[Asterisk-Users] T1-PRI deployment questions...

[Don Pobanz]

I have been on vacation so didn't jump in earlier. Some of what I say 
here has been gone over earlier in this thread but I will repeat the 
results as a summery.

AMI is not lossy, but it is almost always used in conjunction with a 
ones density technique called bit7. Bit7 will change bit 7 to a 1 when 
a word (8 bits) are all zeros. In North America when someone says AMI 
they really mean AMI with bit7. PRI ISDN will not work on an AMI (with 
bit7) T1.

When validating ones density B8ZS does introduce errors but in such a 
way that the far end will know that they are errors and remove them. 
The only reasonable choice for PRI ISDN is B8ZS

Robbed bit signaling uses bit 8 of each word of every 6th frame for 
signaling. This does not introduce errors on the line but it does make 
those bits unavailable for data. Usually data services assumes that the 
8th bit of every work of every frame are not available so that you have 
7 of each 8 bit word. That makes 56Kbit/s of 64Kbit/s of bandwidth 
available. PRI ISDN does not use Robbed bit signaling. Instead it uses 
the D channel.

Slips are caused by a timing problem and have nothing to do with AMI or 
B8ZS.

Framing of either SF (D4) or ESF can be used on a T1 regardless of the 
line coding. (some mistakenly believe that B8ZS must be used with ESF 
and AMI must be used with SF) ESF is preferred because more robust 
performance monitoring is possible while the T1 is in service.

Don Pobanz

Ps. For a while the group I worked with back at the phone company was 
known as 'T1s are us' so I should be familiar with this stuff.  ;)