Somewhere I had heard that the actual switching capacity of local phone companies was far below the actual number of installed lines. Essentially what it boils down to is that one of the basic design assumptions was that a large majority of extensions would always be idle.
Where this would cause problems is when a catastrophe such as an earthquake or hurricane would knock a significant number of phones off the hook at the same time. My recolection on specific numbers is vague, but I belive it was supposed to be that if there were more than 20% of phone recievers knocked off the hook at the same time, the system would be crippled, unable to process incoming or outgoing calls until enough extentions had been hung up.
Does anyone know if this story still holds true? Or is it an artifact of a bygone era, with advances in phone switching technology having increased the network’s capacity to handle such a disaster?
It was and still is true. The switching capacity at your local telco Central Office is far less than the total number of subscriber loops connected to it. I’ve heard numbers anywhere from 10% to 30%, so I don’t know how many phones it actually takes to use up all the capacity.
Also, the problem isn’t so much phones being knocked off the hook, as it is everyone trying to call everyone else at once (most every CO in NYC and DC was swamped on Sept 11, 2001, for example.)
It’s true. We use the Straight Dope Message Board system of customer service. Get a zillion people to sign up for service but hope and pray that no more than 50 actually use it at any one time.
And it’s not exactly true that the system would be “crippled”, just filled to capacity. The trunks are usually the bottleneck. When there are no available trunks, you get what we call a “fast busy” signal, meaning no message trunks available.
Maximum capacity varies quite a bit depending on the LATA in question.
In the UK there is a difference between a Business and a Residential telephone line. A business line is gauranteed a connection into the telephone exchage when the phone goes off-hook whereas a residential customers have to share a (large) number of exhange inputs. It is just possible for a residential customer to pick up a phone and immediately get the engaged tone - before attempting to dial. This means that there are no lines into the exchage available.
My last job was working for a medical center in downtown Chicago, and maybe once a month I’d be trying to make a phone call after 4:00 pm and get a message that “all circuits are currently busy”. Waiting a couple minutes and trying again would usually let me connect.
The actual calculations to determine how much switching and trunk equipment is needed is quite complicated. You have to balance cost against levels of service. This " grade of service " is measured in Erlangs and the link below shows that software is available to help with the calculations.
In one earthquake preparedness show on a local PBS station here in Southern Calfiornia, the fire fighter giving the talk mentioned that after an earthquake if you are out and about and see a pay phone off the hook to put it back on the hook.
A lot of payhones are knocked off during earthquakes and this makes the strained telephone system creak even more.
Yeah, but don’t the phones automatically go dead about a minute after they come off the hook, assuming nobody pushes a button? It seems to me that putting payphones back on the hook would be a bit pointless.
