The AppleTalk Protocols

The Apple Macintosh operating system, as its many supporters will tell you, is extremely easy to learn and use. Apple's AppleTalk network system brings the same kind of simplicity of use to Macintosh connectivity.

Although not an official LAN standard, AppleTalk can be considered a de facto standard: With AppleTalk connectivity options built into every Macintosh, millions of Macs possess ready-made networking capabilities. This has not been lost on Mac aficionados, who have used AppleTalk to link thousands of Macs into efficient, cost-effective LANs.

Networking The Mac

When you consider the kind of work performed by the typical Macintosh user , it's not surprising that Mac users have readily accepted networking. For example, take the desktop publishing environment where the Mac prevails: Few writers are good artists , and vice versa. The nature of their jobs, however, demands that they combine their diverse efforts into a single product.

The ability to share and combine files online means those producing documents with PCs can easily merge graphics and other images with text without having to swap diskettes or "cut and paste" hard-copy images. This means the job gets done faster and more efficiently . Networked Macs are thus the rule rather than the exception in these situations, and AppleTalk is Apple's solution to Mac connectivity.

Apple calls AppleTalk "a comprehensive network system" made up of hardware and software components . An AppleTalk network can consist of many different kinds of computer systems and servers and a variety of cabling and connectivity products. Because it was designed to support a variety of machines, Apple developed a suite of proprietary protocols that permits communication between the varying devices that users might need to attach to an AppleTalk network.

However, AppleTalk is not a network operating system, a media-access control (MAC) method such as Ethernet, or a cabling system (LocalTalk is a trade name of Apple's cabling system). Rather, AppleTalk is a nonstandard suite of protocols that while not fully compliant, still provides most of the functions spelled out by the International Standards Organization's Open Systems Interconnection (OSI) reference model.

As Figure 1 illustrates, the six-layer suite of AppleTalk protocols supports numerous connectivity options, including LocalTalk, Ethernet, and Token Ring. AppleTalk also supports Northern Telecom's Meridian, a now defunct 2.5Mbps twisted-pair network. This set of protocols allows connections of virtually any computing device to an AppleTalk network. Here's how it works.

Figure 1: Apple Computer's six-layer AppleTalk protocol suite, although not fully compliant with the seven-layer OSI reference model, provides many of the capabilities and functions defined by OSI. Here, the two protocol suites are compared side-by-side.

At The Physical Layer

To many network users, the media (or cabling system) that connects PCs into a network is the networkthat's all they ever see of it. Their NOS software operates transparently , having been set up by their network administrator, and their network interface card (NIC) is installed inside their computer, out of sight and mind. In the case of AppleTalk, the original (and only) media users see are Apple's own LocalTalk products.

This scheme, driven by Apple's data-link layer, LocalTalk Link Access Protocol (LLAP), uses proprietary modular plugs and wiring to link Macs and LaserWriter printers into a network.

Since AppleTalk's 1984 release, Apple and other third-party vendors have developed data-link protocols to support Ethernet, Token Ring, and ARCnet networks, which exchange data at 10Mbps, 4Mbps, and 2.5Mbps respectively, all faster than LocalTalk's 230.4Kbps rate.

Despite its relative lack of performance, LocalTalk offers one major benefit these technologies lack: Every Macintosh computer that Apple has manufactured (prior to the iMac) contained the built-in LocalTalk connection; Apple LaserWriters and Apple IIgs computers, as well as many other Apple peripherals, also contained this built-in connection. Apple's Quadra computers and subsequent models come with Ethernet built in.

This ready-made networking option makes LocalTalk an ideal connectivity option for Mac users, particularly those who don't require the better data-exchange performance delivered by Ethernet, Token Ring, or ARCnet. LocalTalk users get most of the benefits of networkingthat is, file and printer sharing, access to electronic mail, and other shared resourceswithout the added costs associated with a network adapter board.

Localtalk's Access Method

LocalTalk, like Ethernet, uses a Carrier-Sense, Multiple-Access (CSMA), media-access scheme to place data packets on the network wire. It does not rely on collision detection (CSMA/CD), as does Ethernet. It uses CSMA/CA, for Carrier-Sense, Multiple-Access with Collision Avoidance.

Stations on a CSMA/CA network, rather than sensing collisions between data packets sent by multiple stations , send out a small (three-byte) packet that signals their intent to place data on the wire. This packet tells all other stations on the wire to wait until the signaling node's data has been sent before they attempt to send data. If collisions between packets are going to occur, they will occur between the preliminary packets, not the actual data packets.

This best effort packet-delivery system, managed by LLAP, does not guarantee that the packet reaches its destination, but it does ensure that all packets delivered are free of errors. The LLAP provides the data-link access specifications and uses a dynamic address-acquisition method that enables AppleTalk's plug-and-play capabilities over twisted-pair wiring.

Localtalk's Limitations

LocalTalk, though convenient , suffers from other limitations besides its slow data-transfer rate. For example, LocalTalk workgroups are limited to 32 nodes over a 1,000- foot cable run. Ethernet and Token Ring both support substantially greater numbers of nodes.

The EtherTalk, TokenTalk, and ARCnet Link Access Protocols (ELAP, TLAP, and ALAP, respectively) manage AppleTalk network access to Ethernet, Token Ring and ARCnet networks. Apple developed EtherTalk and TokenTalk as extensions of the two protocols' industry-standard data-link processes. Standard Microsystems developed ALAP. One of the key responsibilities of ELAP, TLAP, and ALAP is mapping AppleTalk addresses into the standard data-link Ethernet, Token Ring, or ARCnet address required for proper routing of data.

Because the Ethernet, Token Ring, and ARCnet addressing schemes are incompatible with LLAP, AppleTalk node addresses must be translated into the appropriate format; the AppleTalk Address Resolution Protocol (AARP) handles this translation.

One layer up in the AppleTalk stack is the Datagram Delivery Protocol (DDP). The DDP works with the Routing Table Maintenance Protocol (RTMP) and AppleTalk Echo Protocol (AEP) to ensure data transmission across an Internet.

End-To-End Services

The DDP exchanges data packets called datagrams. Datagram delivery is the basis for building other value-added AppleTalk services, such as electronic mail. The DDP permits running AppleTalk as a process-to-process, best-effort delivery system, in which the processes running in the nodes of an interconnected network can exchange packets with each other.

The DDP provides these processes with addressable entitles called sockets, and processes can attach themselves to one or more sockets in their nodes. Once associated with a socket, a process can exchange packets with other nodes via these sockets. Once linked to a socket, the process becomes accessible from any point on the AppleTalk network. It is then called a network-visible entity.

The RTMP provides the logic that routes datagrams through router ports to other networks; it permits routers to dynamically learn routes to other AppleTalk networks in an Internet. The AEP lets nodes send datagrams to any other nodes and to receive a copy, or "echo," of the datagram sent. This confirms the existence of a node and helps measure round-trip delays.

Reliable Data Delivery

The data-delivery group of protocolsthe AppleTalk Transaction Protocol (ATP), Printer Access Protocol (PAP), AppleTalk Session Protocol (ASP), and the AppleTalk Data Stream Protocol (ADSP)guarantee the delivery of data. These protocols can be further broken into two groups, one offering transaction-based services, the other data-stream-based faculties .

Transaction-based protocols use the request-response model typically found in server-workstation interactions. Data stream protocols deliver bi-directional data flow between two communicating nodes.

The ATP directs the AppleTalk transaction processes, in which sockets issue requests that require response (typically, status reports ). ATP binds the request and response to guarantee a reliable exchange. The PAP sets up a connection-oriented service that sends print requests to AppleTalk-compatible printers.

ASP opens, maintains, and closes transactions during a session, while ADSP provides a full-duplex , byte-stream service between any two sockets on an AppleTalk Internet.

At the highest level of AppleTalk are the AppleTalk Filing Protocol (AFP) and the PostScript protocol. The AFP, built on top of ASP, permits users to share data files and applications on a shared server, while PostScript, a programming language understood by Apple's LaserWriter and numerous other output devices, provides a standard way of describing graphics and text data.

The AFP, which conducts the dialog between a user's computer and an AppleShare server, is one of the key AppleTalk protocols. AFP was designed to provide the tools that allow supporting different types of computersthat is, Macs and IBM PCsover an AppleTalk network.

The AFP is also important because any network operating system that is fully compatible with it can operate transparently on any AppleTalk network. In turn , this means that such an NOS can support all AppleTalk-compatible applications.

This tutorial, number 25, was originally published in the August 1990 issue of LAN Magazine/Network Magazine.