1. Physical Layer Dr. K. Raghava Rao Professor, Dept. of ECM

2. Transmission Media Physical layer: Transport a raw bit stream
Characteristics :bandwidth, delay, cost, and ease of installation and maintenance
Physical media
Guided media
Information transmitted on wires by varying some physical property such as voltage or current
Copper wire, fiber optics
Unguided media
Information transmitted wirelessly by electromagnetic waves
Radio, lasers

3. Guided Media
Magnetic Media
Twisted pairs
Coaxial cable
Fiber optics

4. Magnetic Media
Transport data from one computer to another by writing data onto magnetic tape or removable media (e.g., recordable DVDs).
Physically transport the tape or disks to the destination machine, and read them back in again.
More cost effective, especially for applications in which high bandwidth or cost per bit transported is the key factor.

6. Magnetic Media Bandwidth
An industry standard Ultrium tape can hold 200 gigabytes.
A box 60 x 60 x 60 cm can hold about 1000 of these tapes, for a total capacity of 200 terabytes.
A box of tapes can be delivered anywhere in the United States in 24 hours by Federal Express and other companies.
The effective bandwidth of this transmission is 200 terabytes/86,400 sec, or 19 Gbps.
If the destination is only an hour away by road, the bandwidth is increased to over 400 Gbps

7. Magnetic Media
Cost
The cost of an Ultrium tape is around $40 when bought in bulk.
A tape can be reused at least ten times, so the tape cost is maybe $4000 per box per usage.
Add to this another $1000 for shipping (probably much less), and we have a cost of roughly $5000 to ship 200 TB

8. Twisted Pair Cable Oldest, but still most common
Two twisted insulated copper wires about 1 mm thick.
The wires are twisted together in a helical form, just like a DNA molecule.
Twisting is done because two parallel wires constitute a fine antenna.
When the wires are twisted, the waves from different twists cancel out, so the wire radiates less effectively.
Why twisted?
To reduce electrical interference

9. Twisted pair Cable Applications: Telephone system, Ethernet
Repeater needed for longer distances
Repeater: device that extends the distance a signal can travel by regenerating the signal
Twisted pairs can be used for transmitting either analog or digital signals.
The bandwidth depends on the thickness of the wire and the distance traveled.
Adequate performance at low cost

11. Twisted Pair
Category 5 UTP cable with 4 twisted pairs

12. Twisted Pair Category 3 and 5 .
Popular by UTP (Unshielded Twisted Pair)
Twists results in less crosstalk and a better-quality signal over longer distances.
Up-and-coming categories are 6 And 7, which are capable of handling signals with bandwidths of 250 MHz and 600 MHz, respectively (versus a mere 16 MHz and 100 MHz for categories 3 and 5,
Cat Cat 5

13. Coaxial Cable Better shielding than twisted pairs
Span longer distances at higher speeds
Lower error rate
Widely used for
Cable TV
WAN (Internet over cable)

15. Coaxial Cable Two kinds of coaxial cable are widely used.
50-ohm cable-for digital transmission .
75-ohm cable-for analog transmission .
A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely-woven braided mesh. The outer conductor is covered in a protective plastic sheath.
Shielding gives high bandwidth and excellent noise immunity.
The bandwidth depends on the cable quality, length, and signal-to-noise ratio of the data signal.
Modern cables have a bandwidth of close to 1 GHz.

16. Fiber Optics Transmission of light through fiber
Including 3 components:
Light source: Pulse of light=1,
Absence of light=0
Transition medium: an ultra-thin fiber of glass
detector: generate an electrical pulse when light falls on it.

18. Fiber Optics Light Electromagnetic energy traveling at 3108 m/s
Refraction
Critical angle
Reflection

19. Fiber Optics
(Less dense)
cladding
core I
(critical
angle)
cladding
(More dense)
(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.

20. Reflection & Refraction
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.
Refraction is the change in direction of a wave due to a change in its transmission media.
The critical angle is the angle of incidence above
 which total internal reflection occurs.
Total internal reflection is a phenomenon that happens when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
Cladding is one or more layers of materials of lower refractive index, in intimate contact with a core material of higher refractive index.

21. Reflection & Refraction

22. Total Internal Reflection

23. Fiber Optics Thickness of core: 8~10 microns or 50 microns
Two typically light sources:
LED (Light Emitting Diode)
response time=1ns  data rate = 1Gbps
2. Semiconductor laser

25. Fiber Optics
Properties include total internal reflection and attenuation of particular frequencies.
Fiber Optic Networks - can be used for LANs and long-haul.

27. Fiber Optics Vs Copper Wire
Lower
Higher
Bandwidth
5 Km
30 KM
Distance between repeaters
High
Low
Interference -
Smaller/Lighter
Physical
Bi-directional
Uni-directional
Flow

28. Structure of Telephone System
Major components:
1. Local loops (analog twisted pairs going into houses and businesses).
2. Trunks (digital fiber optics connecting the switching offices).
3. Switching offices (where calls are moved from one trunk to another)

30. Modems Computer is digital Telephone line is analog
Need translation device called a modem
Analog
Signal
Digital
Signal
Modem

31. MODEM

32. Modulation A Modem is a Modulator and Demodulator
Modulation is converting outgoing digital device signals into analog transmission line signals
Demodulation is converting incoming analog transmission line signals into digital device signals
Analog
Signal
Digital
Signal
Modem

33. Problems in Transmission Lines
Attenuation is the loss of energy as the signal propagates outward.
Delay distortion is caused by different Fourier components propagating at different speeds in the wire.
Noise is unwanted energy from sources other than the transmitter.
Thermal noise is caused by the random motion of the electrons in a wire and is unavoidable.
Sometimes when talking on the telephone, you can hear another conversation in the background. That is crosstalk.
DC Signaling is subject to strong attenuation and delay distortion.
Hence on telephone lines, AC signaling is use .
Analog signaling consist of varying a voltage with time to represent an information stream. If transmission perfect , the receiver receive exactly same signal, but media is not perfect. For digital data , this difference can lead to errors.

34. Why modulation is necessary ?
Signals are transmitted between a transmitter over some form of transmission medium
But normally signals are not in the form that is suitable for transmission and need to be transformed
Bandwidth requirement
Signals multiplexing
Complexity of transmission system
Preventing noise, interference, attenuation
Modulation is a process of impressing (applying) a low frequency information signals to onto a relatively high frequency carrier signal

35. Modulation techniques
Amplitude modulation: Two different amplitudes of sine wave are used to represent 1's and 0's.
Frequency modulation: Two (or more) different frequencies, close to the carrier frequency, are used.
Phase modulation: The phase of the sine wave is changed by some fixed amount.

37. The number of samples per second is measured in baud
The number of samples per second is measured in baud. During each baud one symbol sent. This n-baud lines send n symbols /Sec.
If symbol consists of 0 volts for logical 0 and 1 volt for logical 1, the bit rate 2400 bps. If voltages 0,1,2 and 3 volts are used, every symbol consists of 2 bits, so a 2400 baud-line can transmit 2400 symbol s/sec at a data rate of 4800 bps.
For example, a 2400-baud line sends one symbol about every microsecond. If every symbol consist of 2 bits , a 2400 –baud line can transmit 2400 symbols/sec at a data rate of 4800 bps.
Similarly, with four possible phase shifts , there are also 2 bits symbol , so again the bit rate twice the baud rate. This technique is called QPSK(Quadrature Phase Shift keying)

38. Bandwidth of a medium is the range of frequencies that pass through it with minimum attenuation. It is a physical property of the medium and measured in Hz.
The baud rate is the number of samples/sec made.
Each sample sends one piece of information, that is, one symbol. The baud rate and symbol rate are thus the same.
All advanced modems use a combination of modulation techniques to transmit multiple bits per baud.
Often multiple amplitudes and multiple phase shifts are combined to transmit several bits/symbol.

39. a)With four possible phase shifts, Used to transmit 2 bits per symbol
a)With four possible phase shifts, Used to transmit 2 bits per symbol. It is QPSK ((Quadrature Phase Shift Keying)
b)Transmit 4 bits per symbol. It is called QAM-16 (Quadrature Amplitude Modulation).
c)Allows 64 different combinations, so 6 bits can be transmitted per symbol. It is called QAM-64.

40. Noise in detected amplitude or phase can result in an error and potentially, many bad bits.
To reduce the chance of an error, standards for the higher speeds modems do error correction by adding extra bits(parity bit) to each sample. The scheme known as TCM(Trellis Coded Modulation)

41. Telephone Modems A telephone line has a bandwidth of Modem standards
3000 Hz (3300 – 300) for voice
2400 Hz (3000 – 600) for data
Modem standards
V.32: 9,600 bps
V.32bis: 14,400 bps
V.34bis: 28,800 ~ 33,600 bps
V.90: download up to 56kbps (56K modem)
V.92: adjustable speed, call waiting, etc.

42. What is DSL? Digital Subscriber Line-New modem technology.
Data transmission is based on digital encoding (digital).
Use digital coding techniques to provide more capacity.
Allows high-speed Internet access over existing twisted-pair and ordinary copper telephone wires.
Provides "always-on" connection.
To transport high-bandwidth data.
A special hardware attached to both the user and switch ends of line.

44. Advantages of DSL High-speed. Secure connection.
No dial-up, waiting or dropped connections.
It's always on connection.
Saves both money and time.
Provides large file transfers.
Multiple workers on a network can connect to a single DSL.

45. What is ADSL? Asymmetric Digital Subscriber Line. Is a form of DSL.
A high-speed Internet access service.
Speed depends on the length and the diameter of the cable and the type of the mode
Requires a special ADSL modem and an Internet service provider (ISP) .

47. What is ADSL?
It is asymmetric since the data coming to your computer from the Internet (download) is faster than the data traveling from your computer to the Internet (upload).
Uses standard telephone lines.
Telephone can be used normally, even when surfing in the Web with ADSL service.
An "always on" service.
Not available to everyone.

48. How does ADSL work?

49. ADSL
ADSL requires a special ADSL modem and subscribers must be in close geographical locations to the provider's central office to receive ADSL service. Typically this distance is within a radius of 2 to 2.5 miles.
ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate).
ADSL divide the available 1.1 MHz spectrum on the local loop into 256 independent channels of Hz each.
Channel 0 is used for POTS. Channels 1–5 are not used, to keep the voice signal and data signals from interfering with each other.
Of the remaining 250 channels, one is used for upstream control and one is used for downstream control. The rest are available for user data.

50. ADSL(Asymmetric Digital Subscriber Line)
Operation of ADSL using discrete multitone modulation.

51. Wireless local Loop Wireless local loop service called LMDS
Local Multipoint Distribution Service.
Like ADSL, LMDS uses an asymmetric bandwidth allocation favoring the downstream channel.
With current technology, each sector can have 36 Gbps downstream and 1 Mbps upstream, shared among all the users in that sector.
IEEE set up a committee called to draw up a standard for LMDS.
The IEEE calls a wireless MAN.

53. LMDS
A tower with multiple antennas , each pointing in a different direction.
Each antenna defines a sector, independent of the other ones.
A single tower with four antennas could serve 100,000 people within a 5-km radius of the tower.
LMDS has a few problems
Millimeter waves propagate in straight lines, so there must be a clear line of sight between the roof top antennas and the tower.
Leaves absorb these waves , so the tower must be high enough to avoid having trees in the line of sight

54. Trunks & Multiplexing
Trunks (digital fiber optics connecting the switching offices).
How to collect multiple calls together and send them out over the same fiber. This subject is called multiplexing.
Telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk.

55. Multiplexing
Multiplexing schemes can be divided into two basic categories:
FDM (Frequency Division Multiplexing)
TDM (Time Division Multiplexing).
In FDM, the frequency spectrum is divided into frequency bands, with each user having exclusive possession of some band.
In TDM, the users take turns (in a round-robin fashion), each one periodically getting the entire bandwidth for a little burst of time.
Advanced FDM applied to fiber optics called WDM (wavelength division multiplexing).
Advanced TDM system used for fiber optics (SONET)Synchronous Optical NETwork.

56. Multiplexing Multiplexor (MUX) Demultiplexor (DEMUX)
Sometimes just called a MUX

57. Multiplexing
Two or more simultaneous transmissions on a single circuit.
Transparent to end user.
Multiplexing costs less.

58. Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium.
Thus, all signals are transmitted at the same time, each using different frequencies.
A multiplexor accepts inputs and assigns frequencies to each device.
The multiplexor is attached to a high-speed communications line.
A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates the multiplexed signals.

61. Frequency Division Multiplexing
Analog signaling is used to transmits the signals.
Broadcast radio and television, cable television, and the AMPS cellular phone systems use frequency division multiplexing.
This technique is the oldest multiplexing technique.
Since it involves analog signaling, it is more susceptible to noise.

62. Time Division Multiplexing
Sharing of the signal is accomplished by dividing available transmission time on a medium among users.
Digital signaling is used exclusively.
Time division multiplexing comes in two basic forms:
1. Synchronous time division multiplexing, and
2. Statistical, or asynchronous time division multiplexing.

63. Synchronous Time Division Multiplexing
The original time division multiplexing.
The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern.
T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.
Drawbacks
If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream.
If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.

64. The T1 carrier (1.544 Mbps).

66. Statistical Time Division Multiplexing
A statistical multiplexor transmits only the data from active workstations (or why work when you don’t have to).
If a workstation is not active, no space is wasted on the multiplexed stream.
A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted.

68. To identify each piece of data, an address is included.

69. If the data is of variable size, a length is also included.

71. Wavelength Division Multiplexing
Prisms form the basis of optical multiplexing and demultiplexing
a multiplexor accepts beams of light of various wavelengths and uses a prism to combine them into a single beam
a demultiplexor uses a prism to separate the wavelengths.

72. Wavelength Division Multiplexing (WDM)
Here four fibers come together at an optical combiner, each with its energy present at a different wavelength.
The four beams are combined onto a single shared fiber for transmission to a distant destination.
At the far end, the beam is split up over as many fibers as there were on the input side.
Each output fiber contains a short, specially-constructed core that filters out all but one wavelength. The resulting signals can be routed to their destination or recombined in different ways for additional multiplexed transport.

74. SONET/SDH Synchronous Optical NETwork Synchronous Digital Hierarchy
SONET is a synchronous system.
It is controlled by a master clock with an accuracy of about 1 part in 109.
Bits on a SONET line are sent out at extremely precise intervals, controlled by the master clock.
The basic SONET frame is a block of 810 bytes put out every 125 μsec.
Since SONET is synchronous, frames are emitted whether or not there are any useful data to send.

75. SONET
The 810-byte SONET frames are best described as a rectangle of bytes, 90 columns wide by 9 rows high. Thus, 8 x 810 = 6480 bits are transmitted 8000 times per second, for a gross data rate of Mbps.
This is the basic SONET channel, called STS-1 (Synchronous Transport Signal-1).
The first three columns of each frame are reserved for system management information.
The first three rows contain the section overhead; the next six contain the line overhead.
The section overhead is generated and checked at the start and end of each section, whereas the line overhead is generated and checked at the start and end of each line.

76. A SONET transmitter sends back-to-back 810-byte frames, without gaps between them, even when there are no data (in which case it sends dummy data).
The remaining 87 columns hold 87 x 9 x 8 x 8000 = Mbps of user data.
However, the user data, called the SPE (Synchronous Payload Envelope), do not always begin in row 1, column 4.
The SPE can begin anywhere within the frame.
A pointer to the first byte is contained in the first row of the line overhead. The first column of the SPE is the path overhead (i.e., header for the end-to-end path sublayer protocol

77. Two back-to-back SONET frames

78. Switching Types of Switching Circuit switching Packet switching
The phone system is divided into two principal parts:
Outside plant (the local loops and trunks, since they are physically outside the switching offices)
Inside plant (the switches), which are inside the switching offices.
Types of Switching
Circuit switching
Packet switching

79. Circuit & packet Switching

80. Circuit Switching
When you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way from your telephone to the receiver's telephone. This technique is called circuit switching.
An important property of circuit switching is the need to set up an end-to-end path before any data can be sent.

81. Message Switching
An alternative switching strategy is message switching.
No physical path is established in advance between sender and receiver.
Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e., router) and then forwarded later, one hop at a time.
Each block is received in its entirety, inspected for errors, and then retransmitted.
A network using this technique is called a store-and-forward network

82. Packet Switching
With message switching, there is no limit at all on block size, which means that routers (in a modern system) must have disks to buffer long blocks.
It also means that a single block can tie up a router-router line for minutes, rendering message switching useless for interactive traffic.
To get around these problems, packet switching was invented.
Packet-switching networks place a tight upper limit on block size, allowing packets to be buffered in router main memory instead of on disk.
In packet Switching individual packets are sent as need be, with no dedicated path being set up in advance. It is up to each packet to find its way to the destination on its own.

83. Timing of events in (a) circuit switching, (b) message switching, (c) packet switching