Wednesday, 23 December 2009

PAP2T vs. SPA2102 VoIP Adapters

I myself have been testing and using the Linksys-Cisco PAP2T-NA and SPA2102-NA ATA's (Analog Telephone Adapter) for a few years now.

When I bought my first PAP2T, I knew nothing about user configurable ATA's (unlocked and open), but I was determined to learn... the hard way, through Internet research and trial and error.

When I first skimmed through the web-based user interface of these ATA's, I became a bit dizzy from the maze of configuration options.  Quite overwhelming and intimidating to the novice, which I was.  Even now, over two years later, I'm still intimidated by some of the advanced options available in these units.

The good news is that the average user doesn't really need to understand anything about most of the options provided.  What you do need to know though is how to navigate to the options that are essential.  And, I will be providing future posts on just that - how to configure the PAP2T-NA and SPA2102-NA with the basic minimum configuration options possible.  And, to keep it simple, I will be specifically illustrating how to configure them with my current preferred VoIP carrier, CallCentric.

What's the difference between PAP2T and SPA2102?
The primary differences of most importance is that the PAP2T does not have a built-in NAT-Router or QOS.  So the PAP2T is really just a configurable Telephone Adapter.  So, from this perspective, it is simpler to configure than the SPA2102 (but, only marginally simpler).  However, because it does not have its own built-in NAT-Router, you will generally be using it behind your existing network router.  And, this very fact does have some drawbacks of its own (VoIP firewall traversal issues).  So, now you may have to fiddle with your local network NAT-Router gateway firewall to port-forward, or port-trigger, SIP and RTP ports required for VoIP services to work behind a firewall.

Whereas, the SPA2102 does have its own built-in NAT-Router and QOS.  This very fact is a huge bonus for the SPA2102 because it automatically takes care of opening SIP and RTP ports and default QOS settings required for your ATA to function without any fiddling with the firewall and QOS options.

How do you decide which ATA is best for you?
If you already have a NAT-Router integral to your Internet Modem gateway, then you may have no choice but to pick the PAP2T-NA.

However, if your Modem does not have a built-in NAT-Router, then I highly recommend you pick the SPA2102-NA.

Even if you already have a separate 4-port NAT-Router, after the Modem, you don't need to stop using it.... just use it "after" the SPA2102.

For the SPA2102 to function at its best, you definitely want it connected "directly after" your Modem.  This way you will benefit from the QOS packet management the SPA2102 provides.  And, you won't have to worry about firewalls blocking the essential SIP and RTP VoIP ports required by the ATA.... the SPA2102 will open the necessary SIP and RTP ports it needs for your VoIP calls.

The SPA2102 does provide one Ethernet LAN port.  So, you can connect your PC directly to this LAN port.  Or, you could connect another 4-port router, or switch, to this port if you have multiple devices on your LAN side.  This is the way I recommend most people use the SPA2102.

However, for myself, I have settled for the following configuration that works quite satisfactory for me:
  • I connect a 4-port Switch directly to my Modem LAN gateway.
  • I connect my SPA2102 to port-1 of the switch.
  • I connect my Linksys WRT54G to port-2 of the switch.
  • All other devices on my LAN are connected via the WRT54G LAN ports.
In the above configuration, the switch is acting as a kind of Internet access splitter between my Modem, SPA2102, and WRT54G.  However, this configuration requires that each device connected to the switch be assigned a unique WAN IP address (public IP).

With my high-speed Internet service, I am allowed to be assigned two dynamic WAN IP addresses.  Therefore, I get one IP address assigned to my SPA2102 and the other IP address assigned to the WRT54G.  Most high-speed Internet services allow up to two dynamic IP addresses with high-speed service... but, you may have to contact their Customer Support to enable the 2nd IP address, which may not be enabled, by default (as it was in my case).

Then, I have my desktop PC connected to one of the WRT54G LAN ports and my wife's laptop through its Wireless-G link.  I then set the WRT54G to limit its "uplink speed" to be 150 Kbps less than my ISP allowed uplink speed.  This ensures that I leave enough uplink bandwidth left over for the SPA2102 VoIP packets to traverse the Modem uplink without being bottlenecked by simultaneous data from the WRT54G data through the Modem.

I do not have any other devices connected to my SPA2102 LAN port... but, I could if I wanted to.  The QOS settings in the SPA2102 throttles "upload" LAN traffic through its LAN port to 128 Kbps (by default).  This is to ensure that there is always enough upload bandwidth available for the VoIP packet traffic.  You can adjust its "uplink speed" to suit your permissible upload bandwidth provided by your ISP.  Actually, I do sometimes connect my PC to the SPA2102 LAN port when I need to view or change any of its web-based configuration settings.

So, if you are only going to be using your ATA behind an existing NAT-Router, the PAP2T-NA is probably your best choice for that scenario.

If you want much more flexibility, go for the SPA2102-NA.  It is also small and compact enough to use as a travel NAT-Router for your laptop.... with the added bonus of being  able to make cheap VoIP calls from your travel NAT-Router while on the road (assuming you have high-speed Internet access).  However, this would also require you to have a small standard telephone to plug into the ATA to make those calls.