Microsoft KB Archive/105153

= AppleTalk Architecture Compared to the OSI Model =

Article ID: 105153

Article Last Modified on 10/31/2006



This article was previously published under Q105153



SUMMARY
The architecture of the Apple Computer AppleTalk operating system is designed for "plug and play." This article shows how AppleTalk architectural components correspond to the OSI Model.



MORE INFORMATION
     OSI Reference                     AppleTalk Architecture Model

7.  Application                                AppleTalk Filing     PostScript 6.  Presentation                               Protocol      | |        |                                                    |         |                       AppleTalk     (ZIP)      AppleTalk   Printer 5.  Session          Data Stream   Zone Inf.    Session    Access Protocol    Protocol     Protocol   Protocol |           |            |         |  |                          |---++-+  |  +--+  |                                  |              |  |  |         |                     Routing Table| AppleTalk    AppleTalk    Name 4.  Transport        Maintenance |   Echo      Transaction  Binding Protocol  | Protocol     Protocol    Protocol |     |    |            |           | 3.   Network         D a t a g r a m  D e l i v e r y  P r o t o c o l                           |      |    |            |           | |     |    |            |           | 2.   Data Link       L i n k   A c c e s s   P r o t o c o l    (LAP) This can be either Ether, Token or Local Talk AP                          |      |    |            |           | |     |    |            |           | 1.   Physical        Ethernet,Token Ring,LocalTalk or other hardware.

PHYSICAL/DATA LINK LEVELS
The greatest difference at OSI levels 1 and 2 is that AppleTalk developed two phases for the data link layer. Phase I and Phase 2 run different protocols and although they do exactly the same thing, the information in the frame fields changes. The Link Access Protocol, which uses this information, differs for each phase to accommodate this change.

A more significant change is that Phase I supported a limited number of physical/logical networks and zones, and did not support token ring networks. Phase 2 increases the number of addressable nodes allowable per network and includes token ring support.

NETWORK LAYER
The network layer is implemented using the datagram delivery protocol (DDP), which provides communication between sockets--the addressable entities within a node. Another protocol, the AppleTalk address resolution protocol (AARP), provides address translations between the hardware (data link layer) and the DDP (network layer).

TRANSPORT LAYER
The transport layer includes four protocols. The routing table maintenance protocol (RTMP) updates the Internet routers with current information about the network. The AppleTalk echo protocol (AEP) is used for maintenance and delay measurements and allows one node to send a datagram to another and have that node echo to the source. The name binding protocol (NBP) provides translations between character names and the corresponding Internet socket addresses on a distributed basis, and without a central database. Finally, the AppleTalk transaction protocol (ATP) provides reliable, sequential, socket-to-socket transmissions, plus "exactly once" transmissions.

SESSION LAYER
Four protocols are also available at the session layer. The AppleTalk session protocol (ASP) opens, maintains, and closes sessions between sockets. The AppleTalk data stream protocol (ADSP) provides reliable, byte-streamed service between two sockets. The zone information protocol (ZIP) maintains an Internet-wide map of the zones within the network, and maps zone names to specific network numbers. Finally, the printer access protocol (PAP) is used for transactions between network devices and Apple LaserWriter printers.

PRESENTATION/APPLICATION LAYERS
Two protocols are defined at the presentation and application layers. The first is the AppleTalk filing protocol (AFP), which handles remote file access. The second is PostScript, a language used by LaserWriter printers for desktop publishing.

