# Physical Layer 1 Analog vs Digital Analog continuous

• Slides: 27

Physical Layer 1

Analog vs. Digital § Analog: continuous values over time § Digital: discrete values with sharp change over time 2

Analog vs. Digital § Can be used in three contexts: information, signal, transmission Digital Analog Text, integers, binary strings Voice, video Signal Square waves Sine waves Transmission Use repeater to boost signal Use amplifier to boost signal Information 3

All Information Encoded Digital § § All information can be encoded in digital data format and become a binary string Digitizing analog data: sampling and quantization We are focusing on digital data (binary strings) for the purpose of this class Benefits of everything going digital - Digital processing, storage, transmission - Zero distortion possible with digital storage & transmission (see later) 4

Signal Decomposition § All signals can be decomposed into harmonic sine waves = + 1. 3 X + 0. 56 X + 1. 15 X 5

Analog Signal vs. Digital Signal § § Digital signal has a wide frequency spectrum - Subject to strong attenuation and distortion - Not good for long distance transmission - Used for short distance transmission such as Ethernet Analog signal is used for long distance transmission - Need modulation technique (more later) 6

Analog vs. Digital Transmission § § § Transmission: Communication of data by propagation and processing of signals Issue: signal distorted and attenuated over distance Analog Transmission - Use amplifiers to boost signal - Amplify both signal and distortion Digital Transmission - Use repeaters to boost signal • receives signal • extracts bit pattern • Retransmits Benefits of digital transmission? 7

Digital Signal 8

Digital Signal § § § Digital signal - Discrete voltage levels Transmission is synchronous, i. e. , a clock is used to sample the signal. - In general, the duration of one bit is equal to one or two clock ticks - Receiver’s clock must be synchronized with the sender’s clock Encoding can be done bit at a time or in blocks of, e. g. , 4 or 8 bits. 9

Encoding Example 1 : Non-Return to Zero (NRZ) 0 1 0 0 0 1 1 0 1 . 85 V 0 -. 85 § § 1 -> high signal; 0 -> low signal Long sequences of 1’s or 0’s can cause problems: - Sensitive to clock skew, i. e. hard to recover clock - Difficult to interpret 0’s and 1’s 10

Encoding Example 2: Non-Return to Zero Inverted (NRZI) 0 1 0 0 0 1 1 0 1 . 85 V 0 -. 85 § § 1 -> make transition; 0 -> signal stays the same Solves the problem for long sequences of 1’s, but not for 0’s. 11

Encoding Example 3: Ethernet Manchester Encoding 0 1 1 0 . 85 V 0 -. 85. 1 s § § § Positive transition for 0, negative for 1 Transition every cycle communicates clock (but need 2 transition times per bit) DC balance has good electrical properties 12

Analog Signal and Modulation 13

Concepts with Sine Wave § Peak Amplitude (A) - maximum strength of signal § Frequency (f) and Period (T) - Hertz (Hz) or cycles per second - T = 1/f § Phase ( ) - Relative position in time § Wavelength ( ) - = v. T 14

Frequency Shift Keying (FSK) 16

Phase Shift Keying (PSK) 17

Medium 19

Physical Media § § § Guided (twisted pair, fiber) vs. unguided (air, water, vacuum) Simplex, half duplex, full duplex Characteristics - Bit Error Rate - Data Rate (what is the difference between data rate & bandwidth? ) - Degradation with distance 20 20

Transmission Channel Considerations Good § Bad Every medium supports transmission in a certain frequency range. - Outside this range, effects such as attenuation, . . degrade the signal too much § Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band. Frequency - Tradeoffs between cost, distance, bit rate § As technology improves, these parameters change, even for the same wire. - Thanks to our EE friends Signal 21

Capacity Limit § Nyquist Theorem: for a noiseless channel of width H, the maximum capacity 2 x H baud rate. § Shannon’s theorem: for a noisy channel of and bandwidth H and signal to noise ratio of S/N, the maximum capability (bps) is H x log(1 + S/N) 22

Copper Wire § Unshielded twisted pair - § Two copper wires twisted - avoid antenna effect Grouped into cables: multiple pairs with common sheath Category 3 (voice grade) versus category 5 100 Mbit/s up to 100 m, 1 Mbit/s up to a few km Cost: ~ 10 cents/foot Coax cables. - One connector is placed inside the other connector - Holds the signal in place and keeps out noise - Gigabit up to a km § Signaling processing research pushes the capabilities of a specific technology. - E. g. modems, use of cat 5 23

Light Transmission in Fiber 1. 0 LEDs Lasers tens of THz loss (d. B/km) 0. 5 1. 3 1. 55 0. 0 1000 1500 nm (~200 Thz) wavelength (nm) 24

Fiber Types § Multimode fiber. - 62. 5 or 50 micron core carries multiple “modes” - used at 1. 3 microns, usually LED source - subject to mode dispersion: different propagation modes travel at different speeds - typical limit: 1 Gbps at 100 m § Single mode - 8 micron core carries a single mode used at 1. 3 or 1. 55 microns, usually laser diode source typical limit: 1 Gbps at 10 km or more still subject to chromatic dispersion 25

Wireless Technologies § § Great technology: no wires to install, convenient mobility, . . High attenuation limits distances. - Wave propagates out as a sphere - Signal strength reduces quickly (1/distance)3 § High noise due to interference from other transmitters. - Use MAC and other rules to limit interference - Aggressive encoding techniques to make signal less sensitive to noise § § Other effects: multipath fading, security, . . Ether has limited bandwidth - Try to maximize its use - Government oversight to control use 26

The Frequency Spectrum is crowded… 27