Other changes There are three NIUs NIUs are the cards inside of the two slot mounting cases not the case themselves in the first picture not two The single card in the left case looks to be either a HDSL or HDSL4 NIU The left card in the left case is a HDSL2 NIU More accurately those are HTU R or H2TU R or H4TU R cards that terminates HDSL data and outputs a DS1 signal functionally similar to older T1 NIU but are not compatible with a T1 system Explanatory information on variants T1C two concantenated T1s etc From a service provider to a customer it would look something like DSX 1 ORB 3kft RPTR 6kt RPTR NIU CSU DSU CPE showing the spacing between the office repeater in the office repeater bay ORB and the first line repeater as 3000 feet 3Kft and spacing between subsequent repeaters as 6Kft Emphasis at least orginally on T1 as being a span system Designed as a short haul interoffice transport for use inside the telephone companies and as part of T Carrier System linking two D1 channel banks D1 bank DSX 1 X DSX 1 ORB RPTR RPTR ORB DSX 1 X DSX 1 D1 bank Links to other analog carrier systems showing T1 development in context