Introduction to Data Communications
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48. ISDN - Integrated Services Digital Network

The Integrated Services Digital Network (ISDN) is a method used to bridge "the last mile" between the Central Office and the premise connection (home). ISDN uses the existing wiring so no new cabling is required.

There are two basic services offered:

ISDN lines can be dedicated lines that are always up and connected or they can be dial on demand (DOD) lines. When the line is required the connection is dialed up and made. The connection time for an ISDN line is very quick, in the order of 0.5 second or so. This can result in a substantial cost saving if used over long distance or paying by the minute. The line charges are only for when data is being transferred and not when it is sitting idle.


ISDN - OSI Layers

The D Channel uses up to the OSI Network Layer while the B channel uses both the Data Link and Physical layers.

ISDN - OSI Model

The line encoding is used by ISDN is standard telecommunication 2B1Q which stands for 2 Binary elements encoded in 1 quaternary. A dibit (digital bit) is represents two binary elements for each voltage change. The following table illustrates the 2B1Q encoding used by ISDN:

Dibit Voltage
10 +3
11 +1
01 -1
00 -3


ISDN Premise Connection

The following diagram illustrates a basic ISDN connection from the Central Office to the premise:

ISDN Premise Connection

The Central Office must have ISDN capabilities in the switch in order to connect to an ISDN premise. The connection from the CO to the premise uses the existing analog phone linges. At the BRI premise, a Network Termination 1 (NT-1) device converts the 2 wire analog line to a 4 wire system called the S/T interface. A PRI rate interface is The S/T interface is a bus topology that terminates in a 100W termination.

A total of 8 ISDN Terminal Equipment 1 (TE) devices are allowed to connect to the S/T Interface. If access is required to traditional analog devices such as a plain old telephone set (POTS) which are called Terminal Equipment 2 (TE2) devices, a Terminal Adapter (TA) can be used to provide access.


ISDN Advantages


ISDN Disadvantages


Introduction to Data Communications
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