Geographically Extended LAN
Authors: Zigmantas L. Budrikis
Venue: Proceedings of the 1999 International Symposium on Parallel Architectures, Algorithms and Networks (1999)

The functional efficiency of an office is enhanced greatly by a shared computer environment. The shared environment is created through homogeneity in computing equipment and software, and by communication among the equipment. The physical extent of the common environment is set by the reach of the LAN that supports the communications, and is generally accepted as being no more than a building, at most a campus. Yet any enterprise or organization that is spread over multiple cities or even countries would be helped by having a single computer environment that would encompass all its locations, provided only that the geographically dispersed environment would sensibly have no greater latencies in computer to computer interactions than prevail in the single campus environ-ment. By name, a LAN appears to be bound to a local area. However only the shared medium subnet is so restricted. A LAN of bridged shared medium segments, can extend over any distance. Therefore a single computer environment for a geographically dispersed organization is a real possibility. The critical requirement is for permanent links of sufficient bandwidth for connecting the bridged segments. The critical issue is the cost of such links.The bandwidth of bridge links will be sufficient when it is same as, or at least comparable to, the bandwidth of the shared media of the LAN segments. A bandwidth of 10 Mbit/sec must regarded as a minimum requirement. Provision of such links over national and global distances on any exclusive basis will invariably prove prohibitive in cost. The only viable possibility is on a statistically shared basis: The average usage of any bridge link would be minuscule compared to the required peak rate and therefore, given suitable access control arrangements, a 10 Mbit/sec communication channel could be shared by hundreds, perhaps even thousands, of LAN bridge links. A necessary condition on the sharing is that a single link should be able to use the whole channel capacity whenever that link is alone with any traffic at the given time, and the whole fraction 1/n-th of the capacity when at the time there are (n-1) other links with traffic. A further necessary condition is that despite the hundred per cent statistical sharing, there should be no losses of information by, for instance, buffer overflow at multiplexing points.There are few communications schemes in which such full statistical and loss free sharing of bandwidth is possible, and none are as yet in existence. An outstanding instance of a possible scheme is in ATM networking, namely using controlled cell transfer (CCT). Protocol standards for CCT have been advanced in a wider context of Controlled Transfer (CT) capabilities in ITU-T and are ready for adoption. They have not however (as yet) been accepted on the grounds that CT would not provide anything beyond what can be obtained from already standardized capabilities. That CCT could give an economic basis for LANs of un limited geographic extent, and hence for truly singular corporate computer environments irrespective of the geographic scatter of the corporation, may still tilt the balance in favour of CT acceptance.