Virtual circuit multiplexing or VC-MUX is one of the two (the other being LLC encapsulation) mechanisms for identifying the protocol carried in ATM Adaptation Layer 5 (AAL5) frames specified by RFC 2684, Multiprotocol Encapsulation over ATM.
With virtual circuit multiplexing, the communicating hosts agree to send only one packets belonging to a single high-level protocol on any given ATM virtual circuit, and multiple virtual circuits may need to be set up.
It has the advantage of not requiring additional protocol-identifying information in a packet, which minimizes the overhead.
This reduction in overhead tends to further improve efficiency (e.g., an IPv4 datagram containing a TCP ACK-only packet with neither IP nor TCP options exactly fits into a single ATM cell).
The chief disadvantage of such a scheme lies in duplication of virtual circuits: a host must create a separate virtual circuit for each high-level protocol if more than one protocol is used.