Some Definitions. Wireless Communication Data is transmitted over the air, modulated onto a carrier signal (e.g., FDMA, CDMA)

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1 Broadband Some Definitions According to International Telecommunications Union (ITU), defined as transmission speed higher than.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. Wireless Communication Data is transmitted over the air, modulated onto a carrier signal (e.g., FDMA, CDMA) 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.

2 Digital Telephony Example Analog signal: T s Digitized signal: (b = 3) Bit rate is b/t s T s 2

3 For digital telephony: Voice quality requires ~4 khz bandwidth T s = 25 µs (f s = 8 khz) b = 8 b bits in T s 8 khz X 8 bits (bit rate 64 kb/s) gives DS0 signal. User-to-network interface: 24 X DS0 Framing bit MUX DS channel DS bits in each T S : 24 X 8 + = 93 DS bit rate: 93 / 25 µs =.544 Mb/s 28 X DS 88 Framing bits MUX DS3 channel DS3 bits in each T S : 28 X = 5592 DS3 bit rate: 5592 / 25 µs = Mb/s T-carrier system: T line carries a DS signal T3 line carries a DS3 signal 3

4 Ethernet Invented in 973 at Xerox PARC IEEE standard (0 Mb/s) created in 985 Used to create Local-Area Networks (LANs) IEEE ethernet identifiers: 0 BASE 5 -- (0 Mb/s, baseband transmission, 500m max. cable length) 000 BASE T -- ( Gb/s, baseband transmission, twisted-pair) Gigabit/0 Gigabit Ethernet (IEEE Standard 802.3): Gb/s links can be transmitted over twisted-pair copper 0 Gb/s links can be transmitted over copper (short lengths) or fiber. 4

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 5

6 Synchronization Methods TX RX Ref. clock CMU only data, not clock, transmitted 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. 6

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 Optical carrier (OC) level OC- OC-3 OC-2 OC-48 OC-92 OC-768 OC-536 OC-3072 Native bit rate 5.84 Mb/s Mb/s Mb/s Gb/s Gb/s Gb/s Gb/s Gb/s * Also known internationally as Synchronous Digital Hierarchy (SDH). 7

8 SONET Ring OC-3 OC-3 Add/Drop MUX OC-48 Add/Drop MUX OC-3 OC-3 working ring OC-48 standby ring OC-48 OC-2 OC-2 Add/Drop MUX OC-48 Add/Drop MUX OC-2 OC-2 Fiber rings can easily be deployed If any one link fails or is down for maintenance, data can still be transmitted. 8

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 , 4.25, 8.5 Gb/s 9

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. 0

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;.25 Gb/s downstream TDMA Burst-mode operation: Switching among fibers requires fast locking at receiver (within ~30 UI).

12 Open Systems International (OSI) Networking Protocol of interest to IC designers 2

13 Characteristics of Broadband Signals & Circuits Primarily digital (i.e., bilevel) operation but high bit rate (multi-gb/s) dictates analog behavior & design techniques. Standard analog circuit applications: Continuous-time operation Precision required in signal domain (i.e., voltage or current) Dynamic range determined by noise & distortion V t 0 ΔV t Broadband communication circuits: Discrete-time (clocked) operation Precision required in time domain (low jitter) Bilevel signals processed V H V t V L V Δt t 3

14 Binary Data Representations (time domain) Non-return-to-zero (NRZ) format (most common): T b unit interval (UI) Return-to-zero (RZ) format: 2 T b 0 0 Higher bandwidth RZ signals require faster circuitry than NRZ, but are more easily synchronized due to more transitions. 4

15 Some Definitions () 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/2 clock cycles transition density = 0.5 Equivalent to density of 00 repeating pattern 5

16 Some Definitions (2) Run Length is the maximum of consecutive 0 s or 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 = 0 bits 6

17 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 s & 0 s Sequence length = 2 n -; max. run length = n Q Q 2 Q 3 D D Q Q 2 Q CK D = Q Q PRBS

18 Definitions of Common PRBS Signals Sequence Sequence Length Run Length Feedback (defined by ITU) D = Q Q D = Q 5 Q D = Q 9 Q D = Q 4 Q D = Q 3 Q D = Q 8 Q D = Q 28 Q 3 Bit error-rate testing (BERT) equipment is programmed to recognize these patterns. 8

19 Decimation Properties of PRBS PRBS: PRBS demuxed into 2 parallel channels Resulting bit sequences are both also PRBS! 9

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

21 Close-up of measured eye diagram: t rise = t fall voltage swing UI (Unit Interval) Zero crossings Zero-crossing width indicates jitter. 2

22 Types of Jitter () 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 22

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: a) Intersymbol interference (ISI) b) Duty-cycle distortion (DCD) c) Periodic jitter (PJ) 23

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

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

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

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

28 c) Periodic Jitter (PJ) 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 Δt Δt 0 PJ = Δt Δt 0 Uncorrelated jitter (e.g., sub-rate PJ due to supply ripple) affects the eye diagram in a similar way as RJ. 28

29 Binary Data Representations in Frequency Domain () 0 A random data signal x(t) can be represented as: p(t) x(t) = where T b k b k p( t kt b ) t [ ] b k,+ is the bit sequence and p(t) is a unit-interval pulse:: P f ( ) =T b sin(πft b ) πft b P(f) 2 3 f T b T b T b 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: S x ( f ) = T b P f ( ) 2 =T b ( ) sin πft b πft b 2 29

30 Binary Data Representations in Frequency Domain (2) S x ( f ) S x ( f ) = T b P f ( ) 2 =T b ( ) sin πft b πft b f T b T b T b Example: 0 Gb/s data signals S x ( f ) random data repeating f (GHz) 00 ps 30

31 Transmission over Copper Ideal transmission line: l l c! c! For l, c 0, transmission line behaves like a constant delay. Lossy transmission line: l r s! l r s! c! g p! c! g p! Series loss r s and shunt loss g p cause attenuation and reduce bandwidth. 3

32 l r s! l r s! c! g p! c! g p! At high frequencies, skin effect causes r s to increase with frequency: H(ω) e Lα ω And dielectric loss causes g p to increase with frequency: H(ω) e Lβω L = transmission line length; α,β are constants For H(ω) = exp L( α ω + βω) 0logH(ω) = 4.34L ( α ω + βω) This results in a very steep drop in a log-log scale 32

33 Effect of High-Frequency Loss in Copper Cable H(f) (db) f (Hz) 33

34 Coaxial cable grounded shield inner conductor (signal) Purpose of outer conductor: Shields region inside from external electromagnetic fields Provides return path Typical 00 MHz: 9 GHz: 22 db/foot 34

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): < 6 MHz Cat 5e UTP: < 00 MHz Cat 6 UTP: < 250 MHz 35

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. 36

37 Transmission over Optical Fiber Snell s Law of Refraction: sin θ sin θ 2 = n 2 n = v v 2 reflected ray n n 2 reflected ray n n 2 refracted ray refracted ray θ θ θ 2 θ θ 2 θ incident ray n 2 > n incident ray n 2 < n 37

38 Total Internal Reflection reflected ray n n 2 refracted ray Let θ 2 = π/2: θ θ 2 θ Then sinθ = n 2 n θ c = sin n 2 n incident ray n 2 < n For θ > θ c, light ray is completely reflected. Total internal reflection 38

39 Optical Fiber Transmission n cladding reflected ray n core n cladding n n 2 refracted ray n cladding < n core θ θ 2 θ Total internal reflection keeps all optical energy within the core, even if the fiber bends. incident ray n 2 < n core cladding 39

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: 000 feet of copper weighs approx. 300 lb. 000 feet of fiber weighs approx. 0 lb. Different light wavelengths can be multiplexed onto a single fiber via Dense Wavelength Division Multiplexing (DWM). 0Gb/s & 40 Gb/s transmission networks are state-of-the art. 40

41 Fiber Loss vs. Wavelength 850nm (LED) 30nm 550nm Commonly-used wavelengths 4

42 Types of Optical Fiber Diameter 25 µm inexpensive; used for shorter distances; dispersion causes jitter. Diameter = 2~8 µm Expensive; used for long distances Optical dispersion compensation; non-uniform n 42

43 Optical Signals 40Gbps NRZ signal Dr 25ps f (GHz) Laser source λ = 550nm f = 93 THz Modulator 93THz 40GHz f λ = v / f 550 λ (nm)

44 Chromatic Dispersion () Chromatic dispersion is due to the fact that different wavelength travel at different speeds. 44

45 Chromatic Dispersion (2) CD is measured in ps/nm. CD = dτ dλ CD is proportional to fiber length: CD = ( 7ps/nm/m) L Relative group Delay, τ (ps) λ (nm) 45

46 Chromatic Dispersion at Different Data Rates 0Gb/s CD=0 CD=600ps/nm CD=600ps/nm CD=2200ps/nm 40Gb/s CD=0 CD=40ps/nm CD=00ps/nm CD=40ps/nm 46

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. 47

48 Eye Diagrams due to PMD DGD=0 (BER<e-5) DGD=0ps (BER<e-5) DGD=5ps (BER=2e-) DGD=20ps (BER=2e-4) DGD=25ps (BER=2e-2) DGD=30ps (BER=2e-2) 48