Virtual circuits and the cable cloud

Tue, 04/30/1996 - 8:00pm
Wendell Bailey

Most of those working in the cable industry should know what virtual circuits are, but have they ever stopped to think about how those circuits might apply to their industry?

Everyone talks about the use of this technique in traditional telephone networks. In that network environment, the idea is that the paths between central offices would be used in a virtual way to conserve resources.

Remember that the idea of a virtual circuit is that there is enough unused time in any communications path that all of the traffic could be arranged so that the total numbers of circuits would exactly equal the amount of time that the communications would consume. Time would not be wasted by unused periods on "dedicated" circuits; it would be used by pieces and parts of other communications needs.

Unused circuit time

The theory teaches that, in the abstract, if there were 100 communicators on one side of a path and one receptor on the other side, and each of the communicators needed a path that was one mile long to reach the receptor, then there would be approximately 100 miles of plant to allow every communicator to send a message to the lone receptor. Imagine instead that a small piece of equipment is placed very near the transmitting end of the 100 paths that are emanating from the communicators.

Also imagine that coming out of that equipment is only one path that travels the remaining distance (let's say seven-eighths of a mile, for example). This would yield a network that has a total of 100 circuits × 1/8 mile = 12.5 miles, and one circuit that is seven-eighths of a mile, for a grand total of 13.375 miles. If the traffic load can all be allocated to timeslots on the one circuit, then this network would serve the same purpose as the one with 100 different paths to the lone receptor.

But what if the traffic load is so high that one circuit will not handle the demands of the 100? Traffic data analysis over many years has shown that there is a relatively large amount of time in multiple circuits that is unused or that carries information that can be gated and delayed by an acceptable amount of time. So even if additional paths were needed from the intermediate point to the receptor to handle all of the traffic demands, there is a good chance that only a couple dozen circuits may be needed, and the savings in physical plant would still be great.

The problem of links between points becomes even more burdensome when it is acknowledged that the 100 communicators would like to send traffic to each other, as well as to the lone receptor. Since the number of links required in such a case is (N-1) × (N)/2 where N is the number of discrete points that need to be connected under this formula (thus, five points yield 10 connections), the needs of the above-mentioned 100 points yields 4,950 connections. The opportunity to save a great deal of plant by using virtual concentration techniques in a large network is even better.

Clouds of circuits

Looking at this concept of virtual circuits, it is apparent that the length of the path from the output of the virtual machine to the receptor is a critical element in how effective this concept is in saving actual plant resources. This, after all, is the physical length that is irreducible, so to speak.

Further, by considering the differences between a typical telco plant (central office with long loops to customer locations) and a cable television plant with the shared part of the plant extending all the way to the property line of the customer, it should be apparent that the use of a version of virtual circuits is more beneficial to cable operators than to telcos.

Think of the design of this beast as a cloud, wherein lies the ability to do virtual circuits, and the customers are connected to physical paths that radiate from the cloud. Imagine that the 100 points above all agreed to be connected to a central point that could switch their traffic to any other point. The total number of links would be exactly 100. A far cry from 4,950 links.

The telco cloud that surrounds the central office has loops that are long. The cable cloud surrounds the entire cable plant, except the drops, which are short (approximately 110 feet per). If an operator wanted to build the most efficient system for using virtual circuits, the larger the cloud (or conversely, the more common plant elements that it surrounds) and the shorter the connecting paths, the better.

A natural advantage

Who knows? Maybe if the cable industry ever gets a chance to provide a large variety of relatively homogeneous traffic elements (such as digital bits representing television signals, data and telephone traffic), perhaps it can find a way to use its natural advantage in the theory of networks to provide the most services to the largest number of customers for the lowest price. The cable television industry is already more than halfway there. The rest, as they say, is a walk.

Contact Wendell Bailey at:


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