Physical Layer (cont

1
Physical Layer (contd)

• 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 (contd)

• Telephone systems (contd)
• structure
• (Fig. 2-14, p. 103)

3
Physical Layer (contd)

• Telephone systems (contd)
• structure (contd)
• (Fig. 2-15, p. 105)

4
Physical Layer (contd)

• Telephone systems (contd)
• structure (contd)
• 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 (contd)

• Telephone systems (contd)
• structure (contd)
• 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 (contd)

• Telephone systems (contd)
• 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 anothers
  business.

7
Physical Layer (contd)

• Telephone systems (contd)
• local loop
• typical configuration to support data
  communications
• ( Fig. 2-17, p. 108)

8
Physical Layer (contd)

• Telephone systems (contd)
• local loop (contd)
• 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 (contd)

• Telephone systems (contd)
• local loop (contd)
• digital modulation techniques
• (Fig. 2-18, p. 110)

10
Physical Layer (contd)

• Telephone systems (contd)
• local loop (contd)
• constellation patterns
• (Fig. 2-19, p. 111)

11
Physical Layer (contd)

• Telephone systems (contd)
• local loop (contd)
• 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 (contd)

• Telephone systems (contd)
• local loop (contd)
• 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 (contd)

• Telephone systems (contd)
• 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 (contd)

• Telephone systems (contd)
• fiber in the local loop
• FTTH (Fiber To The Home)
• FTTC (Fiber To The Curb)
• HFC (Hybrid Fiber Coax)

15
Physical Layer (contd)

• Telephone systems (contd)
• trunks and multiplexing
• FDM (frequency division multiplexing)
• (Fig. 2-24, p.119)

16
Physical Layer (contd)

• Telephone systems (contd)
• 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 (contd)

• Telephone systems (contd)
• trunks and multiplexing (contd)
• 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 (contd)

• Telephone systems (contd)
• trunks and multiplexing (contd)
• TDM (contd)
• DPCM (Differential PCM)
• DM (Delta Modulation)
• (Fig. 2-27, p.123)

19
Physical Layer (contd)

• Telephone systems (contd)
• trunks and multiplexing (contd)
• TDM (contd)
• carrier hierarchy
• (Fig. 2-28, p.124)

20
Physical Layer (contd)

• Telephone systems (contd)
• switching
• from manual to automatic
• schematic illustration
• (Fig. 2-34, p. 131)

21
Physical Layer (contd)

• Telephone systems (contd)
• switching (contd)
• comparison of switching techniques
• (Fig. 2-35, p. 132)

22
Physical Layer (contd)

• Telephone systems (contd)
• switching (contd)
• comparison of switching techniques (contd)
• (Fig. 2-36, p. 134)

23
Physical Layer (contd)

• Telephone systems (contd)
• switching (contd)
• switching hierarchy
• (Fig. 2-37, p. 135)

24
Physical Layer (contd)

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

25
Physical Layer (contd)

• Telephone systems (contd)
• switching (contd)
• space division switches
• (Fig. 2-39, p. 137)
• smaller number of crosspoints
• possible call blocking

26
Physical Layer (contd)

• Telephone systems (contd)
• switching (contd)
• time division switches
• (Fig. 2-40, p. 138)
• the table size and the RAM buffer grow linearly
  with n