Clarifications About Digium R-Series vs Xorcom TwinStar Comparison

Dec 28, 2011 | VoIP PBX News and Blog

The best compliment a manufacturer can receive in any industry is to have another manufacturer publicly compare its products to theirs. It indicates there is a recognized leader in the industry for the specific area being compared. Such is the case with Xorcom and Digium. The latter has finally come out with a resiliency solution for Asterisk and is – through a recent blog post – trying to demonstrate that they now have a more cost-effective and robust resiliency solution than Xorcom.

While Digium undoubtedly has a good solution, let us first look at a few historical facts:

  • Xorcom announced TwinStar for Asterisk PBXs back in August, 2009.
  • Xorcom has well over 500 global deployments in large enterprise customer sites (e.g., Solar City, a former Digium customer, has over 700 phones with 8 T1s supporting 120 simultaneous calls for a total on average of 20,000 calls per day – all managed on Xorcom’s XR3000 series servers with TwinStar).
  • Digium cannot show such a system in action with Switchvox, as Switchvox is not built to manage this amount of traffic and recorded calls. It is no surprise that the Digium R850 mentioned in the post does not work with Switchvox, because Switchvox is not built to manage 8 T1s.

Point-by-Point Comparison

Now back to Digium’s new product. In an effort to separate perception from reality, let’s take a look at each of their points as written in the blog post.


“The combination of Xorcom Astribank (HW Channel Bank) and Xorcom TwinStar (Software for redundancy) is a similar application to the Digium R-Series.”

Saying the Xorcom TwinStar solution is a similar application to the Digium R-Series is quite a stretch as the product strategies are fundamentally very different. Xorcom promotes a manufacturer-supported turnkey hot failover solution for Asterisk-based PBXs. The Digium R-Series is nothing more than a “relay” which requires PRI or FXO cards in each of the PBXs, primary and backup, in order to have a resiliency solution.  See diagram from the Digium R-Series manual below in comparison to a Xorcom TwinStar solution:

R-Series Schematic TwinStar Schematic

Digium R-Series schematic

Xorcom TwinStar schematic

In reviewing the above diagrams one can clearly see significant differences in the two solutions:




T1 Cards Xorcom does not require duplicate T1 cards in each PBX.  That is because Xorcom uses USB 2.0 technology to actually pass phone calls over the USB cables. USB 2.0 has 480 Mbit/s of bandwidth which theoretically can support 2400 simultaneous calls. You would never actually do that but it does answer the question, is USB a good medium for passing phone calls?  The answer is a resounding YES! Digium uses the USB for power and monitoring only, not for passing voice traffic.  A different set of cables connects the telephony interface housed in the server with the PSTN (using basic relay architecture). This strategy forces the integrator to purchase duplicate T1 cards for the servers.
Mixed Interfaces Utilizing Xorcom Astribanks the integrator can have a mix of PRI; FXO and FXS within the same Astribank. Since the Digium R-Series cannot support T1 and FXO within the same unit it forces the integrator to purchase separate units for each interface and duplicate both sets of cards in each PBX. The dollars and hardware start to add up fast.
FXS Resiliency No problem! Since the Digium R-Series does not support FXS technology there is no resiliency support for FXS extensions. 
High Density Astribank + TwinStar Resilient Solution at Customer Site

This Xorcom installation at a customer site supports 512 FXS extensions with full redundancy for power and phones.

Note: This is a photo of an actual installation at a large resort in Branson, Missouri. The configuration includes sixteen 32-port FXS Astribanks (a total of 512 FXS ports) and two fully redundant power supply units that provide redundant power to all the Astribanks.

Cost & Effort

This is the next point from the Digium article:

“The Astribank hardware can run a customer up to four times the cost of R-Series appliances depending on the number of T1 interfaces that are populated in the Astribank. All eight T1 ports are included in the R-Series for $995 list price.”

If all the integrator had to purchase was the R-Series to make this work then it would be possible to agree with the above statement. However, this is not the case. It would appear that the engineering team at Digium does not quite understand how the Xorcom TwinStar solution works.

R-Series Requires Duplicate T1 Cards

R-Series Requires Duplicate T1 Cards

The Digium R850 is actually just a “relay” that switches the T1s when it senses a server failure.  In order for this to work, duplicate T1 cards must be purchased – one set for each server. For example, in order to configure a resilient system for 8 T1s, you would need to purchase either two 8-port T1 cards or four 4-port T1 cards.  See diagram opposite:

The example above gets even more complex if your client has FXO ports.  In addition, FXS is not supported at all for the R-Series resiliency solution.

Instead of purchasing and configuring all the hardware mentioned above, you could easily achieve PBX resiliency using a field-hardened and mature Astribank/TwinStar solution, such as the one described below:

Xorcom Resiliency for 8 E1/T1 ports

Xorcom Astribank 8xPRI Configuration with TwinStar

Xorcom Astribank 8xPRI Configuration with TwinStar

You can purchase two Astribanks, each with a 4-port E1/T1 module (part number XR0056), for $1,390.00 each and add the TwinStar license for $345.00. The total for 8 E1/T1s in this example is $3,470.00. Remember, there are no E1/T1 cards to purchase for the servers, as this is managed by the Astribanks with USB connections.

As you can clearly see, cost is relative. At Xorcom we aim to minimize hardware and concentrate on an intelligent device such as the Astribank that has the capability to manage the telephony interfaces for dual servers and be the watchdog for switching from the primary server to the backup server.  This eliminates the need for telephony interfaces inside the servers. The solution has been in general production for 2.5 years and is mature and stable.

“The Xorcom TwinStar software is an additional cost. The open source tools used for the servers connected to the R-Series is downloadable and is free to use”.

TwinStar software is $345.00 list price, per Astribank. Not exactly expensive for what Xorcom is providing! Besides, how can you compare a set of open source tools (that requires the integrator to build the system), with an out-of-the-box product that passes commercial grade quality assurance both in R&D and in production? How many hours will it take the integrator to achieve such a solution, and how confident will he be that the solution is sound?

In addition, Xorcom has absolutely no hidden fees or charges for our solution.  See this note from the Digium R-Series User Guide:

DRBD Instructions from R-Series User Guide

In addition, TwinStar solutions by Xorcom, when purchased with Xorcom servers, are already synchronized prior to delivery.  If requested, we will even set the static IP addresses for each server and the cluster IP per your network! This allows you to simply connect the servers and start configuring your PBX as there is no software installation to perform…!

See the software instructions below from the R-Series User Guide:

Software Instructions from R-Series User Guide

All software and libraries are installed for you when purchasing a turnkey TwinStar solution from Xorcom.

Hardware Platform

“The Xorcom Astribank channel bank has more parts, such as a power supply, that are prone to fail. Digium utilizes USB power from the servers to power the R-Series which is much less vulnerable”.

For TwinStar implementations Xorcom manufactures and recommends the installation of our Redundant Power Supply unit, which can support up to 16 Astribanks and minimizes any single point of failure.  The list price for the RPS is $975.00.

“R-Series is 1U rack mountable and therefore takes up less space in the rack than the 2U Astribank”.

A quick search of our Web site would have informed the Digium engineering team that the Astribank is a 1U device.


“The TwinStar/Astribank solution can only detect a hardware power failure if the Linux kernel has crashed. It cannot detect if Asterisk has stopped working but is with the R-Series solution (along with loss of power detection)”.

Yes, that is correct. The TwinStar firmware does not detect if Asterisk has stopped working. However, note that Xorcom servers (as well as the most popular Asterisk distributions, including Elastix, trixbox and PBX in a Flash) employ the safe_asterisk script, which is a part of the Asterisk source package, to automatically restart Asterisk. Additionally, if DAHDI stops working, then TwinStar automatically performs the failover.

“If the R-Series were to lose power, the PSTN spans will continue to function through the same ports at the time of power failure. The Astribank will cease to operate at this point and communication with the PSTN is down”.

That is correct, Astribank does not work without a power supply. However, as mentioned earlier, we offer the Redundant Power Supply unit that eliminates call loss from a single power supply failure.

R-Series Compatibility Issues

  • The Digium R-Series solution cannot be used with FreePBX-based systems such as Elastix or trixbox, nor is it suitable for Digium’s Switchvox.
  • Digium suggests replicating the Asterisk configuration files, the Asterisk log files and the voicemail files only. What about the MySQL databases, SIP phone provisioning files, DHCP lease file, and call recordings? What will happen with the CDR records without MySQL replication? It will be necessary to merge the records from two servers. So, their suggestion to keep the replication data on a USB drive does not seem sufficient. This solution does not require special hard disk formatting, but on the other hand is too limited.
  • The Digium R-Series is not a complete resilience solution as presented, but rather a relay component that suits very specific circumstances: a limited number and type of ports, and very specific Asterisk distributions.

Digium, as well as RedFone, provides only about 30% of what is required in order to have a fully supported hot failover solution for Asterisk.

Let me be very clear and not mince my words. Since August of 2009, Xorcom is the only IP-PBX manufacturer in the world that provides a 100%, fully supported, hot failover solution for Asterisk based PBXs.  Period.


Bill Soto
Director of Strategic Alliances, US & Canada
Xorcom USA
Direct: 615-349-2531


If you feel that any of the information in this document is incorrect, please feel free to contact me directly so I may revise it for all our readers.

The Xorcom response to the Digium article published by Nathan Miloszewski is simply an attempt to clarify our functionality. Xorcom has always been a strong supporter of the Asterisk open source community and we believe it to be of the utmost importance to provide accurate information to the community.

Asterisk is a registered trademark of Digium, Inc. All other trademarks are the property of their respective owners.

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