The Physical Layer

1
The Physical Layer

• Chapter 2

2
The Theoretical Basis for Data Communication

• Fourier Analysis
• Bandwidth-Limited Signals
• Maximum Data Rate of a Channel

3
Fourier Analysis
G(t) periodic function G(t) ½ c S an
sin(2pnft) S bn cos(2pnft) an 2/T
G(t)sin(2pnft) dt bn 2/T G(t)cos(2pnft) dt c
2/T G(t) dt
4
Bandwidth-Limited Signals
bandwidth

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

5
Bandwidth-Limited Signals (2)

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

6
Bandwidth-Limited Signals (3)

• Period 1/Frequency
• Harmonics Term of the sine/cosine function
• Bandwidth
• Range of transmitted frequencies without
  a strong attenuation.
• It is a physical property of the
  transmission medium (depends on construction,
  thickness, length, )
• Transmission media are not perfect
• Noise thermal (motion of electrons) or
  induced (by equipments) noise, or crosstalk
  (effect of one wire on another)
• Attenuation loss of energy (amplifiers
  help)
• Distortion change in shape in composite
  signals (different harmonics arrive with
  different delays)

7
Bandwidth-Limited Signals (4)
Compare figure (b)
No transmission

• Relation between data rate and harmonics.

8
Maximum Data Rate of a Channel

• Without noise (Nyquist, 1924)
• max. rate 2B log(L) bits/sec
• where B bandwidth of the channel (after a low
  pass filter)
• L Number of discrete levels (e.g.
  binary L 2)
• ? Example A noiseless binary 3 kHz channel
  cannot exceed the rate 6000 bps.
• With noise (Shannon 1944)
• max. rate (or capacity) C B log(1 S/N)
  bits/sec
• where B bandwidth of the channel
• S/N Signal to noise ratio
• ? Example Extremely noisy channel (N very
  high).
• C B log (1 0) 0 (as expected no data
  can be sent through this channel)

9
Maximum Data Rate of a Channel

• Decibel (dB)
• For measuring signal strength engineers use the
  concept of
• decibel
• dB is negative signal is attenuated
• dB is positive signal is amplified
• dB 10 log10(P1/P2)
• Example P1/P2 ½ ? 10log10(0.5) -3dB
  ?attenuation
• Reason for using dB decibel can be
  added/subtracted easily

Amp
-3 dB
-3 dB
7 dB
1 dB
10
Guided Transmission Data

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

11
Twisted Pair

• (a) Category 3 UTP (unshielded twisted pair).
• (b) Category 5 UTP.
• 100 Hz to 5 MHz
• Two copper wires each surrounded by an insulating
  material
• Suitable for voice and data communication

12
Twisted Pair

• Cables are twisted to minimize noise effects.
• In the past two parallel cables have been used

13
Noise on Twisted Pair Lines
14
Shielded Twisted Pair Cable (STP)
? More expensive than UTP ? Less susceptible to
noise ? Only in IBM installation
15
Coaxial Cable

• A coaxial cable.

16
Fiber Optics

• Refraction of light

17
Fiber Optics

• Critical Angle

Reflection
18
Fiber Optics
Propagation modes Multimode fiber More
distortion (improvement possible) Single mode
fiber Less distortion (beams are more focused)
19
Fiber Optics

• Light sources
• light-emitting-diode (LED), Injection
  laser diode (ILD)

• Light detector
• Photodiode translates light to an
  electrical pulse.
• Response time 1 nsec ? max. data rate
  1Gbps.
• ? Limiting factor is still computing and
  not communication power!
• ? Why? Fiber technology can achieve up to 50000
  Gbps !!!

20
Fiber Optics
Noise resistant fiber optics uses light and
not current, therefore, it is
resistant to external noise. Less
attenuation The signal can travel for miles
without regeneration.
Higher bandwidth currently limiting factor is
signal generation and reception
technology. - Higher cost Fiber-optic
cable is expensive, because of its precise
manufacturing. Also, laser sources are
expensive. - More installation/maintenance
efforts Any cracking of the core
alters the signal. Connections must be perfectly
aligned and matched for core size and
provide a completely light-tight seal. -
More fragile Glass fiber is more easily broken
than wire .
21
Fiber Optic Networks

• A fiber optic ring with active repeaters.

22
Fiber Optic Networks (2)

• A passive star connection in a fiber optics
  network.

23
Wireless Transmission

• Radio Transmission
• Microwave Transmission
• Satellite Communication

24
Radio Communication Band

• The electromagnetic spectrum and its uses for
  communication.

25
Propagation Types
26
Propagation Types

• Surface propagation Waves travel through the
  lowest level of the atmosphere emanating in all
  directions. Distance depends on the amount of
  power in the signal.
• Tropospheric propagation Two modes either from
  antenna to antenna (line-of-sight) or broadcast
  into the upper layers of troposphere (where
  reflection to earth takes place). Latter method
  allows greater distances.
• Ionospheric propagation Similar to last type.
  Waves hit the ionosphere and are reflected back .
  It allows for even greater distance with less
  power.
• Line-of-Sight propagation Very high signals are
  transmitted in straight lines directly from
  antenna to antenna (which should be tall enough
  and facing each other). More sensitive for
  reflection, since signals cannot be completely
  focused.
• Space propagation Satellites are used instead of
  atmospheric reflection. Basically like
  line-of-sight transmission with an intermediary
  (the satellite). Allows very long distances.

27
Frequency Ranges an Their Uses
VLF
Surface propagation Sensitive to noise
LF
Surface propagation Sensitive to attenuation
MF
Tropospheric propagation Absorption by ionosphere
possible (use of line-of-sight helps)
HF
Ionospheric propagation
28
Frequency Ranges an Their Uses
VHF
Line-of-sight propagation
UHF
Line-of-sight propagation
29
Frequency Ranges an Their Uses
SHF
Line-of-sight or space propagation (satellite
microwaves or radar communication)
EHF
Space propagation (mainly for scientific use)
30
Terrestrial Microwaves

• Propagated using line-of-sight transmission.
• Provide the basis for most existent telephone
  systems
• Propagate in one direction only
• ? 2 frequencies are needed e.g. for a phone
  conversation
• Repeaters are used to achieve longer distances
• Repeaters are installed on each antenna
• Signal received on each antenna is converted and
  transmitted to the next antenna

31
Satellite Communication

• Similar to line-of-sight transmission.
• Satellite acts as a supertall antenna.
• No limitation by the curvature of the earth.
• ? longer distances achievable
• Expensive, but leasing time and frequencies on
  existent ones cheap

32
Geosynchronous Satellites

• Fixed satellites are useless They face earth
  antennas only for short time
• ? like a stopped clock is accurate twice a day!
• To ensure constant communication the satellite
  must move at the same speed as the earth.
• One geosynchronous satellite cannot cover the
  whole earth
• ? at least three equidistant satellites are
  needed

33
Frequency Bands for Satellite Communication

• The principal satellite bands.

34
Communication Satellites

• VSATs (Very Small Aperture Terminals) using a
  hub.
• VSAT do not have enough power to communicate
  directly with each other.
• A hub is used relay traffic between VSTAs

35
Modulation

• Modems Convert a digital signal to an analog one
  and vice versa.
• Forms of modulation
• Amplitude modulation
• Frequency modulation
• Phase modulation
• Amplitude Modulation

Amplitude gt 0 means 1, amplitude 0 means 0
36
Modulation

• Frequency Modulation

More frequent means 1, less frequent means 0
37
Modulation

• Phase Modulation

Phase changes
38
Bit Rate Versus Baud Rate

• Bit rate Number of bits transmitted during one
  second
• Baud rate Number of signal units required to
  represent those bits.
• Bit rate equals the baud rate times the number of
  bits represented by each signal unit
• Bit rate Baud rate x Number of bits per
  signal unit
• This means that the bit rate is always greater or
  equal the baud rate
• Bit rate gt Baud rate
• Analogy in transportation
• Bit passenger
• Baud car
• If 100 cars go from A to B carrying one person,
  then 100 person are transported (? Bit rate
  baud rate)
• If, however, each car carries 4 persons, then 400
  persons are transported (? Bit rate 4 x baud
  rate)

39
Phase Modulation Phase Shift Keying (PSK)
Bit rate Baud rate 5
1
0
0
1
1
1 baud
1 baud
1 baud
1 baud
1 baud
1 second
0 (0)
1 (p)
Constellation diagram for 2-PSK
40
4-PSK
Bit rate 10 Baud rate 5
00
11
01
10
10
1 baud
1 baud
1 baud
1 baud
1 baud
1 second
01 (p/2)
10 (p)
00 (0)
Constellation diagram for 4-PSK
11 (3p/2)
41
8-PSK and higher
Constellation diagram for 8-PSK
010 (p/2)
011 (p3/4)
001 (p/4)
000 (0)
100 (p)
101 (5p/4)
010 (7p/4)
110 (3p/2)
16-PSK, 32-PSK, are also possible
42
Quadrature Amplitude Modulation (QAM)

• Idea Combine changes of amplitude with changes
  of phase.
• Example 4-QAM 1 amplitude and 4 phases

01
00
10
11

• Example 8-QAM 2 amplitudes and 4 phases

011
010
001
101
000
100
110
111
43
8-QAM
Bit rate 24 Baud rate 8
101
100
001
000
010
011
111
110
1 baud
1 baud
1 baud
1 baud
1 baud
1 baud
1 baud
1 baud
1 second

• Whenever the amplitude or the phase changes, a
  new symbol is transmitted.
• In general, number of amplitudes less than that
  of phases because amplitudes are more sensitive
  to noise.

44
16-QAM

• There are three popular 16-QAM configurations
• 3 amplitudes, 12 phases (ITU-T recommendation)
• 4 amplitudes, 8 phases (ISO recommendation)
• 2 amplitudes, 8 phases
• Constellation diagrams

45
Multiplexing vs. No Multiplexing
46
Frequency Division Multiplexing (FDM)

• For analog devices (e.g. phones)
• Channels carry the signals with different
  frequencies but at the same time.

47
FDM

• Similar signals as input
• Signals are modulated (e.g. AM, FM) using
  different carrier signals (f1, f2, f3)
• Combined signal is send through the link

48
FDM

• Filters are used to decompose the multiplexed
  signal into its constituents
• Individual signal are then passed to a
  demodulator to generate the original signal

49
Wave Division Multiplexing

• For fiber optic channels only.
• Same idea as FDM, but more reliable than FDM.

prisms
50
Time Division Multiplexing (TDM)

• For digital devices (computers).
• Like FDM, bandwidth of transmission medium high
  enough to accommodate the different signals.
• Conceptual view

? Channel sectioned by time rather by frequency
51
TDM

• Two types of TDM
• Synchronous TDM
• Asynchronous TDM
• Synchronous TDM

• One (or more) static time slot(s) is assigned to
  each input device. If device is not sending data
  its time slot remains empty.
• Time slots are grouped into frames.

52
Synchronous TDM
empty slot

• Different data rates for input devices possible
• ? then more slots for faster
  devices needed.
• ? in this case the faster rate should be
  integer multiples of the other ones.
• If a data rate is not an integer multiples of
  another, e.g. 2.75 times another rate, then bit
  stuffing is used (bits are added by the
  multiplexer and discarded by the demultiplexer)
• Main problem wasted bandwidth because of
  potential empty slots.

53
Asynchronous TDM

• Also frame-based.
• Unlike synchronous TDM
• Number of slots in a frame less than number of
  input lines.
• Slots are not statically assigned to input lines,
  any sending device can use an available slot.

54
Asynchronous TDM
Actual data and addresses, which are needed by
the demultiplexer. In synchronous TDM, there is
no need for addresses (information lies
implicitly in the position of a slot) .
Problem Addresses means overhead!
55
Inverse Multiplexing
Inverse multiplexing is needed for bandwidth on
demand Lines of lower bandwidth are used, if
needed. For example - All lines are used for
telephone calls. - If video transmission is
needed, the video data are broken up and
sent over two or more lines.
56
Hierarchy of Multiplexers
57
Switching

• Three forms of switching
• a) Circuit Switching
• b) Packet Switching
• c) Message Switching

58
Switched Networks

• Main idea Reduce number and length of links.
• Example In the network above, if 7 computers are
  to be connected to 5 ones, we could use dedicated
  links for each pair this would lead to 7x5 35
  links, which will be probably not fully utilized.

59
Circuit-Switched Networks

• Physical path from sender to receiver is
  established at connection time and is held for
  the communicating pair during the whole
  communication. Used mostly in telephone networks.
• Dedicated links for the whole session ? no
  congestion and delays
• Connection setup delays the start of
  communication
• Less utilization of links, if traffic is not
  high.

60
Switches

• A switch is a device (newer are computers) with n
  input links and m output links that creates a
  temporary connection between an input link and an
  output link.

• Example n-by-n folded switch connects n devices
  to each other

61
Circuit-Switched Network

• Circuit switching uses two techniques
• Space division switches
• Time division switches
• Space division switches Paths are separated
  spatially.
• Example Crossbar switch

transistor
? Too many switches needed With n inputs and m
outputs nxm switches are needed.
62
Multistage Switch

• Multistage switches use different crossbar
  switches in such a way that the number of
  crosspoints is minimized.

• 15 inputs and 15 outputs
• For a single crossbar switch 15x15 225
  crosspoints are necessary.
• Now only 3x10 2x9 3x10 78 crosspoints are
  needed.

63
Multistage Switch

• Multistage switches allow for different paths
  between source and destination

A
6
11
B
Blocking Problem In multistage switches the
traffic can be blocked e.g. suppose 6 is
calling 10 (over B) and 11 is calling 6 (over
A), the intermediate switches (A and B) are
occupied and connection cannot be made for 4 and
9.
64
Time Division Switches

• No spatial separation use of time division
  multiplexing (TDM) instead

• Combining TDM with time-slot interchange (TSI)
  leads to switching

65
Time Division Switches

• Time-slot interchange

• TDM Bus An alternative to TSI a bus is used
  instead of a RAM

66
Combination of Space and Time Division Switches

• Space division multiplexing is instantaneous but
  needs a lot of switches and may cause blocking.
  On the other hand time division switching does
  not need switches but causes more delay
  (processing time of TSI). Combining them leads to
  a better design with less delay and blocking
  probability.
• Combinations time-space-time (TST),
  time-space-space-time (TSST), space-time-time-spac
  e (STTS), or other

67
Packet Switching

• Packet switching
• Data is transmitted in discrete units (packets)
  of possibly variable length.
• Packets include the actual data and header
  information (e.g. destination address)
• Packets are transmitted over network node to
  node in each node they are briefly stored
  (store-and-forward).
• Two types of packet switching
• Datagram approach
• Virtual circuit approach
• Datagram approach
• Each datagram (packet) is treated independently
  from other datagrams.
• Datagrams of same message may have different
  paths from source to destination.
• Datagrams of same message may arrive out of order.

68
Datagrams

• Example A message with 4 datagrams.
• ? different paths for different datagrams
• ? at destination, datagrams are out of order.

69
Virtual Circuits

• A single route is chosen at the beginning of the
  session and all packets follow that route.
• Two types
• SVC Switched Virtual Circuit
• PVC Permanent Virtual Circuit
• SVC

Connection release
Data transfer
Connection establishment

• PVC

Permanent connection for the duration of the
lease
70
Circuit Switching vs Packet Switching

• First compare circuit switching to virtual
  circuits
• Path vs route In circuit switching a physical
  path is reserved for the communicating pairs,
  whereas in virtual circuits a route is created.
  Each switch creates entries in its routing table.
• Dedicated vs shared In circuit switching the
  links that make the path are dedicated. In
  virtual circuits, however, the links are shared
  among different connections.
• In general, the difference between circuit
  switching and packet switching can be summarized
  as follows

Item
Circuit Switching
Packet Switching
Call setup
Yes
Not always
Dedicated physical path
Yes
No
Packets/Data follow same route
Yes
Not always
Packets/data arrive in order
Yes
Not always
Is a switch crash fatal?
Yes
No
Bandwidth available
Fixed
Dynamic
Time of possible congestion
At setup time
On every packet
Potentially wasted bandwidth
Yes
No
Store and forward transmission
No
Yes
Transparency (carrier point of view)
Yes
No
Charging
Per minute
Per packet
71
Message Switching

• Message switching is also a store-and-forward
  technique.
• Unlike packets, messages are not limited in size
  and are in general rather big.
• Routers use secondary storage in order to
  temporarily store a message until it is further
  relayed.
• ? in packet switching, packets are stored in
  volatile memory (RAM).
• Technique no more in use.