the physical layer highlights of this chapter highlights of this chapter theoretical basis for data...

The Physical LayerThe Physical Layer

Highlights of this chapterHighlights of this chapter Theoretical Basis for Data Theoretical Basis for Data

CommunicationCommunication The transmission medias (Guided and The transmission medias (Guided and

Un-guided)Un-guided) The Public Switched Telephone Network The Public Switched Telephone Network

(PSTN)(PSTN) The Mobile Telephone SystemThe Mobile Telephone System


The ObjectivesThe Objectives Understand the theoretical basis for Understand the theoretical basis for

data communication, know how to data communication, know how to computer the data rate for specific computer the data rate for specific media.media.

Understand the principles of data Understand the principles of data communication for guided and unguided communication for guided and unguided media.media.

Study the popular telephony Study the popular telephony technologies. technologies.

The Physical Layer (Cont’d)The Physical Layer (Cont’d)

Theoretical Basis for Data CommunicatiTheoretical Basis for Data Communicationon Information can be transmitted on wires by Information can be transmitted on wires by

varying some physical property such as voltvarying some physical property such as voltage or current.age or current.

Fourier Analysis, where f = 1/T is the fundamFourier Analysis, where f = 1/T is the fundamental frequency, aental frequency, ann and b and bnn are the sine and c are the sine and cosine amplitudes of the nth harmonics (terosine amplitudes of the nth harmonics (terms), and c is a constant. ms), and c is a constant.


Bandwidth-Limited SignalsBandwidth-Limited Signals

An example: the transmission of the ASCII character “b” encoded in an 8-bit byte. The bit pattern that is to be transmitted is 01100010.


According to (a), we have the coefficients:According to (a), we have the coefficients:

But, HOW?

root-mean-square amplitudes:root-mean-square amplitudes:

Note: RMS varies according to the harmonics number. (see above figure)

Basic conclusions:Basic conclusions:1. (a) is the most ideal, however, it has the most harmonics;1. (a) is the most ideal, however, it has the most harmonics;

2. (b) is the most simple, and the lest harmonics, but it is difficult 2. (b) is the most simple, and the lest harmonics, but it is difficult to reconstruct the data;to reconstruct the data;

3. (c) – (e) are trade-offs.3. (c) – (e) are trade-offs.

The Physical Layer (Cont’d)The Physical Layer (Cont’d) The definition of Bandwidth:The definition of Bandwidth:

The range of frequencies transmitted without The range of frequencies transmitted without being strongly attenuated is called the being strongly attenuated is called the bandwidth.bandwidth.

The bandwidth is a physical property of the The bandwidth is a physical property of the transmission medium and usually depends on transmission medium and usually depends on the construction, thickness, and length of the the construction, thickness, and length of the medium.medium.

For example, a telephone wire may have a For example, a telephone wire may have a bandwidth of 1 MHz for short distances, but bandwidth of 1 MHz for short distances, but telephone companies add a filter restricting telephone companies add a filter restricting each customer to about 3100 Hz. This each customer to about 3100 Hz. This bandwidth is adequate for intelligible speech bandwidth is adequate for intelligible speech and improves system-wide efficiency by limiting and improves system-wide efficiency by limiting resource usage by customers.resource usage by customers.

The Physical Layer (Cont’d)The Physical Layer (Cont’d)Given a bit rate of b bits/sec, the time required to send 8 bits Given a bit rate of b bits/sec, the time required to send 8 bits

is 8/b sec, so the frequency of the first harmonic is b/8 Hz. is 8/b sec, so the frequency of the first harmonic is b/8 Hz. For voice-grade line, whose bandwidth is 3000 Hz, the For voice-grade line, whose bandwidth is 3000 Hz, the number of the highest harmonic passed through is roughly number of the highest harmonic passed through is roughly 3000/(b/8) or 24,000/b .3000/(b/8) or 24,000/b .

Conclusions: Conclusions: 1. It should be obvious that at data rates much higher than 1. It should be obvious that at data rates much higher than 38.4 kbps, there is no hope at all for binary signals, even if the 38.4 kbps, there is no hope at all for binary signals, even if the transmission facility is completely noiseless.transmission facility is completely noiseless.2. limiting the bandwidth limits the data rate, even for perfect 2. limiting the bandwidth limits the data rate, even for perfect channels.channels.


The Maximum Data Rate of a ChannelThe Maximum Data Rate of a Channel If an arbitrary signal has been run through a low-pass filter of If an arbitrary signal has been run through a low-pass filter of

bandwidth H, the filtered signal can be completely reconstrucbandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling ted by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless (Nyquisthe line faster than 2H times per second is pointless (Nyquist's theorem). If the signal consists of V discrete levels, we have:t's theorem). If the signal consists of V discrete levels, we have:

The maximum data rate of a noisy channel whose bandwidth The maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by is H Hz, and whose signal-to-noise ratio is S/N, is given by ((Shannon's theorem)Shannon's theorem)::

The Physical Layer (Cont’d)The Physical Layer (Cont’d) Guided Media —— the Twisted PairGuided Media —— the Twisted Pair

The most common application of the twisted The most common application of the twisted pair is the telephone system.pair is the telephone system.

Twisted pairs can be used for transmitting Twisted pairs can be used for transmitting either analog or digital signals. The bandwidth either analog or digital signals. The bandwidth depends on the thickness of the wire and the depends on the thickness of the wire and the distance traveled, but several megabits/sec distance traveled, but several megabits/sec can be achieved for a few kilometers in many can be achieved for a few kilometers in many cases.cases.

Figure 2-3. (a) Category 3 UTP. (b) Category 5 UTP.

The Physical Layer (Cont’d)The Physical Layer (Cont’d) Guided Media —— the Coaxial CableGuided Media —— the Coaxial Cable

Two kinds of coaxial cable are widely used. One kind, Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission ohm cable, is commonly used for analog transmission and cable television but is becoming more important and cable television but is becoming more important with the advent of Internet over cable.with the advent of Internet over cable.

The bandwidth possible depends on the cable quality, The bandwidth possible depends on the cable quality, length, and signal-to-noise ratio of the data signal. length, and signal-to-noise ratio of the data signal. Modern cables have a bandwidth of close to 1 GHz.Modern cables have a bandwidth of close to 1 GHz.

The Physical Layer (Cont’d)The Physical Layer (Cont’d) Guided Media —— the Guided Media —— the Fiber OpticsFiber Optics

With current fiber technology, the achievable bandwidth is certainly With current fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps) and many people are looking very in excess of 50,000 Gbps (50 Tbps) and many people are looking very hard for better technologies and materials. hard for better technologies and materials.

An optical transmission system has three key components: the light An optical transmission system has three key components: the light source, the transmission medium, and the detector.source, the transmission medium, and the detector.

Currently available single-mode fibers can transmit data at 50 Gbps fCurrently available single-mode fibers can transmit data at 50 Gbps for 100 km without amplification. Even higher data rates have been aor 100 km without amplification. Even higher data rates have been achieved in the laboratory for shorter distances.chieved in the laboratory for shorter distances.

(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


Wireless Transmission —— The Wireless Transmission —— The Electromagnetic SpectrumElectromagnetic Spectrum

The electromagnetic spectrum and its uses for communication.

The wider the band, the higher the data rate.


The Public Switched Telephone The Public Switched Telephone Network (PSTN)Network (PSTN)

Structure of the Telephone SystemStructure of the Telephone System

Computer Networks PSTN

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


PSTN StructurePSTN Structure

A variety of transmission media are used for telecommunication. Local loops consist of category 3 twisted pairs nowadays. Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely used.

A typical circuit route for a medium-distance call.

In the past, transmission throughout the telephone system was analog, with the actual voice signal being transmitted as an electrical voltage from source to destination. With the advent of fiber optics, digital electronics, and computers, all the trunks and switches are now digital, leaving the local loop as the last piece of analog technology in the system.


Transmit digital signals on PSTN Transmit digital signals on PSTN (Local Loops)(Local Loops)

ISDN

ADSL (Asymmetric Digital Subscriber Line)

Modem


SwitchingSwitching

(a) Circuit switching. (b) Packet switching.


SwitchingSwitching

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


SwitchingSwitching

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


The Mobile Telephone SystemThe Mobile Telephone System three distinct generationsthree distinct generations

1 G: Analog voice.1 G: Analog voice. 2 G: Digital voice. (D-AMPS, GSM, CDMA, and 2 G: Digital voice. (D-AMPS, GSM, CDMA, and

PDC.)PDC.) 3 G: Digital voice and data.3 G: Digital voice and data.

The Physical Layer (Cont’d)The Physical Layer (Cont’d) 3G and the future mobile network3G and the future mobile network

Digital Voice and DataDigital Voice and Data 2 Mbps for stationary indoor users (which will 2 Mbps for stationary indoor users (which will

compete head-on with ADSL), 384 kbps for people compete head-on with ADSL), 384 kbps for people walking, and 144 kbps for connections in cars.walking, and 144 kbps for connections in cars.

3G will give us:3G will give us: High-quality voice transmission.High-quality voice transmission. Messaging (replacing e-mail, fax, SMS, chat, etc.).Messaging (replacing e-mail, fax, SMS, chat, etc.). Multimedia (playing music, viewing videos, films, TV, etc.)Multimedia (playing music, viewing videos, films, TV, etc.) Internet access (Web surfing, pages with audio and Internet access (Web surfing, pages with audio and

video).video). Proposed StandardsProposed Standards

W-CDMA (Wideband CDMA), by Ericsson.W-CDMA (Wideband CDMA), by Ericsson. CDMA2000, by Qualcomm.CDMA2000, by Qualcomm. Both standards are not compatible with GSM.Both standards are not compatible with GSM.

Stepwise Solutions (GPRS, i.e., 2.5G)Stepwise Solutions (GPRS, i.e., 2.5G)


ExercisesExercises Please try to solve these problems:Please try to solve these problems:

1-4, 7-9, 18-20.1-4, 7-9, 18-20.

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