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), a formal agreement between nation states * Protocol (diplomacy), the etiquette of diplomacy and affairs of state * Etiquette, a code of personal behavior Science and technology ...

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. 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 to a telephone line which provides the digital subscriber line (DSL) service for connection to the Internet, which is often called ''DSL broadband''. The ...

s. RFC 2684 describes two encapsulation mechanisms for network traffic: Virtual Circuit Multiplexing and LLC Encapsulation. Either mechanism carries either ''

routed Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netwo ...

'' or '' bridged''

protocol data units In telecommunications, a protocol data unit (PDU) is a single unit of information transmitted among peer entities of a computer network. It is composed of protocol-specific control information and user data. In the layered architectures of c ...

, 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 People * Jim Host (born 1937), American businessman * Michel Host ...

agree on the high-level protocol for a given

circuit Circuit may refer to: Science and technology Electrical engineering * Electrical circuit, a complete electrical network with a closed-loop giving a return path for current ** Analog circuit, uses continuous signal levels ** Balanced circu ...

. It has the advantage of not requiring additional information in a

packet Packet may refer to: * A small container or pouch ** Packet (container), a small single use container ** Cigarette packet ** Sugar packet * Network packet, a formatted unit of data carried by a packet-mode computer network * Packet radio, a form ...

, 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 del ...

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 established within the network between two endpoints. The network, rather than having a fixed data rate reservation ...

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 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 IEEE 802.2 is the original name of the ISO/IEC 8802-2 technical standard, standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical ...

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 8-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by EtherT ...

'' (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