On Distributed Communications Series
VIII. The Multiplexing Station
III. Input/Output Devices
Types of Devices
Input data transducers may be categorized into synchronous and non-synchronous devices. Synchronous devices operate at a constant, fixed, and uninterrupted bit rate. Non-synchronous devices, such as start-stop telegraph systems, are fundamentally synchronous, but need maintain synchronization over only a very short time interval; e.g., a single transmitted character spacing. The time interval between characters is allowed to be highly asynchronous.
Although it is possible to build the Multiplexing Station to work with any arbitrary number of data rate input devices, it was found most convenient to restrict consideration to the few most commonly used standardized data rates which appear to be emerging as the most probable speeds for the future.
We shall briefly review the data rates of some of the synchronous input devices we may expect to see in the future, and then discuss the method to be used to handle non-synchronous channels.
Synchronous Input Devices
An examination of those data terminal devices now in use reveals that all the high-speed devices (those in excess of 600 bits/sec) are synchronous in operation. The only non-synchronous devices found were the low-speed asynchronous systems such as start-stop teletypewriters.
The small allowable plus or minus speed tolerance rate of synchronous devices is sometimes called "rubber." It will be assumed that all synchronous devices used with this network can lock onto timing derived from a remote timing signal, provided that this remote timing signal falls within the allowable timing "rubber." Since all lines in the system within the Multiplexing Station area are essentially "full-duplex," (simultaneously transmitting and receiving), it will be assumed that transmitting synchronization will be derived from the input waveform on the duplexed receiving line. Even when the receiving line is otherwise idle, it will still convey a signal usable for retiming.
There are several key retiming problems which must be considered, including the problem of the time delay caused by phase shift over conventional telephone lines. A conventional telephone circuit might feed the network at a rate of about 2400 bits/sec. We shall have to live with a difference in timing between the Multiplexing Station and the remote input telephone lines, caused by the phase drift, for example, in the case of transmitting a non-classified data stream from a remote device via the conventional telephone plant into the Multiplexing Station. In this discussion, we assume use of only inexpensive frequency control crystals at the Multiplexing Station (accuracy:
- A one-bit interval is equal to 1/2400 sec.
- Clock accuracy at each Multiplexing Station is 10-7.
- At the maximum drift rate, it will take 4167 sec to slip one bit.
- Anticipated drift rate from each input is low and a single time-shared data examination servo-system at the Multiplexing Station could measure and track the timing drift caused by drift on the input line.
- As the Multiplexing Stations must handle data bursts from the high-speed links from the Switching Nodes, buffering equipment is required to unpack Message Blocks at the desired low synchronous stream output clock rate. We circumvent the timing drift problem caused by the slightly different data rates of the various Multiplexing Stations and their devices, by storing entire Message Blocks, equivalent to 866 active bits, to form a second level of "rubber." This permits two subscribers to drift apart in timing by as much as one whole Message Block before a timing discontinuity occurs between the two Multiplexing Stations and their subscribers.
- At the 2400-bit/sec data rate, it would take approximately 40 days to build up to this value of drift.
- At the 19.2-kilobit rate, it will take one-eighth as long, or about five days. As will be later shown, it will probably be desirable to change the Multiplexing Station end-to-end crypto key every 24 hours. Thus, we do not expect synchronization breaks caused by subscriber-to-subscriber drift.
- Therefore, only a modest crystal stability accuracy is necessary in the system to allow synchronous transmitting and receiving devices to track with one another.
Expected Data Rates for Synchronous Devices
While there is an attempt to standardize military digital communications data rates to the 75(2n)-bits/sec series, only a limited number of these speed values are found in common use. The preferred rates--600, 2400, 9600 and 19,200 bits/sec--are used in the following major applications:
- 600 bits/sec provides a conservative data rate capable of being transmitted over long distances over a conventional switched telephone network, with relatively simple modems.
- 2400 bits/sec may be transmitted over several hundred miles of "private line" telephone company circuits, provided the circuits are carefully selected and equalized for data transmission.
- 9600 bits/sec is a convenient speed for high speed digital facsimile transmission.
- 19,200 bits/sec is the lowest standard data rate allowing good voice transmission with relatively simple circuitry. (Other high-quality pulse code modulation systems used for digital voice require pulse repetition rates of 38.4 kilobits/sec, and higher.) At present, the limitation of high-quality digital voice transmission systems is that they suffer from a slight amount of "quantization noise," noticeable only during periods of silence. This may be a detriment in the proposed system, but a simple background squelch circuit may be used to silence these noise periods.
In designing the network to handle non-synchronous (usually teletype) inputs, the first approach considered was that of storing individual bits into separate low-speed buffers to form complete Message Blocks. However, there were several overriding problems:
- The non-synchronous characters contain "stop" bits whose duration is longer than the remainder of the character bits.
- Five-, six-, and eight-bit characters are all in common use.
- Most importantly, the slow-speed teletype bit rates require an excessive time to fill the standard-bit-length Message Block. For example, if two teletype stations wanted to converse with one another, the second station would not receive any text from the first station until the first station had typed almost three complete lines:
The 600-bit/sec modulated-carrier approach allows any conventional-speed teletype signal--whether it be 5-level, 6-level, or more--to be transmitted without providing separate apparatus to interpret each character and strip the start and stop pulses. This modulation approach requires that the bit length of the teletype signal be much longer than the modulation carrier period, 1/600 sec (1.667 ms). Even at teletype speeds of 100 wpm, the bit length is 13.5 ms, compared with 1.66-ms pulse length for the modulated carrier. Thus, only a tolerable 12 per cent time-space distortion is introduced to the teletype waveform. No difficulties are anticipated by this time distortion, as it falls within the acceptable operating range of the conventional mechanical teletype printer. (Auxiliary devices are also available which regenerate teletype signals with time-distortion levels up to 50 per cent--if ever needed.)
Transmitting 60-bit/sec teletypewriter signals by using a 600-bit/sec carrier may strike the reader as an inefficient use of the communications resource. However, the marginal capital investment in terminal equipment necessary for more efficient teletype transmission is in excess of the marginal cost of the communication link's capacity to handle this small additional data rate. It appears economically preferable to use "inefficient" transmission within the distributed network in this case, since it is cheaper--less terminal equipment is required. Transmission of 600 bits/sec in lieu of 75 bits/sec is a negligible price to pay when the cost of 1.5- and 4.5-megabit/sec links are considered (see ODC-X).
While a 600-bit wave train is used by the Multiplexing Stations, this does not mean that full-duplex 600-bit/sec teletype circuits are mandatory outside the Multiplexing Station area. It is feasible to modulate or demodulate the 600-bit/sec teletype waveform for the remainder of the distance to the end-subscriber. Thus, we can use conventional narrow-band, full-duplex, half-duplex, or even simplex telephone or teletype circuits to remote points. Conventional high-frequency radio teletype circuits can also be used, and "binary-stream" transmission maintained to a remote teletypewriter.
The additional equipment required to handle conventional teletype is a cigar-box-size container with the push buttons, generator signaling, and a simple amplitude modulation and detection circuit. The signaling used will be identical in format to that used by the digital telephones, as described.
 Where n is an integer.