The Physical Layer

1
The Physical Layer

• Chapter 2

2
The Theoretical Basis for Data Communication

• Some basic theoretical results relevant to signal
  transmission are introduced in this section.
• Fourier Analysis
• Bandwidth-Limited Signals
• Maximum Data Rate of a Channel

3
Bandwidth-Limited Signals

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

4
Bandwidth-Limited Signals (2)

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

5
Bandwidth-Limited Signals (3)
A voice grade line (for telephone) has a cutoff
frequency near 3000 Hz. If this type of lines
are used for data transmission, we have the
following numbers for some commonly used data
rates

• Relation between data rate and harmonics.

6
The maximum data rate of a channel

• In 1924, Nyquist proved
• If an arbitrary signal has been run through a
  filter of bandwidth H, the filtered signal can be
  completely reconstructed by making only 2H
  samples per second.
• If the signal consists of V discrete levels,
  then maximum data rate 2H log2 V bits/sec
• A noiseless 3000 Hz telephone line cannot
  transmit binary (2-level) signals at a rate
  exceeding 6000 bps.
• In 1948, Shannon carried Nyquist's work further
  to extend it to the case of a channel subject to
  random noise. His major result is
  maximum data rate H log2 (1 S/N) bits/sec
  where S/N denotes the
  signal-to-noise power ratio. For a 3000 Hz
  telephone line with S/N 30dB, the upper bound
  data rate is 30,000 bps.
• In practice, it is difficult to even approach the
  Shannon limit.

7
Guided Transmission Data

• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics

8
Magnetic Media

• Data transmission between two machines takes
  place in the following way
• The source machine writes data onto magnetic
  tapes or floppy disks.
• The tapes or disks are physically transported to
  the destination machine by a station wagon,
  truck, or airplane.
• The destination machine reads the data from the
  tapes or disks.
• A simple calculation
• A standard Ultrium tape can hold 200 GB.
• A box of 60 X 60 X 60 cm can hold about 1000 of
  these tapes, for a total capacity of 200
  terabytes, or 1600 terabits.
• A box of tapes can be delivered anywhere in the
  US in 24 hours by Federal Express and other
  companies.
• The effective bandwidth is 1600 terabits/86,400
  sec, or 19 Gbps.
• If the destination is only an hour away by road,
  the bandwidth is increased to over 400 Gbps. No
  computer network can even approach this.
• The cost is less than 3 cents per gigabyte - no
  network carrier on earth can compete with this !
• Advantages high bandwidth and cost effective.
• Disadvantages off-line, and poor delay
  characteristics.

9
Twisted Pair

• Category 3 UTP.
  Category 5 UTP.

• It is the oldest and still most common
  transmission medium.
• A pair consists of two insulated copper wires
  (about 1 mm thick each). Its most common
  application is the telephone system.
• Twisted pairs can run several km without
  amplification, but for longer distances,
  repeaters are needed.
• It can transmit either analog or digital
  information.
• The bandwidth depends on the thickness of the
  wire and the distance traveled. A few Mbps can be
  achieved for a few km.
• Main advantages adequate performance and low
  cost.

10
Coaxial Cable
This kind of coaxial cable (50-ohm) is used for
digital transmission. The bandwidth
depends on the cable length. For 1-km cables, a
data rate of 1 to 2 Gbps is feasible. It
is widely used for LANs and cable TV.
11
Fiber Optics

• In the 1981, the IBM PC run at a clock speed of
  4.77 MHz. Twenty years alter, PCs could run at 2
  GHz, a gain of a factor of 20 per decade.
• In the same period, data communication went from
  56 kbps (ARPANET) to 1 Gbps (modern optical
  communication), a gain of more than a factor of
  125 per decade !
• Furthermore, single CPUs are beginning to
  approach physical limits, such as speed of light
  and heat dissipation problems. In contrast, with
  current fiber technology, the achievable
  bandwidth is above 50,000 Gbps (50 Tbps)!
• In the race between computing and communication,
  communication won.
• The new conventional wisdom should be that all
  computers are hopelessly slow and networks should
  try to avoid computation at all costs, no matter
  how much bandwidth that wastes!

12
Three components of fiber optics

• Transmission medium
  an ultrathin fiber
  of glass or fused silica.
• Optical transmitter
  LED (Light
  Emitting Diode), or a laser diode, which emits
  light pulses when an electrical current is
  applied. Conventionally, a pulse of light
  indicates a 1 bit and the absence of light
  indicates a zero bit.
• Optical receiver
  a
  photodiode, which generates an electrical pulse
  when light falls on it.

13
How does the fiber optics work?
A principle of physics when a light ray passes
from one medium to another, the ray is refracted
(bent) at the boundary. For angles of incidence
above a certain critical value, the light is
refracted back into the silica none of it
escapes into the air.

• (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.

14
Transmission of Light through Fiber

• Attenuation of light through fiber in the
  infrared region.

15
Fiber Cables

• (a) Side view of a single fiber.
• (b) End view of a sheath with three fibers.

16
Fiber Cables (2)

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

17
Fiber Optic Networks

• A fiber optic ring with active repeaters.

18
Fiber Optic Networks (2)

• A passive star connection in a fiber optics
  network.

19
Wireless Transmission

• Some people believe that the future holds only
  two kinds of communication fiber and wireless
• All fixed (non-mobile) computers, telephones,
  faxes, and so on will be by fiber, and
• all mobile ones will use wireless.

• The following wireless transmissions are covered
  in the text book
• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission

20
The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

21
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.

22
Politics of the Electromagnetic Spectrum

• The ISM bands in the United States.

23
Lightwave Transmission

• Convection currents can interfere with laser
  communication systems.
• A bidirectional system with two lasers is
  pictured here.

24
Communication Satellites

• Geostationary Satellites
• Medium-Earth Orbit Satellites
• Low-Earth Orbit Satellites
• Satellites versus Fiber

25
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.

26
Communication Satellites (2)

• The principal satellite bands.

27
Communication Satellites (3)

• VSATs using a hub.

28
Low-Earth Orbit SatellitesIridium

• (a) The Iridium satellites from six necklaces
  around the earth.
• (b) 1628 moving cells cover the earth.

29
Globalstar

• (a) Relaying in space.
• (b) Relaying on the ground.

30
Public Switched Telephone System

• Structure of the Telephone System
• The Politics of Telephones
• The Local Loop Modems, ADSL and Wireless
• Trunks and Multiplexing
• Switching

31
Structure of the Telephone System

• (a) Fully-interconnected network.
• (b) Centralized switch.
• (c) Two-level hierarchy.

32
A typical circuit route for a medium-distance call

• Each phone has two copper wires going directly to
  the nearest end office. This connection is called
  a local loop. The typical distance is 1 to 10 km.
  
• Each end office has a number of outgoing lines
  (coaxial cables, microwaves, or even fiber
  optics) to one or more nearby switching centers,
  called toll offices.
• Toll offices are connected to sectional or
  regional offices that form a network,
  communicating by high bandwidth intertoll trunks
  (fiber optics).
• Local loops use analog signaling, but most
  intertoll trunks are rapidly being converted to
  digital transmission

33
Major Components of the Telephone System

• Local loops
• Analog twisted pairs going to houses and
  businesses
• Trunks
• Digital fiber optics connecting the switching
  offices
• Switching offices
• Where calls are moved from one trunk to another

34
The Politics of Telephones

• The relationship of LATAs, LECs, and IXCs. All
  the circles are LEC switching offices. Each
  hexagon belongs to the IXC whose number is on it.

35
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.

36
Three forms of modulation

• To transmit signals over the local loop, a
  continuous tone in the 1000 to 2000 Hz range is
  used, called a sine wave carrier.
• There are three ways of modulating it (to
  transmit information)
• Amplitude modulation two different voltage
  levels are used to represent 0 and 1,
  respectively.
• Frequency modulation two (or more) different
  tones are used.
• Phase modulation the carrier wave is
  systematically shifted certain degrees at
  uniformly spaced intervals.

A modem (modulator-demodulator) is a device which
accepts a serial stream of bits as input and
produces a modulated signal as output (or vice
versa). It is inserted between the digit computer
and the analog telephone system. Speed up to
56Kbps.
37
An example of three forms of modulation

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

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

38
Modems (2)

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

39
Modems (3)
(b)
(a)

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

40
Digital Subscriber Lines

• Bandwidth versus distanced over category 3 UTP
  for DSL.

41
Digital Subscriber Lines (2)

• Operation of ADSL using discrete multitone
  modulation.

42
A typical ADSL equipment configuration

• ADSL standard (ANSI T1.413 and ITU G.992.1) 8
  Mbps downstream and 1 Mbps upstream.
• Typically, 512 Kbps downstream and 64 Kbps
  upstream (standard service), or 1 Mbps
  downstream and 256 kbps upstream (premium
  service).

43
Wireless Local Loops

• Architecture of an LMDS system.

44
Trunks and multiplexing

• Physical channels are very valuable, so it is
  worthwhile to multiplex many logical channels
  over a single physical channel.
• Two categories of multiplexing schemes
• FDM (Frequency Division Multiplexing) the
  frequency spectrum is divided among the logical
  channels, with each user having exclusive
  possession of his frequency band.
• TDM (Time Division Multiplexing) the users take
  turns (in round robin), each one periodically
  getting the entire bandwidth for a little burst
  of time.
• An example AM radio broadcasting system.
• The allocated spectrum is about 1 MHz (roughly
  500 kHz - 1500 kHz).
• The allocated spectrum is divided into different
  portions, each for one station (FDM).
• Some stations may have two logical subchannels
  music and advertising. These two alternate in
  time on the same frequency (TDM).

45
Frequency Division Multiplexing

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

• Filters limit the usable bandwidth to about 3000
  Hz per channel.
• 4000 Hz is allocated to each channel to avoid
  interference.
• Channels are raised in frequency, each by a
  different amount.
• Logical channels are combined to transmit over
  one physical channel.
• A widespread standard for FDM schemes
• Group 12 4000-Hz voice channels.
• Supergroup 5 groups (60 voice channels).
• Mastergroup 5 supergroups (CCITT), or 10
  supergroups (Bell System), or up to 230,000
  channels (other standards).

46
Wavelength Division Multiplexing

• Wavelength division multiplexing.

47
Major WDM technological progresses

• 1990 invention of WDM technology. The first
  commercial systems had 8 channels of 2.5 Gbps per
  channel.
• 1998 systems with 40 channels of 2.5 Gbps on the
  market.
• 2001 96 channels of 10 Gbps, for a total of
  960Gbps, which is enough to transmit 30
  full-length movies per second (in MPEG-2).
• Systems with 200 channels are already working in
  the lab.
• The reason that the bit rate of a single channel
  is within 10 Gbps is that it is currently
  impossible to convert between electronic and
  optical media any faster.
• By running many channels in parallel on different
  wavelengths, the aggregate bandwidth is increased
  linearly with the number of channels.
• The bandwidth of a single fiber band is about
  25,000 GHz, so there is theoretically room for
  2500 10-Gbps channels even at 1 bit/Hz (and
  higher rates are also possible).
• All optical amplifiers can regenerate the entire
  signal once every 1000 km without the need for
  multiple opto-electronic conversions.

48
Time Division Multiplexing
In contrast to FDM, which requires analog
circuitry, TDM can be handled entirely by digital
electronics, so it has become far more widespread
in recent years. TDM is mostly used in
interoffice trunks for digital data. Analog
signals from the local loops can be digitized in
the end office by a codec (coder-decoder),
producing a 7- or 8- bit number. The codec
makes 8000 samples per second (125 sec/sample)
because this is sufficient to capture all the
information from a 4-kHz bandwidth. This
technique is called PCM (Pulse Code Modulation).
Virtually all time intervals within the
telephone system are multiples of 125 sec.
49
The T1 carrier (1.544 Mbps)

• Bell System's T1 carrier is a widely spread PCM
  method
• 24 channels of analog (voice) signals are
  periodically sampled on a round-robin basis.
• The resulting sampled analog stream is fed to one
  codec.
• For each sample, 8 bits are generated, with 7 of
  these bits are data (128 discrete levels), one is
  for control.
• bits are generated per sample period () for 24
  channels. These 192 bis plus one extra sync bit
  to form a frame. The sync bits in a series of
  frames take the pattern .
• bits are generated each second, giving a total
  data rate of 1.544 Mbps.

50
Differential pulse code modulation
Statistical techniques can be used to reduce the
number of bits needed per channel. All
techniques rely on the observation the signal
changes relatively slowly compared to the
sampling frequency, so that much of the
information in the 7 or 8 bits digital level is
redundant. Differential PCM Output only the
difference between the current digitized
amplitude value and the previous one. Since jumps
of or more on a scale of 128 are unlikely, 5 bits
should suffice instead of 7. A variation of this
method requires each sampled value to differ from
its predecessor by either 1 or -1, so one single
bit is transmitted (delta modulation), as shown
below.

• Delta modulation.

51
Time Division Multiplexing (3)

• Multiplexing T1 streams into higher carriers.

52
SONET/SDH

• SONET (Synchronous Optical NETwork) is a standard
  optical TDM system from the joint effort of
  Bellcore and CCITT (SDH (Synchronous Digital
  Hierarchy)). Virtually all the long-distance
  telephone traffic in the US and much of it
  elsewhere now uses trunks running SONET at the
  physical layer.
• Four major design goals of SONET
• To make it possible for different carriers to
  interwork, which requires defining a common
  signaling standard with respect to wavelength,
  timing, framing structure, and other issues.
• To unify all digital systems in U.S, Europe and
  Japan, all of which were based on 64-kbps PCM
  channels, but combined in different ways.
• To provide a standard way to multiplex multiple
  digital channels together.
• To provide support for operations,
  administration, and maintenance (OAM).
• An early decision was to make SONET a traditional
  TDM system, with the entire bandwidth of the
  fiber devoted to one channel containing time
  slots for the various sub-channels. Therefore,
  SONET is a synchronous system in the sense that
  bits on a SONET line are sent out at extremely
  precise intervals controlled by a master clock.

53
Two back-to-back SONET frames

• The first three columns of each frame are
  reserved for system management information
• The first three rows contains the section
  overhead.
• The next six contain the line overhead (generated
  and checked at the start and end of each line).
• The first row of the line overhead contains the
  pointer to the first byte of the user data,
  called SPE (Synchronous Payload Envelope), which
  may begin anywhere within the remaining 87
  columns of each frame ( in total) and may span
  two frames.
• The first column of the SPE is the path overhead,
  i.e., the header for the end-to-end path sublayer
  protocol.

54
SONET and SDH multiplex rates
55
Circuit Switching

• Circuit switching
• Packet switching.

• In circuit switching, each switching office has a
  number of incoming lines and outgoing lines.
• When a call passes through a switching office, a
  physical connection is established between
  incoming line and an outgoing line.
• Before any data can be sent, an end-to-end path
  needs to be set up. The setup time can easily be
  10 sec or more.
• Once the connection is set up, the only delay for
  data is the propagation time for electromagnetic
  signal, about 6 msec per 1000 km, and there is no
  danger of congestion.

56
Message Switching

• No physical path is established in advance.
• Each message is sent to a switching office in its
  entirety, stored (and check for errors) there and
  then forwarded later, one hop at a time.
• No limit on message size, so a router needs disks
  for buffering, and one message may tie up a
  router-router link for many minutes.

• (a) Circuit switching (b) Message switching
  (c) Packet switching

57
Packet Switching

• A comparison of circuit switched and
  packet-switched networks.

58
The Mobile Telephone System

• First-Generation Mobile Phones Analog Voice
• Second-Generation Mobile Phones Digital Voice
• Third-Generation Mobile PhonesDigital Voice and
  Data

59
AMPS Advanced Mobile Phone System

• (a) Frequencies are not reused in adjacent cells.
• (b) To add more users, smaller cells can be used.

60
Channel Categories

• The 832 channels are divided into four
  categories
• Control (base to mobile) to manage the system
• Paging (base to mobile) to alert users to calls
  for them
• Access (bidirectional) for call setup and channel
  assignment
• Data (bidirectional) for voice, fax, or data

61
D-AMPS Digital Advanced Mobile Phone System

• (a) A D-AMPS channel with three users (8 kbps per
  user).
• (b) A D-AMPS channel with six users (4 kbps per
  user).

62
GSMGlobal System for Mobile Communications

• GSM uses 124 frequency channels, each of which
  uses an eight-slot TDM system
• (13 kbps per channel after error correction )

63
GSM (2)

• A portion of the GSM framing structure.

64
CDMA Code Division Multiple Access

• CDMA allows each station to transmit over the
  entire spectrum and all the time, rather than
  using FDM and TDM.
• CDMA separates multiple simultaneous
  transmissions using coding theory, which assumes
  that simultaneous multiple signals add linearly,
  rather than being garbled.
• An analogy an airport lounge with many pairs of
  people conversing
• TDM is comparable to all the people being in the
  middle of the room but taking turns speaking.
• FDM is comparable to the people being in widely
  separated clumps, each clump holding its own
  conversation at the same time as, but still
  independent of, the others.
• CDMA is comparable to everybody being in the
  middle of the room talking at once, but each pair
  in a different language.
• The key to CDMA is to be able to extract the
  desired signal while rejecting everything else as
  random noise.

65
CDMA Code Division Multiple Access

• Each bit time is subdivided into m short
  intervals call chips. Typically, there are 64 or
  128 chips per bit.
• Each station is assigned a unique m-bit code
  called a chip sequence.
• To transmit a 1 bit (data), a station sends its
  chip sequence.
• To transmit a 0 bit (data), a station sends the
  negation (ones complement) of its chip
  sequence.
• All chip sequences are pairwise orthogonal, i.e.,
  the normalized inner product of any two distinct
  chip sequences is 0, S T 0.
• If S T 0, then S T 1.
• S S 1, and S S 0.

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

66
Third-Generation Mobile PhonesDigital Voice and
Data

• Basic services an IMT-2000 network should provide
• High-quality voice transmission
• Messaging (replace e-mail, fax, SMS, chat, etc.)
• Multimedia (music, videos, films, TV, etc.)
• Internet access (web surfing, w/multimedia.)

67
Cable Television

• Community Antenna Television
• Internet over Cable
• Spectrum Allocation
• Cable Modems
• ADSL versus Cable

68
Community Antenna Television

• An early cable television system.

69
Internet over Cable

• Cable television

70
Internet over Cable (2)

• The fixed telephone system.

71
Spectrum Allocation

• Frequency allocation in a typical cable TV system
  used for Internet access (total bandwidth 36
  39 Mbps)

72
Cable Modems

• Typical details of the upstream and downstream
  channels in North America.