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Chapter 7. Analog and Digital Signals. Objectives. Describe the characteristics of an analog signal. Describe the characteristics of a digital signal. Explain the benefits of converting an analog voice signal into a digital signal.
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Chapter 7 Analog and Digital Signals
Objectives • Describe the characteristics of an analog signal. • Describe the characteristics of a digital signal. • Explain the benefits of converting an analog voice signal into a digital signal. • Explain how analog signals are connected from a transmitter to a receiver.
Objectives (continued) • Explain how digital signals are coupled from a coder to a decoder. • Explain what Alternate Mark Inversion (AMI) is. • Explain what Manchester coding is. • Explain what differential Manchester coding is.
Objectives (continued) • Explain Non-Return to Zero Level (NRZ-L) and Non-Return to Zero Invert (NRI) signaling. • Explain the correlation between bandwidth and power loss over the local loop.
7.1 Communication Signals and Protocols • A communication protocol in telecommunications will specify: • What type of signal is to be used for communication. • How the signal is to be manipulated. • How the signal is to be placed on the transmission facility. • An analog signal is an electrical signal with continuously varying amplitude. • A digital signal is a signal that can assume one of several discrete states.
7.2 Analog Signal • All electrical signals with varying amplitudes are called analog signals (analog is short for “analogous”). • The transceiver was a device that contained a coil of wire suspended inside a magnet. • The limitations of the transceiver were overcome by the development of the carbon granule transmitter. • Devices that convert a signal from one form of energy to another are called transducers.
7.3 Connecting the Telephone to the Central Exchange • The telephones at our residences and any small businesses connect via one pair of wires to a switching system called the local central office. • Since the switching system is located at the center of the hub, it is called the central office, central exchange, or central. • The pair of wires that connects the telephone to the central exchange is called the local loop.
7.4 Analog Signal in the Local Loop • The telephone receives its power from the central exchange via the line circuit in the exchange. • When a telephone is taken off hook, electric current will flow. • The transmitter of a telephone and the electronic chip that provides the tones for a touchtone dial require about 8 V to function properly. • A varistor in the circuit limits current flow to a maximum of 60 mA because a current of more than 60 mA contributes to the possibility of crosstalk.
Twisted-Pair Wire • Twisting the wires that serve one telephone around each other eliminates crosstalk. • The tighter the twist, the higher-frequency signal it can carry. • Data grade (CAT-5) cable has many more twists per inch than voice grade (CAT-3) cable.
7.5 Coupling Analog Signals from One Circuit to Another • Transformers • Capacitor Coupling • Silicon Controlled Rectifiers (SCRs)
Coupling Analog Signals from One Circuit to Another • When the transmitter of the telephone converts a voice signal into an analog electrical signal, the analog signal is a continuously varying electrical signal. • The analog signal is a continuously varying dc signal. • Current flows in one direction only. • The signal looks like an ac signal that has a center point of 40 mA. • We can use transformers or capacitors to couple voice signals from one circuit to another while isolating the dc voltages of these circuits from each other.
Transformers Used to Couple Voice Signals • The 40 mA of current through the primary winding sets up a magnetic field of a certain strength. • When the local loop transports an analog electrical voice signal to the primary winding of the transformer, the analog signal causes the magnetic field established by the primary winding to vary. • Variations in the magnetic field cause an analog signal to be induced into the secondary winding and into the circuit connected to the secondary winding.
Capacitor Coupling Voice Signals • In the capacitor-coupled circuit, the 40 mA of current in the local loop causes the capacitor to charge to a certain value. • When the local loop circuit transports an analog electrical voice signal, the analog signal causes the electric charge on the capacitor to vary in unison with the changes of the analog signal. • This changing charge on the capacitor is coupled to the next circuit.
Silicon Controlled Rectifiers • Today, we do not use either transformer or inductive-capacitive battery feed circuits for coupling voice signals. • The line circuit that interfaces a local loop to the central exchange includes a codec chip and a hybrid network in the circuit. • The codec chip converts all analog signals received from the local loop to digital signals. • Since the analog voice signal is converted into a digital signal, we cannot use the same techniques to couple the signal from one circuit to another.
Silicon Controlled Rectifiers • The technique used to couple digital signals from one circuit to another is to gate them using silicon controlled rectifiers (SCRs). • Electronic gates are placed between two circuits and are turned on when we wish to connect signals from one circuit to another. • Voice signals at the telephone are converted into analog electrical signals at the telephone. • Analog electrical signals are converted to digital signals at the central exchange. • Digital signals are connected via the PSTN switching network to a receiver for decoding.
7.6 Conversion of Voice into Digital Signals • The standard used in the PSTN to convert analog voice signals into digital signals is pulse code modulation (PCM). • Other processes are available: • Adaptive Differential Pulse Code Modulation (ADPCM) • Predictive Pulse Code Modulation • Digital voice signals are connected from one point to another by connecting the coder portion of one codec via a transmission medium to the decoder portion of another codec.
7.7 Conversion of the PSTN into a Digital Network • Using digital signals to represent voice or data is much more efficient than using analog signals. • Analog signals can be carried only so far by a transmission medium before the signal gets so weak that it must be amplified. This introduces more noise into the signal. • Digital signal regenerators strip all noise out of a signal by regenerating crisp, clean, new 1s and 0s. • Although the circuitry between central exchanges is almost 100% digital, the circuitry that connects our telephone to the central exchange is mostly analog.
7.8 Digital Data over the Local Loop • Integrated Services Digital Network (ISDN) • Asymmetrical Digital Subscriber Line (ADSL)
Integrated Services Digital Network (ISDN) • Provides the ability to place digital data directly into the ISDN equipment on each end of the circuit. • Uses twisted-pair copper wire to connect equipment on the customer’s premises to the local exchange. • ISDN lines do not connect to regular line circuits at the central exchange; they connect to special line interface circuits called ISDN line circuits. • If an ISDN line is to be used for the transmission of a voice signal, The ISDN terminal equipment on the customer’s premises contains a codec, which converts the analog signal into a 64,000 bps digital signal.
Asymmetrical Digital Subscriber Line (ADSL) • This service is classified as a digital service, but in fact uses a modem, and the digital data on the customer’s premises will be used to modulate an analog signal transmitted to the central exchange. • Like ISDN, this ASDL service cannot be interfaced to the exchange using a regular line circuit. • ASDL lines are connected at the central exchange to another ASDL modem. • The ASDL modem in a central exchange is part of a device called a Digital Subscriber Line Access Multiplexer (DSLAM). • ADSL uses high-frequency analog signals, which are modulated by the digital data to be carried.
7.9 Digital Data Coding Techniques • Alternate Mark Inversion (AMI) • Non-Return to Zero – Level (NRZ-L) • Non-Return to Zero – Invert (NRZ-I or NRI) • Manchester • Differential Manchester
7.10 Bandwidth vs. Power Loss • Bandwidth describes the range of frequencies found within a band. • The bandwidth of a signal determines the information carrying capacity of the signal. • When we wish to transfer information over the local-loop twisted pair, we need high-frequency signals to transfer high data rates.
Bandwidth vs. Power Loss • The higher the frequency transmitted, the greater the power loss incurred due to: • Distributed capacitance that exists between the two wires of the local loop. • The inductance in the wire itself. • When a signal is carried by twisted-pair copper wire, it is especially susceptible to interference (noise) from signals in adjacent wire pairs. • It is important to maintain a high signal-to-noise ratio (SNR).
7.11 Summary • Telecommunications requires a transmitter, medium, and receiver. • To ensure accurate transmission and reception of signals: • The transmitter and receiver must use the same protocols. • Protocols specify the rules and procedures that must be followed to set up and maintain accurate, reliable communication.
Summary • The signals used in telecommunications are either analog or digital. • An analog signal is a signal with continuously varying amplitude. • A digital signal assumes one of a number of discrete voltage levels. • The transmitter of a telephone creates analog electrical signals. The local loop was designed to handle these signals efficiently.
Summary • Almost all central exchanges used in the PSTN are digital switching systems. • The line interface to these switching systems contains a codec. • Converts the analog voice signal into a 64,000 bps digital signal. • Uses PCM
Summary • The wider the bandwidth of an analog signal, the more information it is capable of carrying in a given timeframe. • The use of high-bandwidth signals also makes the data more susceptible to interference from noise. • Higher-frequency signals are needed to provide wider bandwidths. • Higher-frequency signals encounter higher power losses when transmitted over twisted-pair copper wire.
