3-1 Physical Layer (contd) l Telephone systems n referred to as PSTNs (Public Switched Telephone Systems) n high coverage n for voice and data n low speed and high bit error rate n an urge for higher efficiency to support data and multimedia applications

3-2 Physical Layer (contd) l Telephone systems (contd) n structure (Fig. 2-14, p. 103)

3-3 Physical Layer (contd) l Telephone systems (contd) n structure (contd) (Fig. 2-15, p. 105)

3-4 Physical Layer (contd) l Telephone systems (contd) n structure (contd) –transmission media l twisted pair l coaxial cable l microwave l fiber optics –advantages of digital trans. over analog trans. l higher accuracy by data regeneration l integrated services l cheaper (only to correctly distinguish a 0 from a 1) l easier maintenance (to track down problems)

3-5 Physical Layer (contd) l Telephone systems (contd) n structure (contd) –three major components of a telephone system l local loops (twisted pair, analog signaling, advanced technologies including DPGS, ADSL and HDSL) l trunks (fiber optics or microwave, mostly digital) l switching offices –CPE (customer premises equipment), e.g. telephones and fax machines, connected by a telephone system

3-6 Physical Layer (contd) l Telephone systems (contd) n politics of telephones (Fig. 2-16, p. 107) n In Feb. 1996, restrictions were removed so that the cable TV, local phone, long distance and cellular companies can enter one anothers business.

3-7 Physical Layer (contd) l Telephone systems (contd) n local loop –typical configuration to support data communications ( Fig. 2-17, p. 108)

3-8 Physical Layer (contd) l Telephone systems (contd) n local loop (contd) –transmission impairments l attenuation: distance and frequency dependent, recovered by amplifiers and equalizers l delay distortion: caused by frequency-dependent propagation speeds, unavoidable l noise: e.g. thermal noise, cross talk and impulse noise –modems l modulator and demodulator l digital data, analog signaling and digital transmission l using modulation to reduce the range of frequencies

3-9 Physical Layer (contd) l Telephone systems (contd) n local loop (contd) –digital modulation techniques (Fig. 2-18, p. 110)

3-10 Physical Layer (contd) l Telephone systems (contd) n local loop (contd) –constellation patterns (Fig. 2-19, p. 111)

3-11 Physical Layer (contd) l Telephone systems (contd) n local loop (contd) –digital modulation standards l ITU V.32: 9600 bps, 16 QAM l ITU V.32 bis: bps, 64 QAM l ITU V.34: bps l ITU V.90: 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

3-12 Physical Layer (contd) l Telephone systems (contd) n local loop (contd) –echo suppresser for long distance transmission (Fig. 2-20, p. 113) l only half-duplex is possible with significant reverse time l designed for human speech, not digital data l in-band pure tone at 2100 Hz to disable the echo suppresser l replaced by echo chancellors

3-13 Physical Layer (contd) l Telephone systems (contd) n 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)

3-14 Physical Layer (contd) l Telephone systems (contd) n fiber in the local loop –FTTH (Fiber To The Home) –FTTC (Fiber To The Curb) –HFC (Hybrid Fiber Coax)

3-15 Physical Layer (contd) l Telephone systems (contd) n trunks and multiplexing –FDM (frequency division multiplexing) (Fig. 2-24, p.119)

3-16 Physical Layer (contd) l Telephone systems (contd) n trunks and multiplexing –WDM (wavelength division multiplexing) (Fig. 2-25, p.120) l highly reliable (completely passive) l desired due to the speed limit on E/O and O/E conversion l can be used in the FTTC architecture

3-17 Physical Layer (contd) l Telephone systems (contd) n trunks and multiplexing (contd) –TDM (time division multiplexing) l digitizing voice by a codec (sampling and quantizing) l PCM (Pulse Code Modulation) l T1 carrier (Fig. 2-26, p.122)

3-18 Physical Layer (contd) l Telephone systems (contd) n trunks and multiplexing (contd) –TDM (contd) l DPCM (Differential PCM) l DM (Delta Modulation) (Fig. 2-27, p.123)

3-19 Physical Layer (contd) l Telephone systems (contd) n trunks and multiplexing (contd) –TDM (contd) l carrier hierarchy (Fig. 2-28, p.124)

3-20 Physical Layer (contd) l Telephone systems (contd) n switching –from manual to automatic –schematic illustration (Fig. 2-34, p. 131)

3-21 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –comparison of switching techniques (Fig. 2-35, p. 132)

3-22 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –comparison of switching techniques (contd) (Fig. 2-36, p. 134)

3-23 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –switching hierarchy (Fig. 2-37, p. 135)

3-24 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –crossbar (crosspoint) switches (Fig. 2-38, p. 136) –simplest and nonblocking –the number of crosspoints grows fast with n 2

3-25 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –space division switches (Fig. 2-39, p. 137) –smaller number of crosspoints –possible call blocking

3-26 Physical Layer (contd) l Telephone systems (contd) n switching (contd) –time division switches (Fig. 2-40, p. 138) –the table size and the RAM buffer grow linearly with n