Newbie question

Answered Question
Dec 11th, 2009

Hi everybody,


I'm quite new to SIP and VoIP telephony in general and I have a (supposedly) simple question to spare.


A customer of mines owns a classic PBX with a spare extention line (FXS) which she would like to connect to a remote office.


I thought this was easy to implement. Looking for a very simple solution, I had the impression that a Cisco SPA 400 Internet Telephony Gateway plus a Cisco SPA 502G IP Phone would do the trick and would even allow for further IP Phones (which are really not needed right now).


Then, I started having a lot of doubts. First of all, every Cisco reference about the SPA 400 seems always referring to the SPA 9000 too, to the point that I'm going to think that a SPA 400 and a SPA 502G wouldn't work without a SPA 9000. Am I right?


What I basically I'm looking for, is a very simple and inexpensive VoIP solution which would allow the remote user to be remotely, 1-to-1 connected to a specific PBX extention line. Basically, I would like that when the remote user picks the handset, he/she would ear the PBX call tone (or, of course, pick the outstanding call in that extention). So, in case the SPA 400 + SPA502G mix isn't the right way to do this, can you head me to the right solution?


Of course, the remote office's LAN will be connected to the main one through a simple VPN. QOS and the like will be activated in the routers in order to give enough bw to this app.


Thank you,


Giampaolo

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Correct Answer by Alberto Montilla about 7 years 1 month ago

Dear Giampaolo;

SPA400 is not the right product in principle. You better go for either SPA3102+SPA2102 (for one phone only) or for SPA8800s (for multiple ports). The operation is as follows. Connect a SPA3102 or SPA8800 FXO port to the PBX extension line. Then on the remote site place the other ATA (SPA2102 or another SPA8800).

You need to setup peer to peer SIP trunks between the gateways (there is a similar post on this community - use search function), one important aspect is that you need static public IP addresses in order to have these devices to talk each other without a proxy.

Regards
Alberto

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rseals Mon, 12/14/2009 - 06:48

Not an expert since the majority of what I've worked with is Avaya and Asterisk with Linksys and Cisco SIP phones.  But to understand what your asking let me make sure I understand your proposed setup

[Legacy PBX] <-> FXS port <-> [SPA 9000/400] <-> VPN <-> IP Phones

You are going to use the FXS as a tie trunk between the spa9000/400 and your PBX so the SPA will be located within hardwire distance?

g.tomassoni Mon, 12/14/2009 - 07:16

This is the "wiring" (with detailed FXS/FXO connection):

     [Legacy PBX's FXS port]<->[FXO port ~~ {SPA 400} ~~ Ethernet port]<->[VPN on Cisco 857]<=>{Internet}<=>[VPN on Cisco 857]<->[IP Phone]

In my previous post, I wrote about a "1-to-1 [virtual] connection" between the PBX extention and the IP Phone. In effect, I later discovered that stuff is referred as "Point-to-Point VoIP" otherwere, which is probably the right name for this brew of rose.

The question I posed is related to the way one can configure a Cisco SPA 400 device and a Cisco SPA 502G IP Phone to work in a Point-to-Point way (if any), such that a Cisco SPA 9000 device wouldn't be needed.

In effect, I later discovered that the Cisco SPA 3102 Voice Gateway supports some kind of point-to-point VoIP operation (see this link). I ordered two of them. I'll attempt to configure them that way.

Do you see any potential problem?

Giampaolo

Correct Answer
Alberto Montilla Mon, 12/14/2009 - 07:16

Dear Giampaolo;

SPA400 is not the right product in principle. You better go for either SPA3102+SPA2102 (for one phone only) or for SPA8800s (for multiple ports). The operation is as follows. Connect a SPA3102 or SPA8800 FXO port to the PBX extension line. Then on the remote site place the other ATA (SPA2102 or another SPA8800).

You need to setup peer to peer SIP trunks between the gateways (there is a similar post on this community - use search function), one important aspect is that you need static public IP addresses in order to have these devices to talk each other without a proxy.

Regards
Alberto

g.tomassoni Mon, 12/14/2009 - 07:30

Alberto,

we posted the same reply at the same time...

Nevertheless, since I'm an absolute begineer in this "analogital" field of knowledge, I'm glad your hint does match the ones I found in the 'net. Now I feel more confortable with the SPA 3102 solution. So, thank you.

Regarding the static ip, of course the IP addresses must be statically assigned at both ends: VoIP will be only one of the services I'm going to deliver to the remote end via the VPN. That remote office have to access the company's billing and accounting application, not even to mention a couple of SMB folders with (very small) Word documents inside. This is why I wrote about QoS in my first post: it will be configured in the two Cisco 857 in order to give priority to the VoIP channel, hopefully keeping latency as low as possible for voice packets, even when the VPN is used also for something else.

Thank you again,

Giampaolo

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