IV. Individual Internet Connections
The simplest and least expensive means to gain direct Internet access (so that your own desktop machine can receive and process IP packets) is by means of a service known as SLIP. This acronym stands for "Serial Line Internet Protocol." The protocol can be used over any serial link, that is, any link over which information is transmitted a bit at a time. This applies, in particular, to telephone links, and this is where SLIP has found its largest audience.
There have been some technical advances since the original SLIP implementation, and another protocol, known as PPP for "Point to Point" protocol, has supplanted SLIP for many applications. For most of the applications discussed in this paper, either SLIP or PPP could be employed. For the present paper we want to limit the total number of acronyms we use. Hence we will use the term SLIP to refer to either of these protocols (with appropriate apologies to the people who developed all the important enhancements contained in the PPP specification).
From the viewpoint of hardware and software requirements, SLIP access and dialup to an Internet host differ very little. Instead of the terminal emulation software discussed in the previous section, there must be a SLIP implementation for the user's personal computer. Implementations exist for Apple Macintoshes and Windows-based personal computers - both in commercial packages and in public domain or shareware offerings. This assures interoperability at the hardware level.
Since SLIP software is newer than the terminal emulation software needed for dialup access to an Internet host, users may find it somewhat harder to install and configure. The hardware required for SLIP access is provided by most presently-marketed models of these machines, and the same type of modem that was recommended for dialup access can be employed for SLIP access as well. The SLIP protocol does entail a certain amount of overhead, in terms of packet headers and error checking at the packet level. The need to transmit this extra information reduces the effective data rate for the end user, so high-speed modems are definitely preferable for this application.
In order to use SLIP to connect to the Internet you must deal with a network service provider who offers this type of service. The network service provider maintains modems to which you dial for connectivity, and provides the electronics necessary to route your network traffic to its proper destination. There are national and international companies who provide this type of service, along with an increasing number of local entrepreneurs. Pricing varies considerably. Although the provider's cost in terms of necessary hardware is comparable to that for simple dialup access, many providers charge substantial premiums for SLIP access. The reason for this is that many service providers view SLIP connections as more complicated for the user to implement and hence more difficult for the service provider to support. The premiums for SLIP service may take the form of vastly higher membership fees for SLIP users or hefty connect-time charges. It is likely that these charges will fall as time goes on and the network service providers become more comfortable with this class of service.
B. The Internet Cloud and Packet Services
SLIP connectivity differs from the stages of connectivity that we have previously discussed in a number of important ways. To explain SLIP connectivity we will first invoke a somewhat odd metaphor that is often used for the Internet, that of an " Internet cloud." This phrase refers to the fact that the Internet is multiply-connected. There is no single wire running from machine to machine on the Internet, but rather a multiplicity of interconnected links. Traffic is directed through the cloud by machines known as routers, which (as their name implies) find the appropriate route for any given packet of information on the network.
A SLIP connection gives your computer a link into the Internet cloud. Once routers on the network know of the existence of your machine (which can be advertised either by your service provider or by broadcast from your own machine), you can receive packets addressed to you, which may originate from anywhere on the network. And of course your machine can direct packets out on the network to any other attached machine.
A major advantage of packet services is that they accommodate increased traffic, even to the point of saturation, with a considerable amount of grace. The way traffic on the Internet works is that one machine sends a packet and waits for the destination machine to acknowledge receipt. If the link between the two machines is uncongested, the packets flow rapidly - with speeds approaching the bandwidth of the intervening link. When congestion occurs, the acknowledgment may be delayed, and the sending machine just responds more slowly.
One can contrast this situation with that which occurs for circuit-based services. Consider, for example, an airline reservation center. There may be 20 ticket clerks taking reservations. If you are caller number 21, you can't do anything but wait for a free line (or circuit). It's not that the clerk simply speaks a little more slowly than if he weren't handling so many transactions; he doesn't speak to you at all. Unlike circuit-based systems, which degrade in this catastrophic manner (so that nothing happens if the link is congested), packet-based systems merely slow down. This makes them more efficient users of the available bandwidth, easier to manage and easier to tune for optimal performance.
Packet services provide certain fundamental economies, having to do with the fact that the infrastructure which supports such services can be shared by a large number of users. In simple terms one can picture a single wire which carries a stream of interleaved packets. Since packets originating from different users can travel along the same wire, these users can share the intrinsic cost of the wire and reduce their individual costs. The Internet involves many such wires, which provide a massive shared infrastructure available to each individual user at an intrinsically low cost.
Packet services can provide a significant level of convenience for the people who use them. The interleaving of packets along a single wire can involve packets from different users, as we have just discussed, or it can involve packets from different applications being run by the same user. In this manner a person using packet-based services can be reading electronic mail while consulting one or more online services, with these activities being displayed at the same time on the same computer screen.
We have already introduced the concept of client/server resources. A remote server manages a particular information resource of interest. Other machines on the network can query that server through appropriate client software. With a SLIP connection to the network (or any other connection which allows your machine to receive IP packets directly), you would typically run the client software directly on your personal computer. This is a very efficient model for the provision of information services. The client software allows you to make the best use of your computing equipment and provides you with the optimal display of the information you retrieve from the network. At the other end of the wire, the server optimizes the organization of remote information and the process of searching through this information to find just what you are looking for. And over the network link, loading is minimized, since client and server communicate only when they have a need to do so. The client/server model creates the illusion for the user that all network resources are local, and it successfully maintains this illusion by using each element of the network in the most efficient way possible.
Examples of information services based on the client/server model include network news (topical discussions on a wide range of issues), Internet gophers (which provide text, graphics and sound resources on thousands of servers around the world) and the World Wide Web (a hypertext-based service providing multi-media resources in hundreds of subject areas). Through this type of software it is now possible to reach discussion groups on thousands of specialized topics, the on-line catalogs of all the world's major libraries, and data and images from NASA, the NSF, the Library of Congress, major art and science museums, and laboratories of the Department of Energy. All-told there are more than a thousand information servers in place on the Internet, with additional servers being added on a daily basis.
In addition to running client/server applications on your Internet-connected computer, you can also log into any machine on the network on which you have a personal account. Having done so, you will encounter the same environment as you would find after dialing such a machine through a modem connection. But this doesn't require a second phone call in addition to the one which established your SLIP link. Instead you are connecting to the remote machine via packets sent along that SLIP link.
Since packets along the SLIP link can be interleaved in arbitrary combinations, it's possible to log into more than one account at a time. The results of your various activities would be displayed in different windows on the screen of your computer. And you can do this while simultaneously running any of the information clients mentioned previously, providing an extremely rich and varied working environment.
This environment, one should emphasize, is typically very similar in appearance to that created by other applications on your personal computer. You have the same type of point-and-click interface as for other applications and the same style of operation to which you have become accustomed in using your machine. No longer does the Internet appear as some strange futuristic, yet anachronistic, foreign world, as sometimes appears to be the case in primitive text-based interfaces to the Internet.
The SLIP service that you obtain from your network service provider does nothing more nor less than allow your machine to exchange packets of information with other machines on the Internet. These other machines may be running information servers of one sort or another, in which case you can interrogate them by running the appropriate client software on your machine. Or the other machines may be machines on which you have personal accounts, in which case you may log into your accounts via the SLIP link.
It's important to recognize that the SLIP link, the machines on which you have accounts and the machines providing remote information services, are all independent of each other. Hence as you use these resources you might be dealing with three completely independent services:
SLIP server. In the present example your personal computer is linked into the Internet cloud through a telephone line which connects your machine to facilities of your network service provider. These facilities, known as SLIP servers, are typically directly wired into the Internet backbone or linked to the backbone through a secondary service provider. This type of connectivity allows your machine to send and receive IP packets to and from the Internet cloud. This enables you to run client applications which process these packets.
Mail host. To send and receive electronic mail on the Internet you must have a mail host, which is a machine permanently connected to the Internet and containing your name registered as a valid Internet mail address. The mail host is logically distinct from the machine that provides your SLIP connectivity and need not be in the same physical location or under the same management as the SLIP machine. You can read your mail either by connecting to the mail host through your SLIP link, much as you would connect via dialup access; or you could access this machine through client software which queries your mail host, as with any other client/server application.
Information servers. A variety of machines on the Internet provide information services through the client/server mechanism. These services include databases and catalogs for a wide variety of topics and information specific to the network itself, such as directories of resources, machines and people. Once you have connectivity into the Internet cloud, you can access any service which is willing to send packets to your machine. Many services, such as the library catalogs, mentioned previously, will respond to requests from any machine on the network. Other services, including the burgeoning category of commercial services, require prior registration.
Your network provider may package together connectivity, a mail host and an information server, but this is not a logical necessity, only a marketing device. Whether or not this is the optimal arrangement for you to have will depend upon your precise needs and the other resources to which you may have access. In the sections which follow we will see what types of resources these might be and how they fit into the picture developed in the present section.
It is also possible that these services might be provided from independent sources. An appropriate model for teachers and students who have accounts on a school-based mail host would be to access this machine through a SLIP link from their homes. The SLIP provider, which could be the school district, a community network, the phone company or some other commercial service provider, would not have to provide any host access for these people, since their school would already provide this service. This type of arrangement is likely to become commonplace as more companies begin to provide host accounts for their employees. These employees would be able to use SLIP access to reach servers or hosts at their place of work, just as would students and teachers with host access in their schools.
Let us reiterate the benefits of SLIP connectivity. SLIP gives you connectivity to the Internet cloud and allows you to access all those services which are provided through the client/server mechanism. Typically, the client software on your machine will resemble other applications on that machine, be it an Apple Macintosh or a Windows-based personal computer. This software will have an attractive graphical interface with simple point-and-click selections for common choices. This makes the Internet as easy to use as anything else in the world of personal computing. Given the richness of the resources available over the Internet, this is a remarkable achievement. These resources currently include elements of multi-media, such as graphics, sound and video. SLIP connectivity allows your computer to display these elements, with the possibility of several remote resources being displayed simultaneously on your machine.
What are the limitations of this marvelous setup? The SLIP model that we have discussed so far has one phone line per person. There is no possibility of a shared infrastructure at a school site, which would lead to higher costs and lower peak performance than in models which do provide such an infrastructure. Individual SLIP access is thus best for sites which have a small number of active users. This could be your home, obviously, or it could be a school site where a few people were just beginning to explore the Internet.
Incidentally, the fact that SLIP works best for isolated individual users suggests a natural vendor to supply this service. This would be a local or long-distance telephone company. So far MCI and AT&T have announced services of this sort, but such services have not yet been offered by any of the regional telephone companies. This is probably a good niche for these companies to fill.
The second limitation of SLIP access has to do with speed. Although 1500 characters per second is rather fast for most readers, the number of character-equivalents required for graphics and sound is large, and SLIP links are fast enough only for a tantalizing glimpse at some of the multi-media resources currently available on the net. It is likely that the greatest growth in Internet services will involve precisely this type of resource, so current SLIP access will probably have to be replaced by something faster, even when it is used for individual network access. These faster services are the subject of sections VI and VIII below. As with SLIP, many of these services are natural ones for local telephone companies to supply.