This read-write object selects the terrestrial data interface type from options including EIA-422/EIA-530(0), V.35(1), EIA-232(2), and several G.703 T1/E1 variants(3-8+), noting that the G.703 selections force the Tx/Rx data rates to fixed values, 1544 kbps for T1 and 2048 kbps for E1. This must match the physical interface cabling and the equipment actually connected to the modem's terrestrial port, and the documented side effect that selecting a G.703 T1 or E1 option automatically forces the data rate objects to a fixed rate means those objects can't be independently tuned once one of those interface types is selected. A site connecting the modem to a T1 multiplexer would set this to one of the T1 options depending on the line coding the mux expects, and would then find the data rate objects automatically locked to 1544 kbps rather than freely adjustable.