ATM is a connection-oriented technology - think: "Build a pipe from host A to host B, use it, then tear it down when you're done"
Ethernet is broadcast-oriented - think:"scream real loud, if the other station hears it, it will respond.
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In order to emulate Ethernet on ATM (LANE: LAN-Emulation) you need some way to convert an Ethernet broadcast into a bunch of separate "pipes" to each station on that emulated LAN.
The system to accomplish that is the LECS, LES, BUS.
LECS - LAN Emulation Configuration Server
LES - LAN Emulation Server
BUS - Broadcast Unknown Server
The LECS sits (usually) on a well-known address. When a host first starts up, it looks to that well-known address and tells it " I belong to this ELAN (Emulated LAN)." The LECS responds by telling the host the address of it's LES and BUS (frequently the same machine/device).
The host then registers with that LES/BUS. All hosts for that ELAN register to that LES/BUS.
When a host, emulating Ethernet, needs to send traffic to another host/device, it gets the destination address from the LES (then uses the system to build a "pipe" (Virtual Circuit) to the destination.
In situations where Ethernet would usually broadcast (like an ARP), the LES/BUS will forward the broadcast on a a group of VCs to each host it knows about. If the desired host hears the request, it responds. If the host is off-ELAN, then (more or less) normal routing protocol-like processes propagate the request to other ELANs, LANs, or segments.
For a couple years, ATM was a hot backbone technology. It WAS the best you could get.
LANE and ELANs were a mechanism to allow a graceful transition from traditional Ethernet (or Token-Ring) to a native ATM environment.
Before ATM could really capture the market, Gig Ethernet hit the street and gained popularity ... passing ATM as a first-choice.
ATM is still widely used in carrier networks, because of it's desirable propagation characteristics.
The LECS exists to tell hosts what it's LES/BUS addresses are. You could configure an ATM LANE network without a LECS, but you'd have to manually configure each host with a specific LES/BUS address.
Hope this helps ... It's just a "scratch the surface" description .. to go deeper would require MUCH more space than provided.
Here's a link that may be helpful: http://www.cisco.com/univercd/cc/td/doc/cisintwk/itg_v1/tr1921.htm
Good Luck
Scott
