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مشاهدة النسخة كاملة : What is LAN Emulation all about?



A7med Baraka
04-15-2009, 08:49 PM
"LAN Emulation" is a work in progress in the ATM Forum. There is not a
complete agreement on all aspects of LAN Emulation, but there is good agreement
on the requirements and general approach. Here's the basics:

The organizations working on it say LAN Emulation is needed for two key reasons
1) Allow an ATM network to be used as a LAN backbone for hubs, bridges,
switching hubs (also sometimes called Ethernet switches or Token Ring switches)
and the bridging feature in routers.

2) Allow endstations connected to "legacy" LANs to communicate though a
LAN-to-ATM hub/bridge/switch with an ATM-attached device (a file server, for
example) without requiring the traffic to pass through a more complex device
such as a router. Note that the LAN-attached device has a conventional,
unchanged protocol stack, complete with MAC address, etc.

LAN Emulation does not replace routers or routing, but provides a complementary
MAC-level service which matches the trend to MAC-layer switching in the hubs
and wire closets of large LANs.

The technical approach being discussed in the Forum among companies with
interest and expertise in this area include three elements:

1) Multicast/broadcast support
Since almost all LAN protocols depend on broadcast or multicast packet
delivery, an ATM LAN must provide the same service. Ideally, this would use
some sort of multipoint virtual circuit facility.

2) Mac address to ATM address resolution.
There are two basic variations being discussed:
a) do an ARP-like protocol to ask for a mapping from Mac address to ATM address
b) send packets to some sort of directory or cache server that sends the
destination ATM address back to the source as a sort of side effect of
delivering the packet.

3) Switched Virtual Circuit Management
It is generally desireable (for scalabilitiy, quality of service, etc) to
set up point-to-point virtual circuits between endpoints that want to
communicate with each other (client to file server, for example) once
the two atm addresses are known. To make this work in the existing legacy LAN
environment, we don't have the freedom to push knowledge or management of
these virtual circuits up above the MAC level (no protocol changes, remember?)
so the logic to resovle for an ATM address and set up a virtual circuit on
demand must be in the LAN Emulation layer. This would include recognising
when an SVC to another ATM endpoint already existed, so that the same circuit
could be used for other traffic.

4) Mac definition
The actual packet format would be some variant of the packets used on existing
networks. For example, a packet leaving an Ethernet to go over a virtual
circuit to an ATM-attached file server would probably be carried directly
over AAL5, with some additional control information.