ISDN Technologies

Today, data traffic dominates voice traffic, and data networks are moving toward transporting voice. Prior to the technology explosion of the late 1990s, however, voice networks were more prevalent than data networks, and engineers focused on sending data over voice networks. Integrated Services Digital Network (ISDN) is a collection of standards that define how to integrate a data architecture into the public switched telephone network (PSTN). ISDN standards define both the hardware and call setup schemes.

In addition to data communications, ISDN provides the following additional benefits:

Multiple traffic feeds - Voice, video, telex, and packet-switched data are all available over ISDN.
Fast call setup - ISDN uses out-of-band signaling (D or delta channel) for call setup. ISDN calls can often be completely set up in less than a second.
The ability to combine data channels for increased bandwidth - With multiple channels, one form of ISDN is capable of 128 Kbps, whereas leased lines usually only provide 56 Kbps in North America.
The ability to purchase digital services directly from the local phone company - You can avoid the expense and hassle of purchasing a dedicated leased line.

Although the benefits of ISDN have largely been eclipsed by standard routers and cable Internet access, ISDN still appears in areas where cable is unavailable or as a dialup backup in case of private WAN failure.

ISDN is not compatible with many other protocols and requires special equipment to operate.

ISDN must run over multiple wires and must have mechanisms for breaking up and reassembling signals.

ISDN and Internet Access
ISDN has been around for many years. Although the business applications were well used, ISDN struggled for many years to fulfill its original promise to home users. Often referred to as "It Still Does Nothing," ISDN was for years a solution in search of a problem. That changed with the advent of the Internet and the ever-increasing need for bandwidth at every house on the block.

ISDN solves two of the major issues with the rising need for broadband connectivity:

It’s already there. ISDN runs over standard phone lines, which means nearly every household is a potential ISDN customer.
It’s relatively fast. ISDN offers more than twice the available bandwidth over the same phone lines not running ISDN.

ISDN Standard Access Methods
ISDN has two types of interfaces: Basic Rate Interface (BR!) and Primary Rate Interface (PR!). Both types have bearer channels (B), which carry data, and delta (D) channels, which carry signal and call control information. The D channel can also carry low-rate packet data (e.g. alarms).

BRI has two bearer (B) channels (64 kbps each) and one delta (D) channel (16 kpbs). BRI is sometimes written as 2B+D.

In North America and Japan, PRI has 23 B channels and one D channel (all channels are 64 kbps). In Europe, PRI has 30 B channels and one D channel. PR! is noted as 23B+D.

Making an ISDN Call
Prior to sending any data or voice traffic over the B channels, ISDN must make a connection using the D channel. The D channel initiates the call by establishing a path between switches and passing information about the source and called numbers. When the destination receives the setup information, it uses the D channel to signal the ISDN switch that it is available. After this connection is complete, the B channels can begin exchanging data and voice.

ISDN Device Types and Reference Points
ISDN specifies both the equipment and the connection points between equipment to ensure compatibility with the PSTN and among ISDN vendors:

Terminal endpoint 1 (TEl ) - Devices that have a native ISDN interface.
Network termination 2 (NT2) - Aggregates and switches all ISDN lines at customer-service sites using a customer-switching device.
Network termination 1 (NT1 ) - Converts signals into a form used by the ISDN line. An NT1 plugs into a standard phone jack.
Terminal endpoint 2 (TE2) - A non-ISDN terminal. TE2s require a terminal adapter.
Terminal adapter (TA) - Performs protocol conversion from non-ISDN (such as Electronic Industries Association/Telecommunications Industry Association (ELAITIA)-232, V.35, and other signals) into ISDN signals.
R - A connection point between a non-ISDN compatible device and a terminal adapter.
S - The connection point into the customer- switching device (NT2). It enables calls between customer equipment.
T - The outbound connection from the NT2 to the ISDN network. This reference point is electrically identical to the S interface.
U - The connection point between NT1 and the ISDN network.

Service Provider identifiers
Your service provider assigns service-provider identifiers (SPIDs) to identify your switch at the central office. A company’s SPID is linked to the services that the phone company ordered. When the central office makes a connection, the SPID tells the switch which services the company is entitled to.

Each channel of the BRI can have a separate SPID configured. SPID requirements depend on both the software revision and the switch type. SPIDs are required only in the U.S.