Packet-switching networks are the basis of high-speed networking, whether local area networking or wide area networking.
A packet-switching network is like a phone line, but different in that wires are not dedicated to one user. When you make a phone call, the phone company allocates a single wire that runs from your phone to the destination, like stringing two tin cans together.
A packet-switching network uses similar wires, but breaks up the traffic of many users into small "data packets" which move independently of each other. The wires of the network are first completely taken up by one data packet, then another packet for a different user, then another. In this way, a single wire is shared between a number of users, any of whom may send data packets to the others.
The three most common types of packet-switching local networks are Ethernet, Appletalk, and token ring. Each uses different types of wires and different conventions for communication. Appletalk is inexpensive but not very fast (though much faster than phone lines). Ethernet is fast but still pretty expensive. Token ring is also fast and somewhat pricey. Other types of local area packet switching networks are known but less common.
If all you could do with a packet switching network was talk to other computers on the same wire, it would not be useful for any but a small range of tasks. Fortunately, networks can be connected to other networks by special-purpose computers called gateways. By using a gateway between wires, computers on different wires may be connected.
The most common type of gateway related to TOPS Terminal is the Fastpath Ethernet-Appletalk Gateway manufactured by Kinetics. A Fastpath has one connection for an Ethernet wire, and one for an Appletalk wire. When connected and configured, the gateway takes packets from Appletalk that are destined for Ethernet and puts them on the Ethernet, and vice versa for Ethernet packets that are destined for the Appletalk network. The Fastpath is thus a gateway between Appletalk and Ethernet networks.
Gateways may be between similar networks (Ethernet-to-Ethernet gateways are very common, and Hayes makes a popular Appletalk-Appletalk gateway called the InterBridge) or different networks (like the Fastpath).
There is no theoretical limit to how many networks can be hooked together by gateways; the largest group, called the ARPANET, extends all over the world and has hundreds of member networks. Visualize the ARPANET as a plate of spaghetti; everywhere two pieces touch, there is a gateway. If your school or business is connected to the ARPANET, you may be able to connect to some or all of these computers directly from your Mac. Packets from your Mac that are destined for Wallamalloo will pass through umpty-million gateways to reach that fair city, and packets from the computer there will take a similar route, kangaroo-hopping through thick spaghetti back to your Mac.
Even though the networks encountered by data packets are very different in the physical characteristics of the electrical signals which carry the packets, the networks may speak the same languages at a more abstract level.
By way of analogy, consider the differences between a home phone and a car phone. Their physical ways of transmitting the data are very different - one modulates electrical signals on a wire, while the other modulates radio waves in space. But at a more abstract level, the data they carry are similar, the sounds of the human voice. Effectively, "voice gateways" run by the phone company let home phones and car phones communicate despite their differences.
Similarly, if computers on different networks talk the same abstract "packet languages", gateways can let computers on the different networks communicate with each other. Packet languages are usually referred to as "protocols".
Protocols rarely come alone; they are usually encountered in families.
The most popular protocol family is called "the Internet protocol suite", or simply "TCP/IP" after its two most significant protocols. Standards exist for talking TCP/IP on most existing networks, and a wide variety of TCP/IP gateways are available.
In the future, all inter-network communications will be done using the Open Systems Interconnection model under development by the International Standards Organization; their protocol family is referred to briefly as "OSI".
Finally, Apple is expanding its own protocol family, which it calls the Appletalk protocols, to various networks. The Appletalk protocols can run on Ethernet networks as well as Localtalk networks.
TOPS Terminal, through TOPS TCP/IP, connects Macs on Localtalk or Ethernet with packet switching networks of any kind, provided they speak TCP/IP. In addition, the phone company's nineteenth-century dedicated-line method is supported through modem control. As OSI protocols become more real, it is not unlikely that new versions of TOPS Terminal will be released which speak these protocols as well.
The ultimate goal of TOPS (A Sun Microsystems Company) is to connect everything to everything else in every useful way, in one vast worldwide orgy of data sharing.
