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POTS has a bandwidth of approximately 4 KHz. The lower sideband cutoff is 300 Hz and the upper cutoff frequency is 4 KHz. This means that all voice signals varying between 300 Hz and 4 KHz, called the passband, are easily passed by the customer premises equipment (CPE, aka telephone), central office (CO) equipment, and transmission systems. The highest frequency in the passband is called the upper cutoff frequency and the lowest frequency in the passband is called the lower cutoff frequency. Everything in between is in the passband of the circuit. Frequencies above and below the passband frequencies are attenuated (suppressed). The voice passband is 300 Hz to 4 KHz and the bandwidth is 4 Khz (technically 3.6 KHz). Audiophiles know that high fidelity sound transmission requires a much broader passband, as true stereophonic reproduction requires the presence of signals as low as 60 Hz and as high as 20 KHz. But Ma Bell decided long ago, in the interest of economics, that a 4KHz voice bandwidth was sufficient to convey enough voice frequencies that voice communications would be intelligible. Grandma may not sound like she is standing right in front of you, but you can still recognize her voice over the telephone. That is, there will be few people who confuse Grandma with Grandpa when talking with them over the phone, although neither sounds like they are sitting next to you.

The voice passband of 4 KHz has worked well for many years. Now, we wish to pass data over the system that was originally designed for analog voice communications. Analog signals, characterized by a continuous change in magnitude with respect to time, require a lot of time to convey intelligent information. Some method of "speeding up" the transmission of information was required. Digitization of analog signals and information was the answer.

Now we have a series of digital bits representing an analog voice signal. These bits can be compressed in time, allowing other bits from another voice signal to be included in the overall bit stream. This technique of "marrying" several different signals into a combined signal is called multiplexing. Additional compression techniques can be used, allowing many different signals to be multiplexed together and transmitted simultaneously in some specified passband.

Narrowband ISDN

Long ago in the Dark Ages, circa 1970, and long before anyone really imagined the proliferation of desktop computers, Integrated Services Digital Network (ISDN) was conceived as a means of transmitting digital data at a whopping 64 Kbps speed. In terms of digital speeds available then, 64 Kbps was blazingly fast. Cold molasses, quietly gripping the sides of the bottle and refusing to budge no matter how hard you beat on the side of the bottle, seems speedier than 64 Kbps today. Yet, some visionaries spoke of a future broadband, or high-speed, ISDN. Only now, with ATM technology, can we really begin to differentiate between two ISDNs: narrowband and broadband.

Narrowband ISDN (N-ISDN) now refers to the original 64 Kbps ISDN service. Many ISDN connections were made in the U.S. and service providers are not in any hurry to abandon profitable legacy systems. So we give it another name while making room for an additional ISDN technology.

Broadband ISDN

Broadband ISDN (B-ISDN) refers to integrated digital services with a bandwidth from l.5 Mbps up to hundreds of Mbps. ATM technology is the father and fiber optic cable is the mother that will make possible the high data speeds necessary to give birth to B-ISDN. The birth of B-ISDN will have a profound impact on every living creature on this planet. Our ability to communicate determines how we evolve, and B-ISDN will revolutionize our ability to communicate.

Summary

Human intelligence is conveyed from person to person through the transfer of information. Information must be properly encoded by the source and decoded by the destination. And information must be converted to the proper type of signal for the transmission medium used from source to destination.

Human perception is inherently analog in nature. Our speech and our hearing, two important ingredients for sending and receiving information between two humans, are analog functions. Yet, digitally encoded information is more efficient in terms of the bandwidth used. So, while humans must necessarily receive and transmit information in an analog fashion, modern communication systems must change the basic human-originated analog information into digital data to accommodate all the information requiring transmission. To preserve precious bandwidth even more, communication systems multiplex and demultiplex the digital data. Still, digital signals are not useful for long-distance communication. The digitally encoded information must be superimposed on an analog signal for efficient long-distance transport. ATM is basically a technique for encoding and multiplexing digital data for transmission and ADSL is basically a technique for multiplexing digitally encoded analog data for transmission.


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