Telecom Goes Digital
ISDN has caused telephone technology to change more in the past decade than it did in its entire first century. And when it comes to developing and implementing ISDN services, Japan has been in the forefront of the digital revolution.by Zack Leatherwood
Open your dictionary to the entry for "telephone," and the explanation will probably describe how sound waves cause a thin diaphragm to vibrate against a carbon chamber, causing variable electric current to flow across the telephone wire, with the pattern of waves converted back to sound at the receiving end by an electromagnet and yet another diaphragm. That was an effective technology when nearly all information sent over phone lines was sound based, but today a growing portion of the information transmitted over telephone lines is in binary form (text, graphics, etc.). In order to send binary data over a normal (analog) phone line, it must be converted to and from analog sound waves by a modem on each end of the transmission -- an inefficient and relatively slow method of sending digital information.
Enter Integrated Services Digital Network (ISDN), a system of sending data over phone lines using end-to-end digital connectivity. While ISDN really shines with fiber-optic networks, it can be used with existing copper phone lines; the only infrastructure change required is for telephone companies to upgrade their switches to handle digital calls. A digital network has two advantages over analog lines: clarity and speed. Digital signals are unaffected by the static and noise that often plague analog transmissions, and they can carry data at significantly higher speeds than analog signals.
Existing phone wiring can be used more efficiently by ISDN than analog services because a different kind of signal is being transmitted. Whereas analog signals are created by varying the voltages sent across the wires (and thus are sensitive to the signal-to-noise ratio), digital signals are sent as a series of positive and negative voltage (binary 0s and 1s). And with ISDN, the same twisted-pair copper telephone line that can carry only a single analog voice, computer, or fax transmission can now carry up to three separate digital transmissions at the same time, through the same line. ISDN is the "magic" that makes this happen. Through multiplexing (a method of combining separate data signals on one channel such that they may be decoded again at the destination), ISDN makes it possible to combine different digital data sources and have the information routed to the proper destination, so that a single user can transmit video, text, and audio data simultaneously through the same connection.
The background of ISDN
ISDN technically refers to a specific set of services provided through a limited and standardized set of interfaces. The ISDN architecture integrates a number of services previously provided by separate networks and adds capabilities not found in standard phone service. Instead of the phone company sending a ring voltage signal to ring the bell in your phone, for example, with an ISDN connection it sends a digital package that may include who is calling, the type of call (data/voice), and what number was dialed (allowing multiple numbers to be used on a single line). In the case of a data call, baud rate and protocol information are also sent, making the connection seem almost instantaneous.The ISDN transmission protocols were originally defined by the CCITT (Consultative Committee for International Telephone and Telegraph; now the Telecommunications Standards Sector of the International Telecommunications Union). Development of ISDN started in the early 1980s, with the original recommendations and initial guidelines for ISDN published in 1984 in CCITT Recommendation I.120. The basic ISDN protocols have been accepted as standard by virtually all the world's communications carriers. The delay in establishing a global ISDN has arisen because there are different ways to implement those protocols -- ways that are not always interoperable.
ISDN builds on groups of standard transmission channels. Bearer (B) channels are high-speed (64K-bps) clear "pipes" that transmit user voice and data. The separate delta (D) channel is a 16K- or 64K-bps packet-switched link that carries call set-up, signaling, and other information. (H channels that carry user information at higher bit rates are available in some implementations.)
The three common types of ISDN service are Basic Rate ISDN (BRI), consisting of two 64K-bps B channels and one 16K-bps D channel; Primary Rate ISDN (PRI), typically 23 64K-bps B channels plus one 64K-bps D channel, for a total of 1.5M bps; and Broadband ISDN (B-ISDN), which can support up to 150M bps but is dependent on a complete fiber-optic network.
ISDN development in Japan
NTT pioneered the development of ISDN services in Japan in 1984 under the so-called Information Network System (INS) plan. The first commercialized INS service, INS-Net 64, was introduced by NTT in April 1988, with connections between Tokyo and Osaka. INS-Net 1500 followed in June 1989, and INS-P (a packet mode ISDN) began in June 1990.Initial growth of ISDN was slow. In June 1993, three months after ISDN services had been made available to all incorporated municipalities in Japan, the number of ISDN subscribers stood at only 200,000 (about 98% INS-Net 64 and 2% INS-Net 1500). One year later, subscriptions had grown by 50%, to 300,000, and in November 1995, NTT reached its 500,000 subscriber goal (over 1 million bearer channels), about four months ahead of the target date. Recent growth has been especially high among personal (non-corporate) users; the number of personal subscriptions nearly tripled in the past year.
NTT's INS-Net conforms to the ITU Recommendation Blue Book and offers both basic and primary rate interfaces. The two INS-Net services currently available in Japan are known as INS-Net 64 and INS-Net 1500.
INS-Net 64 carries simultaneous voice, data, and image transfer over three channels at speeds up to 64K bps. Two 64K-bps B channels carry voice, facsimile, data, and image signals, while one 16K-bps D channel is used as a signaling channel (to control the B channels) and for packet transmissions.
INS-Net 1500 carries simultaneous voice, text, and image data transmissions over 24 channels in a single subscriber line; compatible with the INS-Net 64, it provides additional transmission speeds of 384K and 1.5M bps. INS-Net 1500 provides 23 B (64K and 84K bps) channels plus one 64K-bps D channel, or 24 B channels. Fiber-optic cable is required to achieve maximum speed and capacity.
NTT estimates that about 60% of the INS-Net 1500 service is employed for private branch exchange (PBX) installations, while about 80% of the INS-Net 64 traffic is text-based communications. Over 5,000 G4 facsimiles, which can transmit an A4- or letter-size page of text in just 4 seconds, are currently being used in Japan. As computer use in Japanese corporations rises, the use of ISDN for data transfer (at over 330KB per minute) is also becoming increasingly popular.
INS-Net service enables videoconferencing, which is slowly gaining in popularity for business meetings, and remote monitoring, which is proving useful for banks and stores. Many Japanese banks now use ISDN to monitor their automatic teller machines (ATMs) from a centralized monitoring center after regular banking hours. In the retail arena, the start of packet mode services encouraged 7-Eleven Japan (one of the biggest convenience store chains in Japan) to adopt ISDN in 1991. The company uses ISDN lines for Point of Sale (POS) data transmission between a host computer and the POS terminals of over 4,000 stores throughout Japan.
Nor is the general public left out. ISDN public telephones connected to INS-Net have been in use in Japan since April 1990. These telephones have both an analog port (for connecting a data/fax modem) and a digital port (for connecting a portable ISDN terminal, such as a laptop computer). While ISDN pay phones are not yet in widespread use outside of selected metropolitan areas, NTT has installed hundreds of these phones along streets in business areas, in hotels, and in airports.
ISDN applications
The average residential user will benefit from ISDN services. ISDN makes it possible for a single phone jack to do more than one task, so that a home user's modem no longer need tie up the family phone line. And if computer problems arise, it would even be possible for a help desk technician to connect to a user's home computer for online troubleshooting while simultaneously talking to the computer user.But businesses stand to be big winners in the ISDN sweepstakes. It will be cost effective for companies to replace their numerous existing analog lines with fewer ISDN lines that can handle multiple applications simultaneously. One of the most talked-about INS-Net 64 applications is videoconferencing, with one channel used for voice and the other channel for the display of video pictures, or a shared electronic whiteboard.
Perhaps the greatest attraction of ISDN, though, is that it offers an inexpensive dial-up digital connection. No longer is it necessary to choose between the high cost of leased (dedicated) lines for high-speed digital transmission and the limited data speed and accuracy of using modems on analog lines.
The dramatic increase in data-transmission speeds and line quality being enjoyed by INS-Net users are just the beginning. ISDN empowers a whole new breed of applications that are not practical with an analog connection: telecommuting, low-cost videoconferencing, remote health care, tele-teaching, and LAN-to-LAN connectivity, to name just a few. In tomorrow's dictionaries (which will be conveniently available online via an INS-Net 64 connection), the description of analog telephony may well be relegated to the "history of technology" section.
INS-Net transmission modes
INS-Net permits three modes of communication over a single subscriber line.
Telephone mode: Essentially the same as a conventional analog phone line. Transmits audio signals in 3-KHz band using circuit switching; can be used for G3 facsimile and modem data transmissions.
Digital mode: For high-speed data transfers between INS-Net terminals: 64K bps for INS-Net 64; 384K or 1.5M bps for INS-Net 1500. Full digital transmission; low error rate, high efficiency and performance.
Packet mode (INS-P): Uses packet switching to send comparatively low-density data; can perform transfers between systems operating at different speeds. INS-P is charged by volume of data transferred rather than time online.
INS-NET services and options
Standard services
Sub-addressing: By suffixing a subaddress to the dialed number, the caller
can connect to any one of multiple terminal devices attached in the same
bus-line configuration. Caller ID: Indicates the phone number of the incoming call (if it originates from another INS-Net subscriber).
Advice of charge: Indicates the charges at the end of a call.
Terminal portability: The user can temporarily disconnect a terminal device and reconnect at another location on the same line.
User-to-user-signaling: Messages of up to 128 octets (128 alphanumeric characters) can be sent over the D channel at the initiation/termination of transmission.
Special options
Group security: Restricts access by outside callers to enable establishment of a closed network of specific members.Free dial: Sets up a toll-free number whereby the receiving party, rather than the calling party, pays for the call.
Dial in: Allows assignment of a normal-length telephone number to each terminal device connected to the same line.
Flexphone: This comprises four functions -- call waiting (putting a call on temporary hold to answer a second incoming call), call transfer (transferring a call in progress to another party) three-party service (creating a three-way call), and call deflection (redirecting incoming calls to another phone).
