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LAN Emulation, Virtual LANs And
The Reality Of ATM
by Art Wittmann
When you hear about ATM, you usually hear about LAN emulation and virtual LANs, which many experts think will be part of the next generation of network infrastructure. Virtual LANs let you configure your network much more flexibly, while LAN emulation is a vehicle to support existing LAN hardware and software over ATM -- including virtual LAN technology.
Virtual LANs got their start as part of the switching hub craze, which in turn was a response to expensive routers and was generally nothing more than link-layer bridges designed to get your network more bandwidth. But unlike routers, switches did not offer a way to restrict one part of the network from seeing the broadcasts of another part.
The Choice of a New Generation
Switching hub vendors saw the problem and considered different approaches.
One solution was to implement very basic routing. Routers almost always relate LANs or subnetworks to the physical ports on the router, so taken to its logical limit, every port on the switch would get its own set of network numbers just like a router. That is most definitely undesirable. Network managers do not want to administer a network with that many numbers.
By considering groups of ports on a switch as a virtual LAN, you can put network users into working groups regardless of their physical location. These working groups can share data within the group without worrying about others seeing it, and you can rapidly reconfigure them to meet changing needs.
Since we're talking about switches, normal station-to-station traffic will only be seen by the stations involved a
nyway. But broadcasts and multicasts would normally be propagated to all ports on the switch if the switch knew nothing other than data-link bridging. So virtual LANs are, in effect, nothing more than broadcast and multicast groups.
Virtual LANs Aren't the Perfect Solution
All this is great if you can run your entire network on a single switching hub. The mechanisms for creating virtual LANs are proprietary, and nobody has attempted to retrofit them onto existing backbone technologies like FDDI or Ethernet. These LAN topologies don't have a defined mechanism for identifying the virtual LAN to which a packet belongs.
In other words, there is no way to identify packets for particular working groups without using normal network-layer protocols like TCP/IP or IPX. But, in fact, that is typically how it's done. To move traffic off a switch, or even between virtual LANs on a switch, you'll need a router. In this case, the switch polices the virtual LANs, and most of the bandwidth, but a router moves traffic between working groups and polices the rules for which packets can move between virtual LANs.
But ATM is a different story. ATM is a truly revolutionary technology, not because frames are broken down into small cells, but rather for the way it handles data and data paths. It turns out that ATM's virtual circuits are a natural mechanism for supporting virtual LANs over a backbone. LAN emulation is the key to making this happen today.
The Revolutionary Approach
In routed and bridged networks, each packet contains a fairly lengthy data-link level packet header and a network-layer header. Each time a packet comes by a bridge, the bridge determines what to do with the packet by looking at the source and destination address in the packet header. Routers do the same with each packet's network layer information. The process is repetitive and consumes time and valuable resources.
With ATM, the data path determination and data transmission are separate. When a station wants to communica
te with another, it requests a connection to the destination and starts sending data once the data path is established. As long as there will be some reasonable amount of data transmitted, any burden created by separating the path discovery from data transmission is outweighed by efficiencies in using the discovered path.
But there is just too much legacy networking to expect that ATM will be adopted lock, stock and barrel. The ATM Forum members realized the need to transition from what we have to ATM and came up with LAN emulation as a way to use ATM with existing protocols and equipment.
How LAN Emulation Works
The LAN emulation configuration server takes requests from LAN emulation clients and provides information on the LAN type being emulated and which LAN emulation server to use. In its request, the LAN emulation client must provide its ATM address as well as its 6-byte LAN address. The configuration server then provides the ATM address of the LAN emulation server, to which the client connects in order to join the emulated LAN.
End stations and proxies are treated the same, until they start communicating with the LAN emulation server. The problem is that if the proxy is a bridge, it cannot know the MAC address of all the stations it may serve. The bridge learns about stations over time.
An end station only needs to join the emulated LAN and set up the virtual circuits to communicate, and the virtual circuits used to control the connection. It must inform the LAN emulation server about such things as its desired maximum frame size and LAN type. A proxy client must register as a proxy.
One of the jobs a LAN emulation server may tackle is translating MAC addresses into ATM addresses, or passing MAC addresses on to some device that can translate them.
One solution allowed under LAN emulation for bridges involves a special virtual circuit for transmitting all address resolution requests that the LAN emulation server can't resolve. The bridge can respond if the address is alread
y in its address tables, then employ its normal mechanisms to attempt to resolve the address for the emulation server. Otherwise the bridge does nothing; if it does nothing, the client must use the BUS (broadcast and unknown frame server) to broadcast unknown frames.
The BUS, in addition to assisting in address resolution, helps facilitate the delivery of small numbers of unicast packets-for example, those packets occasionally sent by network management stations, where a virtual connection isn't really warranted. The BUS also delivers broadcasts and multicasts.
The result: LANs running over ATM; and the once proprietary virtual LAN that existed only in the single switch can encompass the entire network. At the same time, ATM allows the backbone of the network to scale to handle almost any amount of traffic.
The technology is not quite here today, but this is where the industry is headed, and by year's end we should begin to see products.
Art Wittmann is a senior editor of Network Computing and associate director of the Computer Aided Engineering Center of the University of Wisconsin-Madison. He can be reached via the Internet at wittmann@engr.wisc.edu
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