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"Optical communications and fiber technology are fast becoming key solutions for the increasing bandwidth demands of the 21st century. This introductory text provides practicing engineers, managers, and students with a useful guide to the latest developments and future trends of three major technologies: SONET, SDH, and ATM, and a brief introduction to legacy TDM communications systems.
There are clear explanations of:
* How ATM is mapped onto SONET/SDH
* The role of IP networking with ATM
* Dense wavelength division multiplexing (DWDM)
* The future direction of convergence of communications.
This concise book features easy-to-follow illustrations, review questions, worked examples, and valuable references. An accompanying CD-ROM provides the key figures in full color, suitable for easy cut-and-paste presentations. UNDERSTANDING SONET/SDH AND ATM is a must-read for communication professionals who want to improve their knowledge of this emerging technology."
IEEE Communications Society
"...an introductory text providing a useful guide to the latest developments and future trends of these three major technologies...includes easy-to-follow figures, review questions, worked examples, and valuable references."
Voice has been one of the primary services in the communications industry. Voice, by nature, is an analog signal. First, an acoustic wave is generated as the vibrating vocal cords and the mouth cavity modulates it into recognizable and distinguishable compounded sounds that we call words. This acoustical signal is converted to an electrical signal by a transducer known as the microphone. The generated electrical signal is also analog; that is, it changes value in a continuous manner with respect to time. At the receiving end, this electrical signal activates the electromagnetic coil of a speaker, another transducer, which reproduces the original acoustical signal.
Initially, telephony entailed few basic functions such as ringing and call initiation (and number dialing), and an analog signal was transmitted over the telephone network. This service became known as Plain Old Telephone Service (POTS) and the telephones were known as POTS telephones. Soon thereafter, the analog signal was converted to a digital one, known as pulse-coded modulation (PCM), to form a binary (or digital) bit stream at 64,000 bits per second, known as digital signal level 0 (DSO).
The circuitry responsible for converting an analog electrical signal to PCM and vice versa is known as a coderldecoder, abbreviated CODEC. Figure 1.1 illustrates an analog voice signal propagated over a twisted pair of wires (left side), also known as tip-and-ring (T&R); it passes through the CODEC circuit and on the right side we obtain a digitally encoded signal.
A CODEC periodically samples the analog signal, and based on a conversion table, it translates each sampled value into a binary representation. There are two different representations or conversion tables. The one, known as the u-law (mu-law), is used in the United States, and the other, known as the a-law (alpha-law), is used in Europe.
The acoustical signal of speaking voice, for all practical purposes, has a maximum frequency of under 4 kilocycles per second, or kilohertz (3.4 kHz).
Although voice (depending on the speaker) may contain higher frequencies, filters remove the frequencies above 3.4 kHz. It is proven that in order to decode PCM perceptually back to the (almost) same voice signal, the analog signal must have been sampled at least twice its maximum frequency content.This is known as Shannon's theorem, developed and proven by Shannon while working at Bell Laboratories. Thus, a CODEC samples the electrical equivalent of analog voice 8000 times per second (2 X 4000), or every 125 us, and it converts each sample into 8 bits PCM. Consequently, in every second there are generated 8000 X 8 = 64,000 bits, or a bit rate of 64 kilobits per second (64 Kbps); see Figure 1.2. This is a 64-Kbps channel and the bit rate is termed DSO.
1.1.1 Adaptive PCM
Besides the 64 Kbps, methods have been developed that use sophisticated digital signal-processing algorithms to compress the 64 Kbps to 32 Kbps, or to 16 Kbps, or even to lower than that. These methods, known as differential PCM (DPCM), adaptive DPCM (ADPCM), and sigma-delta PCM are compression techniques, each identifying the particular algorithm used.
1.1.2 Local Loop
In traditional telephony, the user's equipment is a POTS telephone that transmits an analog signal over a pair of twisted copper wires to the service provider equipment, where the CODEC is located. This pair of copper wires is also known as a local loop cable. Copper wires may be placed underground or on poles. These cables are susceptible to environmental electrical interference and noise, and in long loops and legacy systems chokes or filters had been placed to filter out the high-frequency content (above 3400 kHz) of the analog signal.
1.2 Time-Division Multiplexing
Since the "digitization" era, all communications systems and networks that support voice transport are based on the 8-kHz sampling fate (or an integer multiple of it), on 64-Kbps channels, and on the 125-us "quantum" interval.
In a traditional digital network, the POTS telephone converts the acoustical signal into an electrical signal. The electrical signal is then transmitted over a pair of copper wires to a communications system where the CODEC function is performed. From that point on, the network does not know of analog signals but only of digital, and this is what constitutes an all-digital communications network.
Now that the network has become all-digital, in addition to voice, we can also pass through it digital data (raw digital data, encoded video, encoded sound, etc.). An earlier data service is the Digital Data Service (DDS). This takes advantage of the 64-Kbps channel and uses 56 Kbps for data and the remaining 8 Kbps for "in-band" signaling, that is, bandwidth allocated to the network for its needs and not delivered to the end user.
Another service is the Basic Rate Integrated Services Digital Network (BRISDN or BRI). The BRI uses two 64-Kbps channels, known as channels B, and a 16 Kbps subrate channel, known as channel D, to support a combination of voice and/or data services over a single pair of wires. BRI is defined to support, at the end user's request, any of the following modes: two B channels for voice and a D channel for data; one B channel for voice and the B + D channels for data; two B channels for data and the D for signaling; all channels for data. Notice, however, that BRI can only be supported if both the user has Integrated Services Digital Network (ISDN) equipment and the communications system supports ISDN. The CODEC in this case is located at the end user's equipment. Table 1.1 lists the DSO services and the various formats per service. The formats listed in the table will be explained in a subsequent section.
LEGACY COMMUNICATIONS SYSTEMS: CONCEPTS.
Basic Technology and Services.
Legacy Data Networks.
SONET AND SDH.
Synchronization and Timing.
IP over SONET and DWDM.
ASYNCHRONOUS TRANSFER MODE.
ATM Cell Switching.
Segmentation and Reassembly.
ATM over SONET/SDH.
Management of ATM over SONET.
List of Abbreviations.
About the Author.