Connections Museum - 2020-08-21
In this video, we explore MF signaling! This is part 3 of an ongoing series called "Dial Equipment Appreciation" 0:00 - Intro & History 03:55 - Tones and what they mean 06:55 - MF phone call 07:40 - MF phone call NOW WITH TONES! 08:58 - MF receiver 09:55 - Digits and what they mean 11:30 - Blue Box
Wonderful. I've been trying to encourage several of the collectors with working switches to produce informative videos like this series, to no avail. I'd like to see one explaining the wiring on the back of SxS switch contacts. You are a good teacher. Thank you.
Thank you for that very accurate description of MF signaling. MF signaling was one of the considerations for use as "touch tones", but was rejected due to talkoff (human voice triggering of tone). Regular MF receivers were normally never exposed to the actual voice path.
"Normally" being the operative word. Sending a locally generated 2600hz tone would force a disconnect on the far end, and when removed, the far end would "wink" its return 2600hz, indicating/signalling the availability of an MF reciever. I once learned to actually whistle 2600, and if one walked through a busy airport terminal with banks of businessmen on long distance calls, one could be a dick.
Honestly, I was that dick...then I grew up. 😳
@DavidFromRI That only works on trunks that use 2600hz for supervision. A lot of circuits did/do not use 2600hz for supervision signaling. They may use other methods such as Reverse Battery. A lot of 9-1-1 trunks in use today use a combination of MF with Reverse Battery signaling. If you dial around in Russia you will find a lot of 2600hz SF trunks that you can blue box. Lookup "1VF Signalling" on YouTube and you will find a good modern-day example of blue boxing.
@Tieline Good point, thank you for the clarification. In fact, I couldn't hear 2600 in the discussed video, which kind of makes my post unrelated. My error is a result of the limited scope of my own experience, which is minimal SXS (as a kid), 1XB/5XB, and limited ESS. I was a TSPS operator for a few years though; that was a hoot!
I remember those famous crossbar clicks so well. My exchange was 469 and it was crossbar and if I called someone else on the 469 exchange I would hear the click and it would ring immediately. If I called my grandmother who lived in a sister city on the 423 exchange, I’d hear the first click and then a second click and it would ring. Now I know why. The 469 exchange was sending the MF tones to the 423 exchange during that brief pause. You’re correct, there were no audible tones. Just dead silence for 2 or 3 seconds. I never knew exactly what it was doing. It would also do a longer pause when calling a step by step exchange because I’m guessing it would need to pulse dial the last 5 digits of number since crossbar required pulse dialling.
I'm so glad I found this channel! I am remember back in the early 90's the exchange I was on in 312- 878 I was using pulse because there was a fee for using tone dialing, and that there was an upgrade at the exchange that made tone dialing "free". Also meant my modem connections I'd be using ATDP sequence. :-)
Remember the tones used to dial on a line are not the same tones used to dial on a trunk.
The name "Touch Tone" is a copyrighted name for the tones generated by the telephone set you would have in your home connected to your telephone line. non Bell venders could not use the term and had to use the generic term for the tones DTMF ( Dial Tone Multiple Frequency, Dual Tone Multiple Frequency). Older SXS central offices had to have a lot of equipment added to the switch for TT dialing. This is why the extra fee. Electronic Switching Systems could complete a call faster and cheaper than a dialed call. so no more fee.
It's awesome to see this and then listen to them in action on Evan Doorbell's tapes / channel.
My grandmother used to describe the switch to this system at Ohio Bell back in the day. Very instructive and appreciated!
The 314 Berry box brought back memories. Haven't used one for 32 years.
They were a handy tool that I would have a hard time using now.
Exploding the Phone is a great book. I cant wait to see some more on MF tones and the Phreaks of the past. =) As always great video.
Hi Sara. Really enjoying your videos. Trying to find one that shows how the systems handle thousands of subscriber lines, in terms of termination and digital switching. Is each subscriber's line terminated individually at the exchange, or is it normally multiplexed at the termination point. As someone long fascinated by computer theory, telephone exchanges seem to encompass all of the principles of computing but on a much grander scale in terms of shear heft and widespread use well in advance of large scale adoption of industrial computing elsewhere
I am sooo jealous of y'all's jobs!! I can watch switches go for hours. Keep it coming please!
I would love to hear more recordings from the museum. I love this series so much.
Thanks for making and sharing this bit of video. I've seen lots of stuff about MF signalling but this is a good in-depth explanation. I was waiting for the bit about phone phreaking, and I now look forward to the next video!
Hi folks, In the early 1970s I worked at the then UK Post Office Telecom HQ (now British Telecom) in London on introducing American made MF convertors to allow key phones to replace dial phones on some of our even then rather ancient Strowger electro-mechanical exchanges. Back then I could recognise the digits being pulsed by ear. I also ran some end to end long distance tests to evaluate MF use for slow speed subscriber (aka customer!) data - eg for ordering from printed catalogues that were all the rage back then.
didn't realize the MF system was so old, but that explains how the mechanical switches were able to handle DTMF many years before they went digital and the support for pulse dialing was terminated
I worked the 16B test board in AT&T long lines and there was a MF signal key pad on the board. You could plug the test pad into a speaker and have some fun. I learned to play something very close to the song played before the old Laurel and Hardy films. It was really funny .
The Touch Tone Detection circuits are another world of their own. when you build these circuits you can see all sorts of tones and noises inside the circuit and how the human voice can trigger tone detection. Thats one reason why the tones generate so much volume into the circuit.
The tones come from a two out of five, combination of tones. Today's date is September 9th 2022. That's the number one crossbar office. Number one ESS, electronic switching systems, first introduced in 1964 or 63. My first exposure to number one ESS was in Brooklyn on Bridge Street. As soon as I saw the equipment for the first time I said that's where I want to be, I did not want to be part of old school crossbar equipment. I went to school for number one assess, ETS, electronic translator system, tsps, traffic service position systems, number for ESS, electronic switching systems digital All Digital, and number five ESS first digital fiber optic switching system. Today is September 9th 2022.
When I was young (in the '80s) I was really into telephony, having managed to get a few tours of local (UK) switches; One ancient Strowger system, and a slightly newer TXE4 (common control, reed relays), plus a System X switch (fully electronic, the new shiny - at the time, I think it's since been superseded by fully digital what with the 21CN rollout, eventually there'll be no analog lines left in the UK) that was being trialled at the time. Then as now, I had absolutely no idea how it all worked, but it's still fascinating. Thanks, as ever, for your informative videos!
Standard Touch Tones also called DTMF, thats Duel Tone Muli-Frequency. There are single tones for each row and columb. Pressing anytone generates TWO Tones indicating the Row and Columb to make up the two tones. So, the Rows and columbs, each has its own tone. ROW One has 1, 2 , 3, A, Row two 4, 5, 6 and B, Row Three has 7,8,9 and C, Row 4 has * ,0 ,# and D
I'd love to see an explanation of how the digits stored in the registers are transcoded into MF relay operation. I haven't watched parts 1 and 2 yet cause I need some uninterrupted time to do so, but hopefully that will show how pluses are coded into the register relays.
That's actually a great question, and its something I've struggled with a bit in presenting these videos. One of my "rules" for this series is that I won't ever show a schematic. That may sound crazy, but I really wanted to present these switches in a general way that could be appreciated by most audiences, regardless of how technically savvy the viewers are.
I know there are those among us who would appreciate a more detailed breakdown of circuit operation, including some schematics. I also know that there are folks who watch these videos who may have little to no hands-on experience working with these kinds of machines. So I have to be in the middle somewhere. At least for this series.
I don't have time at the moment to look up a specific answer to your question, but you might find some helpful resources at http://witch.spmh.us/library
@Connections Museum You're right, schematics are helpful and are probably available already. At least the way I learn, it helps to have an explanation to be able to then understand what a schematic is saying.
Not sure you ever took a digital electronics class or computer architecture class, but they explain like what an opcode, in binary, does when it gets to a CPU and then suddenly it makes all sorts of other things make sense.
I recently came into possession of a 6 pole Union Switch and Sign DN-10 neutral relay and it was only after I had held it did I fully understand how one worked and that made all sorts of other stuff make sense.
Also You video on how the Marker decoded office codes with the hard wired terminal boards fully explained that part of the machine and I get it.
Tone signaling both MF and DTMF (TouchTone) was a huge advance. Being a geezer geek it bothered me when the Telcos introduced touchtone phones they charged a premium because as you mentioned tone signaling saved them money on capital equipment.
This is so awesome! I've long been curious about how crossbar worked (former DMS-100 tech here). I do have a dumb question: how does the originating sender get signaled that the terminating sender is ready for it? I know with RP you know the terminating sender is ready when it starts winking...but how does that work with an MF sender? Polarity reversal, a flash, a tone?
Thats actually a very good question! With an MF sender, the far end reverses polarity as soon as the sender signals that it's ready. So the order of operations is: the trunk is seized by the originating end, far end of the trunk finds and attaches to a terminating sender. Sender signals to its trunk that it's ready to go, trunk sends a reversal back towards the originating end.
At about 11:40, the 314A Trunk Test Set from Berry Electronics, Santa Cruz, CA, is demonstrated. Berry Electronics was an independent supplier of test equipment, selling to the telephone companies. It is interesting that the test set's case was painted blue and the test set referred to as a blue box, the same name used by the hackers for a device that exploited the long distance network's tone vulnerability.
As described in the video, MF signaling dates back to the 1940s. The combination of the SF signaling system, which used a tone to represent a trunk's status, on hook or off hook, and the MF system to transmit the called number, made the tone exploit possible.
Initially, long distance calls were handled by telephone operators, who functioned as a sort of gatekeeper, detecting unusual activity. Customer dialing of long distance calls came later, opening the door to use of the exploit without a human monitoring the circuit, other than by a possible technician or by random monitoring to check call quality.
Phone company security employees were aware of the vulnerability as early as 1960. SF could have been migrated to "out of band" signaling, possibly reducing costs, in addition to removing the particular vulnerability. SF signaling units had sophisticated filtering to minimize "talk off", the disconnecting of calls when the audio present happened to simulate the SF tone. The first SF units used vacuum tubes. They were later redesigned with transistor technology. Better than replacing vacuum tube SF units with transistor units might have been to migrate to "out of band" technology.
Besides playing a short burst of the SF tone to disconnect the call and get an MF receiver connected, there may have been other ways to get an MF receiver on line. However, that would have been more difficult and there may have been fixes. Blocking access to the MF receiver would have closed this tone vulnerability, which became a major problem in the 1970s.
As early as the 1950s, AT&T was also well aware that customer dialing would eventually be expanded to all, or nearly all, telephones, and very few calls would be operator handled. Toll free numbers must have been in the planning, at least in the early 1960s. The exploit required dialing a number to get access to the long distance network and then using the tones to divert the call to another number. Toll free numbers were nearly ideal. The number used to access the network would appear in phone company billing records. If that number were toll free, the customer would not be billed. The phone company would have a record of the call, time that it was placed and its duration, just as for any other long distance call, but the call did not appear on the customer's bill. Suspicious calling to toll free numbers was used to track down individuals exploiting the tone vulnerability.
At about 12:25, the "tea cart" is shown. Using 1940s technology, a homemade electronic tone generator might have been that size. However, there were musical instruments, such as pianos, that could be used to produce the required tones, but the piano might not be close enough to the telephone. Small, battery powered, tape recorders became available in the early 1960s. The tones could then be recorded and played back almost anywhere.
Awesome stuff. It brings back such memories. I briefly got to bluebox with my commodore 64 as a young phreaker.
Love that you have your own recording on the announcement machine! Did the original one get deleted/corrupted with time or did you decide to redo it just for fun?
In the museum, how many numbers do you have that will actually put you through to a phone? If you dial a number which would normally connect to a different exchange do you also get the "Not in service" recording?
Not sure what was on that machine originally, before we recorded over it. (It was probably back in the 80s or 90s sometime). It's likely that at the time the museum acquired that machine, there were so many of them in service that overwriting the existing recording wasn't a big deal.
We probably have between 80 and 100 working numbers. Anything non-working should get a not-in-service recording, on the 5XB and 1XB. In the panel, you'll just get a re-order tone.*
* It's slightly more complicated than that. Too much to explain in one youtube comment! :)
@Connections Museum Thanks for the reply! I keep forgetting that the place has been operating for a pretty long time so it's normal for some things to not have been regarded as rare when they were first acquired.
80-100 lines? That's so cool! At first I thought you maybe had 5 lines based on the number of handsets I've seen in some of the videos talking about the switches but now that I think about it those frames have space for a lot more than that so it makes sense.
I really love what you do there, I'm too young to have experienced it, but the network at the time of the Bell monopoly with all the electro-mechanical switches and when it was analog fascinates me and I think your museum is the only place where you can experience it at such a scale. I know there are other places and private collections but those are usually the size of a small PBX so not as cool to me.
I'm very much looking forward to more videos like the phreaking one and in the meantime I'm catching up with all these ones you've been producing during the pandemic.
I'm in Europe but when Covid passes if I get a chance to visit Seattle the Connections Museum will be my first stop :)
@connections Museum -- Sara, I Just saw this cool video, even though you posted it 2 years ago.
Part of what the "new sender" that your crossbar can do is TouchTone as shown at 6:46 of the video. When I was a kid, the Crossbar CO in my home town had TouchTone so it had the new style sender. What is interesting is that your switch uses the old fashioned "buzz" style dial tone, where as the one in my hometown had the modern dial tone associated with touch tone service. Do you have any ideas or historical perspective why Bell Companies changed the sound of the dial tone
loving your videos very interesting content keep em coming.
Actually, back in the day, the MF muting was often imperfect, so I would hear the MF tones in the background before the call connected. The phone system had some crosstalk and on many of the raw Apollo era NASA tapes I can hear faint MF crosstalk into NASA's dedicated circuits from time-to-time.
Love these vids. Keep them coming!
I would love to go visit this museum sometime. This is awesome!
I cannnnnnnot get enough of this stuff.
I was born at the wrong time, I should’ve had a career working on this stuff.
Those test sets are beautiful
If you're looking for an Evan Doorbell tape with a lot of MF phone phreaking, the two "HiFi 914" programs are great.
The MF test set is called a “Berry Box” same as a blue box or in today, a digismart.
If they had gained the technology to do these MFs, then why didn't they replace the tone wheels with this solid-state (or semi-solid-state) technology as well? And what kind of storage were these signals remembered on: core memory? And if they had that kind of technology for tones, then why did it take so much longer to come up with the electronic technology for switching too, especially when this is the same year that the hard disk came out?
(I keep thinking of the 1950s as really dark ages with hardly any technology, but then ya gotta remember: it's when the dang HARD DRIVE came out!)
6:21 sounds awesome. I wonder if that's the same sound used in the intro of the song "Connection" by Elastica? Maybe someone in the band is a phone phreak 🤔😁
I'm A L M O S T there in grasping how the senders and markers store/buffer each digit sequence. It's not easy to wrap your head around how the sender and marker are organized as to how many calls they can handle at once. All I see are a bunch of relays. Are they allocated by rows or sets of rows per call and how is each digit encoded in the relays to then be outpulsed later once connected to the term sender?
Senders and markers can only handle one call at a time, each. The holding time for a marker is about half a second, so in one second it can do the processing for 2 calls. In one minute a marker could process 120 calls. Normal #1 Crossbar offices would have 4-6 originating markers, and 4-6 terminating markers, depending on traffic load. The originating marker is the entire frame (floor to ceiling) seen better in this video https://youtu.be/bQDXI7i0SGY
The holding time for a sender is anywhere from 8-45 seconds depending on how fast the customer dials, or if they bother to dial at all. Each sender can only handle one call at a time, and its only used while the call is in the dialing stage. Once the call is connected, the sender isn't needed and is probably serving someone else. If you expect that 100 people will be dialing at once at any given time, you need 100 senders. If you don't have enough senders to handle the traffic being offered then some unlucky person just won't get a dial tone. Senders are normally housed in relay cabinets. (the old ones were, anyway). You can see me kneeling next to a sender here: https://youtu.be/wH0XArJx3No?t=574 . The racks of relays above the lamp is also a sender, but its much newer, and isn't enclosed in a cabinet.
The encoding is too complex to explain in a youtube comment. Really needs its own video.
In year 2020, on my Yealink VOIP phone, if after dialing a number (that is not part of the digit map), if I press the "send" softkey on the digital display, it generates a sound similar to that of the "ST" MF tone when sending the number to the my Asterisk IP PBX!
good job Sara. brings back old memories on the toll board.
This is great. I can't wait until the museum is open again.
I believe you said the MF tones were two out of six I believe it was two out of five to give you the frequencies I work for the Bell System 36 years number one ss tsps number 4 Ess and 5 ESS fiber optic systems I worked in technical support
The sixth tone is the value 10, used for transmitting KP and ST. The numerical digits themselves are 2-5, yes.
On ham radio keypads there are 0 through 9 and there is also * and # and there are 4 letters A B C and D
I have seen touch tone phones that have a similar keypad
The phone has
FO ( Flash Override ) in the A position
F ( Flash ) in the B position
P ( Priority ) in the C position
I ( Immediate) in the D position
I don't know if the phone company actually used the same tones that are used in ham radio
Have you seen these special phones ?
Yes, those keypads are for the AUTOVON military phone system in the US. Allowed "pre-emption" of calls, where a more important call could break through and interrupt a call already in progress.
Listening to Evan Doorbells recordings has really helped me understand. And they're pretty amusing
I hope you have plenty of 328As...those things are very hard to find and INSANELY expensive if you can find them.
Great. Now I saw what actually 'moved' when I used my blue box.
OK.. So my understanding is that people used pulse dialing, BUT touch tones were used exclusively by the phone company (between whatever dates) to send/receive/connect signals to each other... for some period of time until the tech was eventually upgraded to allow touch tone in the standard home?
Yes, except that MF signaling uses different tone pairs than the touch tone telephone does.
@Connections Museum That explains the end of the song "young lust" from Pink Floyd The Wall, album. Ever hear it? Maybe you could do a quick video sometime, explaining the sounds and telling us the phone number and accuracy of the sounds used?
Hey, there is also a great video by the 8 bit guy covering fracking with sounds generated by the Comodore 64. I used to have the program they used but never really understood what they were for except for the obvious "coin drop" sounds. Maybe that would make a great video too. I like what you're doing with the old equipment
Very good video!!
Larry Hagood - 2021-11-07
Exploding the phone is a great book. Sara, you do an incredible job explaining these systems. Thank you for taking the time to make these videos. I hope to visit the museum someday!