The Physical Layer

1
The Physical Layer
2
The Theoretical Basis for Data Communication

• Fourier analysis
• Niquist chriterium for bandwidth-limited channel
• Shannon maximum data rate of a noisy channel

3
Fourier Transform

• Periodic signals with period T2p/w
• Non-periodic signals

4
Bandwidth-Limited Signals

• A binary signal and its root-mean-square Fourier
  amplitudes.
• (b) (c) Successive approximations to the
  original signal.

5
Bandwidth-Limited Signals

• (d) (e) Successive approximations to the
  original signal.

6
Bandwidth-Limited Signals

• Relation between data rate and harmonics.

7
Band-Limited Channel

• Fourier transform of a typical signal

w
1/Ts
2/Ts
-1/Ts
8
Power Spectrum Density

• Autocorrelation function of signal or noise
• Power spectrum density

9
Filtering

• Channel behaves as a filter
• When the noise is white (uncorrelated)
  Gaussian optimum filter has transfer function
  H(w)X(w).

10
Niquist Theorem

• If the signal bandwidth has width of W, then
  it can be reconstructed by taking 2W samples per
  second.
• Maximum data rate is
• where V is the number of different symbols

11
Niquist Chriterium
Ts sampling interval, ? sampling pulse width
12
Niquist Chriterium
s(t)
t
t
x(t)
t
13
Niquist Chriterium
S(f)
-W
W
f
X(f)
f
-2/Ts
1/Ts
-1/Ts
2/Ts
14
Shannon Theorem

• If the channel bandwidth has width of W, and
  S/N is the signal-to-noise ratio, then the
  maximum data rate is

15
Modulation

• (a) A binary signal
• (b) Amplitude modulation

• (c) Frequency modulation
• (d) Phase modulation

16
Modulation

• Signal is located around carrier frequency w0,
  and its amplitude and phase depend on the data
  symbol in each time slot

17
Modulation Schemes

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

18
Guided Transmission

• Twisted Pair
• Coaxial Cable
• Fiber Optics

19
Twisted Pair

• (a) Category 3 UTP 16 MHz.
• (b) Category 5 UTP 100MHz.

20
Coaxial Cable

• A coaxial cable 1GHz.

21
Fiber Optics

• (a) Three examples of a light ray from inside a
  silica fiber impinging on the air/silica boundary
  at different angles.
• (b) Light trapped by total internal reflection.

22
Transmission of Light through Fiber

• Attenuation of light through fiber in the
  infrared region.
• Bands 25-30THz, and last two bands have
  attenuation less than 5/km

23
Fiber Cables

• (a) Side view of a single fiber.
• (b) End view of a sheath with three fibers,
  diameter 8-10µm.

24
Transmission Devices

• Light emitting diode (LED)
• Semiconductor lasers
• Mach-Zehnder external modulator
• EDFA
• Photodiode

25
Optical Transmitters

• A comparison of semiconductor diodes and LEDs as
  light sources.

26
Wireless Transmission

• Relationship between wavelength and frequency
• 100MHz waves are about 3m long, 1000MHz waves
  are 0.3m long.
• An object distracts those waves, whose length
  is smaller or equal to the object
  dimension.

27
The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

28
Radio Transmission

• (a) In the VLF, LF, and MF bands, radio waves
  follow the curvature of the earth.
• (b) In the HF band, they bounce off the
  ionosphere.

29
Issues in Wireless Transmission

• Radio signals are omnidirectional, and
  penetrate through objects. Throughput is low.
• HF radio and microwave signals are directed.
  Suffer from multipath fading, and are reflected
  against the buildings.
• Above 4GHz, signals are absorbed by the rain.

30
Lightwave Transmission

• Convection currents can interfere with laser
  communication systems.
• A bidirectional system with two lasers is
  pictured here.
• Fog and rain are disruptive too.

31
Communication Satellites

• Geostationary Satellites
• Several kWs. 40 transponders with 80MHz. TDMA.
• Medium-Earth Orbit Satellites
• 24 GPS satellites.
• Low-Earth Orbit Satellites
• Iridium project started by Motorola

32
Communication Satellites

• Communication satellites and some of their
  properties, including altitude above the earth,
  round-trip delay time and number of satellites
  needed for global coverage.

33
Communication Satellites

• The principal satellite bands.

34
Frequency Division Multiplexing

• (a) The original bandwidths.
• (b) The bandwidths raised in frequency.
• (b) The multiplexed channel.

35
Wavelength Division Multiplexing

• Wavelength division multiplexing.

36
Time Division Multiplexing

• The T1 carrier (1.544 Mbps).

37
Time Division Multiplexing

• Multiplexing T1 streams into higher carriers.

38
TDM

• US and Japan T1 1.544Mbps
• 24 channels one sync. bit
• 23 data channels, 7 data bits 1 signalling bit
• Multiplexing degrees 4,7,6
• Others E1 2.048Mbps
• 32 channels
• 30 data channels, 8 data bits, 1 bit signalling
  in every sixth frame
• Multiplexing degree 4, bit rates 2.048Mbps,
  8.848Mbps,

39
SONET and SDH

• Bellcore and CCITT Synchronous Optical Networks
  (SONET), Synchronous Digital Hierarchy (SDH)
• Define frames for bit-rates 50Mbps and up

40
SONET

• Two back-to-back SONET STS-1 frames comprising
  810 bytes

41
Time Division Multiplexing

• SONET and SDH multiplex rates.

42
CDMA Code Division Multiple AccessIS-95
r(t)
43
Walsh-Hadamard Sequences
44
CDMA Code Division Multiple Access(e.g. IS-95)

• (a) Binary chip sequences for four stations
• (b) Bipolar chip sequences
• (c) Six examples of transmissions
• (d) Recovery of station Cs signal

45
The Local Loop Modems, ADSL, and Wireless

• The use of both analog and digital transmissions
  for a computer to computer call. Conversion is
  done by the modems and codecs.

46
Modems
(b)
(a)

• (a) V.32 for 9600 bps.
• (b) V32 bis for 14,400 bps.

47
Higher Bit-rate Modems

• 2400 samples (bauds) per second
• V32 to 14.4Kbps, V34 to 33.6Kbps
• V90 35Kbps upstream, 56Kbps downstream
• V92 48kbps upstream, 56Kbps downstream

48
Digital Subscriber Lines

• Bandwidth versus distanced over category 3 UTP
  for DSL.

49
Digital Subscriber Lines

• Operation of ADSL using discrete multitone
  modulation.
• Up to 8Mbps downstream, and up to 1Mbps upstream.
• Modulation similar to V34, 15 bits per sample,
  4000 bauds per sec

50
Digital Subscriber Lines

• A typical ADSL equipment configuration.

51
Internet over Cable

• Cable television

52
Internet over Cable

• The fixed telephone system.

53
Community Antenna Television (CATV)
antenna
HOME
o o
o o
o o
o o
o o
HEADEND
o o
o o
o o
o o
RF Spectrum
AM-VSB signals
Long chains of RF amplifiers limited
bandwidth, poor reliability.
55 MHz 350 MHz
Sheryl Woodward, ATT Labs-Research
54
Linear Lightwave Revolution
Hybrid-Fiber-Coax Architecture Improved
reliability and performance, BUT to transmit 80
channels of AM-VSB, an optical link must operate
near fundamental limits.
RF Spectrum
80 AM-VSB channels
55 (E 85)MHz 350 MHz 550 (E 606)MHz
Sheryl Woodward, ATT Labs-Research
55
Compressed Digital Video

1. MPEG-3 compresses a video channel to lt5 Mbps.
2. Quadrature Amplitude Modulation (QAM) can be used
   to transmit multiple television channels in a
   single 6 (E 8)MHz RF channel. Around 38Mbps can
   be transmitted through this channel.
3. A much lower Carrier-to-Noise Ratio (CNR) is
   required to transmit these QAM signals than is
   required by AM-VSB.
4. A set top box is required to receive these
   channels.

RF Spectrum
80 AM-VSB channels 30 QAM channels (15
0 video channels)
55 (E 85)MHz 350 MHz 550 (E 603)MHz
750 (E 862)MHz
Sheryl Woodward, ATT Labs-Research
56
Upstream Transmission
Can now offer interactive services
Sheryl Woodward, ATT Labs-Research
57
Upstream Transmission

1. RF band is 5-42 (E 65)MHz, this band can carry
   multiple RF channels. Modulation schemes are QPSK
   or 16QAM
2. 5-15 MHz is plagued with ingress noise.
3. All frequencies suffer from the funnel effect.
4. Up to 10 Mbps transmission per RF channel is
   provided in the standard, but a peak rate of 3
   Mbps is more realistic.
5. Bandwidth is shared.
6. Services can be segregated by RF frequency.
7. For data the standard is DOCSIS (Data Over Cable
   Service Interface Specification).
8. Telephony can be carried over DOCSIS 1.1.
9. A cable modem or set top box resides in the home,
   a CMTS, which coordinates traffic, resides in the
   headend.

Sheryl Woodward, ATT Labs-Research
58
The Mobile Telephone System

• Improved mobile telephone system (IMTS) 23
  channels, 150-450MHz Advanced mobile telephone
  system (AMTS) is cellular system 832x30kHz
  channels, 824-894MHz
• Digital D-AMPS, 30KHz channels, 1850-1990MHz
  Global System for Mobile Communications (GSM)
  124x200kHz channels 890-960MHz Code Division
  Multiple Access (CDMA)

59
GSMGlobal System for Mobile Communications

• GSM uses 124 200kHz frequency channels, each of
  which uses an eight-slot TDM system

60
GSM

• A portion of the GSM framing structure.

61
Control Channels

• Broadcast control channel Base station
  broadcasts control signal
• Dedicated control channel Registration, location
  and connection status of users
• Common control channel
• Paging subchannel announces calls
• Random access channel connection requests
• Access grant channel connection grants

62
3G

• W-CDMA or universal mobile telecommunication
  system (UMTS) compatible with GSM, uses 5MHz.
• CDMA2000 extension of IS-95 uses 5MHz
• Enhanced data rates for GSM evolution (EDGE) uses
  more bits per baud
• General radio packet service (GPRS) is overlay
  packet network over D-AMPS or GSM
• WLAN at 2.5 or 5.7GHz 10-50Mbps WMAN at 10-66GHz
  up to 200Mbps (As oppose to around 50Mbps total
  for D-AMPS and GSM, and 8Kbps and 13Kbps per user.