3Com LANPLEX 2500 EXTENDEDPLEX®2500 EXTENDED User Manual page 81

OSI Reference Model
Application
Presentation
Session
Transport
Network
Link
Physical
Figure 7-2 AppleTalk Protocols and the OSI Reference Model
The AppleTalk six-layer protocol suite is not fully compliant with the OSI
seven-layer reference model. However, AppleTalk provides many of the
functions and services provided by OSI. Note that AppleTalk has no specific
protocols for the application layer, since the lower levels provide printer and
file service.
Physical The physical layer of the OSI protocol stack defines the network hardware.
Connectivity You can use standard network hardware, such as that defined for Ethernet
and Token Ring networks, with AppleTalk. Apple has also defined its own
network hardware, called LocalTalk, which uses a synchronous RS-422A bus
for communications.
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): TokenTalk LAP (TLAP), Ethernet
LAP (ELAP), and LocalTalk Link Access Protocol (LLAP).
The AppleTalk Address Resolution Protocol (AARP), which translates
hardware addresses to AppleTalk addresses, also exists at the datalink layer
AppleTalk
Zone Information
Data Stream
Protocol (ZIP)
Protocol (ADSP)
Routing Table
AppleTalk Echo
Maintenance
Protocol (AEP)
Protocol (RTMP)
Datagram Delivery Protocol (DDP)
TokenTalk ®
Link Access
Protocol
Token Ring
Hardware
AppleTalk Protocols
®
AppleTalk
Filing
Protocol (AFP)
AppleTalk
Session
Protocol (ASP)
AppleTalk
Transaction
Protocol (ATP)
EtherTalk ® LocalTalk
Link Access Link Access
Protocol Protocol
Ethernet
LocalTalk
Hardware
Hardware
7-5
PostScript ®
Printer Access
Protocoo (PAP)
Name Binding
Protocol (NBP)
®
®