ATM is a network technology that combines the features of cell-switching and multiplexing to offer reliable and efficient network services. ATM provides an interface between the network and devices such as workstations and routers. The asynchronous nature of ATM means that bandwidth is made available on demand instead of allocating slots of transmission time to network devices, as it is in a synchronous system employing Time-Division Multiplexing (TDM).
ATM employs fixed-sized cells of 53 bytes each as the basic unit of transmission. Each cell consists of a 5-octet header, identifying the source of the transmission and other information, and a 48-octet payload containing the user data and headers for higher-level protocols. This architecture permits text, voice, graphics, and video to share the same network without any one source dominating network bandwidth.
ATM employs a star topology with an ATM switch acting as the hub of the network. All devices are connected directly to this hub, making network configuration and troubleshooting more straightforward, as well as offering dedicated bandwidth to the central switch.
You can use ATM in two ways on a filer:
The filer can simultaneously support FORE/IP and LANE over User-Network Interface (UNI) 3.0 or 3.1 on the same physical interface.
Note
Data ONTAP uses conventional IP routing table lookups for routing all traffic on a FORE/IP Asynchronous Transfer mode (ATM) interface. For more information, see About Data ONTAP routing.
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.
Many organizations use a LAN for their internal data communications. 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 is required.
LANE is an ATM service that offers the following benefits:
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.
Data ONTAP displays cause code strings when ATM connections for LANE Configuration Server, LANE Server, Broadcast and Unknown Server, or LAN Emulation Client normally or abnormally terminate. They describe the reason for the connection termination or rejection. For more information about these cause codes, see the ATM Forum's UNI 3.0 and 3.1 specifications.
An Emulated LAN comprises a group of ATM-attached devices, logically analogous to a group of LAN stations attached to an Ethernet/802.3 LAN segment. You can configure several Emulated LANs within an ATM network, and membership in an Emulated LAN is independent of where the end station is physically connected. The end station can belong to multiple Emulated LANs.
An Emulated LAN consists of the following components:
A LANE Client communicates with other LANE Clients and with the LANE Service using Virtual Channel Connections (VCCs) in an ATM SVC environment.
The LANE Client is part of an ATM end station or a MAC bridge. It performs data forwarding as well as address resolution, among other control functions. The LANE Client 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 it would over a traditional LAN.
LANE Clients communicate with other clients using the LANE Service and represent users by their MAC addresses. A LANE Client 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 LANE Server, BUS, and LANE Configuration Client Server, can be implemented as part of one or more end systems or as part of the ATM switch. When you implement the service in a distributed fashion over multiple devices, benefits include parallel operation as well as better error recovery through redundancy.
Within the LANE Service, the LANE Server is responsible for coordinating the control functions, while the LANE Configuration Server serves network clients by supplying Emulated LAN configuration information. The BUS forwards broadcast and multicast frames and handles unresolved unicast frames.
The LANE Server performs control services within an Emulated LAN, including registering MAC addresses and resolving these addresses to corresponding ATM addresses when requested. LANE Clients can also request the LANE Server to resolve a route descriptor to an ATM address. The LANE Server responds to the client or forwards the request to other clients, which might be in a better position to service the query.
The LANE Server also coordinates the process of a LANE Client joining an Emulated LAN. There is a single LANE Server per Emulated LAN, and each LANE Server has a unique ATM address.
The LANE Configuration Server maintains information concerning all the Emulated LANs in an administrative domain, and supplies the LANE Client with the ATM address for the LANE Server in the domain. Before joining an Emulated LAN, the LANE Client first exchanges configuration information with the LANE Configuration Server.
Upon successfully communicating with the LANE Configuration Server, the LANE Client receives a list of Emulated LANs that are available to join. There is a single LANE Configuration Server per administrative domain for Emulated LANs within a domain.
The BUS accepts and processes data sent by a LANE Client to the broadcast MAC address `FFFFFFFFFFFF'. The BUS also handles all multicast messages, as well as initial unicast frames sent by a LANE Client 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 LANE Clients within the Emulated LAN.
Although there might be multiple BUSes defined within an Emulated LAN, each LANE Client is associated with only a single BUS per Emulated LAN.
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.
Both incoming and outgoing connections are directed to the least used adapter in a load balancing group. If 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 do not require any configuration. However, you can statically configure or disable UNI load balancing.
LANs use a MAC address to designate the source and destination addresses for end stations. For LANE to function transparently, it must offer similar functionality. In practical terms, this means that each LANE Client has a MAC address, and when more than one LANE Client uses the same network interface, each LANE Client is assigned a different MAC address.
When the LANE Client 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 LANE Client. To do so, it sends an LE_ARP_REQUEST to the LANE Server. The LANE Server can either respond to this request or forward it to other LANE Clients. If the specified MAC address is known anywhere on the Emulated LAN, the originating LANE Client gets an LE_ARP_RESPONSE frame containing the corresponding ATM address.
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, F720, F740, and F760 filers. The software provides Ethernet LANE services, with the capability to configure multiple Emulated LANs on each available network interface.
The current release does not support ATM LANE 2.0, Multiprotocol Over ATM (MPOA), Classical IP (CLIP), or Token Ring LANE services.
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Part No. 210-00474
File: atm2.htm
Last updated: 11/17/03 15:56:18