Before sending your signal to the receiver,it should passed through the narrowband filter(whose band is narrow).the noise appearing at the output of the filter is narrowband noise..

Yes

Radio services that operate on 468 MHz may use both wideband (25 KHz) bandwidth and narrowband (12.5 KHz) bandwith until December 31, 2012. After that date, only narrowband bandwidth will be authorized.

For additional accurate updated information on the FCC's 2013 Part 90 Narrowband Mandate, please see link.

Here are some differences between broadband ISDN and narrowband ISDN:

1) Narrowband ISDN uses 64 kb/s channel, while broadband ISDN uses 100 mb/s channels.

2) Broadband uses call relay, while narrowband uses frame relay.

3) In narrowband, ISDN information carries narrow frequency, while in broadband, ISDN uses a wide band of frequency.

12.5 kHz