ISDN stands for Integrated Services Digital Network. It can provide simultaneous voice and data connectivity or data connectivity at speeds that are between those obtainable with broadband services and those of conventional dial-up. Because it has an extremely long reach from the central office, it is often obtainable in locations where broadband services are not.
Traditional telephone service consisted of a two wires forming a loop with a battery in the telephone set providing current through the loop and both telephone sets. A carbon microphone would vary resistance with sound thus varying current in the loop. A receiver consisting of an electromagnet and thin metallic disk over the electromagnet would flex in response to changes in the magnetic field and reproduce sound in response to variations of current in the loop. A bell which is responsive to a high amplitude low frequency AC signal provided a means to signal the presence of an incoming call. The AC voltage was originally provided by a hand-cranked alternator on the telephone set itself. Since the loop current could assume any value between that permitted by the minimum resistance of the microphone and the maximum, this type of telephone system is referred to as analog.
Numerous innovations improved the functionality of telephones. The batteries in the telephone sets were replaced by a battery supply in the central office. The hand cranked alternator inside the telephone was replaced by ring current generators within a central office. A cord board, a board consisting of a field of jacks and plugs manned by a human operator enabled telephones to arbitrarily be connected to other telephones.
Over the years operators were replaced by automated equipment. Circuits between central offices enabled calls to be made outside of one's local office to a distant central office and provided a new revenue center for the telephone companies, long distance calling. Amplifiers were added to these circuits to allow the volume to remain at usable levels even over very long distances but still a pair of wires was required for each active call.
In the 1950's a method was devised to multiplex up to 24 calls on two pairs of wires using time division multiplexing where the signal was sampled 8000 times a second, turned into a digital value, and that digital value transmitted over one pair of wires. At the receiving end, each time slot was sampled and a rough approximation of the original audio signal reconstructed. This was really the beginning of the digital age for the telephone network. In Europe a slightly different scheme was used that multiplexed 32 channels. One wire pair was used for transmit and one for receive. Thus these systems (T1 and D1 respectively) allowed 24 or 32 calls to be transmitted on just two pairs (4 wires).
Eventually mechanical switches were replaced with electronic switches that digitized the signal on the line as soon as it reached the central office and then switched the call by digitally passing around the data. At this point it made sense to extend that digital signal all the way out to the subscriber rather than leaving the phone line analog and converting in the central office, and this is ISDN. In many parts of Europe ISDN became the standard telephone service whereas in the United States analog remained the standard and ISDN was a more expensive option.
An ISDN line actually has two 64kb/s "B" or bearer channels and one 16kb/s "D" signaling channel. The signaling channel replaces tones and ringing current and loop current used for signaling on an analog line. Instead of closing a loop and allowing current to flow when you go "off-hook" with a telephone, an ISDN set sends a digital message to the central office. Instead of sending touch tones to dial a number, an ISDN line sends the call setup information over the "D" channel in digital form. An incoming call is signaled by an incoming digital message on the "D" channel.
The "B" channels can be used independently acting like two separate telephone lines. Either may carry a voice call or a data call. With data, both "B" channels may be used to carry data simultaneously and bonded into a single 128K data channel. Thus if you have an ISDN line and an ISDN TA (Terminal Adapter) which is the technical term used for what many people and even some manufacturers refer to as an ISDN modem, you can make a 64Kb/s data call, and a voice call simultaneously, or you can bond both channels and make a 128K data call, provided you have an ISP that supports ISDN (we do) access. The reason that "modem" is not a proper ISDN term is that both the incoming signal and line connection are digital, thus there is no "modulation / demodulation" function which is what the term "modem" stands for. The distinction is somewhat artificial because the process of sending digital information over a pair of wires still requires an encoding scheme that is essentially analog in nature.
To use ISDN to access the Internet, you need an ISDN line. You can not just hook an ISDN TA/Modem up to an ordinary analog line. Likewise, you can not connect an analog telephone set to an ISDN line directly, you have to have an ISDN telephone set. However, most ISDN modems have analog ports on them that allow you to connect the ISDN modem to an ISDN line and then plug an analog telephone into one of two jacks which allow analog access to the "B" channels. The modem will do the digital to analog conversion and signaling for the analog telephone set.
The 128K speed provided by ISDN is an intermediate between the faster speeds provided by broadband services and analog 56K dial up modems. The actual surfing experience on an ISDN line is much better than the speed improvement over 56K would indicate. This is because of another factor known as latency. The speed of a data connection refers to how many bytes per second can transverse the connection, latency refers to how long it takes a piece of data to go from one end of the line to the other. In most data circuits latency is measured in milliseconds. In a typical 56K dialup that latency will be around 150 milliseconds, but a typical ISDN will be 20-30ms, about five times faster. This is not real significant when it comes to things like downloading large files, but a typical web page might reference a dozen small elements, images, etc. Your computer first has to download the web page itself, then it has to go back and download every references item, images, Shockwave files, etc.
Early browsers downloaded one file at a time, creating a new connection for each item. Each new connection required a three-way handshake that took nearly one-half second to setup if you had 150ms of latency. Newer browsers and web servers have features such as keep-alive that holds a connection open and sends multiple items over the same connection, the ability to maintain several simultaneous connections, caching, that is locally storing frequently used items so they don't have to be downloaded each time, and pre-fetch where data referenced by links is pre-fetched and cached during periods of link inactivity, that can improve performance over slow links but still the effect of latency is significant. For these reasons, for web browsing ISDN performs much better than analog even if it's only marginally faster.
With an analog connection, 56K is never actually obtainable. Signal loss, noise, and distortion always reduce the maximum obtainable speed to something less than the theoretical maximum, and speeds of 42-48K are typical. ISDN "B" channels are always be 64K but some dial-up connections will be 56K per channel because of limitations of trunk circuits between central offices. Some telephone circuits steal every 8th data bit to indicate on and off-hook status. Modern central offices send that information over a separate data connection known as an SS-7 signaling network or alternatively they use one channel out of the 24 channels on a T1 for signaling. But those that steal every 8th bit for signaling can only send 56K per channel.
ISDN with a single channel you will usually get 64K but may be limited to 56K by interoffice trunking. With dual channels you will either get 128K or 112K. Some ISDN services refer to 64K as data and 56K as voice connections but in reality you can send data or voice over either.
Early ISDN had many issues with vendor compatibility. AT&T (Western Electric), GTE (Automatic Electric), and other switch manufacturers all had their own idea of what ISDN meant. Many early TA's would work only with a particular switch vendor or required manually setting the unit for a given switch type. Most modern units will adapt automatically.
We offer a two week free trial of our ISDN service. If you are considering ISDN we strongly advise that you take advantage of this offer before subscribing to insure that your equipment is compatible with our service prior to subscribing.