Signaling Speed

Author: Edmund A. Laport

The maximum signaling speed, say in telegraphy, is determined by the maximum delay resulting from multi-path signals of appreciable relative intensity. If the signaling speed is such that a prominent delayed wave in the multipath group surpasses 20 percent of the shortest pulse in the signal, there results mutilation because of elongation of the pulse. It is then necessary to reduce signaling speed to maintain accuracy.

On one-hop circuits it might be relatively easy to apply any one or all three of the forementioned principles for reducing multipath and thus maintaining a very high signaling speed; but on a multihop circuit, with different ionosphere characteristics at each point of reflection and the need to use a frequency that is limiting at one of these, the best frequencies would be severely compromised from the standpoint of selective layer and angular penetration. Also, there may be multipath signals with considerable delays coming in at overlapping angles so that there could not be angular discrimination by the antennas.

FIG. 3.9. Computed keying speeds for 25 percent dot elongation, when speed limitation is 10 percent instantaneous height variation only and hop lengths are 1,000 and 1,330 miles, using complementary antennas. (Hallborg data).

As a consequence, such circuits have limited signaling speeds, and the amount of traffic that can be passed over them in a given time is reduced.

Figure 3.9 shows calculated keying speeds versus distance for one particular path, using one particular pair of transmitting and receiving antennas.

The advent of teleprinters in radiotelegraph service brought with it the need for vastly improved standards for telegraph signal distortion because a relatively pure signal must be delivered to a printer to ensure correct response to a code character. A circuit may be performing very well for a certain period on four-channel time-division multiplexed teleprinters; then a sunspot may enter the critical zone of the sun, and the circuit will be disturbed so that the signaling speed may have to be reduced to that of a single printer channel. The tape boards at the central office begin to fill up with delayed traffic, and one who is familiar with the propagation details comes to see sunspots in the form of perforated paper tape containing messages that are waiting for transmittal.