Network Termination Unit is a device for connecting digital ISDN customer premises equipment to ISDN lines allows customer premises equipment to be connected to the switching equipment at the telco’s central office. Read the full article.
What is NTU (Network Termination Unit)?
A NTU is a device for connecting digital Integrated Services Digital Network (ISDN) customer premises equipment to ISDN lines. The Network Termination Unit (NTU) allows customer premises equipment to be connected to the switching equipment at the telco’s central office (CO).
How It Works
Digital ISDN customer premises equipment such as routers and ISDN phones usually have an ISDN S/T interface, while the ISDN termination at the customer end of the local loop usually has a U interface using an RJ-45 connector. The NTU converts the U termination of the ISDN line to one or more standard coding S/T interfaces that are suitable for connecting digital ISDN-ready phones, routers, and other devices to an ISDN line. NTUs work differently than ISDN terminal adapters, which allow you to connect analog telephones, faxes, and similar equipment to your ISDN line. NTUs typically support either point-to-point or multipoint connections over distances of up to 5.4 kilometers on 26 AWG copper twisted-pair cabling.
Built-in U interface
Some newer ISDN-enabled devices have a built-in U interface, making an NTU unnecessary. The following table shows the differences between the Basic Rate ISDN U and S/T interfaces.
Basic Rate ISDN U Interfaces vs. Basic Rate ISDN S/T Interfaces
| Property | U Interface | S/T Interface |
| Number of wires | 2 | 4 |
| Connector | RJ-45 | RJ-45 |
| Zero-to-peak voltage | 2.5 V | 0.75 V |
| Line coding | 2B1Q (or 4B3T in Europe) | ASI (Alternate Space Inversion) |
