Multiprotocol Encapsulation over ATM is specified in RFC 2684. It defines two mechanisms for identifying the

protocol Protocol may refer to: Sociology and politics * Protocol (politics) Protocol originally (in Late Middle English, c. 15th century) meant the minutes or logbook taken at a meeting, upon which an agreement was based. The term now commonly refers to ...

carried in ATM Adaptation Layer 5 (AAL5) frames. It replaces RFC 1483, a standard data link access protocol supported by

DSL modem A digital subscriber line (DSL) modem is a device used to connect a computer or Router (computing), router to a telephone line which provides the digital subscriber line (DSL) service for connection to the Internet, which is often called ''DSL ...

s. RFC 2684 describes two encapsulation mechanisms for network traffic: Virtual Circuit Multiplexing and LLC Encapsulation. Either mechanism carries either '' routed'' or '' bridged'' protocol data units, and DSL modems often include a setting for RFC 1483 bridging. This is distinct from other "bridge modes" commonly found in combined DSL modems and routers, which turn off the router portion of the DSL modem. In '' VC Multiplexing'' (VC-MUX), the

hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County * Host Island, in the Wilhelm Archipelago, Antarctica People * ...

agree on the high-level protocol for a given circuit. It has the advantage of not requiring additional information in a packet, which minimises the overhead. For example, if the hosts agree to transfer IP, a sender can pass each

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The de ...

directly to AAL5 to transfer; nothing needs to be sent besides the datagram and the AAL5 trailer. The chief disadvantage of such a scheme lies in duplication of

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 first established across the network between two endpoints. The network, rather than having a fixed data rate reser ...

s: 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. In ''LLC Encapsulation'' the hosts use a single virtual circuit for multiple protocols. This has the advantage of allowing all traffic over the same circuit, but the disadvantage of requiring each packet to contain

octets Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compos ...

that identify the protocol type, which adds overhead. The scheme also has the disadvantage that packets from all protocols travel with the same delay and priority. RFC 2684 specifies that hosts can choose between the two methods of using AAL5. Both the sender and receiver must agree on how the circuit will be used, and the agreement may involve manual configuration. Furthermore, the standards suggest that when hosts choose to include type information in the packet, they should use a standard IEEE 802.2 ''

Logical Link Control In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface ...

'' (LLC) header, followed by a ''

Subnetwork Access Protocol The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the eight-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by Et ...

'' (SNAP) header if necessary. The AAL5 trailer does not include a ''type'' field. Thus, an AAL5 frame is not self-identifying. This means that either the two hosts at the ends of a virtual circuit must agree ''a priori'' that the circuit will be used for one specific

(e.g., the circuit will only be used to send IP datagrams), or the two hosts at the ends of a virtual circuit must agree ''a priori'' that some octets of the data area will be reserved for use as a type field to distinguish packets containing one protocol's data from packets containing another protocol's data.

External links

ATM Forum

Understanding IPoEoATM and RFC 1483 bridging

Internet Standards

See also

External links