1. Data Encoding Reading Assignment : Stallings Chapter 2, pp. 45 - 60, Chapter 4, pp. 96 - 101, 103 - 105, 107 - 111 Terminology –Data entities that convey meaning –Signals electromagnetic representation of data –Signaling the act of propagating the signal along a suitable medium –Transmission the communication of data by the propagation and processing of signals. –Encoding converting analog or digital data to analog or digital signals

2. Data –analog continuous values on some interval examples –audio / acoustic »sound waves perceived by human »20 Hz - 20 k Hz »above 700 Hz -- slight intelligibility added –video »amount of illumination at each point »time varying analog signal »horizontal and vertical retrace »483 horizontal lines  30 scans / sec »interlace : 241.5 lines  60 scans / sec to avoid flickering –digital e.g., text or character strings ASCII –7 bits + parity bit

3. Signals –analog continuously varying electromagnetic wave medium : twisted pair, coaxial cable, fiber optic cable, atmosphere examples –speech spectrum : 20 Hz to 20 k Hz »telephone transmitter : 300 to 3400 Hz »Either high frequencies of speech are cut off, or »for higher fidelity, convert acoustic signal into an electromagnetic signal over 300 - 3400 Hz. –video signal »horizontal blanking pulse »horizontal sync pulse : to maintain transmitter-receiver synchronization »vertical blanking pulse –Digital e.g., high voltage for 1, low voltage for 0 See chapter 4.

4. –modem (modulator-demodulator) digital  analog (carrier frequency)  digital –codec (coder-decoder) analog  digital  analog methods of transmission –analog amplifier –digital repeater : regenerates digital signal –digital transmission preferred cost dropping for integrated circuits (not as much for analog equipment) Effects of noise, etc. are not cumulative (due to the use of repeaters). effective use of large capacity of medium security and privacy

5. Transmission impairments –Attenuation The signal strength falls off logarithmically with distance for guided medium. Attenuation is an increasing function of frequency. –Decibel notation : 10 log 10 ( P 2 / P 1 ) Considerations –The transmitter generates a signal as strong as possible without causing distortion. –The received signal must be well above noise level. –Delay distortion The signal velocity over a guided medium varies with frequency. critical for digital data –Intersymbol interference »Some of the signal components of one bit position spill over into other bit positions.

6. –Noise unwanted signals inserted between transmitter and receiver types –thermal noise »thermal agitation of electrons in a conductor »across entire frequency spectrum »cannot be eliminated –intermodulation noise »generated by two or more frequencies sharing the same transmission medium »f 1 + f 2 or f 1 - f 2 –crosstalk »electrical coupling between nearby twisted pair »unwanted signals picked up by microwave antennas »same order of magnitude as thermal noise –impulse noise »noncontinuous, irregular pulses/spikes, short duration, relatively high amplitude »electromagnetic disturbances, lightning »affects digital signals most

7. Data encoding techniques –analog signaling carrier signal analog medium : usually band limited modulation –the process of encoding source data onto a carrier signal with frequency f c. –input : modulating signal (baseband signal) –transmitted : modulated signal –four possibilities analog/digital data  analog/digital signal –digital data, digital signals terminology –signal element –signaling rate / baud rate / modulation rate –unipolar signaling »all voltage levels positive or negative –polar signaling »voltage levels both positive and negative

8. Facts –An increase in data rate increases bit error rate. –An increase in S/N ratio decreases bit error rate. –An increase in bandwidth allows an increase in data rate. factors affecting receiver in interpreting digital signals –timing of each bit (clocking) –signal level for each bit position –S/N ratio, data rate, bandwidth –encoding scheme desired properties in encoding scheme –signal spectrum »lack of direct-current (dc) component »lack of high-frequency components, i.e., lower bandwidth »transmitted power concentrated in the middle of the transmission bandwidth –synchronization mechanism for clocking –error detection capability –noise immunity

9. Nonreturn to Zero (NRZ) coding techniques –Nonreturn-to-Zero-Level (NRZ-L) »1 : constant positive voltage »0 : constant negative voltage –NRZI »nonreturn to zero, invert on ones »A transition at the beginning of a bit time denotes a binary 1; no transition indicates a binary 0. –Limitations »presence of dc component »lack of synchronization capability –used for signal input, digital magnetic recording differential encoding –signal decoded by comparing the polarity of adjacent signal elements –advantages : easier to detect a transition –E.g., if the leads of a twisted pair is inverted, NRZI still gives correct results, but NRZL does not.

10. Biphase coding techniques –at least one transition per bit time –Manchester code »1 : low-to-high transition »0 : high-to-low transition »used in Ethernet –Differential Manchester code »1 : absence of transition at beginning of bit period »0 : presence of transition at beginning »differential coding used »used in token ring –mid-bit transition : clocking –advantages »synchronization »no dc component »error detection : absence of an expected transition  error Modulation rate (D) –rate at which signal elements are generated. –Data rate (R) : bits per second –e.g., Manchester encoding »R <= D <= 2 R

11. –digital data, analog signals carrier frequency f c phone lines : 300 - 3400 Hz modulation techniques –Amplitude-shift keying (ASK) »binary 1 : A cos( 2  f c t ) »binary 0 : 0 »susceptible to sudden gain change »1200 bps on voice-grade phone lines »ASK is used in transmission over optical fiber. –Frequency-shift keying (FSK) »binary 1 : A cos( 2  f 1 t ) »binary 0 : A cos( 2  f 2 t ) »f 1, f 2 : f c   »used in 1200 bps modems, high- frequency radio transmission –Phase-shift keying (PSK) »data represented by shifting phase of carrier signal »A cos( 2  f c t + s )

12. –Two-phase differential PSK »binary 1 : A cos( 2  f c t +  ) »binary 0 : A cos( 2  f c t ) –Quadrature phase-shift keying (QPSK) »binary 11 : A cos( 2  f c t +  /4 ) »binary 10 : A cos( 2  f c t + 3  /4 ) »binary 00 : A cos( 2  f c t + 5  /4 ) »binary 01 : A cos( 2  f c t + 7  /4 ) –Combining PSK and ASK »9600 bps modem, 12 phase angles, 2 amplitude values, on 2400 bps baud line