ISDN stands for Integrated Services Digital Network. The services that are integrated are voice and data. They are both delivered over a purely digital circuit-switched network provided by your local LEC (Local Exchange Carrier, your local Telco).
Currently the majority of the telephone system is digital. The only part that remains analog is the wire that runs from your house to your local telco's Central Office (CO). This is referred to in telco-speak as the local-loop. The remainder of the US phone system is digital with service being delivered for the most part over fiber-optic cable. A good deal of copper wire remains, but this is being replaced by fiber every day.
So what is ISDN? Simply put, in California, it's the best deal on the planet. It delivers error free, fast, digital service over a single pair of copper wires. That single pair of copper wires is what probably brings your POTS (Plain Old Telephone Service) line into your home or business now. ISDN brings two combination voice/data channels into your home each with a capacity of 64kbps (64,000 bits per second). These channels can be inverse-multiplexed together to provide a data channel of 128kbps (that's 128,000 bits per second) which will make FTP transfers fly, and allow you to experience Real Audio, and Video the way they were meant to be experienced. It also allows web pages with many graphics images to download much faster.
ISDN service delivered to your home or small office is refered to as BRI, for Basic Rate Interface, service. It is also referred to as 2B+D. This indicates that you get two 64kbps B (Bearer) channels for voice and/or data, and one 16kbps D (Data) channel for call setup and signaling.
The D channel allows extremely fast call connection. No more 20-30 seconds of modem squealing delay for a connection. ISDN connects superfast because all of the call set-up information is sent "out-of-band" on the D channel. POTS phones use "in-band" signaling. An example of in-band signaling is the "call waiting" feature of your current POTS service. When you are on the phone and someone calls you, you hear a series of clicks in the earpiece. The clicks are a signal from the Telco's CO switch indicating that someone else wants to talk to you. This is in-band signaling, and if you were in the middle of an analog modem session, the call most probably would be disconnected. The modem on the other end of the connection would interpret the "clicks" as line noise and at the very least cause an error.
ISDN handles call waiting by sending the signal out-of-band, on the D channel. Smart ISDN devices, like the Ascend Pipeline 25FX ISDN Router, recognize this signal, and can perform a number of alterntive functions to alert you to the presence of another call, without disrupting the call you're on (whether it is voice or data).
If you are using both B channels for a 128kbps data session and an incoming voice (or data) call arrives, the Pipeline 25 will drop one of the B channels from the current session, and use it to answer the incoming call, whether it be voice or data. WHen you finish with the incoming call and hang up, the Pipeline will again add the now-idle B channel back to your data session.
For the price of a regular POTS line (in California) you get TWO voice lines. These lines can be used for data, or as a combination of voice and data. You can use your old POTS devices (including analog modems, FAX machines, and POTS phones) on the ISDN lines.
If you are outside of California, your mileage will vary on ISDN availability and price. Some LECs are charging an outrageous amount for ISDN service, despite the fact that ISDN is easier and cheaper for the LECs to implement and maintain.
Ordering ISDN is easier now thanks in part to Microsoft's "Get ISDN" web site. If you go there and fill in the on-line forms, they send the data to your LEC and start the order process. Buying the right ISDN equipment is crucial to a happy experience with ISDN. The router solution is best, followed not too closely by external "Terminal Adapter" devices such as the Motorola BitSURFR Pro, The 3Com Impact, the US Robotics Courier I-Modem, and others.
If you're a serious web junky, you need ISDN.
Note that 56kbps analog modems are a sham! You will almost never get anywhere near that bandwidth due to line noise between your modem and the LEC's CO switch.
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