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ATM is a a connection oriented protocol. This means that connections are set up prior to transmission of data by either PVC (Permanent Virtual Circuits) or by SVC (Switched Virtual Circuits). Permanent, as the name implies is a point to point type circuit that is down established or torn down at the end of a transmission. ATM transmits its data via cells as opposed to packets. Cells contain more data than a typical packet.
ATM also has prioritization built in so that data can be prioritized based on either user defineable or type of packet transmitted. Many of the Bell companies use ATM in their central offices. Ethernet over ATM allows these companies to map Ethernet VLANS to PVCs and offer ethernet type services over their ATM networks. This provides not only scaling but a myriad of configuration possiblities. Not only that, but Ethernet equipment is less expensive than ATM equipment and this provides them and their end users with savings as well. ATM now understands prioritization schemes of ethernet such as QoS (Quality of Service), etc. EoATM also allows them to offer Ethernet in the last mile for companies in various bandwidth configurations.
I hope this answers your questions. If you have any follow on questions, please let me know.
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