Physical Layer (cont’d)

1. Physical Layer (cont’d) • Telephone systems • referred to as PSTNs (Public Switched Telephone Systems) • high coverage • for voice and data • low speed and high bit error rate • an urge for higher efficiency to support data and multimedia applications

2. Physical Layer (cont’d) • Telephone systems (cont’d) • structure (Fig. 2-14, p. 103)

3. Physical Layer (cont’d) • Telephone systems (cont’d) • structure (cont’d) (Fig. 2-15, p. 105)

4. Physical Layer (cont’d) • Telephone systems (cont’d) • structure (cont’d) • transmission media • twisted pair • coaxial cable • microwave • fiber optics • advantages of digital trans. over analog trans. • higher accuracy by data regeneration • integrated services • cheaper (only to correctly distinguish a 0 from a 1) • easier maintenance (to track down problems)

5. Physical Layer (cont’d) • Telephone systems (cont’d) • structure (cont’d) • three major components of a telephone system • local loops (twisted pair, analog signaling, advanced technologies including DPGS, ADSL and HDSL) • trunks (fiber optics or microwave, mostly digital) • switching offices • CPE (customer premises equipment), e.g. telephones and fax machines, connected by a telephone system

6. Physical Layer (cont’d) • Telephone systems (cont’d) • politics of telephones (Fig. 2-16, p. 107) • In Feb. 1996, restrictions were removed so that the cable TV, local phone, long distance and cellular companies can enter one another’s business.

7. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop • typical configuration to support data communications ( Fig. 2-17, p. 108)

8. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop (cont’d) • transmission impairments • attenuation: distance and frequency dependent, recovered by amplifiers and equalizers • delay distortion: caused by frequency-dependent propagation speeds, unavoidable • noise: e.g. thermal noise, cross talk and impulse noise • modems • modulator and demodulator • digital data, analog signaling and digital transmission • using modulation to reduce the range of frequencies

9. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop (cont’d) • digital modulation techniques (Fig. 2-18, p. 110)

10. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop (cont’d) • constellation patterns (Fig. 2-19, p. 111)

11. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop (cont’d) • digital modulation standards • ITU V.32: 9600 bps, 16 QAM • ITU V.32 bis: 14400 bps, 64 QAM • ITU V.34: 28800 bps • ITU V.90: 56000 bps • trellis coding, e.g. 128 points in the constellation pattern to send 6 data bits and 1 check bit in 1 baud • FDM (to disable noisy bands), compression and error correction to improve performance

12. Physical Layer (cont’d) • Telephone systems (cont’d) • local loop (cont’d) • echo suppresser for long distance transmission (Fig. 2-20, p. 113) • only half-duplex is possible with significant reverse time • designed for human speech, not digital data • in-band pure tone at 2100 Hz to disable the echo suppresser • replaced by echo chancellors

13. Physical Layer (cont’d) • Telephone systems (cont’d) • RS-232-C and RS-449 • standardized by EIA and ITU • physical layer specifications • to connect the computer and the modem • null modems to connect two devices without modems • 20 Kbps over 15-meter trans. limit for RS-232-C • 2 Mbps over 60-meter trans. limit for RS-449 with RS-422-A (balanced transmission)

14. Physical Layer (cont’d) • Telephone systems (cont’d) • fiber in the local loop • FTTH (Fiber To The Home) • FTTC (Fiber To The Curb) • HFC (Hybrid Fiber Coax)

15. Physical Layer (cont’d) • Telephone systems (cont’d) • trunks and multiplexing • FDM (frequency division multiplexing) (Fig. 2-24, p.119)

16. Physical Layer (cont’d) • Telephone systems (cont’d) • trunks and multiplexing • WDM (wavelength division multiplexing) (Fig. 2-25, p.120) • highly reliable (completely passive) • desired due to the speed limit on E/O and O/E conversion • can be used in the FTTC architecture

17. Physical Layer (cont’d) • Telephone systems (cont’d) • trunks and multiplexing (cont’d) • TDM (time division multiplexing) • digitizing voice by a codec (sampling and quantizing) • PCM (Pulse Code Modulation) • T1 carrier (Fig. 2-26, p.122)

18. Physical Layer (cont’d) • Telephone systems (cont’d) • trunks and multiplexing (cont’d) • TDM (cont’d) • DPCM (Differential PCM) • DM (Delta Modulation) (Fig. 2-27, p.123)

19. Physical Layer (cont’d) • Telephone systems (cont’d) • trunks and multiplexing (cont’d) • TDM (cont’d) • carrier hierarchy (Fig. 2-28, p.124)

20. Physical Layer (cont’d) • Telephone systems (cont’d) • switching • from manual to automatic • schematic illustration (Fig. 2-34, p. 131)

21. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • comparison of switching techniques (Fig. 2-35, p. 132)

22. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • comparison of switching techniques (cont’d) (Fig. 2-36, p. 134)

23. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • switching hierarchy (Fig. 2-37, p. 135)

24. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • crossbar (crosspoint) switches (Fig. 2-38, p. 136) • simplest and nonblocking • the number of crosspoints grows fast with n2

25. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • space division switches (Fig. 2-39, p. 137) • smaller number of crosspoints • possible call blocking

26. Physical Layer (cont’d) • Telephone systems (cont’d) • switching (cont’d) • time division switches (Fig. 2-40, p. 138) • the table size and the RAM buffer grow linearly with n