Multiplex systems were originally designed to use the spare capacity available on a circuit for concurrent character or message transmission. In the early years the distributor or multiplex device, in many cases, was a part of the telegraph equipment. A unit is usually called a distributor if it converted the parallel character bits to a serial bit stream for transmission to the line. The character bits are produced as a group when a key is depressed on the keyboard of the telegraph equipment. This group of bits could not be transmitted in parallel simultaneously on a single line; therefore, it is necessary to convert the bit groups to a serial stream, which is the function of a distributor unit. The distributor then becomes a multiplex device since it is connecting more than one telegraph machine to a single line.
1800's:
Duplex operation was very often used, since the capacity of a busy line was practically doubled by the addition of operators and the installation of a little apparatus at the terminals and no change in the line. In 1872 duplex operation was quite common on Western Union lines. In 1874, Thomas Edison showed how to double the capacity of the line by using a quadruplexing technique. Diplexing allowed the transmission of two independent messages in the same direction at the same time. Diplexing was never used by itself, but always in conjunction with quadruplexing. Edison's idea was to send one signal by varying the strength of the signal and the other message by polarity, positive and negative. There were two sounders or relays, one responding to the electric current's strength, the other to signal polarity. The diplex circuit now only had to be duplex-ed by one of the existing methods, and two messages could be sent in each direction at the same time, or four in all. Soon all the most important lines were using quadruplexed operation. In 1878, Western Union had 13,000 miles of quadruplexed lines. There were four operators at each end, two sending and two receiving, all at the same time if necessary. Edison made many contributions to telegraphy, of which quadruplexing and the stock ticker are the most famous. There were other multiplexing methods of transmitting more than one message at a time over a single wire, such as a system implemented in 1873 using a commutator or distributor at each end of the line, running synchronously, so that each user (line) was connected for a fixed interval of time on each revolution of the distributor. Today, this is called "time division multiplexing."
Data Structure:
In the beginning discussions concerning data transmission speeds were expressed in characters per Minute. Following that time period CPM and Words per Minute (WPM) were used to indicate data transmissions speeds. Five characters plus a space is used to represent one word. Then Bits per Second and Baud rates became the norm for indicating data transmission speeds. Data rate speeds of 1200, 2400, 4800 and 9600 bits per second were considered high speed operation in the mid 1950s, 1960s and 1970s. In today’s world nearly all data rates are defined through the use of Bits per Second (bps) or Bytes per Second (BPS), which represents eight bits. Bits or Bytes can be grouped into blocks or packets; and they can represent one individual part in multiplexing schemes. A character, a word, a bit, a byte, a block of data or a packet of data can be multiplexed as one individual unit in Time Division Multiplexing. Baud is not included in this grouping because one baud may represent 1 bit in some cases and a group of bits in other cases. A typical packet contains 1,000 or 1,500 bytes.
Equipment or devices also progressed and development over time. First all data transmission was from point A to B over a limited distance due to degradation of the signal. Relays both standard and multiplexing were implemented to relay, and regenerate the signal, and allow two way duplex transmission and TDM multiplexing. All signaling over the telegraph lines up to this point utilized Direct Current. Vacuum tubes were the next major step in improving data transmission speeds and regeneration of signals. With the implementation of the vacuum tube and carrier systems Alternating Current (AC) could be used for transmission of data signals. This also brought Frequency Division Multiplexing on-line.
Transistors were used to replace vacuum tubes in most communication equipment starting in the 1960s. They were smaller and produced less heat, and also were more reliable. They could be grouped and electrically connected to form a ‘chip’. Examples: below (left), a multiple transistor chip. Below a two transistor “AND’ gate. Then transistors, chips and other electronic units can be placed on an integrated circuit card or board, which is basically what we have today.