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Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

AppleTalk has been the networking protocol for the Macintosh nearly as long as the Macintosh has been around. It provides a way for individual users to share files via AppleShare servers. It also supports printer sharing and remote access. But AppleTalk is showing its age. It was designed in the early 1980s when LANs were primarily for file exchanges and printer sharing. Apple is even downplaying the protocols in favor of TCP/IP. Its Macintosh Operating System (Mac OS) version 8.5 includes the full TCP/IP suite and an enhanced AppleShare server called AppleShare IP that provides file sharing via AFP (Apple Filing Protocol) over TCP/IP. That means Macintosh users can access AppleShare file servers over a TCP/IP network.

Apple will continue to support AppleTalk networking in future operating systems through its transport-independent networking suite called Apple Open Transport, which supports not only AppleTalk but TCP/IP and, eventually, other protocols such as IPX.

Still, AppleTalk is suitable for workgroups of Macintosh users who don't need sophisticated networks that need to support hundreds of users. AppleTalk is installed in many universities and organizations, and will probably not go away soon. In addition, AppleTalk is available in a number of other incarnations on UNIX systems.

AppleTalk is the architecture that defines a set of protocols used by devices to communicate over the network. Originally, AppleTalk was designed to use the LocalTalk cabling system, but Apple added support for Ethernet and token ring networking. AppleTalk was also designed for small local workgroups, not for large LANs and WANs, although Apple made some changes to improve speed.

The early LocalTalk cabling system consists of a set of simple connectors that attach to Macintosh systems and provide plug attachments for LocalTalk cable or telephone wire. Transmission rates are a relatively slow 230.4 Kbits/sec, but the total installation cost is low since network adapter cards and workstation software are not required. LocalTalk was originally called AppleTalk, but Apple changed the name in 1989 to LocalTalk and now refers to the networking protocols as AppleTalk.

AppleTalk Phase 2 was introduced in 1989 with the primary addition of extended addressing. Extended addressing increases the node identification number from 8 bits to 16 bits on EtherTalk and TokenTalk networks only. This overcame the previous restrictions of 254 nodes per network and allowed up to 16 million nodes. LocalTalk is referred to as nonextended AppleTalk and is limited to 254 nodes per cable.

Apple introduced Apple Open Transport in 1995 to provide Macintosh users with a way to use a variety of protocols, including TCP/IP. With support for TCP/IP, Macintosh users gained easy connectivity to the Internet and other TCP/IP networks. See "Apple Open Transport Protocol" for more information.

AppleShare is a centralized file-sharing system that runs on top of AppleTalk on a Macintosh system that has been dedicated for server use. Other Macintosh systems access AppleShare servers through the AppleShare client module. Services provided by AppleShare include file sharing, printer sharing, and electronic-mail handling. Additional modules such as management systems are available. AppleShare servers are accessible via the AppleTalk Filing Protocol.

Apple introduced the Macintosh System 7 operating system with peer-to-peer (distributed) file sharing for AppleTalk environments. This allowed users to share files on their own system (instead of a separate AppleShare server) with other users. However, the file-sharing features in System 7 lacked security and were somewhat deficient. As mentioned earlier, Apple introduced Mac OS 8.5 in late 1998, which includes full TCP/IP support and less emphasis on AppleTalk.

AppleTalk Operation and Protocol Stack

Devices attached to extended AppleTalk networks use two-part addresses that consist of a network number and a node number. The network number occupies two bytes and has a range of 1 to 65,535. The node number occupies one byte and has a range of 1 to 253. This scheme provides a range that allows for over 16 million nodes, although the physical limitation of the cable won't actually support that many nodes. Node addresses are mapped to names that make the network easier to access by humans. These names appear in the Macintosh graphic interface so that users can scan for users or resources they want to contact or connect with. Name mapping is handled by the NBP (Name Binding Protocol), as discussed later.

AppleTalk networking is based around the concept of zones, which are logical areas that include groups of users and resources. A zone may extend across multiple networks. Routers separate zones, and a collection of zones is called an internet. Zones make it easier for users to find services. For example, rather than seeing a list of all users on the network, a user sees only a list of resources in their own zone. A zone might represent a department, a workgroup, or the floor of a building. Zones are assigned names, such as Accounting or Sales, and appear in the Macintosh Chooser. Choosing a zone reveals the names of devices in the zone.

The AppleTalk protocol stack is pictured Figure AppleTalk. It shows each of the individual protocols that make up AppleTalk. Functions available at each layer are briefly described in the following sections, starting with the lowest physical layer and working up to the application layer.

Figure AppleTalk: The AppleTalk protocol stack

The Physical Layer

Physical layer specifications define hardware connections and access methods. Ethernet, token ring, LocalTalk, and FDDI (Fiber Distributed Data Interface) are defined in the AppleTalk physical layer. See the related entries page for related topics.

Data Link Layer Protocols

Link access protocols for Ethernet, LocalTalk, token ring, and FDDI adapters reside in the data link layer. The protocols are called EtherTalk, LocalTalk, TokenTalk, and FDDITalk, respectively. A LAP manager in the data link layer is responsible for hiding the type of network in use from upper protocols. It packages data packets produced by upper protocols for transmission over whatever network the workstation is attached to.

Network Layer Protocols

Protocols in the network layer package data from upper layers and deliver it to the LAP manager in the data link layer. The DDP (Datagram Delivery Protocol) exists in this layer. It packages up to 586 bytes of data in a datagram, inserts addresses and error-checking information in a packet header, and forwards the packet to the LAP manager.

Transport Layer Protocols

The transport layer contains the following four protocols:

  • RTMP (Routing Table Maintenance Protocol)    This protocol is responsible for maintaining address tables and communicating with other routers about the status of the network. This protocol is inefficient on WANs because it sends entire tables across the WAN. A new version called AURP (AppleTalk Update Routing Protocol) reduces the amount of updating that takes place over a WAN connection.
  • AEP (AppleTalk Echo Protocol)    This is responsible for determining whether a destination node is available for a communications session before it begins. The destination node returns an echo datagram to the sender with a response.
  • ATP (AppleTalk Transaction Protocol)    This is responsible for three types of transactions. A TREQ (transaction request) and a TRESP (transaction response) work together, while a TREL (transaction release) closes a transaction session when it is complete. TREQ and TRESP can determine whether requests are lost or delayed, or whether the responder is unreachable.
  • NBP (Name Binding Protocol)    This is responsible for translating the numeric Internet address of a node into a named entity. NBP can broadcast packets over the network to locate the network address that matches the named entity. Nodes listen for the packet and search their name tables, then respond when a match is found.

Session Layer Protocols

The main purpose of protocols in the session layer is to establish and maintain communication sessions between two nodes. AppleTalk contains four protocols in this layer, as described here:

  • ADSP (AppleTalk Data Stream Protocol)    This protocol manages data transmission between two sockets on separate machines, providing a full-duplex byte stream if necessary. With full duplex, both computers can transmit at the same time. Once a connection is established, ADSP manages the flow of data.
  • ASP (AppleTalk Session Protocol)    This is responsible for opening and closing sessions between two nodes and transmits session commands as necessary. ASP calls on NBP to obtain node addresses and ATP to provide transport services for its packets.
  • PAP (Printer Access Protocol)    This protocol is responsible for maintaining communication between a user's workstation and a printer. It talks to PostScript, for example.
  • ZIP (Zone Information Protocol)    This protocol works with RTMP (Routing Table Maintenance Protocol) to maintain a mapping of the network. It creates ZITs (zone information tables) in routers that define network numbers and zone names.

Presentation Layer Protocols

Protocols defined in this layer are related to file translations, file formatting, data encryption, and data compression. The AFP (AppleTalk Filing Protocol) provides access to remote files on the shared disks of network servers. AFP uses ASP in the session layer to establish a communications session, then relies on ADSP to ensure the accuracy of file transfers of the network.

Application Layer

The applications layer does not have protocols of its own. It is where applications reside that interact with the lower protocols in the stack. Depending on the user's interaction, the application makes requests for services from other systems or resources and passes those requests to the presentation layer.

AppleTalk Routing

The most important aspects of AppleTalk Phase 2 network system enhancements were the AppleTalk Internet Router and the addition of hardware/software support for Ethernet and token ring.

The older AppleTalk RTMP (Routing Table Maintenance Protocol) sent full routing information tables (often up to 1MB in size) every 10 seconds. With only a few devices on the network, this strategy was not a problem, but with large interconnected networks, performance problems were excessive due to this overhead.

In 1992, Apple improved RTMP by providing a routing protocol called AppleTalk AURP (AppleTalk Update-Based Routing Protocol). While RTMP is sufficient for small LANs, it is inefficient for WAN connectivity. AURP does not replace RTMP, but complements it. The main difference between the two is that, as long as the Internet is stable, there is little or no routing update traffic over the AURP link. With AURP, routing information is transmitted only when changes actually occur on the Internet, and only the changes are sent. AURP also automates the mapping of addresses, which helps eliminate duplicate addresses when two AppleTalk networks are joined.

AppleTalk routes in an internetwork are selected based on the least number of hops a packet must make to reach its destination. AURP provides an improved routing mechanism and a way to encapsulate AppleTalk into TCP/IP packets so that AppleTalk WAN can be built across a TCP/IP network.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.