I was talking with an old friend from my time at Ma Bell,
> and the subject of Number 5 Crossbar came up.
> Are any of the 5XB offices still in service? If so, where?
Other than the Museum of Communications in Seattle and other similar places, I wouldn't think so, at least on the PSTN in the US and Canada. My possible reasons include:
The originating registers can't hold all the digits to support dialaround to all those long distance carriers.
Finding paper tape for the centralized automated message accounting punches wich record toll calls can be problematic. Finding and maintaining equipment to read the paper tape is more problematic.
Originating a call to another switch would require in-band MF pulsing the originating telephone number to the terminating switch or toll tandem, so caller ID can be maintained.
Users want custom calling features like call waiting.
Users want a clean connection so when they do dial up a modem connection, they can get speeds approaching 56kbps.
Where I live, the central office decommissioned their last #5XBs in the early '90s. Half the customers at that time were on a #1AESS, and I believe we were all cut over to a #5ESS at that time, which we still use today.
Interesting history! I'm reading the "Switching Technologies" book right now and I seem to recall that Bell experimented with common control for things like SxS and even better CC for #1 and #5 Crossbars.
In the end it was all for naught though since the #1 ESS and more importantly #4 ESS spelled the end of crossbar.
I can only speak to the history of Pacific Telephone. In the Los Angeles area they had a whole bunch of both urban and suburban steppers. Bell Labs/Western Electric could never get common control to work for steppers. So, they installed a crossbar switch in each C.O. to handle the common control requirements for regional toll and nationwide direct dialing. Those crossbars were also growth end-office machines.
General Telephone of California wouldn't invest in crossbar. So, Automatic Electric developa piece of poorly working equipment called "The Director," which handled toll for all their steppers. Those directors had a high failure rate in processing toll calls, which made it appear the steppers were failing. It wasn't the steppers, though, because call completion on non-toll calls was very good.
In essence SxS remained pretty much the same until the end. However, the Bell System did implement a number of electronic modern improvements to SxS to improve its performance and efficiency and reduce cost.
Full "senderzation" for SxS was complex, but a partial mode was practical. For plain vanilla calls, the digits were outpulsed as normal to the steppers. But for toll calls and others , they were routed to the tandem switch and possibly translated which made for better trunk usage. Inter-exchange and toll trunks were costly and much Bell System effort was intended to make good use of trunks-- enough to meet service needs of peak demand, but not so much as to be wasteful. Also of course ease of maintenance, debugging for trouble, and network control.
The Bell System history provides many details on this "add-ons" given to SxS over the years. This included things like Touch Tone, ANI, and some AMA. SxS could and did also support Centrex.
Remember when DDD first came out there was no ANI, they used ONI where an operator keyed in the calling number. This continued in use well into the 1970s until ANI became universal. The modern #4 ESS toll switch provided for ONI.
Remember that the type of equipment in a given SxS exchange varied greatly. Some exchanges had many add-ons, but others couldn't even support DDD even into the1970s. (When the Bell System went to cheaper dialed direct rates, customers in exchanges that didn't have DDD still go the cheaper rates).
We forget that electronics were still extremely expensive even in the
1970s, so relay technology remained cost-effective in an existing application. In certain small exchanges or PBXs, SxS remained the cheapest way to go since common control, be it crossbar or ESS required some mandatory overhead which was wasteful if not many lines were served. It took a while to make small switches cost effective with electronics. (The same applied to computers in general, right into the 1980s classic punched card tabulating machines (1940s design) were most cost effective than electronic computers in certain limited applications).
BTW, crossbar could and did handle overseas calls.
Crossbar could also provide Call Waiting and was successfully tried out, but it was too expensive to implement in production service.
New generations of electronics significantly reduced the cost and footprint size of ESS making it desirable to replace electro- mechanical equipment faster than originally planned. For instance, many places needed more switching capacity and going to ESS was cheaper than building a new CO. Technology exploded between 1970 and
1980. ESS also offered special service features that brought in more revenue. Somewhere in that time frame digital switches and trunking as well as SS7 came out as well.
Didn't AE have their own version of crossbar?
********* Moderator's Note *************
As a young, poor student studying under the G.I. Bill at Santa Barbara City College in 1978, it took me about 3 calls to realize that the GTE payphones on campus had a serious security flaw: the phone worked normally with or without a coin in the slot, except that without a coin deposit, the dial didn't function.
The local step exchange had been converted to Touch-Tone(tm) ...