What Is LAN Emulation?

LAN emulation (LANE) is a protocol that allows stations connected to an Asynchronous Transfer Mode (ATM) network to communicate as if they were attached to a conventional IEEE 802-style local area network. Defined by the ATM Forum in the mid-1990s, LANE enables existing Ethernet and Token Ring applications to run over ATM infrastructure without modification, by presenting each host with a familiar connectionless interface while handling the underlying ATM connection management transparently. LANE was a significant architectural bridge during the era when ATM was considered a candidate for high-speed campus and enterprise backbone networking.

The fundamental challenge LANE addresses is the mismatch between ATM's connection-oriented model and the broadcast-capable, connectionless behavior of IEEE 802 LANs. ATM requires a virtual circuit to be established before data can be exchanged, whereas Ethernet delivers frames to a shared medium that all stations can read. LANE resolves this by maintaining a set of server processes that map MAC addresses to ATM addresses and simulate the broadcast and multicast behavior that LAN applications depend on.

Architecture and Components

A LANE domain is organized around three server functions and one client role. The LAN Emulation Configuration Server (LECS) assigns each joining client to a specific emulated LAN. The LAN Emulation Server (LES) resolves MAC addresses to ATM addresses, performing a function analogous to ARP in IP networks. The Broadcast and Unknown Server (BUS) handles broadcast frames, multicast traffic, and initial unicast frames whose destination ATM address has not yet been resolved, forwarding them to all members of the emulated LAN. LAN Emulation Clients (LECs) run on each ATM-attached end station or edge device and implement the protocol that registers with the LECS, queries the LES, and sends data traffic through the BUS or through point-to-point virtual circuits once addresses are known. This architecture is documented in the ATM Forum LANE specification, which defined version 1.0 of the protocol in 1995.

Protocol Operation

When a LEC first joins an emulated LAN, it contacts the LECS to determine which LES serves its assigned emulated LAN. It then registers its MAC address with the LES. To send a frame to an unknown destination, the LEC issues an LE-ARP request to the LES; if the destination MAC is registered, the LES returns the corresponding ATM address and the sending LEC establishes a direct virtual circuit. Until the LE-ARP response arrives, or for broadcast traffic, frames are sent through the BUS, which forwards them to all LECs in the emulated LAN. As analyzed in the IEEE paper on ATM LAN emulation, the BUS can become a scalability bottleneck because all broadcast traffic passes through a single point, a limitation that influenced the design of LANE version 2 and its successor technologies.

Relationship to IEEE 802 Standards

Each emulated LAN corresponds to either an IEEE 802.3 Ethernet segment or an IEEE 802.5 Token Ring segment. The LEC presents a standard IEEE 802 MAC interface to the host operating system, so upper-layer protocols such as IP see no difference between a physical Ethernet and an emulated one. The IEEE 802.1 working group later developed Generalized LAN Emulation concepts that extended these principles beyond ATM to other transport substrates.

Applications

LAN emulation has applications in a range of networking and infrastructure contexts, including:

• ATM backbone migration, allowing legacy Ethernet devices to interoperate with ATM switches
• Enterprise campus networks where ATM was deployed for quality-of-service guarantees
• Multiprotocol networking environments requiring transparent bridging over ATM
• Historical network simulation and laboratory testing of legacy protocol stacks