1 / 48

Some Definitions

Wireline Communication. Network connection is transmitted through physical media (copper or optical fiber). Data is usually sent unmodulated. Multiple channels are aggregated via time-division multiplexing. Wireless Communication.

judson
Download Presentation

Some Definitions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Wireline Communication • Network connection is transmitted through physical media (copper or optical fiber). • Data is usually sent unmodulated. • Multiple channels are aggregated via time-division multiplexing. Wireless Communication • Data is transmitted over the air, modulated onto a carrier signal (e.g., FDMA, CDMA) Some Definitions Broadband • According to International Telecommunications Union (ITU), defined as transmission speed higher than 1.5 Mb/s. • Any connection fast enough to support interactive multimedia. • Any communications method that multiplexes a number of individual channels onto a single, high-speed channel. Prof. M. Green

  2. Digital Telephony Example 111 110 101 100 011 010 001 000 Bit rate is b/Ts Analog signal: Ts 1 0 0 1 0 1 1 1 0 Digitized signal: (b = 3) Ts Prof. M. Green

  3. b bits in Ts 1 0 0 1 0 1 1 1 0 “T-carrier” system: T1 line carries a DS1 signal T3 line carries a DS3 signal For digital telephony: Voice quality requires ~4 kHz bandwidth Ts = 125 µs (fs = 8 kHz) b = 8 8 kHz X 8 bits (bit rate 64 kb/s) gives “DS0” signal. User-to-network interface: 24 X DS0 DS1 bits in each TS: 24 X 8 + 1 = 193 DS1 bit rate: 193 / 125 µs = 1.544 Mb/s DS1 channel MUX Framing bit 28 X DS1 DS3 bits in each TS: 28 X 193 + 188 = 5592 DS3 bit rate: 5592 / 125 µs = 44.736 Mb/s DS3 channel MUX 188 Framing bits Prof. M. Green

  4. Ethernet • Invented in 1973 at Xerox PARC • IEEE 802.3 standard (10 Mb/s) created in 1985 • Used to create Local-Area Networks (LANs) • IEEE ethernet identifiers: • 10 BASE 5 -- (10 Mb/s, baseband transmission, 500m max. cable length) • 1000 BASE T -- (1 Gb/s, baseband transmission, twisted-pair) • Gigabit/10 Gigabit Ethernet (IEEE Standard 802.3): • 1 Gb/s links can be transmitted over twisted-pair copper • 10 Gb/s links can be transmitted over copper (short lengths) or fiber. Prof. M. Green

  5. Networking Wide-Area Network (WAN): multiple LANs connected over a wide geographical area -- made possible by very high-speed optical fibers Metropolitan-Area Network (MAN): Network connection within a metropolitan area Storage-Area Network (SAN): Uses networking techniques to manage very large amounts of data Prof. M. Green

  6. TX RX Ref. clock CMU only data, not clock, transmitted Synchronization Methods • Plesiochronous Digital Hierarchy: • Different parts of network operate at frequencies that are very close (~50 ppm), but not identical. • Such systems require additional functions to compensate for the mismatch by repeating or adding bits. • Reference clocks generated locally (usually with crystal oscillator). • Used in Ethernet protocol. • Synchronous Digital Hierarchy: • All parts of network operate at identical frequencies, accomplished by synchronizing all Reference clocks to the “Stratum” global system of atomic clocks. • Additional functions not required, but jitter requirement is very rigorous. • Used in SONET/SDH protocol. Prof. M. Green

  7. Other Protocols for High-Speed Networks • Synchronous Optical Network (SONET*): • Provides a protocol (standardized by ANSI) for long-haul (> 50km) WAN transmission over optical fiber *Also known internationally as Synchronous Digital Hierarchy (SDH). Prof. M. Green

  8. SONET Ring OC-3 OC-3 Add/Drop MUX Add/Drop MUX OC-48 OC-3 OC-3 standby ring working ring OC-48 OC-48 OC-12 OC-12 Add/Drop MUX Add/Drop MUX OC-48 OC-12 OC-12 • Fiber rings can easily be deployed • If any one link fails or is down for maintenance, data can still be transmitted. Prof. M. Green

  9. Fibre Channel: • Often used for Storage Area Networks (SAN); allows fast transmission of large amounts of data across many different servers. • Serial bit rates of 1.0625 2.125, 4.25, 8.5 Gb/s Prof. M. Green

  10. Some SAN Terminology JBOD: Just a Bunch Of Disks Refers to a set of hard disks that are not configured together. RAID: Redundant Array of Independent (or Inexpensive?) Disks Multiple disk drives that are combined for fault tolerance and performance. Looks like a single disk to the rest of the system. If one disk fails, the system will continue working properly. Prof. M. Green

  11. Passive Optical Network (PON) • Used to replace electronic transmission in “last mile” • Facilitates “Fiber-to-the-home (FTTH)” or “Fiber-to-the-premises (FTTP)” • GPON protocol: • 2.5 Gb/s upstream;1.25 Gb/s downstream • TDMA “Burst-mode” operation: Switching among fibers requires fast locking at receiver (within ~30 UI). Prof. M. Green

  12. Open Systems International (OSI) Networking Protocol of interest to IC designers http://http://en.wikipedia.org/wiki/OSI_model Prof. M. Green

  13. Standard analog circuit applications: • Continuous-time operation • Precision required in signal domain (i.e., voltage or current) • Dynamic range determined by noise & distortion V V t t0 • Broadband communication circuits: • Discrete-time (clocked) operation • Precision required in time domain (low jitter) • Bilevel signals processed V VH Vt VL t t Characteristics of Broadband Signals & Circuits Primarily digital (i.e., bilevel) operation but high bit rate (multi-Gb/s) dictates analog behavior & design techniques. Prof. M. Green

  14. Binary Data Representations (time domain) Non-return-to-zero (NRZ) format (most common): Tb “unit interval” (UI) Return-to-zero (RZ) format: 1 0 1 1 0 1 • Higher bandwidth RZ signals require faster circuitry than NRZ, but are more easily synchronized due to more transitions. Prof. M. Green

  15. Some Definitions (1) Transition Density is the ratio of transitions to the number of unit intervals in a data stream. A high transition density is desirable in a communication system. 6 transitions/12 clock cycles  transition density = 0.5 Equivalent to density of 0011 repeating pattern Prof. M. Green

  16. Some Definitions (2) Run Length is the maximum of consecutive 0’s or 1’s that occur in a data stream. A maximum run length is often specified in a communication system to avoid long periods where no transitions are present. (Also known as Consecutive Identical Digit – CID) Run length = 10 bits Prof. M. Green

  17. Q1 Q2 Q3 D1 1 0 0 1 1 1 0 1 1 1 1 0 0 1 1 1 Q1 Q2 D1 Q3 1 0 1 0 0 1 0 0 0 0 1 1 CK 1 0 0 1 . 23-1 PRBS . . Some Definitions (3) Pseudo-Random Bit Sequence (PRBS) is a repeating pattern that has properties similar to random sequences. • Parameterized by n, number of DFFs in generator. • Gives almost equal number of 1’s & 0’s • Sequence length = 2n-1; max. run length = n Prof. M. Green

  18. Definitions of Common PRBS Signals Bit error-rate testing (BERT) equipment is programmed to recognize these patterns. Prof. M. Green

  19. PRBS demuxed into 2 parallel channels Decimation Properties of PRBS 23-1 PRBS: Resulting bit sequences are both also 23-1 PRBS! Prof. M. Green

  20. Eye diagram Typical broadband data waveform: Length of single bit = 1 Unit Interval (1 UI) An eye diagram maps a random bit sequence to a regular structure that can be used to analyze jitter. Prof. M. Green

  21. Zero-crossing width indicates jitter. trise = tfall Close-up of measured eye diagram: voltage swing 1 UI (Unit Interval) Zero crossings Prof. M. Green

  22. Types of Jitter (1) Random Jitter (RJ): • Originates from external and internal random noise sources • Stochastic in nature (probability-based) • Measured in rms units • Observed as Gaussian histogram around zero-crossing • Grows without bound over time Histogram measurement at zero crossing exhibiting Gaussian probability distribution Prof. M. Green

  23. Types of Jitter (2) Deterministic Jitter (DJ): • Originates from circuit non-idealities (e.g., finite bandwidth, offset, etc.) • Amount of DJ at any given transition is predictable • Measured in peak-to-peak units • Bounded and observed in various eye diagram “signatures” • Different types of DJ: • Intersymbol interference (ISI) • Duty-cycle distortion (DCD) • Periodic jitter (PJ) Prof. M. Green

  24. a) Intersymbol interference (ISI) 1UI < 1UI If rise/fall time << 1 UI, then the output pulse is attenuated and the pulse width decreases. Consider a 1 UI output pulse applied to a buffer: Prof. M. Green

  25. ISI (cont.) 1 0 0 1 1 0 Steady-state not reached at end of 2nd bit t = ISI 2 output sequences superimposed ISI is characterized by a double edge in the eye diagram. Consider 2 different bit sequences: Prof. M. Green

  26. Effect of ISI on measured eye diagram: Double-edge (DJ) combined with RJ Prof. M. Green

  27. b) Duty cycle distortion (DCD) Nominal data sequence Tb 2Tb Data sequence with late falling edges & early rising edges due to threshold shift t = DCD Eye diagram with DCD Crossing offset from nominal threshold • Occurs when rising and falling edges exhibit different delays • Caused by circuit mismatches Prof. M. Green

  28. c) Periodic Jitter (PJ) t1 t0 Timing variation caused by periodic sources unrelated to the data pattern. Can be correlated or uncorrelated with data rate. Clock source with duty cycle ≠50% Synchronized data exhibiting correlated PJ Uncorrelated jitter (e.g., sub-rate PJ due to supply ripple) affects the eye diagram in a similar way as RJ. Prof. M. Green

  29. Binary Data Representations in Frequency Domain (1) where is the bit sequence and p(t) is a unit-interval pulse:: 1 0 Tb t A random data signal x(t) can be represented as: P(f) f If there is equal probability of low or high logic levels (i.e., dc level is 0), the power spectral density of x(t) is given by: Prof. M. Green

  30. Binary Data Representations in Frequency Domain (2) f Example: 10 Gb/s data signals random data f (GHz) repeating 0101 5 10 15 20 25 30 100 ps Prof. M. Green

  31. Transmission over Copper Ideal transmission line: l l For l, c 0, transmission line behaves like a constant delay. c c Lossy transmission line: rs rs l l Series loss rs and shunt loss gp cause attenuation and reduce bandwidth. c gp c gp Prof. M. Green

  32. rs rs l l c gp c gp At high frequencies, skin effect causes rs to increase with frequency: And dielectric loss causes gp to increase with frequency: L = transmission line length;  are constants For This results in a very steep drop in a log-log scale … Prof. M. Green

  33. Effect of High-Frequency Loss in Copper Cable |H(f)| (dB) 108 109 1010 1011 f (Hz) Prof. M. Green

  34. Coaxial cable grounded shield inner conductor (signal) • Purpose of outer conductor: • Shields region inside from external electromagnetic fields • Provides return path Typical loss @ 100 MHz: 9 dB/foot “ @ 1 GHz: 22 dB/foot Prof. M. Green

  35. Twisted Pair + _ • Signal sent differentially. • Twisting gives each line nearly equal exposure to outside interference. • Lighter and less expensive the shielded cable. • Quality specified in # twists/foot Cat 3 unshielded twisted pair (UTP): < 16 MHz Cat 5e UTP: < 100 MHz Cat 6 UTP: < 250 MHz Prof. M. Green

  36. Backplane A circuit board that allows connection of several connectors together, forming a bus. For high-speed signals, the metal traces are considered to be microstrip lines. http://en.wikipedia.org/wiki/Industry_Standard_Architecture Prof. M. Green

  37. Transmission over Optical Fiber n1 n2 n1 n2 reflected ray reflected ray refracted ray refracted ray incident ray incident ray Snell’s Law of Refraction: Prof. M. Green

  38. Total Internal Reflection Let 2 = /2: Then For 1 > c, light ray is completely reflected. Total internal reflection n1 n2 reflected ray refracted ray incident ray Prof. M. Green

  39. Optical Fiber Transmission ncladding ncore ncladding n1 n2 reflected ray refracted ray Total internal reflection keeps all optical energy within the core, even if the fiber bends. incident ray core cladding Prof. M. Green

  40. Advantages of Optical Fibers over Copper Cable • Very high bandwidth (bandwidth of optical transmission network determined primarily by electronics) • Low loss • Interference Immunity (no antenna-like behavior) • Lower maintenance costs (no corrosion, squirrels don’t like the taste) • Small & light: 1000 feet of copper weighs approx. 300 lb. • 1000 feet of fiber weighs approx. 10 lb. • Different light wavelengths can be multiplexed onto a single fiber via Dense Wavelength Division Multiplexing (DWM). • 10Gb/s & 40 Gb/s transmission networks are state-of-the art. Prof. M. Green

  41. Commonly-used wavelengths Fiber Loss vs. Wavelength 850nm (LED) 1550nm 1310nm Prof. M. Green

  42. Diameter  125 µm inexpensive; used for shorter distances; dispersion causes jitter. Diameter = 2~8 µm Expensive; used for long distances Types of Optical Fiber Optical dispersion compensation; non-uniform n1 Prof. M. Green

  43. 25ps -40 40 80 f (GHz) λ = 1550nm f = 193 THz Modulator Laser source f 193THz 40GHz λ = v / f 1550 λ (nm) 0.32 Optical Signals Dr 40Gbps NRZ signal Prof. M. Green

  44. Chromatic Dispersion (1) • Chromatic dispersion is due to the fact that different wavelength travel at different speeds. Prof. M. Green

  45. Chromatic Dispersion (2) • CD is measured in ps/nm. • CD is proportional to fiber length: Relative group Delay, τ (ps) λ(nm) Prof. M. Green

  46. 40Gb/s CD=0 CD=40ps/nm CD=100ps/nm CD=140ps/nm Chromatic Dispersion at Different Data Rates 10Gb/s CD=0 CD=600ps/nm CD=1600ps/nm CD=2200ps/nm Prof. M. Green

  47. Polarization Mode Dispersion • PMD is due to the fact that light travels at different speed across the two orthogonal polarization states. • Output contains two delayed images of the input pulse. Prof. M. Green

  48. DGD=10ps (BER<1e-15) DGD=15ps (BER=2e-11) Eye Diagrams due to PMD DGD=0 (BER<1e-15) DGD=20ps (BER=2e-4) DGD=25ps (BER=2e-2) DGD=30ps (BER=2e-2) Prof. M. Green

More Related