Applenet Hardware Background And Current Status

Applenet Hardware: Background and Current Status CO NFID ENT1AL Authors: Bob Paratore/Jan Sheehan Preface This document presents the current status of and thoughts about the Applenet hardware. Chapter 1 explains local area computer networks, and Chapter 2 describes the current hardware design and implementation of Apple's version of a local network, Applenet. Chapter 3 ends the discussion of current thoughts and designs with an explanation of the personality module, a low-cost way of emulating certain devices. The last chapter of this document, Chapter 4, discusses what developments and user needs future versions of Applenet must consider. 1. Network Concept& ~omputer netw~rks connect computers and related resources together so that 'they can communicate with one another. Each se"parate connection to the communications" medium, called a node, att~ches one or "more computers and related resources to the network. Long-haul computer networks, such as ARPANET, connect computers that are more than a few miles apart; local area computer networks, such as Applenet, connect computers that are only a few thousand feet apart. Networks, whether local or long-haul, need to control how and when the connected computers sommunicate with one another. Networks control communication through choice of structure (topoiogy) and through rules (protocols) implemented in the system's hardware and software. The structure of a network defines how nodes are physically connected to one another. The system protocols are divided into sets of rules for each level of the communication process, and define the structure of packets, how and when a node can transmit or receive a message, and how the network and user software forms and interprets packets. Communication between nodes in a network is either circuit-switched or packet-switched. Circuit-switched networks dedicate a communication line for the duration of a communication. However, packet-switched networks do not dedicate a line. Instead, computers connected to packet-switched networks transmit packets, small envelopes of information. The advantage of packet switching is that many nodes can share the same communication line and transmit virtually simultaneously due to the short length and transmission time of the packet. Usually, local networks are packet-switched rather than circuit-switched. 1.1 Local Network Topologies Some local networks have centralized control. In these networks, one node receives and routes all messages. Common topologies for such networks are the loop and the star (see Figure 1-1). In a star network, all computers connect directly to the controller and send/receive messages only through the controller. In a loop network, the connections of the nodes form a circ~e. A message travels around the loop to the controller and the contrJller routes the message around the loop to its intended destination. These two topologies have one major disadvantage: their dependency on the controller node. …