What Is ISDN

What is ISDN? A frequently ask question regarding ISDN Definition. Integrated Services Digital Network (ISDN) is one of the WAN connection solutions – a solution to make internetwork connection between remote sites.

What is ISDN? As one of WAN technologies ISDN definition describes the technology that provides switched (dialed) digital WAN services in increments of 64 kbps. Data communication between two computers used to be using analog modems to dial a circuit which was the only available method before ISDN technology was discovered. The speeds of analog modems typically did not even exceed 9600 bps when ISDN was created.

What is ISDN? ISDN is widely available mostly part of the world. Even though competing technologies, such as DSL and cable have usurped ISDN in the marketplace for home access to ISPs, ISDN is still a popular technology for dial backup between business sites when a point-to-point or Frame Relay link fails.

ISDN requires that the two endpoint computers have the ISDN equivalent of an analog modem. There are many variations of these ISDN devices, mainly as a result of the fact that ISDN was created as a worldwide standard, so many options were needed to meet the differing needs of the telcos in different parts of the world. The Figure below shows the required ISDN hardware for a typical connection.

ISDN Diagram

Notice that both the home PCs and the router at the ISP use ISDN gear. Routers often use ISDN cards that can be connected directly to the ISDN circuit supplied by the telco. PCs typically use an ISDN device called an ISDN terminal adapter (TA), which is often called an ISDN modem. Because ISDN uses digital signals across the local loop, it does not actually do any modulation or demodulation. However, the term ISDN modem emerged because it was cabled and installed similarly to an external analog modem. So, for the consumer marketplace, the marketing people started calling TAs by the technically wrong but easy-to understand term ISDN modem.

Note that the local loop from the home and the central office (CO) now connects to a device called an ISDN switch. Local phone lines typically connect to a voice switch in the CO. ISDN uses digital signals, so the telco actually must terminate the line from your house in a telco switch that expects digital signals that conform to ISDN specifications.

ISDN Channels

There are two lines type in ISDN namely: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Both BRI and PRI provide multiple digital bearer channels (B channels) over which data can be sent and received. Because both BRI and PRI have multiple B channels, a single BRI or PRI line can have concurrent digital dial circuits to multiple sites. Alternately, you can create multiple circuits to the same remote site to increase available bandwidth to that site.

B channels operate at speeds of up to 64 kbps which are used to transport data, although the speed might be lower, depending on the service provider, or might be based on standards in some parts of the world. For instance, some national standards outside the United States call for 56-kbps B channels.

ISDN uses another channel inside the same single physical line to ask the telco to set up and tear down circuits. The signaling channel, called the D channel, signals new data calls. When a router wants to create a B-channel call to another device using a BRI or PRI, it sends the phone number that it wants to connect to inside a message sent across the D channel. The phone company’s switch receives the message and sets up the circuit. Signaling a new call over the D channel is effectively the same thing as when you pick up the phone and dial a number to create a voice call.

The different types of ISDN lines often are described with a phrase that implies the number of each type of channel. For instance, BRIs are referred to as 2B+D, meaning two B channels, and one D channel. PRIs based on T/1 framing, as in the United States, are referred to as 23B+D, and PRIs based on E/1 framing, typically found in Europe, are referred to as 30B+D. See also ISDN Diva USB device for BRI.

E/1s have 32 DS0 channels, with 1 reserved for framing and 1 used for the D channel when used as a PRI—that leaves 30 DS0 channels as B channels. Table 15-4 lists the number of channels for each type of ISDN line and the terminology used to describe them.

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Suggested reading:

• ISDN network setup and connection,
• WAN Technologies – types and services,
• Windows XP network – secured configuration guideline,
• backup hard drive.
• Basic computer networking diagram