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Communication between a LEC and a LES is defined by the LAN Emulation User to Network Interface (LUNI) protocol. To facilitate this communication, VCCs are set up between the various LAN emulation service components. The VCCs used between the LEC and other LAN emulation service components are referred to as control VCCs. Control VCCs obtain configuration information, send or receive broadcast and unicast frames, and resolve the ATM address of a LAN destination. Control VCCs include Configuration-Direct, Control-Direct, Control-Distribute, Multicast-Send, and Multicast-Forward. VCCs used to transfer encapsulated IEEE 802.3 or 802.5 frames between LECs are known as Data-Direct VCCs.
The BUS and LES must be configured before the LEC can join the ELAN. To join the ELAN, the LEC establishes a connection to the LECS and obtains client configuration parameters, including the LES ATM address. The LEC then establishes a connection to the LES. As the LEC joins the ELAN, it registers a MAC address or route descriptor and an ATM address with the LES. The LEC can also register additional MAC addresses that are associated with the same ATM address. The LEC then uses the LAN emulation Address Resolution Protocol, LE-ARP, with the LES to obtain the BUS ATM address. A VCC connection is then established with the BUS. The BUS, in turn, establishes a multipoint circuit connection for forwarding broadcast and unicast frames to the LEC. At this point, the LEC is operational and can send data to and receive data from other LECs on the same ELAN.
A LEC also maintains a cache of ATM address information for LAN destinations to which it has been connected recently. When the LEC receives a frame for transmission, and the address is available in the cache, the frame is sent directly to the LAN destination. For LAN destinations that are not registered or are unknown, the LEC sends an LE-ARP request to the LES to obtain the ATM address. When the LEC receives a successful ARP_RESPONSE, it establishes a direct VCC to the LAN destination. This Data-Direct VCC is then used by the LEC for sending data.
The LEC forwards all broadcast frames, multicast frames, and initial unicast frames to the BUS. The BUS, in turn, sends the frames to all other LECs in the ELAN.
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