Integrated Services Digital Network

(ISDN)

            A dated communication technology standard for simultaneous high speed baseband transmission for voice and data including video and fax over ordinary twisted pair telephone lines. It is the integration of both analog or voice data together with digital data over the same network.

              ISDN was a concept develop by the Consultative Committee on International Telegraph and Telephone (CCITT).

Interface Components:

NT1
We have mentioned the function of the NT1. It creates the T interface for premise devices (from the U interface).

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.

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.

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.)

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 U-Interface

The U interface connecting to the NT1 and the NT1 in turn creating the internal S/T bus to which up to eight ISDN devices can be connected. This U interface can be either a two−wire or a four−wire connection. In the following discussion, we concentrate on the two−wire connection because it is the more technologically challenging. The four−wire interface requires much less technology and can be delivered over a greater distance.

Physical Interface

 Three powering mechanisms are provided across the Interface:

• The costumer provides power to the NT1
• The carrier provides to the customer from the NT1
• The carrier provides a small amount of keep-alive power on the actual bus lead.

 Applications of the ISDN Interface

Multiple Channels

The plan is to provide access to every possible home device. The original concept was for up to eight devices. After all, you only have two B channels and one D channel to share among eight devices.

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.

Digital Fax

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

Analog Fax

Analog fax machines use a modem, so it has to plug into the telephone (or similar device) that would take the analog modem tones and digitize them at 64 KBps.

Computer/Video Conferencing

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.

Signaling

The primary function of the data channel is to provide for signaling, that is, the setting up and tearing down of the switched bearer channels. At 16 KBps, the data channel has more bandwidth than is needed for signaling alone.

Telemetry

The concept is that many household devices can be connected to the data channel.

Packet Switching

The 16 KBps data channel has bandwidth to spare. Therefore, the local carrier can provide a data service on this excess bandwidth.

ISDN Services

Basic Rate Interface (BRI)

Is the most common bandwidth available for Integrated Services Digital Network. Basic Rate Interface (BRI) uses the telephone twisted pairs and divided them into 3 channels.

Primary Rate Interface (PRI)

The Primary Rate Interface channels are carried on a T-Carrier line or an E-carrier line and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as video conferences.

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 xDSL technology in performance for the cost.