3Com 4007R Implementation Guide - Page 539
Physical Layer Protocols, LocalTalk LAP LLAP
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Key Concepts 539 The AppleTalk six-layer protocol suite does not fully comply with the OSI seven-layer model. However, AppleTalk provides many of the functions and services of OSI. AppleTalk has no specific protocols for the Application layer because the lower levels provide printer and file service. Physical Layer Protocols The Physical layer of the OSI protocol stack defines the connection with network hardware. With AppleTalk, you can use standard network hardware, such as that designed for Ethernet and token ring networks. Apple has also defined its own network hardware, called LocalTalk, which uses a synchronous RS-422A bus for communications. Link Layer Protocols The data link layer provides the interface between the network hardware and the upper layers of the protocol stack. The AppleTalk data link layer includes three link access protocols (LAPs): s TokenTalk LAP (TLAP) s Ethernet LAP (ELAP) s LocalTalk LAP (LLAP) The AppleTalk Address Resolution Protocol (AARP), which translates hardware addresses to AppleTalk addresses, also exists at the data link layer 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 not appear in the reference model. See "AppleTalk Address Resolution Protocol (AARP) Cache" later in this chapter for more information about this protocol. Network Layer Protocols The network layer accepts data from the layers above it and divides the data into packets to send over the network through the layers below it. The Datagram Delivery Protocol (DDP) transfers data in packets called datagrams. Datagram delivery is the basis for building other AppleTalk services, such as electronic mail. With DDP, AppleTalk runs as a process-to-process, best-effort delivery system in which the processes running in the nodes of interconnected networks exchange packets with each other.
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