[Asterisk-Users] Asterisk - fax - spandsp

[Steve Underwood]

Jean-Yves Avenard wrote:

> Hello
>
> On 15/05/2005, at 12:00 AM, Colin Anderson wrote:
>
>> wierd. Im running fc2 2.6.8 smp no problems. Could be timing slips on 
>> your
>>
>> PRI, happened to me until I looked hard at the PRI
>>
>>
> So you turned off Hyperthreading on your linux box or not??
>
> What could I do to check if my PRI has timing slips? I have to say 
> that my knowledge in this area is close to zero. The extent of my 
> knowledge was to connect a TE110P card, configure it and run it. That's it
>
> Jean-Yves

In most cases the slips on T1s and E1s are because people have carefully 
configured their system to cause them. Probably because there is no 
clear explanation around of the right way to do things. Their careful 
configuration generally consists of ensuring they have the following 
lines in their zaptel.conf file (E1 example):

span=1,0,0,cas,hdb3,crc4
span=2,0,0,cas,hdb3,crc4
span=3,0,0,cas,hdb3,crc4
span=4,0,0,cas,hdb3,crc4

This ensures they do not listen to any external source's opinion about 
clock speeds. As they centre of the universe, their Asterisk box expects 
the 4 systems connected to their 4 E1s to fall in line with the clock 
speed of the Asterisk box. (here's a clue: a PSTN switch contains an 
atomic clock as its clock source, and believes everyone else's clock it 
too flawed to listen to).

Other people carefully configure their zaptel.conf file to contain lines 
like (again, an E1 example):

span=1,1,0,cas,hdb3,crc4
span=2,2,0,cas,hdb3,crc4
span=3,3,0,cas,hdb3,crc4
span=4,4,0,cas,hdb3,crc4

and then attach spans 1, 2, and 3 to a PBX and span 4 to the PSTN. So, 
their primary source of clock is some grotty PBX, as it is listed as the 
number 1 option for obtaining a clock. They expect the PSTN to fall in 
line with the PBX's clock. Well, if the PBX has actually locked directly 
to the PSTN by another E1, that might work out. However, in most cases 
the Asterisk box is playing piggy in the middle, and the PBX should be 
syncing to the clock it gets from the Asterisk box.

The right thing to do is to sync to the PSTN. The E1s connected to the 
PSTN should be listed as the lowest numbered clock sources, starting 
from 1. Places you never want to sync to should be set to zero.

Regards,
Steve