On Distributed Communications Series

IV. Priority, Precedence, and Overload

IV. User Control

A tributary communications station is a traffic concentrator. Messages or lines from many individual users are combined at the communications office, thus making most economic use of an expensive and limited communications facility through time-sharing.

Although communications centers of the future can be smaller and more completely automated, we would like to retain human judgment to allow redistribution of the communications resource during crises.

The Communications Control Console

The job and the equipment we describe does not exist today. At each local communications center we propose there be a "responsible" military officer who is charged with local traffic control and who is responsible for changing the gross allocation of the communications resource when necessary.

The Communications Controller is to be an arbiter with the task of allocating the available data capacity or communications resource among the many local users of the communication network.

To aid the Communications Controller to allocate surviving capability equitably among individual demands, a Communications Control Console is proposed. This console provides a picture of both the availability and the demand for communication service, together with devices for its sub-allocation. The commander views his communications capacity (resource), observing present allocation (of currency) and the size of individual demands (spending) for service.

Figure 5 is an illustration of a hypothetical Communications Control Console.[1] A portion of this console is shown in more detail in Fig. 6. The meter in Fig. 6 indicates the summation of total demand by all local subscribers of channel capacity in the common measure of bits per second. The right-hand meter indicates the network's ability to absorb new traffic in bits per second after choking.

Since traffic already in the network is given precedence over new traffic, overload is prevented by the diminution of acceptance of new traffic. Thus, the total input data rate from all local users into the network must be limited until an equilibrium point is reached between the traffic that is admitted into the network and the traffic that the network can rapidly deliver. Each communications center need not use its full allocation of capability for feeding traffic into the network. Conversely, a commander at a particular remote site at a crisis point may handle a heavier-than-normal communications load--if necessary, by requesting nearby stations to reduce their traffic input in order to allow heavier local use of the network capability. Flexibility is thus reserved to allow for the many different centers of various importance which utilize the network.

Further, each individual network user ("subscriber") can ask the Communications Controller for more than his currently allocated share of the communications resource. This change of assignment is effected by the Communications Controller turning a dial, labeled "Allocated" in Fig. 6. This control sets the limit to the total volume of locally-generated traffic (in bits/sec). If, for example, a station was requested to cut its loading to 50 per cent of its allocated capacity, the dial would be set accordingly.

In Fig. 5 two lights are shown above each panel. The green light (left side) indicates situation normal: demand safely under the allocated capacity. The red light (right side) warns that traffic volume is approaching the overload point. Each local Communications Controller in turn can ration his assigned total communications resource among his many competing local users. For the sake of illustration, assume that each user performs a vastly different function; for example, intelligence, early warning, logistics supply, etc. To allow control of these separate, further-subdivided, functional categories, individual panel units similar to the master panel are provided. The first such unit might control the gross volume of intelligence traffic; the second, early warning traffic; etc. Thus the Communications Controller would also have at his fingertips the ability to vary the allocation of data rate to each separate function.

The Communications Controller will not play the console as an organ, since gross changes in loading are slowly occurring phenomena; rather, he will normally leave the controls set to fixed positions, except when a crisis or overload approaches as indicated by the red warning light. He then decides which users with growing demands should deprive others with less important duties, and to what degree. (Every user who desires more communications capability than he is momentarily allocated is always allowed to talk to the Communications Controller to present his case for an increase in allotment of bits.)

The console does not seek to supplant human judgment. It simply provides an automated facility to instantaneously implement the human executive decision. It administers the will of the commander swiftly and without an arbitrary cutoff. No individual function need ever be totally deprived of instantaneous communication, as is necessary with the binary decision rules necessary to enforce today's precedence doctrines.

An Analog Model of the Console

Figure 7 is a sketch of a schematic analog-signal model of the Communications Control Console. This hypothetical implementation is included to suggest that the circuitry required to display the control traffic loading need not necessarily be complex.[2] In Fig. 7, an AC input voltage proportional to local available data rate is inserted at the terminals in the upper-left-hand corner of the illustration. A rotary autotransformer, T-l, is connected to the control knob of the unit. A second voltmeter, V-2, is connected to the output of T-l, to show the reduced amount of data rate that may be utilized locally. Each unit panel is connected to a separate autotransformer. Each autotransformer, in turn, can be connected to lower-level sub-assignment autotransformers. These autotransformers create voltages proportional to the assigned data rate of each of the individual unit panels. Voltmeter V-4 indicates the local bit-rate demand. This signal can be approximated by a simple constant-current summation of binary control signals whose currents are proportional to the bit-rate demand for each data source energized.[3]

For example, if four teletype machines are connected to the system, four separate resistors, each passing a current proportional to the data rate required by each, are used. Each time a teletype machine is energized, an associated relay closes attaching an AC voltage to the resistor associated with the connected teletype. If all four teletypes are in use, the currents passing through all four resistors would be summed, producing a voltage whose value would be approximately equal to the total bit-rate demand of all four individual teletypewriter devices.

On the extreme right-hand side of Fig. 7 it can be seen that each single-unit channel can be subdivided into other smaller sub-assignment panels. These panels can be connected to match the same hierarchical organizational structure of the network feeding the local communications transducers to the communications tributary station. (Nothing, of course, restricts any user from being connected to more than one communications tributary station.)

[1] This is the Priority Control Console described in detail in ODC-VIII.

[2] The all-digital version of this console is described in ODC-VIII, where all signals necessary for the console are included.

[3] In practice, this would be done at the Multiplexing Station which already has equipment to measure activity.

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