Document Sample

Baseband Signaling and Modulation Part 1: Baseband Signaling Part 1 of a 2-part presentation Eric L. Michelsen Inductive Logic If You Could Tell Your Audience Only One Sentence... Transmitting data requires not only the signaling of bit values, but also bit timing. 1 or 0 time sample here & here & here ... If I could tell them a second sentence, it would be: DC is bad. 1/8/2003 Inductive Logic 2 Topics: Baseband Signaling (day 1) • On-Off signaling • Manchester encoding • Antipodal signaling • 4B/5B encoding • Timing recovery • 8B/10B encoding • NRZI • Multi-Level Transition • Multilevel: 2B1Q • A modern line code • DS1 & DS3 Topics: Modulation (day 2) • Cosine review • Communication channels as • Sums of cosines filters • Spectra • Amplitude modulation • Fourier transforms • Amplitude demodulation • Baseband signaling • Quadrature multiplexing • Why cosine waves? • DMT ADSL • Transfer functions 1/8/2003 Inductive Logic 3 Where in the Stack? • Signaling and modulation are ways of transmitting data • They are the lowest sublayers in Layer 1 (physical layer) • In this context, “signaling” means “transmitting data” (not call setup/teardown) 7. Application OSI stack 6. Presentation 5. Session 4. Transport bit serial 3. Network (payload) 2. Link V.35, framing framing framing framing framing bit serial HSSI, (line) SDSL DS1 DS3 ADSL IDSL SONET 1. Physical signaling & modulation electrical optical 1/8/2003 Inductive Logic 4 A Matter of Values • On-Off binary signaling simple indicates 1 (on) or 0 (off) by itself, does not explicitly convey timing works for electrical and optical signals Used by Ethernet 10Base5 and 10Base2 (w/ additional line coding) amplitude 1 1 0 1 1 0 0 0 time bit period 1/8/2003 Inductive Logic 5 Time Is of the Essence • With separate clock and data, the transmitter gives the receiver timing on one signal, and data on another • Requires two signals (clock and data): can be expensive • Data values are arbitrary (no restrictions) • Used by local interfaces: V.35, (synchronous) EIA-232, HSSI, etc. • As distance and/or speed increase, clock/data skew destroys timing sample on clock rising edge of clock sample times centered in data bits data time 1/8/2003 Inductive Logic 6 No Clock: Do You Know Where Your Data Is? • Most long-distance or high speed signaling is self timed: it has no separate clock; the receiver recovers timing from the data itself • Receiver knows the nominal data rate, but requires transitions in the signal to locate the bits, and interpolate the sample points • Receiver tracks the timing continuously, to stay in synch Tracking requires sufficient transition density throughout the data stream • Used in all DSLs, DS1, DS3, SONET, all Ethernets, etc. transitions locate data data time interpolated sample times (bit centers) 1/8/2003 Inductive Logic 7 Timing Recovery • All self-timed line codes provide sufficient signal transitions for timing recovery. Some methods used: Scrambling Return to zero (RTZ) Zero substitution Manchester encoding 4B/5B 8B/10B Multi-level transition 1/8/2003 Inductive Logic 8 All For One ... or Zero • On-Off binary signaling: simple, but not energy efficient (SNR) • At unit distance (A = 1), average energy = A2/2 = 0.5 • For balanced data, DC (Direct Current) ~= 0.5 (bad) • Also known as Non-Return to Zero (NRZ) • Requires sufficient data transition density, or scrambling • Works for electrical and optical signals amplitude 1 distance 1 0 1 1 0 0 0 time bit period 1/8/2003 Inductive Logic 9 Pluses and Minuses • Antipodal binary signaling: energy efficient (SNR) • At unit distance (A = 0.5), average energy = A2 = 0.25 (3 dB better than on-off signaling) • Requires sufficient data transition density, or scrambling • For balanced (or scrambled) data, DC ~= 0 (good) • For electrical signaling only (negative light?) Ethernet 10BaseT, EIA-232, V.35, V.36, HSSI +0.5 amplitude 1 0 1 1 0 0 0 distance time -0.5 Can you say “tip-ring reversal?” 1/8/2003 Inductive Logic 10 NRZI (Non-Return to Zero Inverted) • Data value coded as transition = 1, no transition = 0 • Used in combination with antipodal or on/off binary signaling • With scrambling, DC ~= 0 • Why NRZI? Can you say “tip-ring reversal?” • Requires sufficient data 1s (signal transition) density, or scrambling + 0 ? 1 1 0 1 0 time - Equivalent NRZI signals + 0 ? 1 1 0 1 0 time - 1/8/2003 Inductive Logic 11 Multilevel Signaling: 2B1Q • 4 is better than 2: Encodes 2 Binary bits into 1 Quatenary (4-level) symbol A pair of bits in a single symbol is a dibit • AKA 4-PAM (4-level Pulse Amplitude Modulation) • Requires data transitions, or scrambling • With scrambling, DC ~= 0 • Used in SDSL, IDSL, ISDN BRI • Other PAMs exist: 16-PAM (G.shdsl), 256-PAM, etc. +3 amplitude +1 11 10 01 00 -1 time Usually described as “distance 2”: -3, -1, +1, +3 -3 1/8/2003 Inductive Logic 12 AMI: Alternate Mark Inversion • Bipolar, tri-state (+, 0, and -) mark = 1 • 50% duty cycle RTZ (Return To Zero) space = 0 • Pulses alternate polarity (DC = 0) • Used by DS1 (Digital Service 1, ref. T1.107, T1.403): 2 pair (4 wire) Line rate = 1.544 Mbps, including 8 kbps framing/OAM Payload rate = 1.536 Mbps Generic digital service, can carry T1, PRI, GR-303, Frame Relay, etc. Timing recovery requires at least 2 pulses (ones) every 16 bits B8ZS (Binary 8-Zero Substitution) provides transparency - amplitude + idealized pulse 1 0 1 1 0 0 25% 50% 25% time alternate polarity UI = Unit Interval (bit period) 1/8/2003 Inductive Logic 13 AMI: DS3 • Digital Service 3 (ref. T1.107, T1.404): 2 coax, 75 • RTZ (Return To Zero) pulse, very similar to DS1 • AMI (Alternate Mark Inversion), (DC = 0) • Line rate = 44.736 Mbps, including ~530 kbps framing/OAM • Payload rate = 44.736 x (84 / 85) 44.210 Mbps • Generic digital service: can carry T3, Frame Relay, ATM, etc. • Timing recovery requires at least one pulse every 3 bits B3ZS (Binary 3-Zero Substitution) provides transparency Deliberate bipolar violation, substitutes for 3 zeros - amplitude + 1 0 1 1 0 0 0 0 0 0 X time alternate X bits inserted as needed to make BPVs polarity alternate polarity, to maintain DC = 0 1/8/2003 Inductive Logic 14 Double Time: Manchester Encoding • “Coding” in this sense is applicable to any binary (2-state) signal (on-off, antipodal, FSK, etc.) • Provides a transition in the center of every bit no density requirement High information content: allows rapid timing recovery • DC = 0, exactly (with antipodal signaling) • Data bit is value in last half of bit (or could be first half) • Used in Ethernet 10Base5, 10Base2, 10BaseT • Equivalent to 1B/2B encoding • Not spectrally efficient: requires transmitting 2 signal events for each bit (100% bandwidth expansion) 1 0 1 1 0 0 signal state B time A 1/8/2003 Inductive Logic 15 Enough is Enough: 4B/5B Encoding • Encodes 4 payload bits into 5 line bits • Guarantees transitions; no user data restrictions or scrambling needed • Extra codewords available for control (Idle, SSD, ESD, ...) • More BW efficient than Manchester: 25% expansion Data • DC >> 0 (bad), but used with NRZI or MLT, DC ~= 0 • 0 1 1 1 1 0 Checks line integrity by counting invalid codes 1 0 1 0 0 1 • Used in Ethernet 100BaseTX, FDDI 2 3 1 0 1 0 0 1 0 1 0 1 Control 4 0 1 0 1 0 5 0 1 0 1 1 1 1 1 1 1 IDLE used as inter-stream fill code 6 0 1 1 1 0 7 0 1 1 1 1 1 1 0 0 0 J Start-of-Stream Delimiter, Part 1 of 2; 8 1 0 0 1 0 always used in pairs with K 9 1 0 0 1 1 1 0 0 0 1 K Start-of-Stream Delimiter, Part 2 of 2; A 1 0 1 1 0 always used in pairs with J B 1 0 1 1 1 0 1 1 0 1 T End-of-Stream Delimiter, Part 1 of 2; C 1 1 0 1 0 always used in pairs with R D 1 1 0 1 1 0 0 1 1 1 R End-of-Stream Delimiter, Part 2 of 2; E 1 1 1 0 0 always used in pairs with T F 1 1 1 0 1 1/8/2003 Inductive Logic 16 Twice as Good: 8B/10B Encoding • Encodes 8 payload bits into 10 line bits • Guarantees 3 to 8 transitions per 10-bit codeword • Maximum run-length of 5 • 25% BW expansion (same as Code Group Na me Octet Value Current RD – Current RD abcdei fghj abcdei fghj 4B/5B) D0.0 D1.0 00 01 100111 0100 011000 1011 011101 0100 100010 1011 • 12 control codes (start of packet, D2.0 D3.0 D4.0 02 03 04 101101 0100 010010 1011 110001 1011 110001 0100 110101 0100 001010 1011 end of packet, error, etc.) D5.0 05 101001 1011 101001 0100 • Alternately inverts non-zero-DC : : : : codewords to achieve zero DC Code Group Octet Current RD – Current RD Notes (similar to AMI) Na me K28.0 Value 1C abcdei fghj 001111 0100 abcdei fghj 110000 1011 1 Worst case codeword imbalance is K28.1 K28.2 3C 5C 001111 1001 001111 0101 110000 0110 110000 1010 1,2 1 6/4 K28.3 7C 001111 0011 110000 1100 1 • K28.4 9C 001111 0010 110000 1101 1 Checks line integrity by counting K28.5 K28.6 BC DC 001111 1010 001111 0110 110000 0101 110000 1001 2 1 invalid codes K28.7 : FC : 001111 1000 : 110000 0111 : 1,2 • Used in Gigabit Ethernet, Fiber NOTE 1 — Reserved . NOTE 2 — Conta ins a c omma . Channel (FC), some backplanes 1/8/2003 Inductive Logic 17 Saving Bandwidth: MLT-3 (Multi-Level Transition) • Bipolar, tri-state signal (+, 0, and -) • Like a combination of NRZI and AMI • Transition = data 1, no transition = 0 • Non-zero signals alternate polarity • Cuts bandwidth in half (and SNR as well) • Used by Ethernet 100BaseTX (with 4B/5B and scrambling) - amplitude + 1 0 1 1 0 0 1 1 distance time 1/8/2003 Inductive Logic 18 A Modern Line Code A N • Binary signaling (on and off, not B O dits and dahs) C P • Pulse Width Modulated (PWM) D Q • Return to zero coded (RTZ, vs. E R NRZ or NRZI) F S • Variable rate G T • Self timed H U • Asynchronous at word level I V • Variable length encoding J W • Data compressed K X • Forward error corrected (English) L Y M Z Interesting history of pen and paper 1/8/2003 Inductive Logic 19 Just Do It D O I T 3 1 1 3 7 dah dit minimum size size inter-word inter- inter- space symbol letter space space • Receiver recovers unit time interval from dits and inter- symbol spaces; extrapolates other intervals 1/8/2003 Inductive Logic 20 Data Compression: English size frequency avg. size frequency avg. A 8 .082 .65 N 8 .071 .57 B 12 .014 .17 O 14 .080 1.12 C 14 .028 .39 P 14 .020 .28 D 10 .038 .38 Q 16 .001 .02 E 4 .131 .52 R 10 .068 .68 F 12 .029 .35 S 8 .061 .49 G 12 .020 .24 T 6 .105 .63 H 10 .053 .53 U 10 .025 .25 I 6 .063 .38 V 12 .009 .11 J 16 .001 .02 W 12 .015 .18 K 12 .004 .05 X 14 .002 .02 L 12 .034 .41 Y 16 .020 .32 M 10 .025 .25 Z 14 .001 .01 Avg letter size: 11.2 units English weighted avg letter size: 9.0 (~20% savings) Opt. Eng. weighted avg letter size: 8.6 (within 5%) 1/8/2003 Inductive Logic 21 Baseband Summary Interface Signaling States Transition Coding on-off Ethernet 10Base5, 10Base2 on-off + DC bias Manchester Morse Code on-off RTZ Ethernet 10BaseT antipodal Manchester antipodal none (NRZ, EIA-232, V.35, HSSI antipodal separate clock and data) Ethernet 100BaseTX, FDDI MLT (3-level) 4B/5B, scrambled (electrical) multi-level SDSL, IDSL, ISDN BRI 2B1Q (= 4-PAM) scrambled G.shdsl 16-PAM scrambled DS1, DS3 AMI (3-level) RTZ, BxZS Gigabit Ethernet (optical), Fiber on-off (optical) 8B/10B Channel optical SONET on-off (optical) scrambled FDDI (optical) on-off (optical) 4B/5B, NRZI 1/8/2003 Inductive Logic 22 Baseband Signaling and Modulation Part 2: Modulation Eric L. Michelsen 1/8/2003 Inductive Logic 23 Another Day, Another Sentence Modulation avoids baseband problems of signal overlap and DC error. If I could tell them a second sentence, it would be: Bandwidth is not capacity! If I could tell them a third sentence, it would be: Bandwidth is not capacity! But first, a review of Fourier analysis... 1/8/2003 Inductive Logic 24 Topics: Modulation • Cosine review • Communication channels as • Sums of cosines filters • Spectra • Amplitude modulation • Fourier transforms • Amplitude demodulation • Baseband signaling • Quadrature multiplexing • Why cosine waves? • DMT ADSL • Transfer functions 1/8/2003 Inductive Logic 25 Definitions • Baseband signaling Communicating a signal in its original form for a given medium (e.g., audio) or Communicating a signal with components down to DC (or almost DC) • Carrier modulation Communication based on modifying (modulating) a cosine wave signal Other forms of modulation exist (non-carrier modulation, e.g., PAM, PWM, PCM(?), but that‟s another story) 1/8/2003 Inductive Logic 26 Cosine: A Function of Angle • Basis function for frequency analysis and for modulation 0o - amplitude + 30o o 70 90 180 450 540 angle 270 360 630 720 (degrees) 120o one cycle y y y y 30o 120o 0o 70o x x x x 1 unit 1/8/2003 Inductive Logic 27 Cosine Wave: A Function of Time • Fully characterized by 3 parameters: A Amplitude (e.g., 10 V) f Frequency (e.g., 2 Hz) cosine wave = A cos(f*360t + ) Phase (e.g., 60) = A cos(360ft + ) A = 10 V 10cos(360(2)t + 60o) 60o time 0.25 0.5 1 (sec) f = 2 Hz 132o 204o 60o 240o t=0 t = 0.1 t = 0.2 t = 0.25 1/8/2003 Inductive Logic 28 Sums of Cosines s(t) = A1cos(360f1t) + A2cos(360f2t) + A3cos(360f3t) + ... 1 0.8 0.6 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -0.2 -0.4 -0.6 -0.8 -1 1 0.8 0.6 0.4 0.2 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -0.2 -0.4 -0.6 -0.8 -1 1/8/2003 Inductive Logic 29 Spectrum: A Bar Chart of Cosines • Progressively denser bar charts give way to a simple graph A A f f A A f f 1/8/2003 Inductive Logic 30 Why Cosine Waves? • Cosines are the only basis functions (aka eigenfunctions) of Time Invariant Linear Systems System: produces output from input Linear: if Ia Oa, then kIa kOa and if Ib Ob, then (Ia + Ib) Oa + Ob Time invariant: it does the same thing all the time • If input is a cosine, then output is a cosine of same frequency, but different amplitude and phase • Linear Cosine components of input don‟t interact input is output is cosine of exactly any cosine the same frequency... TILS ...but different amplitude and phase time time 1/8/2003 Inductive Logic 31 Triangles Are Not Cosines • If input is not a cosine, output is not a multiple of the input • Single triangle wave input produces complex output • What a mess! input is a output is NOT a triangle wave triangle wave TILS time time 1/8/2003 Inductive Logic 32 Transfer Functions • A TILS multiplies each input frequency amplitude (& shifts its phase) • The multiplier (and phase-shift) are functions of frequency TILS H(f ) = Aout / Ain or Ain Aout = H(f )Ain Aout time time at frequency, f at same frequency, f • We can graph the amplitude multiplier as a function of frequency, the amplitude transfer function, H(f ): We can graph the phase- shift as a function of H(f ) frequency: the phase transfer function, (f ) (but we won‟t) f 1/8/2003 Inductive Logic 33 Transfer Functions at Work • Since cosine components of the input signal do not interact, each cosine is multiplied by the transfer function at its frequency • Thus, the output spectrum is the input spectrum multiplied by the transfer function, at each frequency • Every TILS has a transfer function, and a transfer function defines a TILS. TILS Input signal Transfer function Output signal spectrum of linear system spectrum 1/8/2003 Inductive Logic 34 The Communication Channel as Filter • Any communication channel is imperfect • A time invariant linear channel is described by its transfer function • A filter is a TILS that passes some frequencies, and blocks others Transfer function H(f ) for a copper loop f Transfer function H(f ) for a copper loop with a splitter f Transfer function for H(f ) a transistor amplifier This is why DC is bad. f 1/8/2003 Inductive Logic 35 The Spectrum of Square Wave Antipodal Signaling • 90+% of energy is in the first lobe • Part of the first, and all of the other lobes can be discarded without much degradation • This is also the spectrum of 2B1Q, and all PAMs A time square wave A fsym 2fsym 3fsym time filtered square wave fsym 2fsym 3fsym 1/8/2003 Inductive Logic 36 Amplitude Modulation • Given a signal, i(t) • And a carrier, cos(360ct) • We modulate the signal onto the carrier by multiplying the two at each instant in time: i(t)cos(360ct) cos(360ct) i(t) x modulator i(t) cos(360ct) = i(t)cos(360ct) 1/8/2003 Inductive Logic 37 Know Your Identity 1. cos(-a) = cos(a) 2. cos(90-a) = -cos(90+a) 3. cos(a+b) = cos(a)cos(b) - cos(90-a)cos(90-b) cos(a - b) + cos(a + b) 4. cos(a)cos(b) = 2 cos(90-a)cos(90-b) 90-b Demonstration of identity #3 cos(90-a) 90-a b Recall that for any right triangle: a a b H•cos(a) cos(a+b) cos(90-a)cos(90-b) cos(a)cos(b) 1/8/2003 Inductive Logic 38 Spectral View of Amplitude Modulation • Modulating a baseband cosine onto a carrier i(t) = cos(360wt) (simple) baseband spectrum: A a cosine of frequency „w‟ w f cos(360ct) carrier spectrum: A a cosine of frequency „c‟ c f Modulated signal spectrum: Using identity #4: A cos(360wt)cos(360ct) = cos[360(c-w)t] + cos[360(c+w)t] f c-w c c+w Pop Quiz: Is a modulator a TILS? 1/8/2003 Inductive Logic 39 Deja View of Amplitude Modulation • Modulating a complicated baseband signal onto a carrier i(t) complicated A baseband spectrum (AM radio BW = 5 kHz) bandwidth f cos(360ct) carrier spectrum A (AM radio carrier = 540 - 1600 kHz) c f Modulated signal spectrum; using identity #4 for each A frequency component f Notice that the modulated bandwidth is c twice the baseband signal bandwidth bandwidth (AM radio BW = 10 kHz) 1/8/2003 Inductive Logic 40 Demodulation: Getting It Back • Given a modulated signal: i(t)cos(360ct) • Multiply by the carrier again: i(t)cos(360ct)cos(360ct) = i(t)[cos(0) + cos(360(2ct))] modulated = i(t) + i(t)cos[360(2ct)] spectrum A f c i(t)cos[360(2ct)] i(t) almost demodulated A spectrum f c 2c filtered and fully filter transfer demodulated A function spectrum f c 2c 1/8/2003 Inductive Logic 41 All Together Now A time Energy Efficient Signaling fsym 2fsym 3fsym A time Filtered Baseband Signal fsym A 2fsym 3fsym Modulated Carrier c 1/8/2003 Inductive Logic 42 Comparison of Modulated and Unmodulated Carrier + - unmodulated carrier modulated carrier 1/8/2003 Inductive Logic 43 Quadrature Multiplexing: Two for the Bandwidth of One • Consider a signal modulated with the wrong carrier phase, off by 90. We attempt to demodulate (recall identity #4): i(t)cos(360ct + 90)cos(360ct) modulated = i(t)[cos(90) + cos(360(2ct) + 90)] spectrum = i(t)cos[360(2ct) + 90] A f c i(t)cos[360(2ct) + 90] attempted demodulated A spectrum f c 2c filter transfer filtered signal A function spectrum f 1/8/2003 Inductive Logic 44 Quadrature Multiplexing: Part Deux • Consider two signals, i(t) and q(t), modulated with two carriers of the same frequency, but different by 90: i(t)cos(360ct) + q(t)cos(360ct + 90) i(t) q(t) baseband spectra A A f f modulated signal spectrum: generally not symmetric A f c 1/8/2003 Inductive Logic 45 Quadrature Demodulation • Given a quadrature multiplexed modulated signal: i(t)cos(360ct) + q(t)cos(360ct + 90) • Demodulate each channel separately, each with its own carrier: carrier for i(t) [ i(t)cos(360ct) + q(t)cos(360ct + 90) ]cos(360ct) = i(t)cos(360ct)cos(360ct) + q(t)cos(360ct+90)cos(360ct) i(t) demodulated A spectrum f c 2c carrier for q(t) [ i(t)cos(360ct) + q(t)cos(360ct + 90) ]cos(360ct + 90) = i(t)cos(360ct)cos(360ct+90) + q(t)cos(360ct+90)cos(360ct+90) q(t) A demodulated c 2c spectrum f 1/8/2003 Inductive Logic 46 DMT ADSL • Discrete Multi-Tone • Up to 255 “separate” carriers, Each carrier is quadrature multiplexed multi-level PAM Two to 15 bits per symbol per carrier (2 - 256 PAM per I/Q axis) Optimum filling of data into the carriers for maximum total SNR All share the same time, frequency, and phase references Lower carriers omitted for baseband voice • Carrier spacing is 4312.5 Hz upstream downstream A baseband voice f 300 Hz 3600 Hz N x 4312.5 Hz 1/8/2003 Inductive Logic 47 DMT ADSL • Two kinds of FEC: “Fast” path (low latency): Trellis Coded Modulation (TCM) “Interleaved” path (higher latency): Reed-Solomon block interleaved • Framing structure built into the modulation • Integral number of bytes per frame, 4000 user data frames per second = N x 32 kbps data rates • G992.1 defines two services: STM and ATM The industry standard is ATM over STM (HEC delineation) No one uses G992.1‟s ATM mode Superframe: 17 ms frame frame frame frame ... frame frame synch 0 1 2 3 66 67 symbol over over over over head Fast bytes head FEC head Interleaved bytes head 1/8/2003 Inductive Logic 48

DOCUMENT INFO

Shared By:

Categories:

Tags:

Stats:

views: | 10 |

posted: | 5/13/2010 |

language: | English |

pages: | 48 |

OTHER DOCS BY pengxiuhui

Docstoc is the premier online destination to start and grow small businesses. It hosts the best quality and widest selection of professional documents (over 20 million) and resources including expert videos, articles and productivity tools to make every small business better.

Search or Browse for any specific document or resource you need for your business. Or explore our curated resources for Starting a Business, Growing a Business or for Professional Development.

Feel free to Contact Us with any questions you might have.