On Distributed Communications Series
VIII. The Multiplexing Station
II. Operations Within the Multiplexing Station
Each of the 1024 separate users, or "subscribers," at each Multiplexing Station is assigned one or more permanent "telephone numbers" or "addresses," and each subscriber is allowed to call any other network subscriber.
Address Designation and Numbering Plan
On the basis of 1024 possible Multiplexing Stations and 1024 subscriber lines at each Station, a numbering plan comprising a four-digit octal-number office code (the Multiplexing Station) and a second four-digit octal-number line code is suggested. This permits a possible expansion to 4096 office codes, each having 4056 line codes.
Each Multiplexing Station may be assigned up to eight separate simultaneous office codes. The office code designations are not permanently assigned to specific Multiplexing Stations. However, each subscriber is assigned a permanent number. Any subscriber can be called by using this permanent number, even if he moves from location to location (provided, of course, that he has informed his nearest Multiplexing Station where he is).
When placing a new call, the calling subscriber's first transmitted Message Block, containing the signaling information needed to set up a "quasi-circuit connection," is sent from the calling Multiplexing Station to the called Station. A determination is made at the called Station as to whether the particular subscriber being called is currently connected to that Station.
If the called subscriber is at the called Multiplexing Station, and if his line is not busy, revertive signaling information is sent back to the calling Station. This revertive information includes the latest line number of the called party, which shall be used to transmit all subsequent Message Blocks constituting the call.
If the subscriber is not at his "home" Multiplexing Station, but has left a forwarding address to be used, this new forwarding address will be reverted back to the calling Station. The calling Multiplexing Station then automatically replaces the call to the new number. Thus, the calling Multiplexing Station always uses an address for Message Blocks which can be rapidly interpreted by the called Multiplexing Station with a minimum of processing time. This simplifies the high-speed handling of Message Blocks during later transmission. Time-consuming table look-up translation search activities which require access to a slow-speed-access store, such as a magnetic drum, are performed only once during the setting up of the initial call.
Push-button dialing is used in the system primarily when calls are placed by humans. The following steps describe the placing of this type call.
- To reach subscriber #2345-1241, the digits 2345-1241 are depressed sequentially on the buttons on the calling telephone, or the data transmitter terminal device. The first four digits comprise the office code; the last four, the line code number.
- A Message Block containing the complete called number is formed by the calling Multiplexing Station and sent to the Multiplexing Station currently responsible for office code number 2345.
- A translator table look-up search is performed at (called) Station 2345 for line code 1241.
- This translator table might indicate, for example, that telephone number 2345-1241 corresponds to the subscriber connected to local line number 0532. Next, line number 0532 is tested. Perhaps, it is found to be busy. Again checking the translator table, a search is made to see if there is an alternate number which may be used; if so, this alternate number is tested.
- The alternate table look-up reveals that local line number 0535 is an assigned alternate. This line is tested and found free.
- Local line number 0535 is then sent signaling waveforms which correspond to "ringing signals."
- Upon detecting a signal that line number 0535 has gone "off-hook" (answered), a revertive signal containing a synchronization starting point for an end-to-end cryptographic generator, together with the modified calling number to be used for subsequent Message Blocks for the duration of the call, is sent back to the originating Multiplexing Station. All subsequent Message Blocks making up the call are immediately directed to the desired subscriber's line in cryptographic synchronization without additional table look-up translation.
The Multiplexing Station takes the outputs of a large number of low-data-rate devices, forms them into Message Blocks and semi-asynchronously time-shares them over a single high-data-rate link into the network via the Switching Nodes. To illustrate the data processing that occurs within the Multiplexing Station, let us consider some of the transformations necessary to transmit an audio signal through the system.
We start from a conventional telephone mouthpiece which transduces speech into an audio voltage. This audio signal is converted by a simple analog-to-digital converter (which may be built into the base of special digital telephones designed for use with this system). The output of the analog-to-digital conversion is a digital stream (whose sampling timing is derived by locking onto the incoming timing signals extracted from the subscriber's line from the Multiplexing Station). The special telephone contains ten push buttons connected into the feedback paths of a simple four-flip-flop, modulo 16, binary counter. To "dial" a number, each of the appropriate push buttons are sequentially depressed. Each button modifies the counter's output to form a repetitive pattern accepted by the local Multiplexing Station. Each button must be depressed for at least one-tenth of a second. This binary signaling stream, because of its highly repetitive pattern, is readily detected and decoded by the Multiplexing Station, permitting rapid separation of signaling from the voice stream of bits.
Signaling operations are generally slow and require complex processing, while the voice stream is handled in a more expeditious "stamping mill" fashion.
The binary stream output data of the digital telephone will depend upon the type of digital modulation chosen. In the following discussions, an output rate of 19,200 bits/sec, using High Information Delta Modulation, is assumed. This synchronous data stream is temporarily stored in the Multiplexing Station. Separate storage space is used for the signaling information and for voice data. Since the Switching Node will only process the standardized packages of 1024-bit Message Blocks, it is necessary to store and convert the low-speed synchronous stream into these standard-format Message Blocks, suitable for transmission. The signaling information, necessary for addressing each Message Block, is stored in a section of memory called the "rubber stamp buffer." About 1/20 of a second of the digitalized audio is stored into one of three consecutively assigned data buffers. The first of these is called the "Phase-A" data buffer. As soon as this buffer is "full of bits," the incoming synchronous stream to which it is connected starts filling the Phase-B data buffer, etc. The Multiplexing Station interleaves and combines the bits in the various positions of the data buffers with bits from the rubber stamp buffer. These form completed Message Blocks and are burst out of the Multiplexing Station into the connected Switching Node whenever the common output link to the Switching Node is free.
Blank Space Suppression
When two people engage in conversation over a fullduplex channel, much more "silence" is transmitted than voice, as only one person talks at any one time and there are interword spaces. Thus, we have chosen not to transmit Message Blocks unless we have detected some sound during the last 1/20 of a second of the Message Block interval. In return for a modest price increment in terms of additional silence detection and suppression equipment, we believe that we can increase the number of simultaneous voice users by a factor of about four, by utilizing the periods of silence in excess of 1/20 of a second.
This means that a 19,200-bit/sec voice stream really loads the network with an average rate of only about 5,000 bits/sec, producing a highly efficient method of transmitting good-quality digital voice.
Choice of an Encryption Scheme
To protect the secrecy of communications, and to help thwart interference, intentional or otherwise, cryptographic transformations are applied to outgoing Message Blocks to make them intelligible and acceptable only by the specific end-addressee. On the basis of analysis in the design of the Switching Nodes, we expect the system to have an extremely low Multiplexing-Station-to-Multiplexing Station error rate (on the order of one Message Block lost or incorrect in every 108). Thus, we are able to use a form of modified auto-key which requires correct reception of the entire preceding string of Message Blocks before the next block can be properly decrypted. In the event of an error, the "connection" would be "knocked down." To obtain a feeling for the frequency of such an occurrence, consider a 19,200-bit/sec telephone used one hour per day. An interruption of the type being considered would occur only about once a year. When such a transient error did occur, either user could dial the other party and the connection would be automatically reinitiated--and synchronized--using a new modified crypto key. (Re-dialing could also be automatically initiated, if later desired.)
Processing An Incoming Message Block
Message Blocks, each containing 1024 bits, arrive at a receiving Multiplexing Station over the 1.5-megabit/sec links from the connected Switching Node. Each Message Block is temporarily stored in the rapid-access store (magnetic core) while the heading address is examined. Each Message Block also contains an end-to-end sequential transmission number (which is encrypted). If the decrypted sequence number matches the next expected sequence number, the incoming Message Block is accepted and the crypto count advanced.
The Message Block is stored in three sequentially assigned data buffers (bands on a magnetic drum similar to those used in transmission). This allows flexibility of timing between the local clock timing and Message Blocks arriving by different paths. The differential propagation time between the transmitting and receiving Multiplexing Stations can vary over wide latitudes. The output of the Output Buffer on the drum after decryption is a continuous binary stream stripped of housekeeping; it reaches the receiving subscriber at the same low synchronous speed, locked to be within allowable tolerance limits, as that used by the transmitting party.
Other Processing Functions
The Multiplexing Station not only transforms digital signals from the telephone microphone into Message Blocks suitable for transmission and back into a continuous digital stream, but also provides the means for concentrating a large number of potential network users. Each subscriber's line is continuously, sequentially, and rapidly tested. Only those users that are transmitting active information are, in effect, connected into the network.
The Multiplexing Station further provides facilities to perform priority control for the various users. It also acts as a terminal point for processing trouble information and performs miscellaneous operations, such as inserting a dummy stream of zeros into the line output devices during periods in which no Message Block transmissions are received from the calling party.
 Having a large excess of possible number combinations imparts a high probability of rapid interception when a network user dials a wrong number.
 This prevents any possible interruptions to the "quasi-circuit" set up between two users by a sophisticated enemy agent who has broken the link-to-link cryptocode and has somehow inserted false signals acceptable to the Switching Nodes.