It is still difficult to get a digital ISDN call connected through without going analog inbetween, or
messed up 64/56kbps as well as "stolen bits" for controll purposes. That is, nothing much
is left of the original data if the 64/46kbps stream is converted to some noisy analog voice,
and then converted back to digital.
That is, the "30 years of promise of ISDN" gives an idea on how well the whole network is, or was,
digitlized.
Note also that one can convert a 8-13kbps voice-coded datastream to analog voice, route it
through consecutive analog plus 64/56kbps digital channels, and back to the same voice-coding
system for the other handset, but then one is back to the "old days", when local calls were OK
but long distance were sometimes, sometimes not, gradually worse or just dropped.
Additionally the digital networks must be synchronized to actually have error-free 64kbps
transfer. GSM was designed to utilize ISDN as well as tolerate transfer between
non-synchronized ISDN networks.
This means one buffer-full of data will disappear or maybe be "repeated twice" every now and
then, not just bit-errors, but messing up frame synchronization, control bits,etc
That is, to move digital data from the east to the west coast through some intermediate nodes and
networks, all have to be synhronized for this not to happen.
Adding buffering "helps" but then the data is delayed, which turns out as voice-echo, same as for
satellite calls (especiilly if 2-3-4 satellite links are used, by accident or on purpose)
This is easy if the call is made from and to the same operator which probably has their
own network,etc "all the way" , but worse between, for example, two small local operators which
use two or even more other larger operators/networks for the long distance parts.
(depending on what time of the day, etc)
Another example are SMS-messages which get delayed minutes or hours instead of
seconds, and sometimes totally disappear due to inter-operator problems (or activites).
(remember my old stories on 10 second back-forth delay to Africa for SMS-messages,
at the time Wap was CRAP using SMS in UK)
Ilmarinen
Another sign of "digitalization", other than the availability of long distance ISDN, is
if the local operator provides ADSL services. When they provide that they have:
- figured out what kind of copper-network they have, moved around T1 connections
to cables where they do not kill ADSL signals, or the other way around
- Bought and installed, once again, new Subscriber equipment, 256-2Mbps packet for former ordinary
just POTS, Plain Old Telepnone customers
- installed an internet backbone, fiber, servers and access points in the same cellar
that all of above is (CISCO + much more)
A third example might be one of my Quack-favorites, this Quack-thread guy whose 56kbps
modem had done a maximum of 28kbps, that is, he had actually never been able to use
the "56kbps" function, just the plain old analog V32bis modem, and not even its 33kbps
speed.
Doubly funny as the max speed for a "56kbps" is 48kbps.
That is, a "56kbs" works by using the 56/64kbps (logartminc) DA-converter in the subscriber
equipment for downstream, modem detects which of those 64,000 levels it transmitted.
However, to actually work, that 64kbps, normally voice-only channel, must not be converted
to analog anywhere else.(and additionally be "good enough")
If that does not work, the modem falls back to plain old analog modem, at 19-28kbps and
if a really good network, 32kbps, but then 48kbps at "56kbps" should work too..
Another funny story was when one USA long distance operator advertized cheaper
long distance calls in the early 90s, by using ADPCM to put two voice channels on
one 64kbps channel. One minor problem, fax modems did not work anymore.
(And AT&T went busy advertising that "we support faxes")
When widely available, the operator has additionally "decentralized" the old switch,
moved out small "parts of it" in small boxes with 0.5-1.5 miles range from "clusters
of homes" |
