3-1

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

3-2

Physical Layer (cont’d)

Telephone systems (cont’d) structure

(Fig. 2-14, p. 103)

3-3

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

structure (cont’d)

(Fig. 2-15, p. 105)

3-4

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)

3-5

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

3-6

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.

3-7

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

local loop– typical configuration to support data communications

( Fig. 2-17, p. 108)

3-8

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

3-9

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

local loop (cont’d)– digital modulation techniques

(Fig. 2-18, p. 110)

3-10

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

local loop (cont’d)– constellation patterns

(Fig. 2-19, p. 111)

3-11

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

3-12

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

3-13

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)

3-14

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)

3-15

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

trunks and multiplexing– FDM (frequency division multiplexing)

(Fig. 2-24, p.119)

3-16

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

3-17

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)

3-18

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)

3-19

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

trunks and multiplexing (cont’d)– TDM (cont’d)

carrier hierarchy

(Fig. 2-28, p.124)

3-20

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

switching– from manual to automatic

– schematic illustration

(Fig. 2-34, p. 131)

3-21

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

switching (cont’d)– comparison of switching techniques

(Fig. 2-35, p. 132)

3-22

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

switching (cont’d)– comparison of switching techniques (cont’d)

(Fig. 2-36, p. 134)

3-23

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

switching (cont’d)– switching hierarchy

(Fig. 2-37, p. 135)

3-24

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

3-25

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

3-26

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