ISDN(Integrated Services Digital Network)

Integrated Services Digital Network (ISDN) is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switch telephone network

ISDN is a public switch telephone network system, which also provides access to packet switch network, designed to allow digital transmission of voice and data over ordinary telephone copper wire, resulting in potentially better voice quality than an analog phone can provide. It offers circuit-switched connections (for either voice or data), and packet-switched connections (for data), in increments of 64 kbps.

ISDN’s ability to deliver at minimum two simultaneous connections in any combination of data, voice, video, and fax, over a single line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of most people's complete communications needs (apart from broadband internet access and entertainment television) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.

The world's telephone companies conceived ISDN in the early 1980s as the next generation network. The existing voice networks didn't deal well with data for the following reasons:

·         One had to use modems to transmit data.

·         The data rates were around 9600 bps.

·         Connections (worldwide) were unreliable.

ISDN Interfaces

The customer interface I.45x specifies the Basic Rate Interface (BRI). It was intended to become the standard subscriber interface.

The BRI specifies two bearer channels and a data channel. The two bearer channels would bear the customer's information. The initial concept had this as being everything from analog telephone calls (digitized) to teleconferencing data and these would be switched channels. The only difference between a conventional telephone circuit and a bearer channel is that the bearer channel would be 64 Kbps all the way to the customer. (Note in the current network, your analog telephone circuit is digitized to 64 Kbps at the local Telco office before being switched across the network. It is then turned back into analog at the far end before being delivered to the called party.)

Now with BRI, we have not one, but two such telephone circuits.  Since it is digital, we have switched digital 64 Kbps to (theoretically) anywhere in the world.

The problem with the existing Telco network is that the signaling information shares the telephone channel with the user information. With plain voice circuits, a customer doesn't notice or care. With the advent of modems, this represents a loss of channel bandwidth, and with digital transmission, it means a loss of usable bits per second. The customer is therefore stuck with 56 Kbps, instead of the actual channel rate of 64 Kbps.

The BRI interface therefore specifies a multifunctional data channel at 16 Kbps that could handle signaling (its primary function) and network data (X.25) when not needed for signaling. BRI is therefore referred to as 2B + D, two bearer channels and a data channel.

ISDN Interface Components

NT1:

It creates the T interface for premise devices (from the U interface). In the original CCITT concept, the NT1 was provided by the Telco as part of the ISDN service. The U interface was therefore only the concern of the Telco with open networks (which concerned North America at the time). It is now important to understand on general principles.

NT2:

This device would do the switching, permitting more than the standard eight devices to share the T bus by creating perhaps multiple S buses. Therefore, ISDN terminal equipment (TE) device can't really tell if it is connected to an NT1 or NT2.

TE1:

The terminal equipment type 1 (TE1) is a standard (there is that word again) ISDN terminal that is capable of dealing with the B and D channels. In other words, it can interface with the S/T bus.

TE2:

The terminal equipment type 2 (TE2) is a standard device having an RS−232 or V.35 interface. (In ITU parlance, this is called a V−series interface.) It may be intelligent, but it doesn't have an ISDN interface capable of handling the D and B channels.

TA:

The terminal adapter (TA) is the semi−intelligent device that lets a TE2 connect to the S/T ISDN interface. The primary function of the TA is to run the ISDN interface for our TE2. The functionality varies widely due to the manufacturers. Some are simple and support only one

TE2:

Others support two TE2s and an analog telephone. The TA need not be a stand−alone box; it can come on a PC card and plug into the computer's internal bus.  With the proper software to run the card, this instantly creates a TE1 out of your computer.

Thus, the BRI then was designed for small offices or home offices (SOHOs).

Applications of the ISDN Interface

Digital fax:

Fax machines now have to be digital. Therefore, the Group IV fax standard specifies 64 Kbps fax operation.

Telephone:

The obvious starting point is the telephone, which is now a digital   telephone. Instead of the telephone conversation being analog from the handset to the central office where it becomes digitized, the conversation can be digitized directly at the source and passed digitally all the way through the network to the other end.

Computer/video conference:

Our computer or video conferencing equipment can use one of the 64 Kbps or bond both bearer channels together for a 128 Kbps digital channel across the network.

Primary-rate ISDN

Primary Rate Interface (PRI) provides 23B + D and, in this case, all are 64 Kbps channels.

PRI is frequently used for a PBX interface where the full signaling capability of the D channel is needed

Signaling in D channel

The signaling packets on the D channel are the same for BRI and PRI.

As we indicated, Signaling System 7 (SS7) is the mechanism for managing the network and it's logical to simply let the D channel use SS7 packets

Conclusion:

ISDN, therefore, was a great technology−driven service that didn't really solve a business (or home need. It is little wonder that ISDN is not widely implemented or used, but there are, as we have noted, some clear exceptions. The most notable is video conferencing. Internet access is also a possibility, but ISDN can't compete with DSL technology in performance for the cost. The ISDN primary rate is used extensively in call centers, utilizing computer telephony integration to maximize their efficiency. PRI is also used in PBX applications, where the digital PBX can make use of the network control and status information provided by the PRI.
Source:
www.wikipedia.com
Broadband Telecommunications Handbook second edition by Regis J. "BUD" BATES