My company has a Nortel PBX, with a full panel of extensions. However, we need to add more handsets. In addition, our traders have asked for ‘Turrets’ (more of this in a later post). The obvious route is to go VoIP - with the precondition that the Nortel PBX should be unaffected (and repluggable if things don’t go according to plan).

Solution : Add an Asterisk PBX in between our telecoms provider (Paetec) and the Nortel PBX, which is currently delivered by some sort of T1. Requirements : Rhino 2 T1 card (installation in another post).

Problem #1 : No access to Nortel programming interface. Solution : Talk to Paetec, armed with the /etc/dahdi/system.conf man page (and the asset tag from the T1 where it arrives in the room : importantly, this is marked ESF/B8ZS)

Unfortunately our telephone provider (while helpful) doesn’t speak ‘Asterisk’ - but they were prepared to acknowledge that the T1 coming in was not PRI : it carried two types of ‘analog’ circuit :

• A Paetec TecPath (which, after a lot of experimentation with dahdi_tool) proves to speak ‘E&M;_Wink). They described it as a “TecPath Trunk Group with Wink Dial Tone”.

• A number of Loopstart lines. They described this as “Business Lines, i.e. Terminal Lines, Incoming only, Loopstart, with Clocking from Paetec.

Interfacing Asterisk to Paetec was pretty straight-forward, using the (straight-through) cable that the Nortel was already using. Plus the following /etc/dahdi/system.conf :

span=1,1,0,esf,b8zs
span=2,0,0,esf,b8zs,yellow
#span=2,0,0,esf,b8zs`

Loopback testing :

#clear=1-24 #clear=25-48

Connection to Paetec

Works for all other numbers (group)

e&m;=1-14

Works for 212-xxx-yyyy incoming group

fxsls=15-24

Connection to Nortel

Simulation of all other numbers (group - included CallingID)

e&m;=25-38

Simulation of 212-xxx-yyyy incoming (outgoing) group

fxols=39-48

Interfacing Asterisk to the Nortel was more tricky, since it seems to require a T1 Crossover (not a 568A/568B patch panel crossover).  Somehow, I guess, Paetec must detect what the recipient requires, since a straight cable (but not a patch panel crossover) worked to Asterisk...  Very strange (I didn't test the T1 crossover between Paetec and Asterisk).

For Asterisk, the _/etc/asterisk/chan_dahdi.conf_ looked like this :

rxgain=0.0 txgain=0.0 busydetect=yes

usecallerid=yes hidecallerid=no usecallingpres=yes

context=from-paetec-tecpath

group=1 immediate=no signalling=em_w channel=>1-14

context=from-paetec-main

group=2 immediate=no signalling=fxs_ls channel=>15-24

context=from-nortel-tecpath

group=3 signalling=em_w channel=>25-38

context=from-nortel-main

group=4 signalling=fxo_ls channel=>39-48

FYI, the E&M; signalling seems to be able to provide the last 4 digits of the number called in to, whereas the Loopstart doesn't.  This explains why my test dial plan (in /etc/asterisk/extensions.conf) from Paetec looks like this  :

[from-paetec-tecpath]

exten => _X.,1,Verbose(“NEW CALL FROM:[${CALLERID(number)}] FOR:[${EXTEN}]”) exten => _X.,n,Dial(SIP/2101,60,r) exten => _X.,n,Hangup

[from-paetec-main] exten => s,1,Verbose(“Business Line Group CALL FROM:[${CALLERID(number)}] FOR:[${EXTEN}]”) exten => s,n,Dial(SIP/2101,60,r) exten => s,n,Hangup

```

where in the Business Line Group, no CALLERID or EXTEN information is available.

Now, on to cross-linking everything…