1. The Physical Layer Highlights of this chapter Highlights of this chapter Theoretical Basis for Data Communication Theoretical Basis for Data Communication The transmission medias (Guided and Un- guided) The transmission medias (Guided and Un- guided) The Public Switched Telephone Network (PSTN) The Public Switched Telephone Network (PSTN) The Mobile Telephone System The Mobile Telephone System

2. The Physical Layer The Objectives The Objectives Understand the theoretical basis for data communication, know how to computer the data rate for specific media. Understand the theoretical basis for data communication, know how to computer the data rate for specific media. Understand the principles of data communication for guided and unguided media. Understand the principles of data communication for guided and unguided media. Study the popular telephony technologies. Study the popular telephony technologies.

3. The Physical Layer (Cont’d) Theoretical Basis for Data Communication Theoretical Basis for Data Communication Information can be transmitted on wires by varying some physical property such as voltage or current. Information can be transmitted on wires by varying some physical property such as voltage or current. Fourier Analysis, where f = 1/T is the fundamental frequency, a n and b n are the sine and cosine amplitudes of the nth harmonics (terms), and c is a constant. Fourier Analysis, where f = 1/T is the fundamental frequency, a n and b n are the sine and cosine amplitudes of the nth harmonics (terms), and c is a constant.

4. The Physical Layer (Cont’d) Bandwidth-Limited Signals Bandwidth-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.

5. The Physical Layer (Cont’d) According to (a), we have the coefficients: But, HOW? root-mean-square amplitudes: Note: RMS varies according to the harmonics number. (see above figure) Basic conclusions: 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 to reconstruct the data; 3. (c) – (e) are trade-offs.

6. The Physical Layer (Cont’d) The definition of Bandwidth: The definition of Bandwidth: The range of frequencies transmitted without being strongly attenuated is called the bandwidth. The range of frequencies transmitted without being strongly attenuated is called the bandwidth. The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium. The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium. For example, a telephone wire may have a bandwidth of 1 MHz for short distances, but telephone companies add a filter restricting each customer to about 3100 Hz. This bandwidth is adequate for intelligible speech and improves system-wide efficiency by limiting resource usage by customers. For example, a telephone wire may have a bandwidth of 1 MHz for short distances, but telephone companies add a filter restricting each customer to about 3100 Hz. This bandwidth is adequate for intelligible speech and improves system-wide efficiency by limiting resource usage by customers.

7. The Physical Layer (Cont’d) 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. For voice-grade line, whose bandwidth is 3000 Hz, the number of the highest harmonic passed through is roughly 3000/(b/8) or 24,000/b. Conclusions: 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 transmission facility is completely noiseless. 2. limiting the bandwidth limits the data rate, even for perfect channels.

8. The Physical Layer (Cont’d) The Maximum Data Rate of a Channel The Maximum Data Rate of a Channel If an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless (Nyquist's theorem). If the signal consists of V discrete levels, we have: If an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless (Nyquist's theorem). If the signal consists of V discrete levels, we have: 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 (Shannon's theorem): 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 (Shannon's theorem):

9. The Physical Layer (Cont’d) Guided Media —— the Twisted Pair Guided Media —— the Twisted Pair The most common application of the twisted pair is the telephone system. The most common application of the twisted pair is the telephone system. 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, but several megabits/sec can be achieved for a few kilometers in many cases. 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, but several megabits/sec can be achieved for a few kilometers in many cases. Figure 2-3. (a) Category 3 UTP. (b) Category 5 UTP.

10. The Physical Layer (Cont’d) Guided Media —— the Coaxial Cable Guided Media —— the Coaxial Cable Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television but is becoming more important with the advent of Internet over cable. Two kinds of coaxial cable are widely used. One kind, 50-ohm cable, is commonly used when it is intended for digital transmission from the start. The other kind, 75-ohm cable, is commonly used for analog transmission and cable television but is becoming more important with the advent of Internet over cable. The bandwidth possible 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. The bandwidth possible 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.

11. The Physical Layer (Cont’d) Guided Media —— the Fiber Optics Guided Media —— the Fiber Optics With current fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps) and many people are looking very hard for better technologies and materials. With current fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps) and many people are looking very hard for better technologies and materials. An optical transmission system has three key components: the light source, the transmission medium, and the detector. An optical transmission system has three key components: the light source, the transmission medium, and the detector. Currently available single-mode fibers can transmit data at 50 Gbps for 100 km without amplification. Even higher data rates have been achieved in the laboratory for shorter distances. Currently available single-mode fibers can transmit data at 50 Gbps for 100 km without amplification. Even higher data rates have been achieved 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

12. The Physical Layer (Cont’d) Wireless Transmission —— The Electromagnetic Spectrum Wireless Transmission —— The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication. The wider the band, the higher the data rate.

13. The Physical Layer (Cont’d) The Public Switched Telephone Network (PSTN) The Public Switched Telephone Network (PSTN) Structure of the Telephone System Structure of the Telephone System Computer NetworksPSTN (a)Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

14. The Physical Layer (Cont’d) PSTN Structure PSTN 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.

15. The Physical Layer (Cont’d) Transmit digital signals on PSTN (Local Loops) Transmit digital signals on PSTN (Local Loops) ISDN ADSL (Asymmetric Digital Subscriber Line) Modem

16. The Physical Layer (Cont’d) Switching Switching (a) Circuit switching. (b) Packet switching.

17. The Physical Layer (Cont’d) Switching Switching Timing of events in (a) circuit switching, (b) message switching, (c) packet switching.

18. The Physical Layer (Cont’d) Switching Switching A comparison of circuit-switched and packet-switched networks.

19. The Physical Layer (Cont’d) The Mobile Telephone System The Mobile Telephone System three distinct generations three distinct generations 1 G: Analog voice. 1 G: Analog voice. 2 G: Digital voice. (D-AMPS, GSM, CDMA, and PDC.) 2 G: Digital voice. (D-AMPS, GSM, CDMA, and PDC.) 3 G: Digital voice and data. 3 G: Digital voice and data.

20. The Physical Layer (Cont’d) 3G and the future mobile network 3G and the future mobile network Digital Voice and Data Digital Voice and Data 2 Mbps for stationary indoor users (which will compete head-on with ADSL), 384 kbps for people walking, and 144 kbps for connections in cars. 2 Mbps for stationary indoor users (which will compete head-on with ADSL), 384 kbps for people 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 video). Internet access (Web surfing, pages with audio and video). Proposed Standards Proposed 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)

21. The Physical Layer Exercises Exercises Please try to solve these problems: Please try to solve these problems: 1-4, 7-9, 18-20. 1-4, 7-9, 18-20.