The modem was a major breakthrough in computer-mediated communication (CMC). It allowed computers to communicate by converting their digital information into an analog signal to travel through the public telephone network. However, emerging technologies such as ISDN lines and cable modems have enabled a much faster means of communication and user access to information.
WHAT is ISDN ?
ISDN, which stands for Integrated Services Digital Networks, is a system of digital phone connections which allow data to be transmitted simultaneously across the world using end-to-end digital connectivity. Most of the world's existing telephone network is already digital. The only part that typically isn't digital is the section that runs from the local exchange to your house or office. ISDN makes that final leg of the network digital.
ISDN transmitted data can be compressed , which increases transmission capacity and speed. With the increased capacity, it becomes possible to transmit all types of information through the phone lines, including audio, video, images, and data.
ISDN SPEED
Commonly available modems have a range of speed from 14 - 56 kilobits per second (kbps), but are limited by the quality of the analog connection and routinely go no faster than 45 kbps. ISDN allows multiple digital channels to be operated simultaneously through the same regular phone line wiring. The change comes about when the telephone company's switches can support digital connections. Therefore, the same physical wiring can be used, but a digital signal, instead of an analog signal, is transmitted across the line.
This scheme permits a much higher data transfer rate than analog lines. ISDN can operate at speeds up to 128 kbps (thousand bits per second), which is five or more times faster than today's analog modems. This can dramatically speed the transfer of information over the Internet or over a remote LAN connection, especially rich media like graphics, audio and video.
The Internet is constantly expanding and as a direct result, access speeds are falling. To help resolve this problem, Internet Service Providers have started offering ISDN based Internet services. This results in both increased speed and reliability. ISDN sets up calls in just 300 milliseconds, compared to the 15 seconds that an analog modem takes. In terms of reliability, digital lines are virtually error free. Analog calls, on the other hand, are subject to interference, which may corrupt the data and cause the modem to fall to slower speeds.
INTEGRATED SERVICES
Because it's designed for use in a home or small office, ISDN allows lines to be shared by voice telephones, fax machines, and computers. If an incoming call originates from another ISDN device, the digital "ring" signal identifies the type of device that places the call. Only the fax machine would answer an incoming fax. An incoming voice call would ring the phone.
Through an ISDN network, the same phone line that could previously carry only one voice, or one Internet, or one fax "conversation" can now support as many as three separate "conversations" all at once. Two voice, fax or PC "conversations," and one data "conversation" can take place at the same time, through the same ISDN connection.
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Integrated Services means that with ISDN you can talk, receive and send data, and transmit images, video or faxes all on the same line, at the same time. |
HOW DOES ISDN WORK ?
One of ISDN's strengths lies in the fact that it handles all types of information. Unlike some other digital communications technologies, ISDN handles all types of information: voice, data, studio-quality sound, still and moving images. They are all digitized, and transmitted at high speeds in the same flow of data.
The basic ISDN-to-user connection, called a Basic Rate Interface (BRI) contains three separate channels, or "pipes." Two of these channels, the Bearer (or B) channels carry user "conversations" from a telephone, a computer, a fax or almost any other device. The D channel does the administrative work, such as setting up and tearing down the call and communicating with the telephone network, but can also carry user data transmissions. Having two B channels allows the user to make two calls simultaneously.
ISDN lines also allow variable, responsive transmission speeds. Two or more channels can be combined into a single larger transmission channel. Channels can be assembled as needed for a specific application (a large videoconference, for example), and then broken down and reassembled into different groups for other applications (normal voice or data transmissions). Combining B channels in this manner is called inverse multiplexing, or bonding.
Bearer channels (or B channels) transmit user information at relatively high speeds, while separate Delta channels (or D channels) carry call setup, signaling and other information.
EMERGING TECHNOLOGIES
With the explosion in popularity of the Internet and multimedia, content publishers, hardware manufacturers and telecommunications companies are working vigorously to create services, equipment and networks that will bring the future of interactive education into schools, homes, and businesses.
Over the next several years, the bandwidth bottleneck faced by residential and small business users will be alleviated with new technologies such as an Asymmetric Digital Subscriber Line (ADSL) which works over regular telephone wires and cable modems which work over coaxial cable television wiring. Both of these services offer multiple megabits per second, but it will take time to deploy these technologies on a widespread basis.
CABLE MODEMS
Similar to a telephone modem which sends and receives signals over the telephone network to a PC, a cable modem sends and receives signals over a cable TV network. An important difference between telephone and cable TV networks is capacity. Because telephone networks were built to carry only voice signals, the capacity or "bandwidth" is very limited. Cable TV networks were designed to deliver full-motion video and, as a result, have much greater bandwidth. This significantly greater bandwidth enables the cable TV network to deliver more information persecond.
Typically, cable modem speeds are in the 1.5 - 3 Mbps (million bits per second) range.
HOW DOES A CABLE MODEM WORK?
CONCLUSION
Despite ISDN's potential, the infrastructure needed for widespread adoption of the technology is not in place. But the numbers are increasing. Prophet Research and Consulting predicts that more than 4 million ISDN PC Cards will be sold by the end of the decade.
By contrast, several European and Asian countries already have an established ISDN infrastructure in place. In these countries ISDN has provided the bandwidth and reliability of fixed leased lines (56Kbps), but at a lower cost. In the United States, however, the cost of leased lines has been relatively inexpensive, and the analog telephone service is of a higher quality. And considering the less-than-unified telecommunications industry, the widespread use of ISDN has been slow to implement.
Cost is another consideration. ISDN can be an expensive technology to implement. The phone company charges up to several hundred dollars to run a line to a business or residence. A monthly fee is paid to the phone company (residential fees average about $30 to $40 and businesses can expect fees in the $150 to $350 range). An ISDN PC card or terminal adapter can run up to $600, and then there are the Internet Service Provider's (ISP) charges. ISDN subscribers must pay a monthly fee to the ISP as well as additional charges if they use both B channels at one time.
Cable companies appear to be in better position to provide high bandwidth service. They already have an established customer base, and can provide a dedicated connection to the internet (which also frees up the telephone line).
In addition to Internet access, the cable TV industry also envisions providing its subscribers with video-enriched interactive opportunities that take advantage of the bandwidth currently used to deliver video. Much of what we see on the World Wide Web (WWW) today is designed to the constraints of slower telephone networks and analog modems. Most Web sites are comprised of text-based pages with minimal, static graphics. In the cable modem environment, the WWW can include sites with full-motion video, animated text and audio, all at very high speed.
REFERENCE LINKS
Detailed information regarding ISDN and cable modems, such as typical installation costs, and equipment needed, is available from local phone and cable companies and on the Internet.
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