More commonly stands for National ISDN, less commonly for Narrowband ISDN.
Narrow-ISDN (to distinguish it from Broadband ISDN). ISDN by any other name.
Narrowband Integrated Services Digital Network: Services include basic rate interface (2B+D or BRI) and primary rate interface (30B+D - Europe and 23B+D - North America or PRI). Supports narrowband speeds at/or below 1.5 Mbps.