Many organizations use a LAN for their internal data communications needs. Examples of these LANs include Ethernet/IEEE 802.3 and IEEE 802.5 (Token Ring). However, LANs typically offer a connectionless service, while ATM is always connection-oriented. This means that to use LAN-based applications using ATM, some form of LANE (LAN Emulation) is required.
In addition to the issue of connection-based versus connectionless service, LANs differ from ATM in the following ways:
This means that the address is constant, independent of the network to which the interface is connected.
LANE is an ATM service that offers the following services:
This permits applications running on ATM-connected end systems to interoperate with those running on traditional LAN-based devices. These LAN-based end systems can also communicate with each other across the ATM network.
This means that users cannot communicate between ELANs without the appropriate routing or bridging methods.
An Emulated LAN consists of the following components:
The LEC communicates with other LECs and with the LANE Service using Virtual Channel Connections (VCCs) in an ATM Switched Virtual Circuit (SVC) environment.
An ELAN comprises a group of ATM-attached devices, logically analogous to a group of LAN stations attached to an Ethernet/802.3 LAN segment. Several ELANs can be configured within an ATM network, and membership to an ELAN is independent of where the end station is physically connected. The end station can belong to multiple ELANs.
LANs use a MAC address to designate the source and destination addresses for stations. For LANE to function transparently, it must be capable of offering similar functionality. In practical terms, this means that each LEC has a MAC address, and when more than one LEC uses the same network interface, each LEC is assigned a different MAC address.
When the LEC needs to send data to another MAC address, it must first resolve that address to an ATM address, thus enabling it to establish a data direct VCC to that LEC. To do so, it sends an LE_ARP_REQUEST to the LES. The LES can either respond to this request, if it can, or it can forward it to other LECs. If the specified MAC address is known anywhere on the ELAN, the originating LEC gets an LE_ARP_RESPONSE frame containing the corresponding ATM address.
The LEC is part of an ATM end station or a MAC bridge and performs data forwarding as well as address resolution, among other control functions. The LEC supplies higher-level software with an Ethernet/IEEE 802.3 MAC layer interface that enables LAN-based application software to communicate over ATM networks just as they would over a traditional LAN.
LECs communicate with other clients using the LANE Service, and represent users by their MAC addresses. A LEC employs separate VCCs for data and control communication, including LE_ARP requests for address resolution. User data, intended for another end station, is encapsulated in IEEE 802.3 frames.
The LANE Service, consisting of a LES, BUS, and LECS, can be implemented as part of one or more end systems or as part of the ATM switch. In cases where the service is implemented in a distributed fashion over multiple devices, benefits include parallel operation as well as better error recovery through redundancy.
Within the LANE Service, the LES is responsible for coordinating the control functions, while the LECS serves network clients by supplying ELAN configuration information. Finally, the BUS forwards broadcast and multicast frames, and handles unresolved unicast frames.
The LES performs a number of control services within an ELAN, including registering MAC addresses and resolving these addresses to corresponding ATM addresses when requested. LECs can also request the LES to resolve a route descriptor to an ATM address. In either case, the LES responds to the client or forwards the request to other clients, which might be in a better position to service the query.
The LES also coordinates the process of a LEC joining an ELAN. There is a single LES per ELAN, and each LES has a unique ATM address.
The LECS maintains information concerning all the ELANs in an administrative domain, and supplies the LEC with the ATM address for the LES in the domain. Before joining an ELAN, the LEC first exchanges configuration information with the LECS.
Upon successfully communicating with the LECS, the LEC receives a list of ELANs that are available to join. There is a single LECS per administrative domain for ELANs within a domain.
The BUS accepts and processes data sent by a LEC to the broadcast MAC address `FFFFFFFFFFFF'. The BUS also handles all multicast messages, as well as initial unicast frames sent by a LEC before the ATM address has been resolved.
The BUS thereby offers services that emulate the shared medium capabilities typical of a LAN. The BUS does this by serializing the frames and retransmitting them to the appropriate LECs within the ELAN.
Although there might be multiple BUSes defined within an ELAN, each LEC is associated with only a single BUS per ELAN.
The User-Network Interface (UNI), which serves as an interface point between ATM end systems and the ATM switch, supports both automatic adapter failover and load balancing across multiple adapters connected to the same physical ATM switch. This means that the UNI signaling module automatically detects which adapters are connected to a single physical network and places all adapters connected to that network in a failover group.
With respect to load balancing, both incoming and outgoing connections are directed to the least used adapter in a load balancing group. In cases when an adapter in a load balancing group fails, the connections on that adapter are automatically transferred to another adapter.
UNI load balancing and adapter failover does not require any configuration. However, you can statically configure or disable UNI load balancing, if desired.
This release of Data ONTAP supports the following features and standards:
The software works with the FORE OC3 ATM network interface and on Network Appliance F520, F630, and F760 filers. The software provides Ethernet LANE services, with the capability to configure multiple ELANs on each available network interface.
The current release does not support ATM LANE 2.0, MPOA, CLIP (Classical IP), or Token Ring LANE services.
Copyright © 1999, Network Appliance, Inc. All rights reserved.
Part No. 210-02334
File: atm2.htm
Last updated: 04/26/99 08:11:23