🚀 go-pugleaf

In article <4c1q90$n37@mercury.near.net>, <fgoldstein@bbn.com> writes:
> 
> No, RB's "wet hay" is more accurate.  The 56k restriction comes about
> because 
> a) T1 carrier using old-fashioned AMI signaling can't allow the
> all-zero octet to be transmitted, so they ALWAYS set it to 1 during a 
> call and allow you to send your info on the 7 other bits, thus 56k;
> this is fixed via B8ZS encoding, and
> b) Sometimes "inband" robbed-bit signaling is used where SS7 is not in
> place, and in North America, this low-order (eighth) bit is set to 1
> on every sixth sample (the other 5 are passed cleanly, but who's to 
> coordinate this end-to-end?) during a call and used for call signaling
> otherwise.  
> 
> These are both only noticeable for interswitch calls.
> -- 
> Fred R. Goldstein   k1io   fgoldstein@bbn.com   +1 617 873 3850
> Opinions are mine alone; sharing requires permission.
> 

Fred, can you help me understand what the yellow alarm on a T-1 for
"15 zeroes" is? I thought an AMI T-1 could not take 15 consecutive
zeroes before possibly slipping sync. It needs those 1's to make
marks. Alternate Mark Inversion (AMI) means successive marks are
different polarity, unless they are the same polarity, which is
an AMI violation, which is used to mean something else. I don't 
think a 1 is automatically inserted by the transmission gear as 
you suggest in a), else there would be no need for a "15 zeroes"
alarm. B8ZS within an ESF allows bi-polar violations within an 
ESF frame to identify the all zeroes octet by inserting two marks
with the same polarity within the octet. On non-ESF T-1s, a 
bi-polar AMI violation is used to indicate the start of a new 
193 bit non-ESF T-1 frame.

Also, in b) aren't many of the channels completely inert with 
regards to the "inband" robbed-bit signaling? If only three or 
four channels are affected, then 64kbps DOSBS should work on the
remaining unaffected channels. This would be OK unless two DOSBS
channels are adjacent and both want to send all zeroes, which 
would cause the yellow alarm. This, I think, is the reason so 
many carriers don't like DOSBS, not because it deprives them of
charging more for ISDN data calls.

Thanks for your contributions on this subject. I believe DOSBS 
users should be encouraged to attempt 64kbps DOSBS first and 
only revert to 56kbps DOSBS when 64kbps is "noisy". Of course, 
if all you care about is calling an ISP who refuses to offer a 
separate 64kbps DOSBS Directory Number for users on the same 
switch or users able to reach the ISP inter-switch on clear 
channels, well then 56kbps/B-channel DOSBS is going to have to
be the limit to that ISP.

Jeffrey Rhodes at jcr@creator.nwest.attws.com

ps. with 64kbps DOSBS using V.120 error correction, would the 
infrequent "robbed" bits in b) be corrected anyway? Half the 
robbed bits will match the DOSBS data stream, so a V.120 packet
would usually get 11 out of 12 octets thru unharmed. How long 
is a V.120 packet and is there a Hamming code included to 
correct transmission errors?