On Distributed Communications Series
VIII. The Multiplexing Station
IV. User-to-User Considerations
The Quasi-Real-Time Circuit Illusion
Consider the distributed network as being a large "black box" connected between two subscribers. Thus, if one looks at the transfer function of this box from the viewpoint of either subscriber, it will appear that the black box is a simple delay line; an illusion is created of a quasi-real-time link connecting two terminal points with the following specific properties:
- A fixed time delay exists between input and output. (When setting up a call, a maximum expected time delay is purposely inserted that is longer than the worst-case longest expected differential propagation time between the input and the output caused by Message Blocks arriving by different paths.)
- The output bit rate tracks the input bit rate to within about 10-6 or 10-7 time drift. Rather than insert extra bits or delete bits, when the output drifts relative to the input, we have chosen the alternative of modifying the overall delay time between input and output.
- A slight, very slow variation of this fixed overall time delay occurs between input and output.
- The overall "quasi-circuit" exhibits a basic very low error rate (on the order of one error in 108 bits) between any two subscribers.
- In the rare event of an error, all subsequent transmissions of this single call become "garbage." Both parties and the nearest trouble-monitoring point will be immediately informed of the error and the quasi-circuit will be knocked down. It will be necessary to re-establish the call, unless we choose to re-establish calls automatically, which has not been felt to be necessary.
One unique property of the network is that the signaling data is of identical pulse shape form and characteristics as the transmitted data, allowing great freedom in inserting additional signaling information even after the circuit has been set up. This is expected to be most useful when building future computer-to-computer Systems, because the signaling information can be intermixed in the data stream by a computer connected into the network.
Signaling digits are expected to conform to the following restrictions:
- Each signaling digit shall be a repetitive binary pattern and shall continue for a time longer than 8/75 sec (one drum revolution).
- Each signaling digit shall last no longer than 3 sec.
- Signaling digits may follow one another with a delay equal to or greater than 8/75 sec.
- Any pause greater than 20 sec between punching signaling buttons, shall require the subscriber to redial.
- Addresses shall generally be comprised of eight digits, of which four shall comprise local exchange (Multiplexing Station) number and four digits shall be the line number.
The repetitive digital pattern used for signaling can be created by a set of three flip-flops connected in a feedback counter circuit, or by a simple ring counter. Depressing signaling buttons would modify the feedback connections of the counter. For example, Fig. 5 shows repetitive waveforms which can be created by three flipflops whose feedback patterns are changed. Ten separate waveforms are shown; these are used to represent the numeric digits 0-9, and are essentially comma-free codes having the property of being easily decoded. (Also shown are five additional states not used since they could be confused with the numbers 1, 3, 4, 6, and 7, in the event of a phase reversal.)