Since the inception of the plain old telephone system (POTS) in the 1880s, it has formed the backbone of the communications world. Reliant on twisted pairs of copper wires bundled together for its operation, there has not really been any quantum jump in its transmission mode, except for its transition from analogue to digital at the end of the 1970s. Of the total bandwidth available on the copper wires, the voice portion, including the dial tone and ringing sound, occupies about 0.3 %—that is, the remaining 97.7 % is unutilized This seems to be poor resource management as prior to the advent of the Internet, telecommunication companies (telcos) have not really sought to explore better utilization of the bandwidth through technological enhancements—for example, promoting better voice quality and reducing wiring by routing two neighboring houses on the same line before splitting the last few meters. Two possible reasons could be cited for this. Advances in microelectronics and signal processing necessary for the efficient and cost-effective interlinking of computers to the telecommunications network have been rather slow (Reusens, van Bruyssel, Sevenhans, van Den Bergh, van Nimmen, & Spruyt, 2001). Also, up to about the 1990s, telcos were basically state-run behemoths which had little incentive to come out with innovative services and applications. With deregulation and liberalization of the telecommunication sector introduced in the 1990s, the entire landscape underwent a radical change that saw telcos instituting a slew of services, enhancements, innovations, and applications; in parallel, there was a surge in technological developments facilitating these. Prior to the advent of the Internet, POTS was used mainly for the transmission of voice, text, and low resolution graphics—the latter two are in relation to facsimile machines which became popular in the late 1980s. The POTS network is, however, not able to support high bandwidth applications such as multimedia and video transmission. Because of the ubiquity of POTS, it makes sense to leverage on it for upgrading purposes in order to support high bandwidth applications rather than deploy totally new networks which would need heavy investments. In recent times, asymmetric digital subscriber line (ADSL) has emerged as a technology that is revolutionizing telecommunications and is fast emerging as the prime candidate for broadband access to the Internet (Tan & Subramaniam, 2005). It allows for the transmission of large amounts of digital information rapidly on the POTS.
Attempts by telcos to enter the cable television market led to the beginnings of ADSL (Reusens et al., 2001). They were looking for a way to send television signals over the ubiquitous phone line so that subscribers could use this line for receiving video. An observation made by Joseph Leichleder, a scientist working at Bellcore, that there are a plethora of applications and services for which faster transmission rates are needed from the telephone exchange to the subscriber’s location rather than for the other way around (Leichleider, 1989), led to the foundations of ADSL. Telcos working on the video-on-demand market soon recognized the potential of ADSL for sending video signals on the phone line. The video-on-demand market, however, did not take off for various reasons: telcos were reluctant to invest in the necessary video architecture as well as upgrade their networks for the transmission of video signals, the quality of the MPEG video stream was rather poor, and there was competition from video rental stores that were proliferating in many countries and leasing out the videos inexpensively (Reusens et al., 2001). Moreover, the hybrid fiber coaxial (HFC) architecture for cable television, launched in 1993, posed serious competition. At about this time, the Internet was becoming a buzzword, and telcos were quick to realize the potential of ADSL for fast Internet access. Field trials began in 1996, and in 1998, ADSL started to be deployed in several countries.
Excessive interest by telcos towards ADSL has more to do with the fact that the technology offers speedy access to the Internet as well as provides scope for delivering a range of applications and services while offering competition to cable television companies entering the Internet access market. Obviously, this means multiple revenue streams for telcos and maximizing shareholder value.
Key Terms in this Chapter
SNR: Standing for signal-to-noise ratio, it is a measure of signal integrity with respect to the background noise in a communication channel
Broadband Access: This is the process of using ADSL, fiber cable, or other technologies to transmit large amounts of data at rapid rates
Frequency Division Multiplexing: This is the process of subdividing a telecommunications line into multiple channels, with each channel allocated a portion of the frequency of the line
CAP: Standing for carrierless amplitude phase modulation, it is a modulation technique whereby the entire frequency range of a communication line is treated as a single channel and data transmitted optimally
Forward Error Correction: It is a technique used in the receiving system for correcting errors in data transmission.
DMT: Standing for discrete multitone technology, it is a technique for subdividing a transmission channel into 256 subchannels of different frequencies through which traffic is overlaid
Splitter: This is a device used to separate the telephony signals from the data stream in a communications link.
Twisted Pairs: This refers to two pairs of insulated copper wires intertwined together to form a communication medium.
MPEG: This is an acronym for moving picture experts group, and refers to the standards developed for the coded representation of digital audio and video
Bandwidth: Defining the capacity of a communication channel, it refers to the amount of data that can be transmitted in a fixed time over the channel, it is commonly expressed in bits per second
ADSL: Standing for asymmetric digital subscriber line, it is a technique for transmitting large amounts of data rapidly on twisted pairs of copper wires, with the transmission rates for downstream access much greater than for the upstream access
QAM: Standing for quadrature amplitude modulation, it is a modulation technique in which two sinusoidal carriers which have a phase difference of 90 degrees are used to transmit data over a channel, thus doubling its bandwidth
Modem: This is a device that is used to transmit and receive digital data over a telecommunications line.