1 Rick Graziani Long Distance Communications (Carriers, Modulation, and Modems) Computer Networks and Internets by Douglas Comer

Rick Graziani, 2 Other information For most of this chapter we will rely on other sources. Comer does a good job in explaining “what happens” but does not provide enough information to see “how it works.” Sources used for this presentation:

Rick Graziani, 3 Digital and Analog Bandwidth Bandwidth = The width or carrying capacity of a communications circuit. Digital bandwidth = the number of bits per second (bps) the circuit can carry used in digital communications such as T-1 or DDS measure in bps T-1 -> Mbps Analog bandwidth = the range of frequencies the circuit can carry used in analog communications such as voice (telephones) measured in Hertz (Hz), cycles per second voice-grade telephone lines have a 3,100 Hz bandwidth

Rick Graziani, 4 Digital and Analog Bandwidth Available at

Rick Graziani, 5 Digital and Analog Bandwidth Digital Signals digital signal = a signal whose state consists of discrete elements such as high or low, on or off Analog Signals analog signal = a signal which is “analogous” to sound waves telephone lines are designed to carry analog signals

Rick Graziani, 6 Sound Waves

Rick Graziani, 7 Analog Signals, Modulation and Modem Standards A perfect or steady tone makes a wave with consistent height (amplitude) and pitch (frequency) which looks like a sine wave. (Figure 4-15) A cycle or one complete cycle of the wave The frequency (the number of cycles) of the wave is measured in Hertz Hertz (Hz) = the number of cycles per second

Rick Graziani, 8 Transmission Terminology (whatis.com) Broadband transmission = In general, broadband refers to telecommunication in which a wide band of frequencies is available to transmit information.band Because a wide band of frequencies is available, information can be multiplexed and sent on many different frequencies or channels within the band concurrently, allowing more information to be transmitted in a given amount of time (much as more lanes on a highway allow more cars to travel on it at the same time). Baseband transmission 1) Describing a telecommunication system in which information is carried in digital (or analog) form on a single unmultiplexed signal channel on the transmission medium. This usage pertains to a baseband network such as Ethernet and token ring local area networks.Ethernettoken ringlocal area network Narrowband transmission Generally, narrowband describes telecommunication that carries voice information in a narrow band of frequencies.band More specifically, the term has been used to describe a specific frequency range set aside by the U.S. Fcc for mobile or radio services, including paging systems, from 50 cps to 64 Kbps.FccKbps

Rick Graziani, 9 Carrier Signal or Analog Wave = An electronic signal used to modulate data in broadband transmission, usually a sine wave. Carrier Signal

Rick Graziani, 10 Three parts of any analog wave are: 1. amplitude - the height of the wave 2. frequency - the pitch of the wave 3. phase - the shift or position of the wave These are the three parts we can modulate or change the carrier signal or wave! Modulate = Change More in a moment. Carrier Signal

Rick Graziani, 11 Telephone Lines, Modems, and PSTN Voice grade telephone lines and equipment are designed to transmit tones between 300 and 3,400 Hertz bandwidth = 3,100 Hz or 3.1 KHz “most” of our human voice falls into this range Economics dictated the size of this bandwidth (Keyboard example) The “maximum” number of cycles (highest frequency) of an analog signal over voice grade telephone lines is 3,400 Hz (cycles per second)

Rick Graziani, 12 Telephone Lines, Modems, and PSTN Modem MOdulator/DEModulator converts analog signals to digital and digital signals to analog used for transmitting digital information between computers over voice- grade telephone lines Computers use transmission interface standards such as RS-232-C using positive and negative voltages which form square waves, whereas the PSTN is designed to carry analog signals (sine waves)

Rick Graziani, 13 Modulation modulation = 1. the process of varying the characteristic of an electrical carrier wave (analog, sine wave) as the information on that wave varies Three types of modulation 1. amplitude modulation 2. frequency modulation 3. phase modulation 2. the process of converting digital signals to analog

Rick Graziani, 14 Amplitude Modulation (AM) a modulation technique to vary the height the electrical signal (the sine wave or carrier wave with modems) to transmit ones and zeroes, while the frequency of the wave remains constant different amplitudes for 0’s and 1’s a.k.a. amplitude shift keying, ASK Figure 4-22 frequency for each bit remains constant volume = amplitude Amplitude Modulation

Rick Graziani, 15 Different amplitudes for 0’s and 1’s, while the frequency of the wave remains constant Full duplex different amplitudes and frequencies are used for different directions Disadvantage Voice-grade telephone lines are susceptible to distortions which affect amplitudes, as volume fades, the amplitude lowers Amplitude modulation only effective for low speed transmissions Amplitude Modulation

Rick Graziani, 16 Frequency Modulation a modulation technique to vary the frequency of the sine wave (or carrier wave) to transmit ones and zeroes, while the amplitude remains constant different frequencies for 0’s and 1’s a.k.a. frequency shift keying, FSK Figure 4-23 two separate frequencies for ones and zeroes Frequency Modulation

Rick Graziani, 17 Full Duplex requires a minimum of four frequencies, two frequencies for each direction i.e. CCITT V.21 for 300 baud modems: Originating Sending Modem Modem 1270 Hz Hz 1070 Hz Hz loss of amplitude will not cause errors in transmission Frequency Modulation

Rick Graziani, 18 Looking at waves Wave Generator Tone Generator Adjust the amplitude and frequency of a sign wave Modulation

Rick Graziani, 19 Frequency Modulation Conceptually: If voice-grade telephone lines can transmit a “maximum” of 3,400 Hz (cycles per second), between 300 Hz and 3,400 Hz, AND If one cycle = 1 bit, Then a maximum of 3,400 bits per second can be transmitted over voice grade telephone lines? (Hold that thought!)

Rick Graziani, 20 Phase Modulation (PM) a modulation technique to vary the phase of the sine wave (or carrier wave) to transmit ones and zeroes, while the amplitude and the frequency remains constant sine waves repeat themselves indefinitely shifting the wave breaks the wave abruptly and starts it again a few degrees forward or backward A different phase shift, 0 to 360 degrees, is used to transmit one or more bits Phase Modulation

Rick Graziani, 21 A different phase shift, 0 to 360 degrees, is used to transmit one or more bits Full Duplex requires a minimum of two frequencies, one frequency for each direction Phase Modulation

Rick Graziani, 22 Bits per second vs. Baud and High-speed modems So far, discussed transmission of one bit at a time, via high or low amplitude, high or low frequency, phase shift or no phase shift older modems sent only one bit per signal change, bps = baud baud rate = the number of these signal changes per second What if we could transmit more than one bit with each signal change (baud), amplitude, frequency of phase shift? Remember, voice-grade phone lines limit transmission to 3,400 Hz or 3,400 bps with 1 cycle per bit

Rick Graziani, 23 Dibit Modulation 2 bits per baud, per cycle Two bits or dibit modulation: 00, 01, 10, 11 Using Amplitude Modulation use four different amplitudes (wave heights) Using Frequency Modulation use four different frequencies Using Phase Modulation use four different phases Dibit Amplitude modulation

Rick Graziani, 24 Amplitude Modulation (AM) Frequency Modulation (FM) Phase Shift Keying (PSK) Summary of Modulations

25 Rick Graziani Multiplexing

Rick Graziani, 26 Multiplexing Multiplexing and WAN (Wide Area Networks) The ability to establish, maintain and terminate multiple wide area system-to-system connections over a single wide area link. Data/Voice systems to Data/Voice systems LAN to LAN Terminal to Host

Rick Graziani, 27 Multiplexer (mux) = A device which allows several devices to share the same communications circuit (cable, airwaves, etc.). Common Types of Multiplexing Time Division Multiplexing (TDM) Statistical Time Division Multiplexing (STDM) Frequency Division Multiplexing (FDM) Multiplexing Adtran TSU (T1) Multiplexer

Rick Graziani, 28 Adtran T3SU 300 (T3) Multiplexer Blackbox Multiplexer Multiplexing

Rick Graziani, 29 Time Division Multiplexing = A multiplexer which allows devices to transmit information (data/voice) over the circuit by quickly interleaving information. Train Example: Five Accordion Manufacturers with 20 box cars of accordions needed to get to their destination ASAP SF to New York Three solutions 1. Build 5 sets of tracks 2. Build 1 set of tracks and have 5 separate trains 3. Build 1 set of tracks and share a single train (multiplexing) Time Division Multiplexing

Rick Graziani, Build 1 set of tracks and share a single train with the box cars lined up as: Company Box Car A 1 B 2 C 3 D 4 E 5 A 6 B 7 etc. Time Division Multiplexing

Rick Graziani, 31 Each source connected to the TDM mux has the entire bandwidth for a portion of time. TDM constructs a “frame” consisting of one or more time slots for each input source. TDM scans each input source for data during its designated time slot. If the source has no data to transmit, TDM mux inserts null data and the time slot is wasted. Time Division Multiplexing

Rick Graziani, 32 The TDM channel or circuit must be able to handle the sum of the data rates of all its input sources plus overhead (later). TDM can handle input sources with different data rates. A slower device may be assigned one time slot, where a faster device may be assigned two or more time slots. Time Division Multiplexing

Rick Graziani, 33 Multiplexing where input devices share the bandwidth of the circuit by dividing the link into many separate frequencies. Involves modulating the signal from digital to analog and any other modulation techniques such as TCM. Each user has the full bandwidth of the circuit at all times. Frequency Division Multiplexing

Rick Graziani, 34 More later… T1 Multiplexing is covered in later in CCNA semester 4 and in CCNP Remote Access. This would be another excellent project topic.

35 Rick Graziani Chapter 6 – Long Distance Communications (Carriers, Modulation, and Modems) Computer Networks and Internets by Douglas Comer Instructor: Rick Graziani, August 1, 2002 CST 311 Introduction to Telecommunications Includes material from other sources. University transferable option of CIS 181 at Cabrillo College