ISDN also specifies what are known as reference points between different types of ISDN equipment. These are letters that are used to define logical interfaces between equipment like an NT1 and TE1 device, for example. The letters R, S, T, and U identify the different reference points, but I’ve always found it easier to remember them using the term “RUST”. The reference points described below are also illustrated in the figure below.
- The R reference point is used to describe the logical connection between non-ISDN devices (TE2) and a Terminal Adapter TA. If it helps you remember, just think of TE2 equipment as being “Retro”, since it predates ISDN standards.
- The U reference point is used to describe the logical connection between the service provider’s network and an NT1 device. Since an NT1 device is only found at the customer premises in North America, I always find it easiest to remember by designating it “United States”.
- The S reference point is used to describe the logical connection between ISDN-aware equipment (like TE1s and TAs) and an NT1 device. Because TE1s and TAs are ISDN-aware, I use the term “Standard” to remember these connections.
- The T reference point is used to describe the logical connection between an NT1 and an NT2 device. In my mind, this has always looked like a type of trunk link, so I remember the interfaces using the term “Trunk”.
As mentioned earlier, many vendors implement both NT1 and NT2 capabilities with a single device, and their interfaces are electronically identical. As such, the S and T reference points are often referenced by the combination S/T.
