Editing Optical telegraph (section)

==As first data networks==
The optical telegraphs put in place at the turn of the 18th/19th centuries were the first examples of data networks.<ref>Holzmann & Pehrson, p. 214</ref> Chappe and Edelcrantz independently invented many features that are now commonplace in modern networks, but were then revolutionary and essential to the smooth running of the systems. These features included [[control character]]s, [[routing]], [[error control]], [[Flow control (data)|flow control]], [[Message precedence|message priority]] and [[symbol rate|symbol rate control]]. Edelcrantz documented the meaning and usage of all his control codes from the start in 1794. The details of the early Chappe system are not known precisely; the first operating instructions to survive date to 1809 and the French system is not as fully explained as the Swedish.<ref>Holzmann & Pehrson, pp. 210–216</ref>

Some of the features of these systems are considered advanced in modern practice and have been recently reinvented. An example of this is the error control codepoint 707 in the Edelcrantz code. This was used to request the repeat of a specified recent symbol. The 707 was followed by two symbols identifying the row and column in the current page of the logbook that it was required to repeat. This is an example of a [[selective repeat]] and is more efficient than the simple [[go back n]] strategy used on many modern networks.<ref name="Holzmann & Pehrson, p. 216">Holzmann & Pehrson, p. 216</ref> This was a later addition; both Edelcrantz (codepoint 272), and Chappe (codepoint 2H6)<ref group=note>The notation here follows that given in Holzmann & Pehrson (p. 211). The two digits represent, respectively the angle of the left and right indicators. Vertical pointing up is "1" and each successive 45° from this position increments this number. "H" means the regulator is in the horizontal position and "V" the vertical.</ref> initially used only a simple "erase last character" for error control, taken directly from Hooke's 1684 proposal.<ref>Holzmann & Pehrson, pp. 214–215</ref>

Routing in the French system was almost permanently fixed; only Paris and the station at the remote end of a line were allowed to initiate a message. The early Swedish system was more flexible, having the ability to set up message connections between arbitrary stations. Similar to modern networks, the initialisation request contained the identification of the requesting and target station. The request was acknowledged by the target station by sending the complement of the code received. This protocol is unique with no modern equivalent.<ref name="Holzmann & Pehrson, p. 216"/> This facility was removed from the codebook in the revision of 1808. After this, only Stockholm would normally initiate messages with other stations waiting to be [[Polling (computer science)|polled]].<ref name="Holzmann & Pehrson, p. 216"/>

The Prussian system required the Coblenz station (at the end of the line) to send a "no news" message (or a real message if there was one pending) back to Berlin on the hour, every hour. Intermediate stations could only pass messages by replacing the "no news" message with their traffic. On arrival in Berlin, the "no news" message was returned to Coblenz with the same procedure. This can be considered an early example of a [[token passing]] system. This arrangement required accurate clock synchronisation at all the stations. A synchronisation signal was sent out from Berlin for this purpose every three days.<ref>Holzmann & Pehrson, p. 188</ref>

Another feature that would be considered advanced in a modern electronic system is the dynamic changing of transmission rates. Edelcrantz had codepoints for faster (770) and slower (077). Chappe also had this feature.<ref>Holzmann & Pehrson, pp. 215–216</ref>