This read-only Counter tallies the number of frames discarded for carrying an unknown protocol in the egress direction, used only in SIW (service interworking) for PDUs that end up being dropped. An administrator monitors this on service-interworking channels to detect traffic carrying a protocol the interworking function does not recognize or support. For example, a rising xmtFramesUnknownProtocols count on an SIW channel would indicate that channel is receiving traffic in a protocol the interworking translation cannot handle and is therefore dropping.