Much like TCP/IP and IPX/SPX, AppleTalk can run over a variety of network technologies. Four main media-access specifications are implemented at the Data Link layer. These include:
EtherTalk. To allow AppleTalk protocols access to Ethernet networks, the Data Link layer specifies a protocol referred to as the EtherTalk Link Access Protocol (ELAP). ELAP first encapsulates upper-layer data into a Ethernet SNAP frame, and into a standard 802.2 frame.
TokenTalk. To allow AppleTalk protocols to access Token Ring / 802.5 networks, the Data Link layer specifies a protocol referred to as TokenTalk Link Access Protocol (TLAP).
LocalTalk. LocalTalk is a proprietary protocol developed by Apple that was originally designed as a workgroup network technology. Local Talk uses a media access method somewhat similar to Ethernet referred to as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Unlike Ethernet, when nodes on a LocalTalk network wish to communicate, they first go through a handshaking process, which reserves the network for their exclusive use. Collisions can still occur if more than one pair of systems attempts to begin the handshake process at the same time. LocalTalk does not use static MAC addresses – the Data Link layer assigns these addresses dynamically. The protocol used at the Data Link layer is referred to as LocalTalk Link Access Protocol (LLAP)
FDDITalk. To allow AppleTalk protocols to access FDDI networks, the Data Link layer specifies a protocol referred to as FDDITalk Link Access Protocol (FLAP).
Remember that the Data Link specification listed above are only used with the AppleTalk suite, and not when a Macintosh system uses other protocol stacks such as TCP/IP or IPX/SPX.
