Key Concepts 539The AppleTalk six-layer protocol suite does not fully comply with the OSIseven-layer model. However, AppleTalk provides many of the functionsand services of OSI. AppleTalk has no specific protocols for theApplication layer because the lower levels provide printer and file service.Physical Layer ProtocolsThe Physical layer of the OSI protocol stack defines the connection withnetwork hardware. With AppleTalk, you can use standard networkhardware, such as that designed for Ethernet and token ring networks.Apple has also defined its own network hardware, called LocalTalk, whichuses a synchronous RS-422A bus for communications.Link Layer ProtocolsThe data link layer provides the interface between the network hardwareand the upper layers of the protocol stack. The AppleTalk data link layerincludes three link access protocols (LAPs):n TokenTalk LAP (TLAP)n Ethernet LAP (ELAP)n LocalTalk LAP (LLAP)The AppleTalk Address Resolution Protocol (AARP), which translateshardware addresses to AppleTalk addresses, also exists at the data linklayer because it is closely related to the Ethernet and token ring LAPs.AARP is usually included in the definition of each LAP, so it does notappear in the reference model. See “AppleTalk Address ResolutionProtocol (AARP) Cache” later in this chapter for more information aboutthis protocol.Network Layer ProtocolsThe network layer accepts data from the layers above it and divides thedata into packets to send over the network through the layers below it.The Datagram Delivery Protocol (DDP) transfers data in packets calleddatagrams.Datagram delivery is the basis for building other AppleTalk services, suchas electronic mail. With DDP, AppleTalk runs as a process-to-process,best-effort delivery system in which the processes running in the nodes ofinterconnected networks exchange packets with each other.
