V. LAN Connectivity through SLIP Links
In this section we address a fundamental limitation of all the stages of connectivity that we have discussed so far - their lack of scalability. Specifically, we have noted that schemes which require one phone line per user would lead to very large costs if they were implemented for whole-school sites. Significant economies can be achieved by having a number of people share a common communications link. First of all, higher bandwidth links are typically cheaper per unit of bandwidth than lower bandwidth links. Secondly, the traffic of many users can be interleaved on a single line, as with packets on the Internet itself, and this leads to more efficient use of the line. Finally, the line will be in use more often when several people are sharing it, which adds a further economy.
The advantage of shared infrastructure applies to facilities other than communications, and this fact has been a determining one in the architecture of most business systems sold today. By linking machines together in a local area network (LAN), one can let them share mass storage units (such as disk drives, compact disk players or tape drives) through devices known as file servers. Similarly, the LAN allows for the sharing of printers, scanners and other peripheral devices. This arrangement leads to additional economies in the area of maintenance and software upgrades, and allows for remote monitoring and maintenance of the network and of devices attached to the network. These economies have made this architecture the norm for most new office systems.
Within a local area network users can have access to electronic mail and to information servers on the local network. For the purposes of the present paper, LAN architecture is attractive because it makes it simple to link a local electronic mail system into the mail system of the global Internet. Once a connection from the LAN to the Internet has been established, all devices on the LAN can access information servers external to the LAN, and people outside of the local establishment can query the local information servers.
For the purposes of design it is useful to consider a hierarchy of networks. At any given site there may be a number of devices linked together as part of a local area network. The LANs in a given region will typically be linked together (whether by conscious design or through the facilities of a regional network service provider) to form a metropolitan area network. And, finally, one can picture the various metropolitan area networks as being joined to form a larger wide area network. In this terminology the Internet itself is simply a big wide area network. Specifically, the Internet is a collection of networks, all of which use the IP protocol.
Schools, like businesses, have begun to embrace LAN technology. It is therefore not unusual to find new computer installations in a school to be linked to a local area network. As schools master the use of this technology they will achieve the benefits that university and business users have found - economies of shared resources and ease in monitoring and maintaining the system.
Let's assume, then, that your school's computers are connected by a local area network. Devices on that network communicate by exchanging packets over the network, just as we have discussed in the previous section. If you want to connect your school LAN to the Internet, machines on the LAN must be able to receive and process IP packets. This might entail some modification of the software used on your present computers, but most network vendors offer this capability for their systems.
What additional pieces of hardware does your school need for this stage of connectivity? First of all you need a router, to select packets with addresses which are not part of the local area network and send them to their appropriate destinations on the metropolitan or wide area network. There exist routers appropriate for SLIP connectivity which sell for close to $1000 at current educational discounts. It's also possible to use a specially configured personal computer as a router or to adapt a local information server for this purpose. Separate routers have the advantage of requiring less effort to configure and maintain and being more stable in operation than routing software running on some general purpose device. For this reason a conservative recommendation would be to purchase a separate router.
The least expensive link of such a system to the Internet involves using a high-speed (14,400 bits per second) modem attached to your school router, with external network traffic flowing over a standard voice-grade analog telephone line. At the other end of the line you need to have a network service provider willing to support your LAN via SLIP or PPP. The network service provider's cost here is somewhat higher than for individual SLIP access, since the service provider must provide routing to all machines on your local area network. This requires slightly more effort on the part of the service provider's router. More significantly, a LAN connection is likely to tie up the phone line for much longer periods of time than would an individual connection. Many network service providers allocate an individual port and phone line for each LAN connection. This allows your LAN to have connectivity to the Internet whenever you want it and for as long a period as you want it. In fact, unless telephone charges discourage this type of arrangement, you can use the LAN's SLIP connection for 24 hour a day connectivity. This arrangement contrasts with what is typically done for dialup access to an Internet host, where a given port and phone line will be shared among a number of different users. The use of dedicated ports and phone lines does increase the cost of the service. Since this is a relatively new business, one finds a range of prices, but a not atypical figure runs to around $200 a month. This is probably higher than the true cost of the service and can be expected to fall eventually to perhaps as little as one quarter of this figure.
This type of connectivity allows many users at a school to share a single phone line - an arrangement which has its own pluses and minuses. Obviously we are saving money on telephone costs, but also we are forcing many people to share a single channel which is itself quite limited in bandwidth. This is a tenable arrangement if the school LAN includes a mail host and a local information server. That way much of the network traffic will remain inside the school, and the single phone line will have to handle only the occasional packets which must pass outside of the school. In the long run that single line will have to be upgraded to handle a higher bandwidth, but the general architecture of a LAN with a single connection point can still be maintained. The means of achieving that higher bandwidth are the subject of the sections which follow. They all share the common architecture that we have described in the present section. Hence we regard the architecture of this section as conveniently scalable and upgradable.