1993 << Back To The X Files nº 1

__ \ / |_ / \ e n o n | o u n d a t i o n presents: \ / *------ the \ / / \ files ------* / \ Fall/1993 Issue: 1 INTRODUCTION TO BASIC DIGITAL TECHNOLOGY Written by: Erik Turbo File: 1 of 6 1. Analog to Digital Conversion 1.1 Introduction Data communication is simply sending information from one location to another by coded signals. There are three minimum components all communincations systems have regardless of how complex or simple the system is. Those three components are: 1. a transmitter 2. a receiver 3. a transmission path between the transmitter and receiver Information is sent over communication systems from one location to another via data signals. There are two basic types of signals: > Analog > Digital On the next few pages you will read about analog and digital technology. Analog technology is used by customers that do not require sending infor- mation at high speed. Currently Analog services are cheaper than digital services but this will not always be the case. Analog technology does not offer the reproduction quality nor the vast services which are offered with digital technology instead of analog. Digital technology has only become available in the 80's. 1.2 Overall Configuration And Digital Concepts Central Office 1 Central Office 2 _______________ ________________ | D4 | | D4 | | _____ | | _____ | __________ | | |=====|=====================| | | | | | | | FXO | | | | FXS | |====== | TELPHONE | | | | | | | | | |__________| | |_____|=====|=====================| |_____| | | | | | |_______________| |________________| The FX Circuit configuration above contains groups of components that comprise most Special Service Circuits. These components include the following: > Switching Equipment > Facilities: - Exchange - Interoffice > Facility Terminal Equipment > All three component groups exist in either an Analog or Digital environment 1.3 Analog Signal Characteristics An Analog signal is a continously varying voltage and current quantity representing the human voice. The amplitude represents the loudness and the number of cycles per second represents the voice frequency: Characteristics of the Analog signal are: 1. The signal is continuous in time. 2. All values are permitted in the positive maximum limits defined. 3. In Analog transmission systems, the sound being transmitted is represented by the shape of the Analog signal. 1.4 Analog Signal Generation __________________ | | \ | | / \ | | / | | VARIABLE | | SOUND --> |======| RESISTOR |=====| ----> REPRODUCED WAVE | | | | SOUND / | | \ WAVE 1 / 2 | 3 | 4 \ ------------------ Analog Signal Generation The generation of an Analog signal takes the following steps: 1. When a person speaks into the transmitter of a telephone set, changes in the air pressure, sound waves, and sensed by the diaphragm (2) 2. The diaphragm repsonds to changing air pressure and changes circuit resit- ance by compressing carbon in the transmitter. 3. The change in resistance causes current flow to fluctuate, creating an electrical wave analogous to the sound wave. 4. Fluctuating current flows through the coil and vibrates the receiver diaphragm, which reproduces the sound wave. AN ANALOG SIGNAL IS A CONTINOUSLY VARYING REPRESENTATION OF A SOUND WAVE. 1.5 Analog Waves - Components An Analog signal is composed of amplitude and frequency. These components define the sound wave an Analog signal represents. The amplitude and frequency are two characterisitcs of the analog signal that can be varied to convey information. Amplitude is the measure of the volume or loudness of the Analog signal. Amplitude is the relative strength of the signal. Frequency is the number of cycles in a unit of time. 1.6 Frequency Frequency is related to the pitch of a sound. Frequency is measured in Hertz (Hz) - The number of cycles or oscillations per second. Frequency and amplitude relationships: 1. Low frequency, low amplitude - whispering at a low pitch. 2. Low frequency, high amplitude - yelling at a low pitch. 3. High frequency, low amplitude - whispering in a high pitch. 4. High frequency, high amplitude - yelling at a high pitch. 1.7 Analog Signal Impairments > Loss - Attenuation > Noise - Unwanted Electrical Signals > Distortion - Frequency Characteristic Changes Attenuation (Loss) and Amplification. The ideal transmission channel will deliver an accurate replica of the original signal to the receiving terminal. Three major problems affect the transmission of Analog signals: 1. LOSS - Weakening of the signal 2. NOISE - Unwanted electrical signals that interfere with the information signal. 3. DISTORTION - Changing of the frequency characteristics of the signal. 1.8 Analog Signal Attenuation and Amplificiation > The signal is continuously attenuated, or weakened, as it progresses along the transmission medium. > The signal is then amplified at intervals to compensate for the attenuation. > Line loss can be overcome by properly spacing amplifiers in the circuit. 1.9 Amplifying Distorted Signals > The Analog signal is also affected by noise and distortion. > Analog signals pick up noise as they travel through the network. > Noise and distortion change the shape of the Analog signal. > Amplifiers are designed to reproduce all of the variation of the Analog signal, the amplifier cannot distinguish between the voice, noise, and distortion components of the Analog signal. > The amplifier amplifies the entire input signal, thus the noise is amplified along with the original signal. > As the signal path increases in length and more amplification is needed, more noise is introduced. > The effects of noise and distortion is cumulative along the Analog trans- mission system. 1.10 Analog Signal Disadvantage > The major disadvantage of Analog transmission systems is the cumulative nature of transmission impairments. > Loss can be overcome by amplification to increase the sigmnal to its original value. Noise is also amplified. > Once introduced, the effects of noise and distortion cannot be eliminated. > Digital transmission systems solve the "Analog Problem." 1.11 Digital Transmission Concepts Digital Signal Definition A Digital signal is a discontinuous signal in the form of pulses. Good examples would be flashes of light, telegraph clicks, and dialing pulses. A transmitted Digital signal generally represents a series of on/off pulses, transmitted at a steady rate and amplitude. 1.12 Digital Signal Regeneration Digital transmission systems solve the basic "Analog Problem" of cumulative effects of noise and distoriton by regenerating rather than amplifying the transmitted signal. The regenerative repeater detects the presence of a pulse, (signal), and creates a new pulse, (signal), based on a sample of the existing signal. The regenerated signal duplicates exactly the signal originally transmitted. This eliminates the cumulative effects of noise and distortion inherent in Analog facilities. Distortion is not amplified as it is an Analog signal, it is omitted when the signal is regerated. 1.13 Comparison of Analog and Digital Signals The Analog signal is a signal that varies in a continuous manner over a wide range of amplitude and time. As you know, in Analog transmission, amplifiers were used to boost the strenght of the signal. With Analog signal transmission, the line noise is amplified along with the signal at each repeater point. Thus, as the distance increases, so does the distortion. The Digital signal is a series of pulses, all having a specified amplitude and duration in time. A Digital signal has only a discrete number of states, 0 or 1. This on/off state simplifies the process of detecing and regerating the digital bit stream. Instead of amplifying the signal, a rengenerator produces a fresh signal based on a sample of the existing signal. By using this method, noise does not accumulate. At each repeater location, the incoming Ditital signal is regenerated into the correct 0 or 1 signal. While the associated line noise is ignored. 1.14 Analog to Digital Conversion - Overview 1 1 0 1 0 1 1 /\ ______ __________ ________ _ _ _ _ _ / \ /==| | | | | | | | | | | | | | | | \/ |SAMPLE|==| QUANTIZE |===| ENCODE |_| |_| |___| |___| |_| |__ ANALOG |______| |__________| |________| SIGNAL Analog to Digital Conversion Converting an Analog signal to a Digital signal requires the steps of sampling, quanitizing, and encoding. > Sampling In the sampling process, portions of a signal are used to represent the whole signal. Each time the signal is sampled, a Pulse Amplitude Modulation (PAM) signal is generated. In order to accurately reproduce the Analog signal (speech), a sampling rate of at least twice the highest frequency to be reproduced is required. Because a majority of voice frequencies are less than 4 KHz, and 8 KHz sampling rate has been established as the standard. > Quantizing In order to obtain the Digital signal, the Pulse Amplitude Modulation (PAM) signal is measured and coded. The amplitude or height of the PAM is measured to derive a number that represents its amplitude level. > Encoding The decimal (Base 10) number derived in the quanitizing step is then converted to its equivalent 8 bit binary number. The output is an 8 bit "word" in which each bit may be either a "1" (pulse) or a "0" (no pulse) This process is repeated 8,000 times a second for a telephone voice channel service. 1.15 Filtering The range of frequencies in the human voice approximates 50 Hz to 20,000 Hz. Telephone transmission systems are arranged to transmit Analog signals between 200 Hz and 4,000 Hz. Extreme frequencies below 200 Hz and above 4,000 Hz are removed by a process called Filtering. 1.16 Sampling The sampler measures the filtered Analog signal 8,000 times a second, or once ever 125 microsecons (u sec.) The value of each of these samples is directly proportional to the amplititude of the Analog signal at the time of the sample. The sampling process is called Pulse Amplitude Modulation (PAM) 1.17 Quantizing Quantizing is essentially matching the PAM signals to one of the 255 numbers on a segmented scale. The quantizer measures the amplitude or height of each PAM signal coming from the sampler and assigns it a value from -127 to plus 127. 1.18 Pulse Code Modulation (PCM) Encoding Encoding involves the conversion of the number that was determined in the quantizing step, to a binary number. each quantized PAM signal is conerted ito an 8-bit binary "word" in which each bit may be either a "1" (pulse) or a "0" (no pulse). The 8-bit "word" represents the binary equivalent of the number from the quantizing step. 1.19 PCM Encoding Example If the Pulse Amplitude Modualation (PAM) signal measures +45 on the quantizing scale, the output of the encoding step is the 8-bit word "10101101" (ie: the binary equivalent of +45.) 1.20 Digital to Analog Conversion At the receiving terminal the following occurs: > The Digital pulses are converted back to the original Analog signal. > The Pulse Code Modulation (PCM) signals are decoded to the Pulse Amplitude Modulation (PAM) signals they represent. > The succession of PAM signals are passed through a filter, thereby reconstructing the orignal analog wave form. 1.21 Conclusion Some customers are still using analog technology for services like FX lines, POT lines, WATTS lines and voice services. These customers feel they don't need the high speed of the quality of digital services. Currently the Analog services are cheaper than the Digital services, although this could change. In our fast paced environment many customners want higher speed communications with top quality. Digital technology provides this and allows our customers to send data and voice communications simulataneously. Our jobs will be influenced greatly by the new services our customers want, which only digital technology can provide. You will learn about the services which only digital technology can offer later on in this manual. NOTICE Not for use or disclosure outside the NYNEX Corporation or any of its subsidiaries except when rightfully stolen. ------------------------------------------------------------------------------ EOF ---------------- Xenon Foundation Productions 1993 -------------------EOF ------------------------------------------------------------------------------