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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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LANs. Each separate LE layer needs a different MAC address, but can share the<br />

same physical ATM connection (adapter).<br />

LAN emulation server<br />

The basic function of the LE server is to provide directory, multicast, <strong>and</strong> address<br />

resolution services to the LE layers in the workstations. It also provides a<br />

connectionless data transfer service to the LE layers in the workstations, if<br />

needed.<br />

Each emulated LAN must have an LE server. It would be possible to have<br />

multiple LE servers sharing the same hardware <strong>and</strong> code (via multithreading),<br />

but the LE servers are logically separate entities. As for the LE layers, an<br />

emulated token-ring LAN cannot have members that are emulating an Ethernet<br />

LAN. Thus, an instance of an LE server is dedicated to a single type of LAN<br />

emulation. The LE server can be physically internal to the ATM network or<br />

provided in an external device, but logically it is always an external function that<br />

simply uses the services provided by ATM to do its job.<br />

Default VCs<br />

A default VC is a connection between an LE layer in a workstation <strong>and</strong> the LE<br />

server. These connections can be permanent or switched.<br />

All LE control messages are carried between the LE layer <strong>and</strong> the LE server on<br />

the default VC. Encapsulated data frames can also be sent on the default VC.<br />

The presence of the LE server <strong>and</strong> the default VCs is necessary for the LE<br />

function to be performed.<br />

Direct VCs<br />

Direct VCs are connections between LE layers in the end systems. They are<br />

always switched <strong>and</strong> set up on dem<strong>and</strong>. If the ATM network does not support<br />

switched connections, you cannot have direct VCs, <strong>and</strong> all the data must be sent<br />

through the LE server on default VCs. If there is no direct VC available for any<br />

reason, data transfer must take place through the LE server. (There is no other<br />

way.)<br />

Direct VCs are set up on request by an LE layer. (The server cannot set them up,<br />

because there is no third-party call setup function in ATM.) The ATM address of<br />

a destination LE layer is provided to a requesting LE layer by the LE server.<br />

Direct VCs stay in place until one of the partner LE layers decides to end the<br />

connection (because there is no more data).<br />

58 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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