Backward Compatibility Re: 202401100645.AYC Re: IPv4 address block

Mon Jan 15 19:12:26 UTC 2024

On 1/15/24 09:37, Abraham Y. Chen wrote:

> 2)    Allow me to share with you an almost parallel event in the PSTN, 
> to illustrate how tough is to achieve the replacement of a working 
> service, even under an environment with very strict backward 
> compatibility disicpline:
> 
>      A.    The Decadic (rotary) Dialing (DD) technique worked well on 
> the telephone loop to a certain limit of distance for many years that 
> enabled the automatic telephone switching systems. But, it could not go 
> beyond the CO (Central Office).
> 
>      B.    So, Bell Labs studied the use of DTMF (Dual Tone 
> Multi-Frequency) or commonly known as Touch-Tone as trademarked in US, 
> etc. The work started in mid 1940s.

Actually, Bell had a multifrequency interoffice signaling system long 
before Touch-Tone was introduced to the public. Many of us old-timers 
were *very* familiar with this, much to the discontent of the Bell System.

>      F.    Initially, AT&T offered the DTMF service for free (well, 
> covered by the rental of the new phone) to encourage that adoption. 
> Then, they raised the monthly service rate for lines still requiring DD 
> receiver hoping to gracefully forcing the subscribes to wean from using 
> DD phones.

In the early days of deployment, DTMF was not free. It was typically $1 
more per month. IIRC, there was at one time an upcharge for 12-button 
vs. 10-button Touch-Tone pads. I have never seen a tariff with an 
upcharge for pulse dialing.

>      G.    Guess what, the inertia of the huge DD phones out there in 
> the field accumulated through near one century made the strategy 
> impossible. That is, many subscribers would rather to pay one extra 
> dollar or so a month to enjoy having the old DD phone around, either to 
> avoid paying a new DTMF phone or just for the antique look of the DD 
> phone. This also created a nightmare of three types (DD, DTMF and 
> combined) line cards in the CO.

With step-by-step, XY, or panel offices the DTMF receiver was an add-on 
that buffered the digits and outpulsed them at rotary dial speed. Pulse 
dialing always worked. Crossbar was also an add-on but with a crossbar 
marker the delay of converting to pulse was avoided. By the time ESS 
came around both pulse and DTMF were built in.

Again, when and where was there ever an upcharge for pulse dialing?

>      H.    As this went on, a version of phone with DTMF dial pad but 
> sending out DD pulses appeared on the open market (thanks to the 
> deregulation / break up the Bell System). Such novelty phones really 
> gave phone companies a hard time about the transition plan.

The purpose of these phones was actually the opposite. It allowed a 
"modern" keypad-equipped phone to function on older lines not equipped 
with a Touch-Tone receiver. In GTE territory with Strowger switching, 
the digits from DTMF phones were buffered in the CO and outpulsed as 
rotary dialing. Bang out the number with Touch-Tone and you could hear 
the tick-tick of the digits being sent while you waited.

These days people get upset with post-dial delay of more than a couple 
of seconds. It used to be substantially more, especially with 
interoffice calls.

>      I.    In the meantime, IC technology advanced to have single chip 
> capable of both dialing techniques (even further integrated other 
> traditional line card functions onto the same chip) making the 
> transition plan moot.

TTBOMK, every common BORSCHT chip accepts both.

>      J    Nowadays, almost every line card type chip handles DTMF as 
> advertised. But, if you try a DD phone on it, chances are, it works anyway!

Yes, because TTBOMK, telco central offices have always accepted pulse 
dialing and still do. SIP ATAs, on the other hand, mostly don't, with 
the exception of some older Grandstream units.

-- 
Jay Hennigan - jay at west.net
Network Engineering - CCIE #7880
503 897-8550 - WB6RDV