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What is the architecture of ISDN
ISDN stands for Integrated Services Digital Network. It can be used to bridge the Central Office’s local loop and the premise connection (home). It is a network in which digital switching links are used to connect digital signals.
Integrated Services refers to ISDN’s ability to deliver two simultaneous connections, in any merging of data, voice, video, and fax, over an individual line. There are several devices that can be connected to the line and sent as required.
An ISDN is a network expanding from telephony ISDN, which supports end-to-end digital connectivity to support a wide area of services such as voice and non-voice services, to which users have to make by a definite set of standard multipurpose user network interfaces.
ISDN Services
There are two essential ISDN services and they are as follows −
Basic Rate Interface (BRI) − It includes 2B + D channels which represent 2 Bearer channels of 64 kbps each for data and one D channel of 16 Kbps for handshaking and control. It can have a separate track for handshaking and control is known as “out of band” signaling.
Primary Rate Interface (PRI) − It consists of 23B + D channels that stand for 23 Bearer medium of 64 kbps each for data and one D medium of 64 Kbps for handshaking control. The Bearer channels can be ensured in any merger as required.
ISDN Structure
Given below is the structure of ISDN and OSI Model −
OSI Model ISDN Network DC Channel Data Link Physical B Channel
The D channel supports the OSI Network layer while the B channel uses both the Data Link and Physical Layer.
ISDN uses the line encoding in standard telecommunication 2B1Q, which represents 2 Binary elements encoded in 1 quaternary. A dibit (digital bit) defines two binary elements for each voltage change.
The following table displays the 2B1Q encoding used by ISDN −
| Dibit | Voltage |
|---|---|
| 10 | +3 |
| 11 | +1 |
| 01 | -1 |
| 00 | -3 |
ISDN Premise Connection
The following figure shows a basis ISDN connection from the Central Office to the premise −
The Central Office must have ISDN capabilities in the switch to link to an ISDN premise. The connection from the Central Office to the premises uses the current analog phone lines.
At the BRI parameter, a Network Termination 1 (NT-1) device changes the 2 wire analog line to a 4 wire system referred to as the S/T interface.
The S/T connection is a bus topology that removes in a 100W termination. An NT-2 device modifies the T interface into an S interface.
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