Functional devices:

ISDN standards define different types of devices. Each device type has certain functions and reposibilities, but may not represent an actual physical piece of device. For that reason. the standards call them functional devices.

LE: The network device that provides ISDN services is the LE. ISDN protocols are implemented in the LE, which is in the network side of the ISDN local loop. The functions of LE can be further broken down to local termination (LT) and exchange termination (ET). LT handles the termination of the local loop, while ET handles switching funtions. LE is also reponsible to distinguish user data on the D-channel from signalling data and routes the user data towards its destination.

NT1: Network termination type 1 or local loop terminator equipment, represents the termination of the physical connection between the customer site and LE. Its reposibilities include performance monitoring, timing, physical signalling protocol conversion and power transfer.

NT2: Network termination type 2 are those devices providing customer site switching, multiplexing and/or concentration. This includes PBX, multiplexers,host computers, terminal controllers and other CPE for voice and data switching. NT2 is used only in ISDN PRI.

TE: Terminal equipment refers to end-user devices such as an analog or digital telephone, X.25 DTE, ISDN workstation, etc. Terminal equipment type 1 (TE1) are those devices that use the ISDN protocols and support ISDN devices. Eg: ISDN telephone or ISDN workstation. Terminal equipment type 2 (TE2) are non-ISDN compatible devices such as analog telephones.

TA:  A terminal adapter allows a non-ISDN device to communicate with the network. TA allow analog phones, X.25 DTEs, PCs and other non-ISDN devices to use the network by providing any necessary protocol conversion.
 
 

 
 
 
ISDN Reference Points:

The ISDN reference points define the communication protocols between the different ISDN functional devices. The importance of reference points is that different protocols may be used at each reference point. Four protocol reference points are commonly defined for ISDN called R, S, T, and U. The figure given above shows the functional devices and reference points used in ISDN.

The R reference point is between non_ISDN terminal equipment(TE2) and a TA. The TA will allow the TE2 to appear to the network as an ISDN device.

The S reference point is between ISDN user equipment (i.e. TE1 or TA) and network termination equipment (NT2 or NT1).

The T reference point is between the customer site is betwewn customer site switching equipment (NT2) and the local loop termination (NT1).

The U reference point is not a ITU standard and is defined by FCC. It is between the NT1 and the LE.