On Distributed Communications Series

II. Digital Simulation of Hot-Potato Routing in a Broadband Distributed Communications Network

IV. Time Scale and Scaling Factors

The network is a highly parallel system in which separate simultaneous transactions are taking place, as in a three-ring circus. In order to simulate this network on a computer, which is a pure serial device capable of processing only one event at a time, it is necessary to examine each link and each node sequentially. Perfect simulation of events can only be performed if the assumed sequential time intervals of examination are so short that not more than one interacting change of state takes place during any single time interval. This approach, of course, is quite wasteful of computer time.

Fortunately, it was found that good simulation could be performed with a very coarse "grain size" of time. We have standardized upon this time unit as a standard "time-frame." All other times are scaled in terms of this basic value. The following is a scaling of real-world times in time-frame units.

tf = 0.33 milliseconds - 1 time-frame.
ts = station processing delay time; variable from 0 to 0.66 ms
tp= pipe filling plus message-length time; variable from 0.66 to 2.64 ms; 2 to 8 time-frames.
te = message length; 0.66 ms; 2 time-frames.

The values assumed here are for links up to about 150 miles in length, transmitting 1024-bit Message Blocks at a data rate of about 1,500,000 bits per second. A period of 3000 time-frames is equivalent to one second of real-world time. Since it takes about nine minutes to simulate one second of real-world time, the ratio between real-world time and simulation time is about 1:540. Each message is held at each node for a single time-frame. If a link is free, the message may move after the count. It takes from two to eight time-frame counts to move from one node to the next. The random value within the limits of tp is re-chosen whenever a new network is defined; the greater the value chosen, the greater the geographical length of the assumed communication link.

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