The DUS 31 is the first digital unit (we know) which supported multi-standard on the same DU. As you can run both LTE and GSM on the same unit, it basically combines a DUL and a DUG.
The GSM back-haul is no longer exposed via E1/T1 TDM lines, but uses Ericsson PacketAbis over IP, which is basically the L2TP/HDLC based protocol stacking that the SIU 02 or the TCU would have traditionally generated.
The unit has an internal 8GB USB Mass Storage device, as a Micron Embedded USB Mass Storage Drive (e230). You can think of it as a USB thumbdrive, but not with an exterenal USB A plug, but with a 9-pin 2-row 2.54mm pitch receptacle, which can be plugged directly onto USB 2.0 headers on mainboards.
The partition table looks as follows:
Disk ./dus31.dd: 7.56 GiB, 8120172544 bytes, 15859712 sectors Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x00000000 Device Boot Start End Sectors Size Id Type ./dus31.dd1 4 1999999 1999996 976.6M 21 unknown ./dus31.dd2 2000000 13999999 12000000 5.7G 21 unknown
The DUS31 combines the serial UART (traditionally "LMT A") and the management ethernet port (traditionally "LMT "B) in a single RJ45 connector. You need a special breakout cable to split those two signals. As it is a purely mechanical/electrical adapter without any active parts, it's relatively easy to build one DIY.
TODO: connector layout
We have created a small OSHW circuit board design of a break-out board, its design files can be found at https://git.osmocom.org/osmo-small-hardware/tree/dus-lmt-breakout