About The Apple Talk

APPLE TALK
AppleTalk, a protocol suite developed by Apple Computer in the early 1980s, was developed in conjunction with the Macintosh computer. AppleTalk's purpose was to allow multiple users to share resources, such as files and printers. The devices that supply these resources are called servers, while the devices that make use of these resources (such as a user's Macintosh computer) are referred to as clients. Hence, AppleTalk is one of the early implementations of a distributed client-server networking system. This chapter provides a summary of AppleTalk's network architecture.
AppleTalk was designed with a transparent network interface. That is, the interaction between client computers and network servers requires little interaction from the user. In addition, the actual operations of the AppleTalk protocols are invisible to end users, who see only the result of these operations. Two versions of AppleTalk exist: AppleTalk Phase 1 and AppleTalk Phase 2.

AppleTalk Phase 1, which is the first AppleTalk specification, was developed in the early 1980s strictly for use in local workgroups. Phase 1 therefore has two key limitations: its network segments can contain no more than 127 hosts and 127 servers, and it can support only nonextended networks.

AppleTalk Phase 2, which is the second enhanced AppleTalk implementation, was designed for use in larger internetworks. Phase 2 addresses the key limitations of AppleTalk Phase 1 and features a number of improvements over Phase 1. In particular, Phase 2 allows any combination of 253 hosts or servers on a single AppleTalk network segment and supports both nonextended and extended networks.

AppleTalk Network Components
AppleTalk networks are arranged hierarchically. Four basic components form the basis of an AppleTalk network: sockets, nodes, networks, and zones. Below figure illustrates the hierarchical organization of these components in

an AppleTalk internetwork. Each of these concepts is summarized in the sections that follow.

Figure: The AppleTalk internetwork consists of a hierarchy of components.

Sockets
An AppleTalk socket is a unique, addressable location in an AppleTalk node. It is the logical point at which upper-layer AppleTalk software processes and the network-layer Datagram-Delivery Protocol (DDP) interact. These upper-layer processes are known as socket clients. Socket clients own one or more sockets, which they use to send and receive datagrams. Sockets can be assigned statically or dynamically. Statically assigned sockets are reserved for use by certain protocols or other processes. Dynamically assigned sockets are assigned by DDP to socket clients upon request. An AppleTalk node can contain up to 254 different socket numbers. Figure illustrates the relationship between the sockets in an AppleTalk node and DDP at the network layer.
Figure : Socket clients use sockets to send and receive datagrams.

Nodes
An AppleTalk node is a device that is connected to an AppleTalk network. This device might be a Macintosh computer, a printer, an IBM PC, a router, or some other similar device. Within each AppleTalk node exist numerous software processes called sockets. As discussed earlier, the function of these sockets is to identify the software processes running in the device. Each node in an AppleTalk network belongs to a single network and a specific zone.

Networks
An AppleTalk network consists of a single logical cable and multiple attached nodes. The logical cable is composed of either a single physical cable or multiple physical cables interconnected by using bridges or routers. AppleTalk networks can be nonextended or extended.