SPAN in details (for Asterisk)

SPAN is the trunk parameter defined in /etc/dahdi/system.conf [ traditionally /etc/zaptel.conf ] for the PRI interface.

Trunk can be T1/E1.

span = [span num], [timing], [line build out], [framing], [coding]

e.g span = 1,0,0,esf,b8zs

This means, the specified trunk is mapped to SPAN 1 of the T1 card. It uses internal clock of card as time source as second parameter is 0. LBO is 0 and it doesn't require to be changed most of the time as cable is not going to be more than 133ft from smart jack. Framing is esf. Coding is b8zs.
[ Also remember, each PRI card should have their own independent clock source ]
In brief, you can say, this particular trunk is esf,b8zs with internal clock as time source.


I would like to write about some other parameters defined in /etc/dahdi/system.conf

span = 1,0,0,esf,b8zs
bchan=1-23 #B-Channel defination
dchan=24 #D-Channel defination

Since bchan+dchan=24, it is T1 trunk configuration.
For E1 trunk, bchan+dchan=31

Also note that FRAMING and CODING is main factor that can distinguish between T1 and E1 trunk.
--------------T1----------------------E1
Framing -- esf / d4------------ ccs / cas
Coding ---- b8zs / ami -------- hdb3 / ami
-----------------------------------------


When you talk about Trunk, you also need to specify the switch type. Switchtype parameter is defined in /etc/asterisk/chan_dahdi.conf . It can be National or dms100 or 5ess

Default Switch: National
Nortel Switch : dms100 [ but it's weird that in my case, i had to use 5ess switchtype to interface my Asterisk Server with Nortel Switch]
Dialogic Fax Server Switch : dms100

So, PRI with following configuration can be said as T1 trunk with esf,b8zs signalling and National switchtype.

#/etc/dahdi/system.conf
span = 1,0,0,esf,b8zs

#/etc/asterisk/dahdi.conf
switchtype = National



Wonderful explaination at :
http://www.asteriskguru.com/tutorials/pri_zaptel.html
http://www.asteriskguru.com/tutorials/pri.html