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

[John Harragin]

Thanks to everyone for all the info...

One more question. Until now, I had succesfully avoided isdn throughout my
computer career... other than having the notion that it was a fairly
troublesome line with data rates several times faster than pots.

So far from ACC (my pri vendor) my T1-PRI must be b8zs/esf. On the form I
have to fill out most of the pri enteries are listed under ISDN/PRI so I'm
not really sure which questions are particular to pri.

My usage will 23 voice (with an occational fax) plus the d channel. What is
the relationship between pri & isdn - is it considered isdn? If so, what do
I need to know about that?

John



-----Original Message-----
From: asterisk-users-admin at lists.digium.com
[mailto:asterisk-users-admin at lists.digium.com]On Behalf Of Don Pobanz
Sent: Monday, June 02, 2003 9:42 AM
To: 'asterisk-users at lists.digium.com'
Subject: RE: [Asterisk-Users] T1-PRI deployment questions...


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.  ;)


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