The ISDN Reference model
ITU-T use a model with a system of alphabetical reference points to describe the connection point of equipment on the customer side of the NTE. It is useful to have an understanding of the model as it is often used to identify functional configurations.

The picture above illustrates the functional groupings and the various reference points which can be defined as follows:

NT1 Network Termination 1.
The connection point to the network. This provides line transmission termination, line maintenance functions and performance monitoring, timing, power feeding, multiplexing and contention resolution.

NT2 Network Termination 2.
Used to identify Customer Premises Equipment (CPE) that can provide a LAN, multiplexer or switching function.

TE1 Terminal Equipment 1.
This identifies terminal equipment that complies with ISDN Interface specifications and can be connected directly to the NT1 or the SIT bus.

TE2 Terminal Equipment 2.
This identifies non ISDN equipment which is not compliant with ISDN interface specifications and therefore cannot be connected directly to the BT Line Box or the SlT bus.

TA Terminal Adapter.
The TA can be used to interface TE2 equipment to the ISDN.

The Q, R, S, T and U Reference points are described in the following paragraphs.

Having looked at the reference model it is now possible to relate the various functional blocks and reference points to physical points within the customer's premises.

The figure above shows how the Reference Model can be related to BRA in a customers premises. The PRA reference points are described later.

The U Interface
This is a single pair (two wire) connection between the Local Exchange (LE) and the customers premises (subs line). This is terminated at the customers premises with a BT Line Box (NT1) which is the ISDN equivalent of the standard telephone interface.

There are two variants of the Line Box, the NTE6C and the newer NTE8, both provide two RJ45 sockets for connecting to the line. The Line Box also provides In/0ut Links and a Long/Short switch. The setting of these is dependent upon whether extension wiring is used and, if used, the overall length of the wiring.

Echo cancellation is used to allow two simultaneous transmission over the single single copper pair. A layer one protocol is used to convert the bit stream into a waveform for transmission between the local exchange and the NT1 . 3B2T is currently used in the UK between the NTE6 and IMUX. Where an embedded line card is used 2B1Q is used between the NTEB and the local exchange.

The Q Interface
This reference point is used to identify a Q SIG digital interface to another ISPBX.

The T Interface
The T interface identifies the connection between the NT1 and NT2 which for example would be a BRA PBX. Terminal equipment can be connected to the PBX via an S bus. As the use of NT2 equipment is optional, when there is terminal equipment connected to the NT1 this interface is often described as the S/T interface.

The S Interface
The S interface is normally found on the customers premises side of the NT2. It provides a passive bus connection for up to 8 equipment's.
The S or S/T interface is a four pair (eight wire) connection between the NT1 and the customers terminal equipment. One pair is used for transmit and a second pair to receive. The other two pairs are optional, allowing for various power feeding arrangements between the NT1 and terminals. Although ITU-T makes no recommendation on specific use of the optional two pairs they should still be provided to allow flexibility in system design.
The length of the bus is determined by the number of devices connected to it. There are three wiring configurations that may be used for the passive bus:

Point to Point - A single Terminal Equipment is connected to the NT1 which can be up to 1000m away.

Point to Multipoint - This provides access for multiple terminal equipment. There are two options available with the length of the bus being determined by the varying delays experienced by signals:

(a) Short Option - Any number of sockets may be connected randomly to the bus but a maximum of eight devices can be connected at any time. Using high impedance cable (150 ohms) the bus can be up to 200 metres in length, however, when 75 ohm cable is used the bus length is limited to 100 metres.

(b) Extended Option - With this option the bus can range between 100m and 1000m dependent upon the cable used, but would typically be 500m long. Any number of sockets can be connected to the bus but only four devices can be connected at any time all the terminal equipment's are clustered within 50m of the end of the bus.

Terminal equipment's may be connected to the network by using one of the three options. In all cases the bus (cable) must be terminated at the end furthest from the NT1. This is achieved by using a socket that includes 100 Ohm resistors connected across the transmit and receive pairs.

The different wiring options provide the customer with a great deal of flexibility. Numerous sockets connected to the same BRA can be located in different rooms. This enables up to eight devices to share the same access point and to be moved without the necessity for changes in the wiring.

In the UK the wiring between the NT1 and the terminal equipment is the responsibility of the customer. This flexibility in wiring will not only provide the customer with adaptability but will also produce cost savings in the long term.

The R Interface.
The R interface is the reference point for the connection of non ISDN compliant equipment to the S/T Bus.

Due to replacement costs, many customers may be reluctant to change all of their existing equipment for ISDN compliant equipment. The TA enables them to continue using their existing equipment by providing the means of connection to the ISDN.

One side of the TA connects to the S/T bus and the other can provide a variety of interfaces, for example, V24/V28,X21/V11 for data or an analogue port for a fax machine.


2 B Channels at 64 Kbits.sec = 128 Kbits/sec
1 D channel at 16 Kbits/sec = 16 Knits/sec
Total Channel rate (2B + D) = 144 Kbits/sec

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Last revised: October 30, 2022