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...

The Modem (Part 1)

Older readers might associate the word "modem" with the dial-up noise. That noise is the sound of the modem, communicating with another modem. The first tones set a speed they can communicate at. Then the SYN-ACK sets other parameters like the bit number and parity. Then they check the rate, set up duplex for simultaneous communication and viola! throughput. For younger readers it may mean nothing at all. It's just a very annoying noise. A modem is a device that modulates an analog carrier signal to encode digital information and demodulates the signal to decode the transmitted information. The word is a Portmanteau of Modulate and Demodulate.

modem (ˈməʊdɛm) — n computing a device for connecting two computers by a telephone line, consisting of a modulator that converts computer signals into audio signals and a corresponding demodulator

Most sources cite the first modems as being invented in "the 1950s" a reference to the DEWline (Distant Early Waming) or sometimes specifically will refer to the PC modem which was invented in 1977 by Dennis C. Hayes and Dale Heatherington. Neither of these were the first modem. There were modems long before there were computers and even long before there was a cold war.

The modem evolved from news wire services in the 1920s. In order to transmit data over a phone line it had to be converted (modulated) into an analog signal so it could be multiplexed on the telegraph wire. To receive it, the same analog signal had to be converted (demodulated) back into signals the telegraph could receive.  This allowed a much more efficient use of the available spectrum. While the goal as multiplexing, those modulate/demodulate processes are comparable to those of later modems. Note how much that sounds like the A-to-D and D-to-A conversions you do in digital audio.

The Philosophy of PCM

In short, PCM (pulse code modulation) is a digital schema for transmitting analog data. It and it's derivations are how most electrical communications work today. According to the National Inventors Hall of Fame three different men invented PCM (pulse code modulation.) A similar patent was filed by John R. Pierce in 1945 under U.S. Patent # 2,437,707. The three of them had worked at Bell Labs, and jointly published a unifying document "The Philosophy of PCM" in 1948, a victory for geek inventors everywhere. So how did we get here?

It starts with the telegraph. TDM (time-division multiplexing) was patented in 1853 by Moses G. Farmer, [pictured] a telegraph operator who was employed as a technician to maintain a network of telegraph lines north of Massachusetts. Farmer was trying to interlace samples from multiple telegraphy sources, and convey them over a single telegraph cable. He at least proved the viability of future duplex and quadruplex systems. This was borne out by the work of W. M. Miner in 1903. He successfully multiplexed multiple telegraph signals. He managed to achieve a sample rate of 3500–4300 Hz.

But this was not PCM. It was PAM (pulse-amplitude modulation) In PAM the data is encoded in the amplitude of a series of signal pulses. PCM reuires an A-to-D conversion (analog to digital) First the analog signal amplitude is sampled at a known time interval. The amplitude of the signal at each sample is rounded off to a power of 2 (2,4,8,16 etc) a process is called quantization. This is represented by binary digits three, four, five, or six binary digits (aka bits) respectively. So the output is now just a string of binary numbers. See the chart below:

Decimal	Binary number
0	0000
1	0001
2	0010
3	0011
4	0100
5	0101
6	0110

In 1926 Paul M. Rainey at Western Electric patented a facsimile machine which transmitted its signal via PCM. It was using a 5-bit encoded PCM. It was never sold commercially but the idea was taking hold. This was it's first use transmitting an image. Though telegraph and fax are both both used to transmit text, the fax sends an image with text on it. The telegraph sends dots and dashes directly correlating to alphanumeric characters. A fax can send non text data. You can fax a xerox image of your middle finger which I have done on occasion. Alec Reeves invented PCM on his own in 1937 unaware of any prior work. He was a British expat working at the International Telephone and Telegraph in France. He patented the concept in 1938 but no working model. His U.S. patent # 2,272,070 was granted in 1943. More here. He lived long enough to see it used in WWII by the allies, though it was so top secret he probably didn't know until after the war. SIGSALY went into service in 1943 and was in military-only use until 1946.