Jim,
On 07/01/14 05:43, Jim Lux wrote:
On 1/6/14 8:36 PM, Magnus Danielson wrote:
Bob,
It works the other way around. The standard Bell handset (103A I believe
the designation was) has the 300-3400 Hz response, and with not so fancy
analogue filtering, you can handle 4 kHz and thus 8 kHz sampling rate.
The ITU-T G.711 A-law (where naturally the americans wanted their own,
so u-law appeared) does non-linear pseudo-dynamic compression into 8
bits. T1s cram 24 channels into one frame, and adding 1 bit for framing,
giving 24*8+1=193 bits per frame, giving the 1544 kb/s rate. 193 being a
prime have caused a bit of headache over the years. In Europe, cramming
30 channels into a bundle was preferred, and allowing 2 bytes for
framing and signalling. In T1, you do signalling by bit-stealing every
6th LSB on a channel. Caused some grey hairs for modem designers back in
the day, and followed along over into the ISDN, as the primary rates was
over E1 and T1. T1 also had three different line-encodings, of which
only one was really transparent to all binary combinations.
Oh the joy of early digital telephony. Many lessons where hard to learn.
Synchronization was only one of them.
Don't forget the length of ATM cells.. 53 bytes.. because of how big
France is.
No, that's not it. It's a design-by-committee thing. As I recall it, the
Europeans wanted a 32 byte payload, as then you throw in a 32-byte E1
into it, but this was judged to small for datacom which the North
American side wanted, that wanted a 64 byte payload. Since no agreement
could be done, they went half-way and made it 48 bytes payload, so both
would be equally annoyed. Toss a 5 byte header that people where
agreeing on and we have the lovely 53 byte (prime number again!) ATM
cell size.
That is only the start of the trouble. The real troubles was that they
designed really stupid leaky-bucket algorithms amongst other things. The
leaky bucket algorithms I saw didn't have a very smooth rate behaviour
at higher speeds. When I set up a channel over the pan-European ATM
pilot using G3-signaling (Group 3 fax signalling that is, first
signalling protocol of ATM), I also discovered that each operator had
their own definition of what made up 1 Mb/s of ATM stream, so I got the
lowest rate along the line... we had ordered 24 Mb/s, so it was more an
interesting experience than failure.
They also ended up being forced to do the synchronization as in the
PDH/SDH, so they just used the same to get the traffic shaping decent.
Magic how the history re-occurs as we see the same occurring again with
MPLS (being viewed as the ATM again but let's not call it that and lets
make the cells... eh... packets longer, I keep referring to it as
tag-switching).
Synchronous Ethernet and Telecom profile PTP continues the PDH/SDH
synchronization tradition.
Cheers,
Magnus
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