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The definition page on Wiktionary for "Isochronous" offers this example of the word in a sentence:

Their entire national telephone network is isochronous, with a clock distribution tree radiating from a single, protected cesium reference clock.

I know it's just an example but is it true that the telephone network is isochronous?

I was always under the impression that it's non-isochronous since telephone conversations happen in live time at any time - and they can't be constrained to any particular clock. If the Wiktionary article is correct, why is this?

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    It's just an example. I don't know of any country doing that. The time source used almost universally (in the US) by telcos is GPS. And it's used for records keeping; the network really doesn't care. – Ricky Jun 11 '14 at 16:53
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The definition of "isochronous" is that of something that happens at a regular interval. Wikipedia:

In telecommunication, an isochronous signal is a signal in which the time interval separating any two significant instants is equal to the unit interval or a multiple of the unit interval. Variations in the time intervals are constrained within specified limits.

A voice call is isochronous as it is transferring bits of your voice at regular intervals.

The second part of the sentence

with a clock distribution tree radiating from a single, protected cesium reference clock.

seems to indicate that the transport network carrying voice calls relies on a centralised clock. SDH would be an example of a technology that does rely on synchronised clocks and using caesium clocks as a source of that clock is fairly common. A Caesium clock will only give you a frequency, which is what is required for operating an SDH network. GPS could be an alternative source that would also provide absolute time, i.e. date, time of date and so forth. It's not uncommon to use both, with GPS as primary and Caesium as a backup. Using multiple clocks also allows you to detect drift (someone could attempt tampering with GPS). Without accurate clocks you will get frame slips in your SDH network effectively dropping bits in your payload.

Again, as put by Wikipedia:

"Isochronous" is a characteristic of one signal, while "synchronous" indicates a relationship between two or more signals.

From a technical perspective perhaps the "telephony network" should refer to the transport network that is carrying those calls, in which case it might not be isochronous (SDH definitely isn't) but when just looking at the calls themselves they are isochronous. Regardless of underlying carrier you will have bits coming in at regular intervals, representing the audio stream, thus making the voice calls isochronous.

I agree that the sentence is a bit weird but depending on your interpretation it isn't necessarily incorrect.

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Answer: It isn't.

The closest we can get to this is that many providers use GPS for NTP stratum 1 sources. Since many providers are moving to VoIP we do emulate TDM circuts over IP to interconnect with the PSTN and comply with standards, and codecs rely on this. Everyone is keeping their own time though, if you understand my meaning.

I have worked with various networks that have poor time keeping; Due to humans being lazy and prone to mistakes the concept of the telephone network being isochronous is in fact almost imposible.

I have encountered on several occasions other SIP carriers when tourbleshooting issues between us they are running packet captures are specific times of day and missing the VoIP traffic we want to capture and test or measure because their clocks on their gateways aren't accurate!

Maybe this was true 50 years ago (although I failt to see how?) but it certainly is not true today, especially with the rise of VoIP where everyone has their own timing mechanisms (I'm thinking NTP sources as I write that).

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