SUBJECT E7)

ATM Forum LAN Emulation V1.0 essentially provides for an emulation of an 802.3 (Ethernet) or 802.5 Token Ring network. FDDI is also supported. LANE provides a Layer 2 switching service which is mostly transparent to existing 802 networks and network stacks above the Datalink layer. One of the main purposes of LANE V1.0 is to allow ATM to be used in backbone campus networks to interconnect existing legacy networks, that is, provide a means for LANE compatible Layer 2 bridges and switches to take advantage of high-speed interconnection, transparent to the end systems, allowing every end system to benefit from the higher aggregate bandwidth. LANE systems include the LAN Emulation Client (LEC), LAN Emulation Server (LES) and Broadcast Unknown Server (BUS). ATM LANE V1.0 provides for "best effort" service only.

Classical IP over ATM is an IETF protocol which uses ATM's high speed ability to provide for better "wires" between IP members. The classic logical IP subnet (LIS) is implemented over an ATM switching network using an ATMARP server to replace the broadcast ARP service. IP over ATM is a Layer 3 switching service, in which IP and ARP datagrams are encapsulated in AAL5 using IETF RFC1483 LLC/SNAP encapsulation as the default. RFC1577 (IP over ATM) provides for "best effort" service only however, work is proceeding in the IETF for mapping the Resource Reservation Protocol (RSVP) over ATM. When in place, this will enhance classical IP for supporting RSVP signaling for obtaining differentiated QoS over an ATM network.

The purpose behind the ATM Forum's LAN-Emulation effort is to allow existing applications (i.e., layer-3 and above protocol stacks) to run *with no changes* over ATM. Thus, the mapping for all protocols is already defined. In a PC environment, such applications tend to run over an NDIS/ODI/etc. interface. The LAN-Emulation effort aims to be able to be implementable underneath the NDIS/ODI-type interface.

In contrast to LAN-Emulation, the IETF's scheme will allow IP to make better use of ATM capabilities (e.g., the larger MTU sizes), and for unicast traffic will be more efficient than having the additional LAN-Emulation layer.

For better or worse, I think both are going to be used. So, vendors may have to do both. The worse part is extra drivers (or extra code in one driver that does both). The better part is that all existing LAN applications can use one (LAN Emulation), and over time (as their mapping to ATM is fully defined) can transition to use the other (IETF Scheme).

NOTE: I suggest reading RFC 1932, "IP over ATM: A Framework Document", which summarizes many of the early discussions in the IP over ATM working group, especially with regard to various models of how to do IP over ATM, including the Classical approach. Also check out some slides of an interop presentation Crenville Armitage gave on Classical IP/ATM: http://gump.bellcore.com:8000/~gja/interop95/interop95.html

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