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: bps, 64 QAM
ITU V.34: bps
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

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