The current status of the ATM Forums LAN emulation standard development

The current status of the ATM Forum`s LAN emulation standard development

The LAN emulation standard will allow organizations to hook up their legacy LANs to an ATM network without giving up existing applications

John Keene

Interphase Corp.

As it runs across fiber or copper, asynchronous transfer mode networking technology will bring voice, video and data transmissions across the world and to the desktop. ATM has several advantages over existing technologies because voice, video and data simultaneously move at megabyte to gigabyte speeds across a seamless local area network/wide area network. In communications applications, ATM has inspired a convergence of technologies that blurs the distinctions between telephony and data networks. For today`s fiber and copper networks, ATM allows a clear migratory path from their current installations.

For examples, the University of North Carolina at Chapel Hill and the University of Hawaii are experimenting with ATM to develop networks that will support telemedicine and teleradiology applications. UNC`s applications involve health care, which link physicians for medical imaging and remote consultations. Ultimately, these services will extend to other disciplines, linking healthcare professionals to patients` homes.

The University of Hawaii is using ATM to extend to remote areas the medical school resources. The university recently tested its telemedicine capabilities with GTE, Newbridge Networks and Apple Computer. In the demonstration, GTE provided fiber-optic lines and Newbridge ATM switches to the University of Hawaii medical facilities in Manoa, which ran a test of remote medical consultation and diagnosis for an ill patient between the university and the Queens Medical Center in Honolulu. The physicians were able to discuss the case over video, exchange medical images--including X-rays and lab reports--and search the medical database for literature on the condition. The application ran on Macintosh computers linked by Ethernet. Eventually, an ATM network could be used to connect clinics and doctors throughout the archipelago.

During the May Interop/Networld and Supercomm shows, more than 30 telecommunications and data communications companies that comprise the ATM Forum joined together to demonstrate that ATM applications can run across public networks and that basic connectivity is an achieved goal, not a future possibility.

There is still much work to be done, however. Nine subworking groups ascertain the activities of the ATM Forum and are proposing new applications.

These subworking groups are charged with determining the standards for ATM operation in a variety of situations, including interactions between public and private networks and how to adapt ATM to current networking technologies. Like fiber distributed data interface, ATM will likely be used as a backbone technology by many organizations, which raises additional issues of making Ethernet or token-ring local area networks and current FDDI networks connect to and interact with the ATM network.

One subworking group of the ATM Forum in particular, LAN emulation, is writing the standards to allow existing applications to move across an ATM network, whether it be fiber- or copper-based. In other words, the Ethernet LAN is not aware that its messages are going across an ATM network, but the Ethernet message protocols are read as ATM and run across the fiber as an ATM message.

The LAN emulation standards allow ATM-station-to-ATM-station connectivity; Macintosh-level driver compatibility; and both transparent and source route bridging. This month, the completed standard is expected to go to the entire ATM Forum for an acceptance vote. To understand what impact this standard will have on companies that have mixed media networks, major issues must be addressed, including the need for Ethernet networks to communicate with ATM and how ATM network developments by companies with legacy LAN networks will be affected.

Most companies contemplating ATM have high-performance networks in place, often with fiber at the backbone. Early implementers of this technology will look to leverage their existing investment in fiber technology while evolving their organizations to ATM networks.

What is LAN emulation? It provides for all existing LAN applications to run over ATM. In addition, it allows the use of ATM as a backbone to interconnect existing legacy LANs. It also provides for the interconnection of ATM-attached servers/workstations and to those on legacy LANs. Finally, LAN emulation allows multiple emulated LANs that are logically separate to use the same physical ATM network. LAN emulation is crucial for any organization that has Ethernet or token-ring networks. FDDI connectivity requires the same types of solutions that exist today among FDDI and Ethernet and token ring. For example, ATM connectivity to FDDI requires a bridge with an ATM port and an FDDI port that converts an emulated token ring or emulated Ethernet frame type to an FDDI frame type or vice versa.

Significant differences among Ethernet, token ring and ATM make LAN emulation crucial. For example, ATM does not support broadcast and uses a 20-byte address. Ethernet supports broadcast and has a 6-byte Macintosh address. LAN emulation will combine these technologies.

One function LAN emulation will not serve is to transfer Ethernet to token ring or vice versa because they differ at the frame level. The end system of an emulated LAN transmission has to know what frame it is receiving (token ring or Ethernet); it does not work on a frame-by-frame basis. For this reason, ATM will still require translation devices--some kind of bridge--for networks to talk to each other.

The ATM Forum has determined the following requirements for the LAN emulation standard:

Based on user network interface 3.0 signaling specification.

Require a high-performance, highly scalable facility backbone.

Provide for protocol-independent switching across emulated LANs (can bridge between them instead of route).

Work with high-performance, low-latency, distributed workgroup environments, such as client/server.

Provide seamless internetworking with legacy LANs via bridges.

Work with permanent virtual circuits only, switched virtual circuits only, or in combination.

What LAN emulation will not fix or solve today are existing Ethernet/token ring/FDDI bridging problems, nor will it allow a station to receive all frames on a logical LAN. It will also not support existing Mac-layer protocols such as carrier sense multiple access/collision detection, which provides for collision avoidance in Ethernet networks, station management for FDDI rings or token management for token-ring networks. Also outside the scope of this standard is server-to-server LAN emulation.

The LAN emulation standard that is expected to gain approval by the ATM this month will allow organizations to hook up their legacy LANs---whether copper- or fiber-based or both---to an ATM network without giving up existing applications. The standard allows ATM to be used as a backbone to interconnect legacy LANs and ATM-attached hosts. In the future, however, organizations that wish to exploit the power of ATM at the desktop level will have to migrate to fully ATM networks to connect directly and take advantage of new ATM applications. u

John Keene is technical director at Interphase Corp., Dallas.

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