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
ATM Adaptation Layer 5 (AAL5) is an ATM adaptation layer used to send variable-length packets up to 65,535 octets in size across an Asynchronous Transfer Mode (ATM) network.
Unlike most network frames, which place control information in the h ...
(AAL5) frames specified by {{IETF RFC, 2684,
Multiprotocol Encapsulation over ATM Multiprotocol Encapsulation over ATM is specified in RFC 2684. It defines two mechanisms for identifying the protocol carried in ATM Adaptation Layer 5 (AAL5) frames. It replaces RFC 1483, a standard data link access protocol supported by DSL modem ...
.
With
virtual circuit
A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is established within the network between two endpoints. The network, rather than having a fixed data rate reservation ...
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. For example, if the hosts agree to transfer IP, a sender can pass each datagram directly to AAL5 to transfer, nothing needs to be sent besides the datagram and the AAL5 trailer. This reduction in overhead tends to further improve efficiency (e.g., an
IPv4
Internet Protocol version 4 (IPv4) is the fourth version of the Internet Protocol (IP). It is one of the core protocols of standards-based internetworking methods in the Internet and other packet-switched networks. IPv4 was the first version d ...
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. Because most carriers charge for each virtual circuit, customers try to avoid using multiple circuits because it adds unnecessary cost.
It is commonly used in conjunction with
PPPoA
In computer networking, the Point-to-Point Protocol over ATM (PPPoA) is a layer 2 data-link protocol typically used to connect domestic broadband modems to ISPs via phone lines. It is used mainly with DOCSIS and DSL carriers, by encapsulating PP ...
which is used in various
xDSL
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric dig ...
implementations.
Internet Standards