Crossbar's Hidden Superpower: Alternate Routing!
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- čas přidán 22. 10. 2020
- Let's talk about what happens when a call can't be routed via the first choice route, and how electromechanical switches handle error conditions.
This video is part 4 of the "Dial Equipment Appreciation" series. Please appreciate this dial equipment :)
• Dial Equipment Appreci... - Věda a technologie
From an old telephone engineer: Very good presentation, easy to understand and accurate. Some of those same principles happen on the Internet, but way too fast to ever notice as a user.
So basically the engineers who developed the protocols for the routing infrastructure most likely learned some things from learning how telephone switches like this utilized alternative routing capabilities and thus the base logic for internet data routing protocols like RIPv1, RIPv2, and OSPF were most likely based on the alternative routing functions of these telephone switching systems.
@@byebyedarren It probably is! It all evolved from this. BGP is really interesting, and a lot is said on the "honor system" of it. Hey, the world runs with it and not much issues arise .. well with BGP at least...
The most important principal that carried over is the "well, that's weird!" which I've said in front of internet routers doing BGP carrying gigabit/sec of traffic when they did something that i'm sure means they're broken.
Wow, I remember some of those sounds from my youth. I also remember another case of hardwired logic, an old Chyron (that put characters onscreen, like for station ID) that had all of its fonts done in wire wrap on giant PCB cards. I remember from "An Engineering & Science History of the Bell System" that wire wrap was invented by accident at Western Electric when a switchboard that was supposed to be wrapped and soldered was removed from service and it was discovered that it had worked just fine even though someone forgot to solder the connections. The wire wrap job on the Chyron boards was a rats nest, and I was happy to let the Chyron service guy take care of those.
Hey thats neat. Thanks!
These Videos have some magic to me i can't really describe. This 50s era relay technology just really fascinates me because of how much logic they actually contain without any software. I keep finding myself going bank to these videos. You also have a really calm way of explaining these very complex topics in an easy to understand way.
I'm also looking forward to the 5XB explanation some day and maybe I'll get around to visiting this place some day in the future.
Great ! NOW you tell me ! How come I didn't know this back in 1971 ?!?!?! LOL 😂😁
It's not the sounds I did not know about. It was the alternate routing. And I don't have the outside phone number for the CM
4:39 Boop!
Great job keep doing what you're doing, your educating the entire world how early communication systems work. I have a total of 36 years, starting out with Western than the name changes after the messenger, my primary focus was on number one electronic switching system, tsps traffic service position systems, number for ESS, the first all electronic switching system, and number 5ss first Electronics switching system with fiber optics. And I was asked to work in our technical support, until I retire. My employment started in February of 1963 with Western Electric, retired 36 years service. Today's September 26th 2022. Even though I never worked in step or panel or number one crossbar or number five crossbar other than running cable and doing all the grunt work, still enjoyed it at the time I was learning a little bit about those systems but never really worked as a specialist. So great education for me to see what happened prior to my arrival, on the scene.
I never knew about this style of switching... I'm absolutely in love with it!! That entire wall of contactors... Very cool! Glad to see someone taking care of these machines.
Today in 2020 CAMA trunks are the last holdout of the old Bell System. Today we use them to route 9-1-1 calls.
Not long before it's all SIP
This is really interesting to see in operation! Also, that's a cute old rolleiflex on the desk there too.
Can totally relate to this from my time in a 4A Toll XBar with Electronic Translator that provides the marker with alt route information ( among many other things)
Great video, understandable and professional narration. Simply loved it! Very educational!
You seem to have forgetten the 4M toll crossbar like they had in New York City, Chicago and Philadelphia, but its a great video, thank you
Hi Bryan. I didn’t forget 4M. I just didn’t list it because I figured most people wouldn’t notice the distinction. You probably know this already, but for those who don’t, 4M was a modified version of the original No. 4 Toll Crossbar. “M” stood for “modified”. It was modified to match the configuration of the later 4A or 4 “Advanced”. The need for toll crossbar systems was very great, so the first ones were installed in a “beta” configuration. They were then modified later on and the “M” was added to the name.
@@ConnectionsMuseum Thank you for that information, do you guys have a 4A or any toll switch at the museum?
@@magesnz unfortunately we don’t. There probably just wasn’t enough room to store and display one.
Absolutely incredibly video, thank you so much for documenting this!! This is wonderful old tech and you are doing a wonderful job as custodian, curator and educator.
Woot! Been wating for this vid :) great presentation!
Love these videos! Wish I could go back to the museum and visit right now....... *sigh*, But alas it will have to wait till this whole pandemic goes by the wayside.
But these are the next best thing! It almost feels like being there!
Hope the #5 crossbar behaves on video, as the wirespring maker was giving me problems when I was there in 2018.
Every video that Sarah makes is awesome!
Another fascinating video. So cool how the system can switch the call to an alternate route so fast.
For as many of these systems as there once was, it makes me sad that there aren't museums all over the country where we can play with them and learn in person..
Yeah, it’s pretty sad that nobody thought to save more of them. There were plenty to go around!
interestingly, modern internet routing in principle works this exact same way and these mechanical switches are honestly great at visualizing how it works. If a route is "too busy" or rather saturated in internet terms, data packets may be sent through a different route (which may be through a physically different cable or location). Thus when downloading large files, such as video, or multiple files at once, like with games, different parts of the large files or even different files may be sent along different routes and since that can cause the data to arrive out of order, data packets are numbered so the receiver can piece them back together in the right order - this is all part of the TCP protocol. When this re-rerouting fails for one reason or another, that's when you get timeouts and connection failure, just like with phone calls. This is most obvious when you watch streaming video, like here on youtube, where the video suddenly stop and buffer because the stream is sent through a sub-optimal path or the path doesn't have enough remaining bandwidth. Routers generally figure out the optimal path based on response times rather than channel hunting like with the mechanical switching, but the basic principle is the same - the goal is to find a path from A to B
Response times, and data from other routers (BGP at the edge, generally EIGRP/OSPF/IS-IS within a network) that may or may not include things like link bandwidth, a reliability index (observed packet loss), or, yes, RTT.
This is all done transparently by routers, neither the sending side nor the receiving side really know nor care. One of them packages a stream of bytes (a file) into TCP segments, and packs that into an IP packet for routers to make their calculations. The other side just unwraps the packets and uses the TCP sequence number to ensure order. QUIC does the same thing, but is built upon UDP without some of the baggage that TCP has.
Generally, one TCP connection tends to take the same path through the internet, it's not an intentional choice for large data transfers or long-running connections to have separate routes.
The nice thing about routers is that they almost always have their path allocations laid out, it's called the Routing Information Base (RIB) for the higher-level logic, and the Forwarding Information Base (FIB) for what it actually does. The RIB may contain multiple routes to a destination (prefix), but only the most optimal route is installed into the FIB and actually utilized. Gateway protocols (BGP/OSPF/IS-IS/etc.) update the RIB as new data comes in about the state of links. Fun fact: every BGP enabled router on the internet (excluding iBGP, different purpose) more or less has a complete map of the internet, because they all synchronize eventually. BGP contains path data (what systems were in the path between the originator or a prefix and the router receiving it's information), which helps route selection. The "global internet routing table" is the theoretical RIB managed by internet BGP, and it's... Huge. But every BGP enabled router has an entire copy of it, to route traffic from wherever, to wherever.
I absolutely love these videos ❤️ I used to manage an Avaya (and before that, a Lucent/AT&T Definity) PABX, and hearing all these terms (Trunk Groups, AAR, etc) applied to real-life hardware is fascinating and illuminating! I just wish I lived nearer to Seattle 😔 Keep up the good work, folks 🙏
Thank you!
Awesome ! AWESOME videos !
Wow! This video was a TREAT! 💛
Fantastic!! Love it. Thank you very much for doing all that work and sharing it. Do you also have CO cord boards on display?
Excellent.
Is the crybaby a shared piece of equipment similar to the Audichron where your call just gets connected to that shared circuit, rather than holding crossbar equipment busy to play to the crybaby tone? Is the busy tone also the same thing?
The Audichron was usually shared between multiple offices in a city, but the crybaby tone generator was per-office. It was comparatively small (only 2 or 3 mounting plates high), and much less expensive than an Audichron, so there would be less need to share it.
While you're connected to the crybaby tone, there are three major crosspoints that are being held up: the line link (for your line), the district link (for your district junctor), and the office link (for the trunk to the crybaby generator).
The crybaby is just another destination...another trunk on the office link, just like the hundreds of other trunks that leave your office. Some go to other telephone switches. Others go to announcement machines. Still others go to the crybaby.
Busy tone is different. The busy tone is a *function* of your district junctor or incoming trunk circuit, not a destination, like the other things. So you don't really "route" a call to a busy signal. You merely operate a relay, which cuts in the busy signal tone to the trunk you're already sitting on. Ringing tone is the same way. It is not a destination, but a function provided by relays in the trunk circuit you are using.
not many know what the tandem did good job
Sarah, I Love your videos! I have a question about the panel system. Panel was before dad's time, he worked on 1xb n 5xb. Switchman dad taught us kids the difference between the crossbar busy and overflow signals. Destination line busy was a slower beep and "overflow" was a faster beep. A knowledgeable person knew the difference. Question: Was the panel system capable of returning an overflow signal to the calling party or was it just busy whether it was destination or trunk?
Course-worthy presentation. I've heard that crybaby tone. So THAT's what happened to the call!
Great Analog Computer
@ 15:23 - It sounds like Kitty Carlisle, singing her No Such Number tone!
" Before the widespread use of recorded intercept announcements, the Phone Company hired Kitty Carlisle to answer misdialed calls and intone a “pleasing, yet distinctive and arresting” noise to the calling party. A distinctive tone was desired, which could be easily remembered and associated with the idea of disconnecting and then dialing the number again. "
Where did the link to Kitty Carlisle come from? I found the bit you quote on (what I assume is) your website, but the April 1941 Bell Labs article merely notes "various combinations of existing tones were tried... [then] an entirely new sound was developed."
I know you probably get asked this often, but where did you acquire your wealth of knowledge about old telco equipment?
The easiest way to learn is to work on this equipment with your hands. I'm fortunate to live here in Seattle, and volunteer at the museum. I enjoy working on these old machines very much, so naturally, I spend a lot of time doing it.
The switches need constant maintenance, and as we add more and more equipment, they need more and more circuits wired in. I'm (mostly) the one who does all of that!
@@ConnectionsMuseum all of our (now all retired) really good switch techs had worked with the vendor, installing the switch in the office they worked. No way the five day training that became standard could ever match that.
@@ConnectionsMuseum Your enthusiasm and dedication to preserving and demonstrating this equipment is a huge asset to us.
@@boballmendinger3799 Sarah here, responding personally: For my day job, I am an instructor at a large tech company. I am disappointed at the general trend of less training/faster training/computer-based training. I find that it produces less confident, less capable people, but it makes management happy by lowering costs, and giving the appearance that we're doing more with less money. The whole industry is going that way, and in my opinion, its a mistake.
@@ConnectionsMuseum I could not agree with you more. I find today most technical people know absolutely nothing about what they're supposed to be experts at, including the Geek Squad and things like that, I was with a friend of mine I said to him let me just go over to these people and ask a simple question and see how they respond. When I asked the question something very technical they had no idea even when I was talking about, but they try to give you a snow job well we'll have to take them and have our technicians look at it blah blah blah blah. Very disappointing I spent many many years in training with Western Electric AT&T, starting with, number one ESS, electronic switching system, tsps traffic service position systems, that's when we upgraded manual operators to electronic switching systems, ETS, electronic translator systems, number for ESS, First digital electronic switching system, number five ESS, Electronics switching digital fiber optic system. Many many years in school and I was an expert in my field at the time. Not bragging, just making a comment. Today is September 12th 2022. My employment started with Western Electric in February 1963, have a total of 36 years service.
awesome video!
Just a question, when this kind of system was in use, was the route of last resort sent to a busy tone or sent to an operator or a circuit that reports the problem? or was it always sent to a crybaby tone?
Just keep thinking about because i don't actually know but would be cool if you could clarify.
Good question! The route of last resort was used if all of the first and second choice trunks were busy. That would usually be an announcement or operator, depending on local practice. If all the announcement trunks were used up, then the route of last last last resort was to just leave you sitting there with no audio at all until you eventually hung up. That didn't happen often though, as it would need to be a major failure to use up all of the previous possible routes.
Priceless videos. Great job. Forever grateful.
/S/ Your biggest crybaby!
I was taught it was re-order tone but it matters not.
Was any of this call routing compatible with GETS(Government Emergency Telecommunications Service)? Or did GETS come along later after this equipment was retired.
I’m a retired X-bar 5 centrex switchman. When they tore my office out I salvaged my MTFs like link. If anyone can use it please contact me!
Hi Robert, i’m not sure what you mean by “MTFs like link”. Did you mean to type something else?
I'm familiar with ARS on definiy pabx
i kind of want to know a tad more about the crybaby.
Great video but the voiceover was much too quiet compared to the other sounds. Please make the voice track volume match the volume of other sounds in future videos if you could
Thanks. Will do!
more like a router than cpu
Was this potentially what Evan door bell was getting on his tapes when there was a "wah waaa wah waaa" "woo woo woo woo woo woo" (alarm sound) or similar sound to this crybaby?
Almost certainly this is what he was talking about. The "no such number" tone had the nickname "crybaby" because of its peculiar sound.
@@ConnectionsMuseum Awesome thanks for the fast reply. Keep up the great work you do and the videos as they are really interesting :)
I just came from Evan doorbell's CZcams video, lol