The Physical Layer

1
The Physical Layer

• The Theoretical Basis for Data Communication
• Transmission Media
• Wireless Transmission
• Multiplexing Technologies
• Switching Technologies
• The Telephone System
• Narrowband ISDN
• Broadband ISDN and ATM
• Cellular Radio
• Communication Satellites

2
The Theoretical Basis for Data Communication

• Information can be transmitted on wires by
  varying some physical property such as voltage or
  current.
• The theoretical basis for data communication
• Fourier Analysis
• Bandwidth-Limited Signals
• The Maximum Data Rate of a Channel

3
Fourier Series

• Any reasonably behaved periodic function, g(t),
  with period T can be constructed by summing a
  (possibly infinite) number of sines and cosines

where f1/T is the fundamental frequency and an
and bn are the sine and cosine amplitudes of the
nth harmonics (terms).
4
Harmonics
b1
a1
The 1st harmonic
0
0
T
-a1
T
-b1
a2
b2
The 2nd harmonic
0
0
T
-a2
T
-b2
b3
a3
The 3rd harmonic
0
0
-a3
T
T
-b3
5
Relation Between a Binary Signal and Its Harmonics
A data signal (e.g., a character with 8 bits)
that has a finite duration T can be handled by
just imagining that it repeats the entire pattern
over and over forever.
6
Bandwidth-Limited Signals

• The larger n is, the higher the frequency nf of
  the nth harmonic.
• All transmission facilities diminish different
  Fourier components by different amounts, thus
  introducing distortion.
• Usually, the amplitudes are transmitted
  undiminished from 0 up to some frequency fc (in
  Hertz, Hz) with all frequencies above this cutoff
  frequency strongly attenuated.

7
Baud Rate and Bit Rate

• The signaling speed
• the number of times per second that the signal
  changes its value (e.g. its voltage).
• measured in baud.
• A b baud line does not necessarily transmit b
  bits/sec
• each signal might convey several bits
• For example, if the voltages 0, 1, 2, 3, 4, 5, 6,
  and 7 were used, each signal value could be used
  to convey 3 bits, so the bit rate would be three
  times the baud rate.
• we assume that baud rate bit rate

8
Relation Between Bit Rate and Harmonics

• Given a bit rate of b bits/sec, the time T
  required to send 8 bits (for example) is 8/b sec,
  so the frequency f of the first (i.e., n1)
  harmonic is b/8 Hz.

0
8 bits
b bits
0
1 sec.
T
9
Relation Between Bit Rate and Harmonics (Cont.)
N 1 2 n
Frequency (Hz) b/8 2b/8 nb/8
nb/8? fc ? n ? fc/ (b/8)

• The number of the highest harmonic passed through
  is fc/(b/8) or 8fc/b, roughly. That is, the 1st,
  2nd, 3rd, ..., and (8fc/b)-th harmonics could
  pass through without diminution.

10
The Ordinary Telephone Line with fc3000 Hz
11
The Maximum Data Rate of a Channel

• Nyquists Theorem (noiseless channel)
• H bandwidth
• V discrete levels
• Maximum data rate 2H log2 V bits/sec
• Shannons Theorem (noise channel)
• H bandwidth
• S/N signal to noise ratio
• Maximum number of bits/sec H log2 (1 S/N)

12
Signal-to-Noise Ratio

• S/N
• S signal power
• N noise power
• dB
• 10 log10 S/N
• an S/N ratio of 1000 is 30 dB

13
Transmission Media

• Magnetic Media
• magnetic tape and floppy disk
• Twisted Pair
• UTP (Unshielded Twisted Pair) and STP (Shielded
  Twisted Pair)
• Coaxial Cable
• Baseband (50-ohm) and Broadband (75-ohm)
• Optical Fiber
• Multimode Step Index Fiber, Multimode Graded
  Index Fiber, Singlemode/ Monomode Fiber

14
Twisted Pair

• Unshielded Twisted Pair(UTP), telephone wire,
  subject to external electromagnetic interference
• Category 3 UTP, up to 16 Mbps
• Category 5 UTP, 125MHz, 155Mbps
• Category 5e UTP, 250MHz, 155/622Mbps, 1000based-T
• Category 6 UTP, 622Mbps, 1.2/2.4Gbps,
  1000based-T
• Repeater (Hub, switch hub)
• Shielded Twisted pair(STP), reduced interference,
  more expensive (IBM 1980)

15
Twisted Pair
16
Coaxial Cable

• Baseband Coaxial Cable, 50-ohm, used in Ethernet
• RG-11, RG-58(10base-2, thin)
• Broadband Coaxial Cable, 75-ohm, used in Cable TV
  (RG-57)
• In CATV, each channel has 6 MHz bandwidth
• 300-400 MHz

17
Optical Fiber

• Use optical signals instead of electrical signals
• Light sources LED (?????)
• Semiconductor Laser
• Optical Fibers
• Single Mode
• Multi Mode
• Stepped Index
• Graded Index
• Detector
• Photoelectric Diodes

18
Optical Fiber
19
Optical Fiber

• Greater capacity gt 50,000 Gbps (theoretically),
  1T Gbps (Lucent)currently in Lab, a few Gbps in
  use.
• Small size and lighter weight.
• lower attenuation constant over a wide range.
• Electromagnetic isolation lower error rate

20
Optical Transmission System
21
Optical Fiber
22
multi-Mode

• ??????? 50um 100um
• 50 /62.5 um
• ??????125um 140um?
• ?????????????????,????????????,????????????????60
  0MB/KM????2KM????300MB???????,????????????,???????

23
Single-Mode

• ????????
• ?????? 20?30??
• ??????? 5um 10um,??????125um?
• ?? ?????,??????????

24
Example Optical Lans
25
Wireless Transmission

• The Electromagnetic Spectrum
• Radio Transmission
• Microwave Transmission
• Infrared and Millimeter Waves
• Lightwave Transmission

26
The Electromagnetic Spectrum
f (Hz) 104 105 106 107 108 109 1010 1011 1012
1013 1014 1015 1016
V very U ultra S super E extremely T
tremendously
L low M medium H high
27
Wave Properties

• Radio, Microwaves, Infrared, and Visible Light
• can all be used for transmitting information
• AM, FM
• UV, X-rays, and Gamma Rays
• would be even better due to their higher
  frequencies
• hard to produce and modulate
• do not propagate well through buildings
• dangerous to living things

28
Radio Transmission

• Radio waves
• easy to generate
• can travel long distances
• penetrate buildings easily
• ommidirectional
• at low frequencies, the power falls off sharply
  with distance from the source
• at high frequencies, radio waves tend to travel
  in straight lines and bounce off obstacles

29
Propagation of Radio Waves
HF and VHF bands
VLF, LF, and MF bands
Earth
Earth
Radio waves follow the ground
Ionosphere
30
Microwave Transmission

• Microwaves
• travel in straight lines (over 100 MHz)
• can be narrowly focused (by a dish)
• the transmitting and receiving antennas must be
  accurately aligned with each other.
• do not pass through buildings well.
• can be absorbed by water/rain
• widely used for long-distance telephone
  communication, cellular telephones, TV
  distribution

31
Infrared and Millimeter Waves

• widely used for short-range communication.
• TV remote controller
• do not pass through solid objects.
• Bad limited distance
• Good security
• candidate for indoor wireless LAN
• cannot be used outdoors (due to sun shines)

32
Lightwave Transmission

• Each side needs its own laser and its own
  photodetector.
• The lasers strength, a very narrow beam, its
  weakness.
• Difficult aiming at far distance
• offers high bandwidth
• easy to install

33
(No Transcript)
34
The Telephone System
35
Multiplexing Technologies

• Basic Concept
• Multiplexing and Demultiplexing
• Multiplexing Schemes
• Frequency Division Multiplexing (FDM)
• Wavelength Division Multiplexing(WDM)
• Time Division Multiplexing (TDM)

36
MUX and DEMUX
1
2
3
37
Frequency Division Multiplexing (FDM)
Multiplexing
3
3
2
3
2
1
2
60
64
68
72
Frequency (kHz)
1
1
300
3100
60
64
68
72
Frequency (Hz)
Frequency (kHz)
38
Wavelength Division Multiplexing
39
Wavelength Division Multiplexing
40
Time Division Multiplexing (TDM)
41
The T1 Carrier (1.544 Mbps)
Multiplexing is done byte for byte
42
Multiplexing T1 Streams Onto Higher Carriers
Multiplexing is done bit for bit
43
SONET(bellcore)/SDH(CCITT)

• A synchronous TDM system
• Four major goals
• for different carriers to interworks
• to unify the U.S., European, and Japanese digital
  systems
• to provide a way to multiplex multiple digital
  channels together
• provide supports for operations, administration,
  and maintenance (OAM)

44
Multiplexing in SONET
...
Scrambler
Electro-optical Converter
31 Multiplexer
41 Multiplexer
45
A SONET Path
Source Multiplexer
Destination Multiplexer
Repeater
Repeater
Multiplexer
Section
Section
Section
Section
Line
Line
Path
46
SONET and SDH Multiplex Rate
47
Switching Technologies

• Circuit Switching
• need to setup an end-to-end path before any data
  can be sent
• Store-and-Forward Switching
• Message Switching
• no limit on block size
• Packet Switching
• Virtual Circuit

48
Circuit Switching
NNI
UNI
UNI
NNI
Physical copper connection set up when a call is
made
UNI User Network Interface NNI Network Node
Interface (Network-Network Interface)
49
Packet Switching
1
1
A path is not necessarily established when a
connection is made.
50
Packet Switching(out of order)
1
1
A path is not necessarily established when a
connection is made.
51
Circuit and Packet Switching A Comparison
Item Dedicated cooper path Bandwidth
available Potentially wasted bandwidth Store-and-f
orward transmission Each packet follows the same
route Call setup When can congestion
occur Charging
Circuit-switched Yes Fixed Yes No Yes Required At
setup time Per minute
Packet-switched No Dynamic No Yes No Not
needed On every packet Per packet
52
Timing of Events
Data
Circuit Switching
Message Switching
Packet Switching
53
Virtual Circuit
A path is established when a connection is made.
Physical links are shared.
Packets queued up for subsequent transmission
54
Crossbar Switches

• In a switch with n input and n output lines
• n2 intersections (crosspoints)

0 ? 4 1 ? 7 2 ? 6
Inputs
Outputs
55
Space Division Switches
Multistage switches with many fewer crosspoints
k crossbars
k crossbars
56
Time Division Switches
Time slot interchanger
n input lines
n output lines
0
1
2
3
4
5
6
7
4
7
6
3
0
5
2
1
1
7
Counter
2
RAM buffer of n k-bit word
6
5
5
0
4
n word mapping table
3
3
6
2
7
1
4
0
57
Narrowband ISDN

• ISDN (Integrated Services Digital Network)
• attempts to replace the analog telephone system
• provides end-to-end digital connectivity
• supports a wide range of services, including
  voice and non-voice services,
• lacks the necessary bandwidth for VOD
• 2BD provides a single 144-kbps digital channel
  for Internet access

58
ISDN Channels
59
ISDN Channel Combinations
Including echo bit, synchronization bit,
balance bit
60
Broadband ISDN and ATM

• for high-speed transfer of voice, video, and data
  through public networks
• digital virtual circuit
• fix-sized packet (cell)
• 155 Mbps
• Asynchronous Transfer Mode (ATM)
• the target transfer mode solution for
  implementing a B-ISDN.

61
STM and ATM
Synchronous Transfer Mode
Frame Signal
Asynchronous Transfer Mode
Cell Header
Cell
62
ATM Transmission Medium

• Normally, fiber optics
• all ATM links are point-to-point and
  undirectional
• multicasting is achieved by having a cell enter a
  switch on one line and exit it on multiple lines
• two parallel links are required for full-duplex
  operation

63
ATM Switches

• Synchronous, bidirectional lines
• common goals
• as low as discard rate as possible
• never reorder the cells on a virtual circuit

Cells are switched
Switching fabric
64
ATM Switch Design Issue

• What to do if the cells arriving at two or more
  input lines want to go to the same output port in
  the same cycle?
• pick one cell to deliver and discard the rest
• provide input queue
• provide output queue

65
Input Queueing at ATM Switches

• Head-of-line blocking

66
Output Queueing at ATM Switches

• Output queueing is generally more efficient than
  input queueing

67
The Knockout Switch
68
The Banyan Switch
69
Routing Within a Banyan Switch
70
Cells Colliding in a Banyan Switch
71
The Batcher-Banyan Switch
72
Batcher-Banyan Switch Example
73
Cellular Radio

• Paging Systems
• Cordless Telephones
• Analog Cellular Telephones
• Digital Cellular Telephones
• Personal Communications Services

74
Paging Systems and Cordless Telephones

• Paging Systems
• older systems 150174 MHz
• modern systems 930932 MHz
• Cordless Telephones
• CT-1 or CEPT-1 analog
• CT-2 (originated in England) digital, one-way
• CT-3 or DECT digital, two-way, roaming over base
  stations

75
Analog Cellular Telephones

• Push-to-talk Systems (half-duplex)
• a single channel only for both sending and
  receiving
• IMTS (Improved Mobile Telephone System)
• full-duplex, FDM
• high-powered (200-watt) transmitter
• 23 channels spread out from 150 MHz to 450 MHz
• impractical due to the limited capacity
• AMPS (Advanced Mobile Phone System)
• introduces the concept of cells

76
Advanced Mobile Phone System

• A geographic region is divided up into cells with
  1020 km across.
• AMPS uses relatively small cells and reuses
  transmission frequencies in nearby cells
• less power (hand-held telephones with 0.6 watt),
  smaller and cheaper devices
• Base stations are connected to an MSC (Mobile
  Switching Center) or MTSO (Mobile Telephone
  Switching Office), in turn connected to a
  second-level MTSO, and so on.
• handoff within 300 msec
• Security Problem (32 bit serial number, 10 bit
  Tel. No)

77
Frequency Reuse
B
C
G
B
A
C
G
F
D
A
E
F
D
B
E
C
G
A
F
D
E
the base station with the strongest signal
78
Digital Cellular Telephones

• Two systems in the USA
• compatible with the AMPS frequency allocation
  scheme standard IS-54 and IS-135
• dual mode analog and digital
• 30 kHz channels with 48.6 kbps for 3 users
  simultaneously (13 kbps/user)
• digital signaling and digital voice encoding
• direct sequence spread spectrum standard IS-95
• GSM (Global Systems for Mobile communications)
  originated in Europe
• Smart card

79
Personal Communications Services

• called PCS in the USA and PCN (Personal
  Communications Network) everywhere else
• People only have one telephone number.
• Microcells 50100 meters wide
• low power (1/4 watt)
• a lot of small base stations (telepoints)

80
Communication Satellites

• Geosynchronous Satellites
• Low-Orbit Satellites

81
Geosynchronous Satellites

• Satellites are apparently motionless
• Each download beam can be focused on a small
  geographical area (diameterhundred km)
• VSATs (Very Small Aperture Terminals)
• low-cost microstations
• hub is used to relay traffic between VSATs

82
Properties of Satellite Communications

• The larger round-trip distance introduces a
  substantial delay
• Satellites are inherently broadcast media
• satellite broadcasting is much cheaper
• encryption is essential when security is required
• The cost of transmitting a message is independent
  of the distance traversed

83
Low-Orbit Satellites

• Satellites zip into and out of view quickly
• as soon as one satellite went out of view,
  another would replace it
• Iridium project uses 77 (later 66) satellites
• Both the cells and the users are mobile

84
(No Transcript)
85
Homeworks

• 5, 23,33,35,
• 37,38,44, 47