The 2Wire/Pace i3812 iNID (intelligent Network Interface Device) is needed for FTTN-BP installations (difference between FTTN & FTTP-BP explained in this thread: »FTTN and FTTN-BP? ).
It differs from an "IG" (Inside Gateway, such as 2Wire/Pace 3600, 3800, 3801) in the face the the iNID is an outdoor device, mounted on the outside of the residence either adjacent to or in place of the existing AT&T NID (Network Interface Device). The iNID terminates the 2 VDSL2 bonded pairs, houses the VDSL2 pair bonded modem, and then outputs any combination of IPTV, Internet, VoIP, and/or POTS. An "IG" is installed inside the house, like a typical modem, and terminates the VDSL2 signal inside and all devices using the IG connect to the IG inside
The output ports include a single Ethernet port (not typically used; disabled by default), an HPNA/Coax port (typically used to feed IPTV service inside the house), an HPNA/Twisted Pair port (typically used to feed Internet service inside the house via the 2Wire/Pace i38 Inside Access Point), and support for 2 voice lines, either 2 POTS lines, 2 VoIP lines, or 1 VoIP & 1 POTS line
The i3812 iNID does also need power, so an iPSU (Power Supply Unit for iNID) is installed indoors, typically across the wall from the iNID itself, and power wire is ran between the iPSU and the iNID to carry the power. If VoIP service is included, the iPSU also is installed with a Backup Battery.