larry@uunet.uu.net (Larry Lippman) (04/12/90)
In article <6176@accuvax.nwu.edu> Robert.Savery@f666.n285.z1.fidonet.org (Robert Savery) writes: > >I saw a phone once ( Bell system 2500 ) that had a push to talk switch > >in the handset..... > When I was in the Air Force, I worked in a whole building of these > things. I can't remember who made them, but as the whole system was > older than the hills, I'm sure they were Bells. While Western Electric and the Bell System had the lion's share of the *fixed* installation military telephone communications market, Stromberg Carlson (div of General Dynamics) and Automatic Electric (later a div of GTE) were also well represented. I personally have a "souvenir" red secure telephone that I "rescued" from a DPDO scrap pile many years ago; this is a 4-wire device with PTT handset made by Stromberg Carlson. > As the entire building was a restricted area, quite often we'd be > discussing classified information when the phone rang. The PTT switch > was an added precaution to ensure that the person calling in didn't > hear anything they weren't supposed to. Since the phones were on > unsecure lines, you could never tell when Ivan might've been > listening! While the PTT switch may have appeared to benefit security, this was most likely not its intended purpose. Chances are the stations which had the PTT handsets were 4-wire with connection to AUTOVON and/or dedicated command circuits. While AUTOVON requires a 4-wire circuit and 4-wire station set, a PTT handset is not required. However, dedicated command circuits (i.e., "hotlines" which ring a predefined set of stations without requiring dialing), which are invariably 4-wire, often create a fixed conference arrangement with a large number if stations. The PTT handset is used to eliminate what could be a horrendous amount of background noise if the common receive path summed the ambient noise from all of the station transmitters. In addition, dedicated command circuits usually have a multiplicity of alternate routing possibilities to assure reliability. Such routings may range from landline to HF to UHF to troposcatter to microwave. In the particular case of the HF and UHF options, the PTT handset assures positive transmitter control instead of relying upon VOX. Since the telephone station set does not know what communication medium it is feeding at a given time, a PTT arrangement assures compatibility with all of the above media in a manner which is transparent to the user. Furthermore, many military fixed installation station sets are multiple-line, in which case if one line requires a PTT switch, then its operation carries over to all lines appearing on such a station set - even if they don't require it (like an AUTOVON line). While I have seen multiple-line station sets with an internal switching relay for 2-wire/4-wire operation depending upon the line selected, I have never seen one which enabled or disabled a PTT switch - i.e., the PTT switch was always enabled. Incidently, 4-wire stations with PTT handsets are not limited to the military, NASA or the FAA. While they are now being replaced with alternate methods of communication, for many years 4-wire conference circuits were extensively used by interstate trucking companies for coordination of dispatch, tracing and OS&D operations. This type of 4-wire conference circuit was commonly called a "full-period line" (somewhat of an anachronism) or "hoot-n-holler line" :-) [really!]. The most common implementation of this circuit involved one or more 4-wire station sets (sometimes with PTT, sometimes not, at discretion of facilities design engineer) at each location, along with 106-type loudspeakers in close proximity to the stations. The net result was that every station could be heard on *every* other station and *every* loudspeaker. When one station wanted to call another, they would pick up their telephone and announce in a loud voice whom they were trying to reach. The recipient would hear their name or location on the loudspeaker and then pick up their station to converse. Some of these circuits later used selective signaling (but not selective *talking*) such as SS1 or later variants to eliminate the loudspeaker. I betcha at least one TELECOM Digest reader has at one time worked for an interstate trucking company and used a "hoot-n-holler" line. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {boulder|decvax|rutgers|watmath}!acsu.buffalo.edu!kitty!larry <> TEL 716/688-1231 || 716/773-1700 {utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 || 716/773-2488 "Have you hugged your cat today?"