energy is spread out, or dispersed, at the receiver
Fiber is specified as XX Mhz-km. So a specification of 800Mhz-kmmeans you could have a bandwidth of 400Mhz over a 2km distanceor 1.6Ghz over a 0.5km distance.
Dispersion has two components: modal and material
Traditional Baseband
Coax
Terminator
Transceiver
Transceiver Cable
Host
CATV Systems
Headend
Amplifier
Splitter
NetworkInterfaceUnit
Translator
"Forward" Signal"Return" Signal
Twisted Pair
• Just copper wire where each two wires (“pairs”) have been twisted around each other in the cable. {Phone wire}
• Rejects common mode noise
• Minimizes antenna characteristics
• Shielded or Unshielded refers to a ground sheath around the whole cable.
• Cat 3 vs Cat 4 vs Cat 5
Design
Backbone
Distribution
Daisy Chain Home Run Bus
Riser Systems
Wireless Overview
• Radio– Mobile– Cellular
• Microwave
• Satellite
• Infrared/Laser
Modulation
ASK - Amplitude Shift Keying {varying signal strength}
FSK - Frequency Shift Keying {varying signal frequency}
PSK - Phase Shift Keying {don't ask!}
{NB the above three methods are usually applied to signal carriers}
PCM/PWM - Pulse Code Modulation/ Pulse Width Modulation{good for fiber}
Others
"Modification of a transmitted signal to encode information (bits)"
Basic Network Concepts
Circuit switching - {basic TELCO service. Guaranteed responsebecause resources are guaranteed. Inefficient for some applications}Virtual-circuit packet-switching - {divide the info into packets tomultiplex}Datagram packet-switching - {like the US Mail....}
Connectionless vs Connection-oriented{At the Link layer, do we do acknowledgements? At the network layer,doall the packets have to follow the same route?}
Multiplexing - {single media, multiple independent 'circuits'}{putting multiple 'sessions' on a single media}
Switching
• Circuit Switching– Guaranteed resource– No size limit on information sent
• Packet Switching - Divides the information into packets; restricts sizes; also sharing of resources
– Virtual Circuit // Connect-oriented– Datagram // Connection-less
Multiplexing
TDM - {time division multiplexing} {low overhead, inefficient}
FDM - {frequency division multiplexing}
STDM - { statistical time division multiplexing} {some overhead, more efficient, may FAIL}
Multiplexing Examples
TDM
STDM
ABCD
BADCBADCBADCBAD
2400 2400
2400 2400
9600
4800
ABCD
BACACBDCBDBABBD
FDM
Multiplexing w/ Packets
Like STDM, except NO "ROUND ROBIN"
Framing & Synchronization
Synchronous - Sender and receiver somehow share a common clock.good for longer runs of data; more efficient but requires the clock signalsomehow be sent along with the data
Asynchronous - Sender and receiver use different clocks so data runshave to be shorter. Doesn't require the extra clock signal
Synchronous vs Asynchronous - Framing & clocksIsochronous vs Aperiodic - Characteristics of traffic {beware ofconfusion as each writer may mix terms}
{Here synchronization refers to the sender's and receiver's clocks}{Frames are packets added signal needed to transmit them on Physical Layer}
Specific Framing
Bit Stuffing - Used to ensure special framing and/or control characters are not sent in the data. A problem because the 'clock' isusually continuous but data may not be there, so we have to know whenthe line is idle and when a frame starts {frame delimiter}
Manchester Encoding - Example of combining clock with data toform a single signal -- no separate line is required. It does require twicethe bandwidth of the original signal
ManchesterEncoding
Ensures for each bit there is a clock transition. Data values (0 or 1)are encoded by positive or negative clock transitions in the middle ofthe bit time. Transitions are made at bit edges if needed so that thecorrect transition can be made in the middle of the bit.
TELCO Architecture
Users
Users
Users
Users
Trunks
CO
CO
CO
CO
TELCO Trunking
Older, Analog World: Frequency Multiplexing
Current World: Digitizing & Time Multiplexing
ISDNIntegrated Services Digital Network
"Think of defining ISDN by looking at the two pairs of words"
BRI • 2B+D Basic Rate Interface B = 64kbps each D = 16kbp for TELCO signaling
PRI • 23B+D
ISDN Nets
"User"
GTE
AT&T
Definition points
BISDN
•Not broadband in the sense of FDM, but rather:
“So fast, we can carry a lot of stuff."
RF LANsEthernet (or slower) over microwave
Pushed by Apple and Motorola
PCS
An umbrella for several new digital cellular services.