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

• MPEG-3 compresses a video channel to lt5 Mbps.
• 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.
• A much lower Carrier-to-Noise Ratio (CNR) is
  required to transmit these QAM signals than is
  required by AM-VSB.
• 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

• RF band is 5-42 (E 65)MHz, this band can carry
  multiple RF channels. Modulation schemes are QPSK
  or 16QAM
• 5-15 MHz is plagued with ingress noise.
• All frequencies suffer from the funnel effect.
• Up to 10 Mbps transmission per RF channel is
  provided in the standard, but a peak rate of 3
  Mbps is more realistic.
• Bandwidth is shared.
• Services can be segregated by RF frequency.
• For data the standard is DOCSIS (Data Over Cable
  Service Interface Specification).
• Telephony can be carried over DOCSIS 1.1.
• 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.