BASICALLY ... AN ISDN SYSTEM IS DIVIDED INTO THE FOUR PARTS :
  1. COT -> CENTRAL OFFICE TERMINAL. THE TELEPHONE COMPANY'S LOCAL FACILITY THAT PROVIDES TELEPHONE SERVICE IN YOUR AREA.
  2. TA -> TERMINAL ADAPTER, THE DEVICE THAT CONNECT BETWEEN ISDN AND TELECOMMUNICATION EQUIPMENT.
  3. NTI -> NETWORK TERMINAL 1, THE DEVICE THAT CONNECT YOUR ISDN AND WORKS AS A CONVERTER BETWEEN AN ISDN U-INTERFACE AND AN ISDN S/T INTERFACE, SOME ISDN ADAPTORS HAVE A TA ALREADY BUILD INTO THEM (IT'S CALLED SMART ISDN) THIS IS EASIER AND LESS EXPENSIVE THAT AN EXTERNAL NT-1, BUT ... MAY PREVENT YOU FROM CONNECTING OTHER EQUIPMENT, TO YOUR ISDN LINE.
  4. TE -> TERMINAL EQUIPMENT, SUCH AS ISDN PHONE, ISDN TELEFAX, PC, ....