This read-write field selects which ATM UNI signaling protocol version, UNI 3.0, UNI 3.1, or IISP, Interim Inter-Switch Signaling Protocol, this port uses for ATM call control signaling. An admin must set this to match exactly what the connected ATM switch or CPE expects, since UNI 3.0 and 3.1 have incompatible message formats in places, and a mismatch would prevent SVC call setup from working even though the physical ATM layer is fine. For example, connecting to an older ATM switch that only supports UNI 3.0 requires setting lportAtmProtocol to uni-30(1) rather than the newer uni-31(2) default many devices ship with, or SVC signaling between the two devices will fail.