This numeric field encodes protocol-specific connection identification for the source endpoint, whose internal structure depends entirely on the endpoint's protocol: for Ethernet it's a VLAN ID, for Frame Relay it's a DLCI, and for ATM it's a packed value with 12 bits of VPI and 16 bits of VCI. An admin must interpret and construct this value differently depending on circuitEndPoint1Protocol, since the same 32-bit field means something structurally different for an Ethernet circuit than for an ATM one. For example, on an ATM circuit an admin would compute this value by shifting the VPI into the upper 12 bits and combining it with a 16-bit VCI, whereas on a Frame Relay circuit they'd simply enter the DLCI number directly.