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Chapter 1 Overview
Avaya M770 M-ACCF/SF ATM Access Modules User’s Guide 7
Figure 1.2 illustrates the VCCs active among LAN Emulation Components.
Figure 1.2 VCCs in LAN Emulation Components
Frame Ordering
There are two paths for unicast frames between a sending LAN Emulation Client
and a receiving client: one via the BUS and one via a data direct VCC between them.
For a given LAN destination, a sending client is expected to use only one path at a
time, but the choice of paths may change over time. Switching between those paths
introduces the possibility that frames may be delivered to the receiving client out of
order. Delivery of out-of-order frames between two LAN endpoints is
uncharacteristic of LANs, and undesirable in an ATM emulated LAN. The flush
protocol ensures the correct order of delivery of unicast data frames.
Flush Protocol
When switching between paths, the sender first transmits a flush message down the
old path and suspends further transmission to that LAN destination. When the
flush message is returned by the receiving client (via control VCCs), the sender
knows that all previous messages for that LAN destination have been processed
and it can start using the new path.