[ppml] 2005-1 and/or Multi6

Edward Lewis Ed.Lewis at neustar.biz
Thu Apr 14 14:58:00 EDT 2005

Admittedly OT...but I'm curious if what I've learned "way back when" is untrue.

At 11:17 -0700 4/14/05, Owen DeLong wrote:

>The reason modems haven't advanced beyond 56k is because there is
>no market to support the research to do so.  Beyond 56k, there
>is ISDN, DSL, Frame Relay, ATM, etc.  Modem research stalled
>because 56k is fast enough for those rare circumstances in which
>a modem is still the best form of communication, and, those
>circumstances are becoming increasingly rare, thus lowering the
>potential value of said market.  It has much more to do with
>the inability to create a market for the product than the ability
>to achieve improvement.

My recollection is that 56k represents the number of bits a DS0 is 
allocated in a T-1 trunked voice frame, once you discount the DS0 
channel's signalling bit.  Being a life-long networker and not a 
bell-head, I'd be curious if that wasn't the reason for the limit.

The way it was taught to me, and repeated by me in class, from 
Tannenbaum's text book.:

24 DS0's were sampled a 7 bits every 8000 times per second at CODEC's 
and then assembled into a T-1 frame.  7b x 8000/sec = 56kbps (where 

24 of these w/1 signalling bit, plus one more frame-wide signalling bit:
        = {(24 x [7+1]) + 1 = 193} * 8000/sec = 1.544 Mbps or 1,544,000 bps.

T-1's don't all use the same framing, the above is for voice, but a 
voice frame is appropriate for a modem.

I always thought that the 56k limit was the fastest obtainable bit 
rate over trunked telephone lines because of the CODECs.  With 
compression, higher effective bit rates may be possible for 
applications, but the real physical limit was 56k - or so I have been 
led to believe.  (Compression is not always possible, true random 
data cannot be compressed.)

8000/sec is the limit of reasonable sampling for a 4KHz bandwidth 
which is optimized for human hearing.  SONET also uses 8000/sec for 
it's basic frequency.

I've also been told that in some regions, fiber was used that lowered 
the bandpass to about 3300 or 3200 KHz, meaning that even 56kbps 
would be unobtainable.  I've also been told that the higher rates of 
?DSL are possible because the non-customer end of the circuit 
terminates before the trunking CODEC.

Edward Lewis                                                +1-571-434-5468

If you knew what I was thinking, you'd understand what I was saying.

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