The Migration to IP-Based Transport

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1 CDE #47920 ANALOG VS. DIGITAL TRANSMISSION The Migration to IP-Based Transport By Guy Ball Part one of a two-part series L INTRODUCTION ocal Government and Public Safety agency communications systems today consists of a hybrid of analog and digital radio and transmission networks. However, the stage has been set for a continued migration to digital IP-based technologies. Examples of this trend can be seen with planned deployment of FirstNet between 2017 and 2019 as well as upgrades to existing transmission infrastructure in order to support reliable high speed broadband data. In Part 1, we examine a brief history our nation s analog telephone networks and how they migrated from circuit switched analog based transport systems to packet switched IP-based transmission. Additionally, we will define what analog and digital signals are, as well as provide a basic description of how analog signals are converted to digital and vice versa. We will see that the invention of digital technologies before the 1950s had to wait for invention of the components to support it. However, starting about 2009, customer demands for high-speed broadband data became the driver stimulating research and development of 4G LTE technology. For example, operators of wireless networks experienced data traffic about equal to voice traffic by Q4 of However, by Q1 of 2013 it had increased by a factor of 500 causing traffic congestion in the existing 2G and 3G networks. 1 In Part 2 of the article, we will begin with a more in-depth discussion of the basic performance characteristics of analog and digital technologies and compare the differences between the two in a transmission network. Furthermore, we will analyze the imperfections of the digital Vocoder, the electronic device used to convert analog speech signals to digital bit streams for transmission over digital radio. The vocoder has been an ongoing issue for the Public Safety Project 25 (P25) committee as a result of a detailed report released by a federal group of engineers in 2008 comparing the audio quality of analog and digital networks. The report specifically dealt with problems firefighters were having using their digital P25 two-way radios in high noise environments. We will then consider some possible scenarios where analog radio may have the advantage compared to digital radio. In closing, the importance of synchronization in digital radio and transmission networks will be examined. In summary, we will review the advantages of digital IP-based wireless and transmission networks. In this article we will take a look at a brief history of our nation s analog telephone networks and how they migrated from circuit switched analog-based transport systems to packetswitched IP-based transmission. We will see that the invention of digital technologies before the 1950s had to wait for invention of the components to support it. However, starting about 2009, customer demands for high-speed broadband data became the driver stimulating research and development of 4G LTE technology. For example, operators of wireless networks experienced data traffic about equal to voice traffic by Q4 of However, by Q1 of 2013 it had increased by a factor of 500 causing traffic congestion in the existing 2G and 3G networks. 1 We will then go back to the basics and define what analog and digital signals are, as well as provide a basic description of how analog signals are converted to digital and vice versa. Additionally, we will give a brief overview of the basic performance characteristics of the two technologies and compare the differences between the two in a transmission network. 34

2 KJPARGETER/SHUTTERSTOCK.COM FIGURE 1: BLOCK DIAGRAM OF THE NORTH AMERICAN (AT&T) TDM HIERARCHY AND MULTIPLEX PLAN Furthermore, we will analyze the imperfections of the digital vocoder, the electronic device used to convert analog speech signals to digital bit streams for transmission over digital radio. The vocoder has been an ongoing issue for the public safety Project 25 (P25) committee as a result of a detailed report released by a federal group of engineers in 2008 comparing the audio quality of analog and digital networks. The report specifically dealt with problems firefighters were having using their digital P25 two-way radios in high noise environments. We will then consider some possible scenarios in which analog radio may have the advantage compared to digital radio. In closing, the importance of synchronization in digital radio and transmission networks will be examined. We will also review the advantages of digital IP-based wireless and transmission networks. THE MIGRATION FROM ANALOG TO DIGITAL TRANSMISSION In the first half of the 20 th century, public safety and telecommunications networks were dominated by analog radio and transmission systems. Multi-billions of dollars were invested in the equipment for such networks. If those same networks were installed today, there is no doubt that analog technology would have limited use. Radio systems and transmission links would transmit digital bit streams and not analog. Additionally, most transmission media would be incapable of transmitting analog signals without conversion to digital bit streams first. Today, voice signals become digital bit streams resembling computer data. Pulse Code Modulation (PCM), the process of converting analog to digital signals, was first invented in However, the technology at that time was not widely understood or appreciated. Even if it had been appreciated, PCM had to wait for the invention of transistors and integrated circuits starting in the 1950s and a better understanding of digital devices in general. The first working PCM systems, referred to as T1-carrier (i.e. DS-1 or Digital Signal 1), were developed by AT&T for digital transmission of multiplexed telephone calls. The system could transmit up to 24 telephone calls simultaneously over a single transmission line of copper wire with a data rate of million bits per second. By 1968, more than a half-million channels were in-service. The T1 carrier was just the beginning. The next step was the introduction of higher order PCM systems, which combined FIGURE 2: BASIC CONCEPT OF CAPSNET NETWORK USING MICROWAVE BACKHAUL multiple T1 electrical signals into higher bit rates for transmission over coaxial cable, microwave radio and later to light pulses for transmission over fiber optic cables (See Figure 1). In Figure 1, we can see that 4 x Mbit/s DS1 (T1) signals can be multiplexed (or combined) to a higher order bit rate of Mbit/s or a DS2 (T2) signal. Likewise, 7 x DS2 signals can be multiplexed to a Mbit/s or a DS-3 (T3) signal. This was referred to as Time Division (digital) Multiplexing (TDM), developed to gradually replace the existing analog version of multiplexing known as Frequency Division Multiplexing (FDM). Prior to TDM multiplexing, the analog FDM version of multiplexing combined numerous voice channels by assigning each one a different frequency within the main channel. Technology, however, never stands still and today the migration to newer more efficient methods of data transport continues. For example, voice and data traffic established over the first TDM-based multiplex systems were required to be circuit switched. Circuit-switched traffic required a fixed circuit to be entire duration of a call. By contrast, modern transmission systems are migrating to a packet-switched IP based system of transport. Packets of data do not require a fixed circuit, as opposed to circuit switching, and can be transported to the destination on independent transmission paths. The consolidation of voice (using voice over IP or VoIP) and data traffic based on the transmission of packet data results is a more efficient method of transport requiring fewer network elements. The reduced number of network elements increases the overall hardware reliability. In addition to improved hardware reliability, transmission delay (latency) is reduced through the network, allowing for the introduction of real-time data applications such as video conferencing. The planned upgrades to the California Public Safety (CAPSNET) microwave network are a prime example of migration from circuitswitched hybrid/legacy TDM-based transmission to a packet-switched IP-based network. CAPSNET, used primarily to support first responder voice and radio communications for all of the state of California s public safety agencies, was still based on circuit-switched traffic over a hybrid analog and legacy TDMbased microwave network as of In that same year, a roadmap was developed to PSC January/February

3 FIGURE 3: ANALOG SIGNAL FIGURE 4: DIGITAL SIGNAL upgrade the antiquated microwave network to a state of the art Internet Protocol (IP)/ Multiprotocol Label Switching (MPLS) system that will carry existing radio control circuits, along with new voice, video and data applications. 2 Figure 2 shows the basic concept of the CAPSNET network providing connectivity between remote mountaintop radio transceivers, supporting radio communication between first responders and dispatch/emergency communications centers. Starting in the 1950s, the decision of whether or not to convert the existing analog transmission links to digital was determined by economics. Vast sums of money were invested in the nation s analog telecommunications networks at that time. Therefore, business decisions were made based on which parts of the network would benefit the most technically and economically from an analog to digital upgrade. Analog technology was still less expensive at the time but the cost of digital equipment continued to drop as time went on. The same model applies today as economics plays a major role in influencing our business decisions in an unprecedented period of austerity and tight state general fund budgets. By the 1970s and 1980s it was not uncommon at large telecommunications facilities to have a mixture of analog- and digitalequipment technology. During that time, new markets emerged and manufacturers began researching and developing equipment designed to act as an interface between the various analog and digital transmission technologies. Since that time, there has been a global migration from traditional analog to digital transmission systems as the cost of the technology continues to drop. over a range of frequencies. Any enthusiast knows that sound consists of a range of frequencies between about 30 to 15,000 hertz (cycles per second) per second or up to 20,000 Hz for people with very good ears. Sound cannot be heard below 30 or above 20,000 Hz. A high-fidelity FM radio channel, for example, requires a minimum range of frequencies from 20 to 20,000 Hz to be transmitted. In the interest of economy, the telephone company transmits a frequency range of 300-2,400 Hz only or just enough to make a person s voice recognizable and intelligible. 3 Figure 3 shows a voice signal occurring as an analog signal. Digital signals, on the other hand, are signals that represent a sequence of discrete values as opposed to a continuously varying analog signal. A logic signal is a signal with only two possible values. Figure 4 shows a digital signal where the voltage takes on the two discrete values of 0 volts (logic 0), or +5 volts (logic 1). 4 Digital signals can be digitized voice or data communication between computers. Communications networks today can be designed to carry either analog or digital signals. This applies to all forms of communications, such as public safety land mobile radio, commercial-fixed and cellular networks, fiber optic cables, microwave radio links and satellites, etc. FIGURE 5: BLOCK DIAGRAM OF ANALOG CELL PHONE WITH LAPTOP COMPUTER THE BASICS OF TRANSMITTING DATA OVER ANALOG RADIO In the past, most telephone channels and communications systems were analog, capable of transmitting a certain range of frequencies. Therefore, the modem was in common use at that time converting digital bit streams to the analog signals required for transmission over analog carrier systems. Figure 5 shows a laptop computer connection through a modem to a legacy analog cellular phone. The laptop generated digital data streams consisting of binary ones and zeros, which were then converted to analog tones via the modem. The analog tones then modulated the transmitter carrier frequency of the analog cellular phone. Once detected at the receiver, the analog tones passed through a second modem that converted the signal back to digital data for transport to another computer for additional processing (i.e. display, printing, query to National Crime Information Center). 5 THE BASICS OF TRANSMITTING VOICE OVER DIGITAL RADIO Analog voice signals can also be sent over digital radio systems. However, they must first be converted to a digital signal. This is done by sampling the voice frequency and DEFINING ANALOG AND DIGITAL SIGNALS Analog means that the amplitude of the transmitted signal varies over a continuous range. Both the sound you hear and the light you see are considered analog signals spread 36

4 FIGURE 6: BLOCK DIAGRAM OF DIGITAL RADIO TRANSMITTER (TX) AND VOCODER CIRCUIT FIGURE 7: PULSE AMPLITUDE MODULATION (PAM) then changing the sampled information to ones and zeros, which then modulate the transmitter carrier frequency of the digital radio transmitter. At the receiver, the process is reversed to convert the digital data back into analog voice signals. This electronic circuit, also found in public safety land mobile radio digital systems, is called a vocoder. Figure 6 shows the vocoder circuit responsible for conversion of the traditional analog voice signals into digital data (A/D) consisting of positive and negative binary signals. It is of critical importance to public safety communications to evaluate the performance of the vocoder and its ability to accurately convey a continuously variable analog signal such as speech into a digital form in high-noise environments. However, even as periodic improvements are made to the vocoder, the original signal can never be reproduced exactly due to quantization noise, a property referred to in digital communications as granularity. Let s now discuss the process of how analog signals are converted to digital in order to better understand quantization noise and the origin of the phenomenon. In order to convert an analog signal such as speech into a digital pulse train, a circuit must first sample it at periodic intervals. The simplest form of sampling produces pulses, the amplitude of which are proportional to the amplitude of the analog signal (See Figure 7). This process is called Pulse Amplitude Modulation (PAM). After the analog signal has been quantized and samples taken at specific points, as in Figure 7, the result can be coded in to a binary word. For example, the public safety P25 radio system architecture uses a specific method of digitized voice-speech coding called Improved Multi-Band Excitation (IMBE TM ). Figure 8 shows an example of analog to digital speech coding for description purposes. Note that the red line on the graph of Figure 8 shows an example of an analog signal applied to the vocoder input. The blue line represents the digital pulse train created to represent each sample taken as a result of the PAM previously discussed. Note that when viewing the graph starting from left moving to the right, the red line (depicting the voltage of the analog signal) starts to rise. Also note that as the red line is rising, the blue line (representing the coded digital pulse train) stays flat and coded 100 even as the voltage continues to rise and does not change until the analog voltage reaches a predetermined value due to the rounding functionality of the coding process. The areas on the graph showing the red analog signal voltage varying as the digital coding remains constant are where quantization noise is occurring in the vocoder circuit. The question therefore arises of how often we should sample a signal in order to be able to reconstruct it satisfactorily from the samples on the receiver end. The less frequently we can sample it, the fewer number of pulses we have to transmit per second in order to send the information, or conversely, FIGURE 8: EXAMPLE OF ANALOG TO DIGITAL SPEECH CODING the more information we can transmit over a given bandwidth. The answer to this question is dictated by limitations of bandwidth. If we sample the analog signal at more frequent intervals, quantization noise is reduced, however the data rate required for transmission is increased. Therefore, there is a trade-off between reduction of quantization noise and the data rate required for transmission. Part 2 of the article will begin by comparing the transmission characteristics of analog and digital signals. Then we will describe the improved performance characteristics of digital radio compared to analog radio. Furthermore, scenarios will be outlined where analog radio may have the advantage when compared to digital radio. In conclusion, we will examine the need for synchronization of digital data networks. Part 2 of this article will appear in the March-April issue of PSC magazine. 1 LTE: An Overview of the Technology and the Benefits to Public Safety, Guy Ball, PSC Magazine, November/ December 2016, Vol 82, No.6. 2 Public Safety Microwave Network Strategic Plan, California Technology Agency Public Safety Communications Office, March Telecommunications and the Computer, 2 nd edition, James Martin, Published Selected Articles from, The Lenkurt Demodulator, PCM, Published March Understanding Wirelesss Communications in Public Safety, Kathy J. Imel and James W. Hart, P.E., March 2000, Revised: August Mobile Network Synchronization Plan, Viag Interkom (company renamed to O2), Guy Ball and Thomas Angerer, Published September Guy Ball is a Telecommunications Services Engineer specializing in VHF, UHF, and 800MHz frequency coordination and microwave transport systems at APCO with 35 years of experience in the cellular industry as a microwave/rf transmission planner for international and domestic corporations such as Hutchison Whampoa Ltd of Hong Kong, Telus Communications of Canada, and the AT&T Corp. in the United States. PSC January/February

5 1. Pulse Code Modulation (PCM), the method invented in 1937 to convert analog signals to digital signals, had to wait for which devices to be invented starting in the 1950s before it could be introduced to the market? a. Capacitors b. Vacuum Tubes c. Transistors and Integrated Circuits d. Resistors 2. What was the name of the first working PCM system developed by AT&T to multiplex and transmit 24 simultaneous telephone calls using a data rate of Mbit/s over a single transmission line? a. Frequency Division Multiplexing b. T-Carrier c. Pulse Code Modulation d. Digital Transmission 3. What data rate is required to multiplex 4 x (DS-1) Mbit/s signals to a DS-2 mutliplex rate according to the North American (AT&T) TDM multiplex hierarchy? a. 64 Kbit/s b Mbit/s c Mbit/s d Mbit/s 4. Which description best defines circuit switched telecommunications traffic? a. Analog and digital signals requiring a fixed amplitude to be CDE EXAM #47920 b. Analog and digital signals requiring a temporary circuit to be c. Analog and digital signals requiring a fixed circuit to be d. Analog and digital signals requiring a temporary circuit to be established for video conferencing. 5. What type of telecommunications traffic does not require a fixed circuit and can be transported to the destination on independent transmission paths? a. Packet Switched b. Circuit Switched 6. Which range of frequencies are transmitted by the telephone company which is just enough to make the human voice intelligible? a. 30 to 15,000 hertz b. 30 to 20,000 hertz c. 300 to 2,400 hertz d. 20 to 20,000 hertz 7. Which statement best describes the definition of a digital signal? a. Signals that represent a sequence of discrete values as opposed to a continuously varying signal. b. Signals that vary in amplitude over a continuous range 8. What modulation technique samples analog signals at periodic intervals and creates a sequence of pulses which are proportional in amplitude to the analog signal? a. Pulse Code Modulation (PCM) b. Pulse Amplitude Modulation (PAM) c. Frequency Modulation (FM) d. Quadrature Partial Response Modulation (QPR) 9. What is the name of the circuit found in public safety land mobile radio which samples the amplitude pulses and converts them to a digital bit streams? a. Mixer b. RF Amplifier c. Modulator d. Vocoder 10. What is the name of the method used in public safety P25 radio systems to convert voltage samples taken of the analog signal to a digital bit stream? a. Digital Baseband Modulation b. Frequency Division Multiplexing (FDM) c. Improved Multi-band Excitation (IMBE TM ) d. Dynamic Transcoder Method (DTM) FOR CREDIT TOWARD APCO RECERTIFICATION(S) Each CDE article is equal to one credit hour of continuing education 1. Study the CDE article in this issue. 2. Answer the test questions online (see below for online exam instructions) or on the exam page from the magazine article (photocopies are not required). 3. Add/upload your CDE article information and certificate of achievement in the My Classes Taken section of APCO s Training Central at trainingcentral. Questions? Call us at (386) You can access the CDE exam online! To receive a complimentary certificate of completion, you may take the CDE exam online. Go to net/login/index.php to log in or create your username and password. Enter the CDE article in the search box, and click on the 2018 Public Safety Communications Magazine Article Exams, then click on enroll me and choose Analog vs. Digital Transmission (47920) to begin the exam. Upon successful completion of the quiz, a certificate of achievement will be available for download/printing. IF YOU WOULD LIKE TO USE THE CDE ARTICLES FOR ANYTHING OTHER THAN APCO RECERTIFICATIONS AND NEED A PRINTED COPY OF THE CERTIFICATE: Complete the written exam and submit the following: 1. Answer the exam questions online, and fill out the form below. Photocopies are acceptable, but please don t enlarge them. 2. Mail to: APCO Institute 351 N. Williamson Blvd. Daytona Beach, FL Payment of $15 Name: Organization: Address: Phone: Method of Payment (US funds only) Check Purchase Order (attach copy) New Jersey Original Purchase Order Only Credit Card (circle one) VISA MASTERCARD DISCOVER AMEX Card #: Expiration Date: Name on Card: Cardholder s Address: Cardholder s Address: 38

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