To better appreciate connection-oriented ATM, it is helpful to review connectionless systems. LAN architecture, whether Ethernet, token ring, or FDDI, share certain characteristics. Each station is connected to the network via an adapter card, which has a driver, above which is a protocol driver, such as TCP/IP. In traditional LANs, such as Ethernet, the driver protocol is connectionless, meaning that the protocol driver simply provides a packet with a source address and a destination address and sends it on its way. Being joined by a common medium, each station will see the packets of data put on the wire by each of the others, regardless of whether the packet is passed sequentially, as in a ring topology, or broadcast, as with Ethernet. The primitive from the station to the wire, or from the protocol to the adapter, is simply "send packet."
Once the packet has been sent, according to the specifications of whatever LAN is being used, the adapter knows that the packet is visible to all stations on the network. Each station has an adapter card, which processes the packet and examines the destination address. If the address applies to that machine, the adapter does a hardware interrupt and accepts the packet. If not, the adapter parses it. Again, this is called connectionless because no logical connection to the recipient address was made, the packets were simply addressed and put onto the network.
A LAN network such as Ethernet offers very few services, because all an Ethernet card can do is take a packet and send it. Being connectionless, it can provide no guarantees or similar features. For example, it can't determine the status of the target machine. This is why developers rarely write applications directly to Ethernet. Rather, protocol drivers are used to enter sequence numbers, verify packet arrival (retransmitting, if necessary), partitioning big messages into smaller ones, and such—with all of these services adding time to the transmission, and with none of them able to provide end-to-end quality of service guarantees.