[asterisk-dev] NFAS Implementation Spanning Multiple Servers

Matthew Fredrickson creslin at digium.com
Sat Feb 4 09:38:31 MST 2006

On Feb 2, 2006, at 7:13 PM, John Todd wrote:
> Just thinking out loud here, but why couldn't this work with some type 
> of modified TDMoE (*) driver setup that handles just the D channels?  
> The system with the "primary" D channel would then be told which 
> machines had which "trunk groups", and then messages for each of those 
> systems would be split off using a single DS0 TDMoE connection to each 
> of the respective platforms.  While I am not able to say that this is 
> an "easy" solution, it certainly seems like it could re-use quite a 
> few of the existing components, protocols, and configuration 
> methodologies that Asterisk currently utilizes. Basically: add the 
> ability for the D channel for an NFAS group to come in via TDMoE.  Add 
> the ability to split out D channels for certain sets of trunks onto a 
> TDMoE channel.  Just like I can take a single PRI into an Asterisk 
> server and then transfer/divvy calls out to another set of PRIs, so 
> too could (with some patches) the system just send the messages to the 
> appropriate trunk slave box as long as the master system has a mapping 
> of PRI-trunk-to-chassis relationships.

This is the easy part.  The D-channel message redistribution to the 
appropriate machine is not that hard.  The difficult part is how you're 
going to setup all your phones and such behind it.  Inbound calls 
shouldn't be too complicated.  Outbound calls are going to be more 
complicated, at least to try to make this look like a single switch.  
For NFAS/PRI it's not so bad, but I think that it may be a bit more 
complicated with ISUP/SS7.  I'm not sure if there is a good way to 
change bearer channels if there's a problem with one of the bearer 
channel boxen.

> I think requiring local LAN domain connectivity of a cluster of NFAS 
> boxes is reasonable, so TDMoE is a contender.

It probably wouldn't be TDMoE.  We don't want to actually carry the b 
channel data to the other boxes anyway, so there's not really a use in 
using TDMoE just for signaling messages.  I was thinking more along the 
lines of a SIGTRAN style protocol, possibly UDP based with Q.921/LAPD 
running on top of it.

Matthew Fredrickson

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