Thursday, June 12, 2014

The Modem (Part 2)

Let's discuss SYN-ACK. I really breezed past it yesterday and it was actually important to understanding how the modem came to be. Below is a simplified summary of the TCP 3-way handshake: (SYN, SYN-ACK, ACK) More here.
  • Host A sends a TCP synchronize packet (SYN) to Host B 
  • Host B receives A's synchronize packet (SYN)
  • Host B sends a synchronize-acknowledgement (SYN-ACK)
  • Host A receives B's synchronized-acknowledgement (SYN-ACK)
  • Host A sends acknowledge (ACK)
  • Host B receives acknowledgement (ACK). 
  • the TCP socket connection is now established!
This whole sequence comes from the modem. That is if you tend to agree that the modem descended from the work of Jean Maurice Émile Baudot. He patented a 5-bit telegraph code in 1870. In 1874 he invented a telegraphic system of TDM (Time-Division Multiplexing.) He used synchronized clockwork-powered switches at the transmitting and receiving ends, and was able to transmit five messages simultaneously. But because the timing was controlled mechanically the telegraph operator had to enter characters at a steady 30 wpm. The receiving end was a bit more forgiving, as those signals were temporarily stored on a set of five electromagnets, before being decoded to print the corresponding character on paper tape.

If not, then for you the first modem was probably a crude device that transmitted data as audio via a loudspeaker into the earpiece of the phone. This wooden box with a speaker and a mic were formally known as an acoustic coupler. They were sensitive to external noise, and variances in the shape and size of handsets. The dialing done by modern modems now was done then by human fingers on a rotary dial. They were also wholly technologically unnecessary. But directly connecting the the phone network was illegal. Inexplicably such systems were still in use well into the 1990s for mobile use such as in Telecommunications device for the deaf (TDDs.)

It was 50 years after Baudot's work that George Stibitz invented the first relay computer in 1937. AT&T put a full scale model into production in 1940. This relay computer connected Computer in New York to a teletype at Dartmouth College in New Hampshire over a phone line. It was the first remotely accessed computer.  The computer was a "complex number computer" an early digital computer. This required an A-to-D conversion.

AT&T built five of these complex number computers for the military. A total of 7 existed. They were used by the military to interpolate linear sequences for aiming anti-aircraft guns. It had no electronic memory, not even Baudot's simple magnets, but instead used his five-channel paper tape in a loop. Tapes had to be swapped to allow the computer to employ different mathematical functions. Post war yet different tapes allowed them to be re-purposed. (In this sense it was reprogrammable)

In 1949 Dr. George Valley of M.I.T.'s Lincoln Laboratory had recommended computerized networking  to operate our radar stations guarding the northern air approaches to the United States. Manual (i.e. human) identification, warning and control was slow compared to computers. This project was farmed out to IBM. An experimental subsector in Massachusetts was online in 1955. The SAGE (Semi-Automatic Ground Environment ) air-defense system used modems to connect their sites.  They combined analog radar signals so they could be sent from one computer to another. (Ken Olsen from MIT designed a series of transistorized computers to run the system.) It's modems were used to communicate data over the public switched telephone network or PSTN. The groundbreaking for the SAGE System was held at McChord AFB in 1957. By December of 1961 they had networked 78 SAGE radar stations and DEW Line sites.

In 1960 the Federal Communications Commission issued the Carterphone decision causing an explosion of private development.  The commercial potential was clear, by 1962, the first commercial modem was being manufactured by AT&T the Bell 103. It was the first modem with full-duplex transmission, frequency-shift keying or FSK, and had a speed of 300 bits per second or 300 bauds. in 1972, Vadic introduced the VA3400 which was capable of full duplex operation at 1200 bit per second. In 1976 AT&T came out with their own model capable of the same. Then in 1977 the Hayes modems debuted on the market and the modern era had begun...