Isdn

Continuing ISDN------
For enabling digital communication the singal should come in bits..10001 like that..this is decoded in 2b1q format..ie 2bits and one qua: .for this bits are grouped into 2 .Each two bits are represented by a particular voltage level.So there are 4 voltage levels available for representing the bits.they are +3,-3,+1,-1. If the bits are 00-->-3 v 01-->-1v 10-->+1,11-->+3 v.Thus only single voltage level is required to represent the signal.
The Components in ISDN architecture.

• Network Terminal Interface
• TE-1
• TE-2
• TA

TE-1 category of instruments are ISDN compatible instruments,like digital telephone,g4fax,Pc with isdn card.
TE-2 category of instruments are ordinary phone,ordinary fax ,pc etc
Exchange line is first connected to a network terminal interface .To this TE-1 Equipments can be connected.But TE-2 equipments cannot be connected directly to this.For connecting them an adapter should be there.that is called TA(Terminal Adapter).
So TA+TE-2=TE-1
Functions of Terminal adapter is to make the analog devices ready for connecting to Terminal Equipt-1.
Usually the NT will be provided by the operated itself.
There are Two channels involved in the isdn communication
They are Bearer Channels(B Channels) and Data Channels(D Channels).The bearer channels carry the voice,data,image,video...information and D channels carry the controls required for enabling the communication.
The bit rate of B channel is 64kbps and that of D channel varies from 16 kbps to 64 kbps.
depends upon the type of isdn connection provided.
Then comes the types of isdn connections

• BRI
• PRI

Basic Rate Interface and Primary Rate Interface.BRI supports 2 ISDN B channels and 1D Channel.so(2*64+16)=144kbps... and for PRI supports 30 B Channels and 1 D channels .So for a PRI connection the D channel has to carry the control and signal information of 30 B Channels.So there will be more than 1 NT for this.

For further reference pls go to

http://public.swbell.net/ISDN/overview.html