Physical Layer contd l Telephone systems n referred

Physical Layer (cont’d) 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 1

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

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

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

Physical Layer (cont’d) l Telephone systems (cont’d) n structure (cont’d) – three major components of a telephone system l 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 5

Physical Layer (cont’d) l Telephone systems (cont’d) 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 another’s business. 6

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

Physical Layer (cont’d) l Telephone systems (cont’d) n local loop (cont’d) – transmission impairments l l l 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 l l l modulator and demodulator digital data, analog signaling and digital transmission using modulation to reduce the range of frequencies 8

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

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

Physical Layer (cont’d) l Telephone systems (cont’d) n local loop (cont’d) – digital modulation standards l l 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 11

Physical Layer (cont’d) l Telephone systems (cont’d) n local loop (cont’d) – echo suppresser for long distance transmission (Fig. 2 -20, p. 113) l l 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 12

Physical Layer (cont’d) l Telephone systems (cont’d) 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) 13

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

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

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

Physical Layer (cont’d) l Telephone systems (cont’d) n trunks and multiplexing (cont’d) – TDM (time division multiplexing) l l l digitizing voice by a codec (sampling and quantizing) PCM (Pulse Code Modulation) T 1 carrier (Fig. 2 -26, p. 122) 17

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

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

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

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

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

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

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

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

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