physical layer - courses.cs.washington.edu · •bottom-up through the layers: •followed by more...
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Physical Layer
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Lecture Progression
• Bottom-up through the layers:
• Followed by more detail on:• Quality of service, Security (VPN, SSL)
Computer Networks 2
Application - HTTP, DNS, CDNs
Transport - TCP, UDP
Network - IP, NAT, BGP
Link - Ethernet, 802.11
Physical - wires, fiber, wireless
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Where we are in the Course
•Beginning to work our way up starting with the Physical layer
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Physical
Link
Network
Transport
Application
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Scope of the Physical Layer
•Concerns how signals are used to transfer message bits over a link• Wires etc. carry analog signals• We want to send digital bits
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…1011010110…
Signal
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Topics
1. Coding and Modulation schemes• Representing bits, noise
2. Properties of media• Wires, fiber optics, wireless, propagation
• Bandwidth, attenuation, noise
3. Fundamental limits• Nyquist, Shannon
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Coding and Modulation
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Topic
•How can we send information across a link?• This is the topic of coding and modulation• Modem (from modulator–demodulator)
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…1011010110…
Signal
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A Simple Coding
• Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0• This is called NRZ (Non-Return to Zero)
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Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
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A Simple Modulation (2)
• Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0• This is called NRZ (Non-Return to Zero)
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Bits
NRZ
0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0
+V
-V
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A Simple Modulation (3)
• Problems?
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Many Other Schemes
•Can use more signal levels• E.g., 4 levels is 2 bits per symbol
•Practical schemes are driven by engineering considerations• E.g., clock recovery
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Clock Recovery
•Um, how many zeros was that?• Receiver needs frequent signal transitions to decode bits
•Several possible designs• E.g., Manchester coding and scrambling (§2.5.1)
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1 0 0 0 0 0 0 0 0 0 … 0
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Ideas?
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Answer 1: A Simple Coding
• Let a high voltage (+V) represent a 1, and low voltage (-V) represent a 0
• Then go back to 0V for a “Reset”• This is called RZ (Return to Zero)
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Bits
RZ
0 1 1 1 0 0 0 1
-V
+V
0
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Answer 2: Clock Recovery – 4B/5B
•Map every 4 data bits into 5 code bits without long runs of zeros• 0000 11110, 0001 01001, 1110 11100, …
1111 11101• Has at most 3 zeros in a row• Also invert signal level on a 1 to break up long runs of 1s
(called NRZI, §2.5.1)
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Answer 2: Clock Recovery – 4B/5B (2)
•4B/5B code for reference:• 000011110, 000101001, 111011100, …
111111101
•Message bits: 1 1 1 1 0 0 0 0 0 0 0 1
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Coded Bits:
Signal:
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Clock Recovery – 4B/5B (3)
•4B/5B code for reference:• 000011110, 000101001, 111011100, …
111111101
•Message bits: 1 1 1 1 0 0 0 0 0 0 0 1
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Coded Bits:
Signal:
1 1 1 0 1 1 1 1 1 0 0 1 0 0 1
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Modulation vs Coding
•What we have seen so far is called coding• Signal is sent directly on a wire
•These signals do not propagate well as RF• Need to send at higher frequencies
•Modulation carries a signal by modulating a carrier• Baseband is signal pre-modulation• Keying is the digital form of modulation (equivalent to
coding but using modulation)
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Passband Modulation (2)
•Carrier is simply a signal oscillating at a desired frequency:
•We can modulate it by changing:• Amplitude, frequency, or phase
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Comparisons
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NRZ signal of bits
Amplitude shift keying
Frequency shift keying
Phase shift keying
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Philosophical Takeaways
●Everything is analog, even digital signals
● Digital information is a discrete concept represented in an analog physical medium○ A printed book (analog) vs.○ Words conveyed in the book (digital)
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Simple Link Model
• We’ll end with an abstraction of a physical channel• Rate (or bandwidth, capacity, speed) in bits/second
• Delay in seconds, related to length
• Other important properties:• Whether the channel is broadcast, and its error rate
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Delay D, Rate R
Message
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Message Latency
• Latency is the delay to send a message over a link• Transmission delay: time to put M-bit message “on the wire”
• Propagation delay: time for bits to propagate across the wire
• Combining the two terms we have:
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Message Latency (2)
• Latency is the delay to send a message over a link• Transmission delay: time to put M-bit message “on the wire”
T-delay = M (bits) / Rate (bits/sec) = M/R seconds
• Propagation delay: time for bits to propagate across the wire
P-delay = Length / speed of signals = Length / ⅔c = D seconds
• Combining the two terms we have: L = M/R + D
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Latency Examples
• “Dialup” with a telephone modem:• D = 5 ms, R = 56 kbps, M = 1250 bytes
• Broadband cross-country link:• D = 50 ms, R = 10 Mbps, M = 1250 bytes
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Remembering L = M/R + D
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Latency Examples (2)
• “Dialup” with a telephone modem:• D = 5 ms, R = 56 kbps, M = 1250 bytes
• L = (1250x8)/(56 x 103) sec + 5ms = 184 ms!
• Broadband cross-country link:• D = 50 ms, R = 10 Mbps, M = 1250 bytes
• L = (1250x8) / (10 x 106) sec + 50ms = 51 ms
• A long link or a slow rate means high latency: One component dominates
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Bandwidth-Delay Product
•Messages take space on the wire!
•The amount of data in flight is the bandwidth-delay (BD) product
BD = R x D• Measure in bits, or in messages• Small for LANs, big for “long fat” pipes
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Bandwidth-Delay Example
•Fiber at home, cross-country R=40 Mbps, D=50 ms
110101000010111010101001011
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Bandwidth-Delay Example (2)
•Fiber at home, cross-country R=40 Mbps, D=50 msBD = 40 x 106 x 50 x 10-3 bits
= 2000 Kbit= 250 KB
• That’s quite a lot of data in the network”!
110101000010111010101001011
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Media
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https://www.merriam-webster.com/dictionary/media
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Types of Media
•Media propagate signals that carry bits of information
•We’ll look at some common types:• Wires• Fiber (fiber optic cables)• Wireless
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Wires – Twisted Pair
•Very common; used in LANs and telephone lines• Twists reduce radiated signal
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Category 5 UTP cable with four twisted pairs
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Wires – Coaxial Cable
•Also common. Better shielding for better performance
•Other kinds of wires too: e.g., electrical power (§2.2.4)
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Fiber
• Long, thin, pure strands of glass• Enormous bandwidth (high speed) over long distances
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Light source(LED, laser)
Photo-detector
Light trapped bytotal internal reflection
Optical fiber
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Fiber (2)
•Two varieties: multi-mode (shorter links, cheaper) and single-mode (up to ~100 km)
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Fiber bundle in a cableOne fiber
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Signals over Fiber
• Light propagates with very low loss in three very wide frequency bands• Use a carrier to send information
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Wavelength (μm)
Attenuation(dB/km)
By SVG: Sassospicco Raster: Alexwind, CC-BY-SA-3.0, via Wikimedia Commons
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Wireless
•Sender radiates signal over a region• In many directions, unlike a wire, to potentially many
receivers• Nearby signals (same freq.) interfere at a receiver; need to
coordinate use
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Wireless Interference
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WiFi
WiFi
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Wireless (2)
•Unlicensed (ISM) frequencies, e.g., WiFi, are widely used for computer networking
802.11b/g/n
802.11a/g/n
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Multipath (3)
•Signals bounce off objects and take multiple paths• Some frequencies attenuated at receiver, varies with
location
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Wireless (4)
•Various other effects too!• Wireless propagation is complex, depends on
environment
•Some key effects are highly frequency dependent, • E.g., multipath at microwave frequencies
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Limits
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Topic
•How rapidly can we send information over a link? • Nyquist limit (~1924)• Shannon capacity (1948)
•Practical systems are devised to approach these limits
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Key Channel Properties
•The bandwidth (B), signal strength (S), and noise (N)• B (in hertz) limits the rate of transitions• S and N limit how many signal levels we can distinguish
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Bandwidth B Signal S,Noise N
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Nyquist Limit
•The maximum symbol rate is 2B
•Thus if there are V signal levels, ignoring noise, the maximum bit rate is:
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R = 2B log2V bits/sec
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
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Claude Shannon (1916-2001)
•Father of information theory• “A Mathematical Theory of
Communication”, 1948
•Fundamental contributions to digital computers, security, and communications
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Credit: Courtesy MIT Museum
Electromechanical mouse that “solves” mazes!
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Shannon Capacity
•How many levels we can distinguish depends on S/N• Or SNR, the Signal-to-Noise Ratio• Note noise is random, hence some errors
•SNR given on a log-scale in deciBels:• SNRdB = 10log10(S/N)
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0
1
2
3
N
S+N
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Shannon Capacity (2)
•Shannon limit is for capacity (C), the maximum information carrying rate of the channel:
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C = B log2(1 + S/N) bits/sec
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Shannon Capacity Takeaways
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C = B log2(1 + S/N) bits/sec
• There is some rate at which we can transmit data without loss over a random channel
• Assuming noise fixed, increasing the signal power yields diminishing returns : (
• Assuming signal is fixed, increasing bandwith increases capacity linearly!
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Wired/Wireless Perspective (2)
• Wires, and Fiber• Engineer link to have requisite SNR and B
→Can fix data rate
• Wireless• Given B, but SNR varies greatly, e.g., up to 60 dB!
→Can’t design for worst case, must adapt data rate
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Engineer SNR for data rate
Adapt data rate to SNR
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Putting it all together – DSL
•DSL (Digital Subscriber Line, see §2.6.3) is widely used for broadband; many variants offer 10s of Mbps• Reuses twisted pair telephone line to the home; it has up
to ~2 MHz of bandwidth but uses only the lowest ~4 kHz
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DSL (2)
•DSL uses passband modulation (called OFDM)• Separate bands for upstream and downstream (larger)• Modulation varies both amplitude and phase (QAM)• High SNR, up to 15 bits/symbol, low SNR only 1 bit/symbol
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Upstream Downstream
26 – 138kHz
0-4kHz 143 kHz to 1.1 MHz
Telephone
Freq.
Voice Up to 1 Mbps Up to 12 Mbps
ADSL2:
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Phy Layer Innovation Still Happening!
● Backscatter “zero power” wireless
● mm wave 30GHz+ radio equipment
● Free space optical (FSO)
● Cooperative interference management
● Massive MIMO and beamforming
● Powerline Networking
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All distilled to a simple link model
• Rate (or bandwidth, capacity, speed) in bits/second• Delay in seconds, related to length
• Other important properties:• Whether the channel is broadcast, and its error rate
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Delay D, Rate R
Message