One of the biggest questions in the world of Asterisk is: “How many concurrent channels can be sustained with an Asterisk server?” – while many had tried answering the question, the definitive answer still alludes us. Even the title of this post says “3500 concurrent channels with Asterisk” doesn’t really say much about what really happend. In order to be able to understand what “concurrent channels” really means in the Asterisk world, let us take a look at some tests that were done in the past.

Asterisk as a Signalling Only Switch

This scenario is one of the most common scenarios in the testing world, and relies upon the basic principle of allowing media (RTP) to traverse from one end-point to the other, while Asterisk is out of the loop regarding anything relating to media processing (RTP). Examine the following diagram from one of the publicly available OpenSER manuals:

Direct Media Path between phones via a SIP Proxy

As you can see from the above, the media path is established between our 2 SIP endpoints.

This classic scenario had been tested in multiple cases, with varying codec negotiations, varying server hardware, varying endpoints, varying versions of Asterisk – no matter what the case was, the results were more or less the same. Transnexus had reported being able to sustain over 1,200 concurrent channels in this scenario, which makes perfect sense.

Why does it make sense? very simple, as Asterisk doesn’t manage or mangle RTP packets, Asterisk performs less work and the server also consumes less resources.

Asterisk as a Media Gateway

Another test that people had done numerous times is to utilize Asterisk a Media Gateway. People used it as a SIP to PSTN gateway, SIP to IAX2 gateway, even as a SIP to SIP transcoder gateway. In any case, the performance here varied immensly from one configuration to another, however, they all relied on a simple call routing mechanism of routing calls between endpoints and allowing Asterisk to handle media proxy tasks and/or handle codec translation tasks.

Depending on the tested codec, I’ve seen reports of sustain over 300 concurrent channels of media on a single server, while other claim for around the 140 concurrent channels mark – this again mostly relied on various hardware/software/network configurations – so there is nothing new in there.

These tests tell us nothing

While these tests are really nice in the theoretical plane of thinking, it doesn’t really help us in the design and implementation of an Asterisk system – no matter if it is an IVR system, a PBX system or a time entry phone system for that matter – it simply doesn’t provide that kind of information.

The Amazon EC2 performance test

In my previous post, Rock Solid Clouded Asterisk, I’ve discussed the various mathmatics involved in calculating the RoI factors of utilizing Cloud computing. One thing the article didn’t really tell us, did it really work?

Well, here are some of the test results that we managed to validate:

• Total number of Asterisk based Amazon EC2 instances used: 24
• Total number of concurrent channels sustained per instances (including media and logic): 80
• Average length of call: 45 seconds
• Total number of calls served: 2.84 Million dials
• Test length: approximately 36 hours

According to the above data, each server was required to dial an approximate 3300 dials every hour. So, let’s run the math again:

• 3300 Diales per hour
• 55 Dials per minute
• As each call is an average of 45 seconds, this means that each gateway generates 20 calls
  per second, and within 4 seconds fills the 80 channels limit per server.

According to the above numbers that we’ve measured, each of the Amazon EC2 instances used was utilized to about 50% of its CPU power, while consuming a load average of 2.4, which was mostly caused by I/O utilization for SIP and RTP handling.

Conclusion

When asking for the maximum performance of Asterisk, the question is incorrect. The correct question should be: “What is the maximum perfromance of Asterisk, utilizing X as the application layout?” – where X is the key factor for the performance. Asterisk application performance can vary immensly from one application to another, while both appear to be doing the exact same thing.

When asking your consultant or integrator for the top performance, be sure to include your business logic and application logic in the Asterisk server, so that they may be able to better answer your question. Asterisk as Asterisk is just a tools, asking for its performance is like asking how many stakes a butcher’s knife can cut – it’s a question of what kind’a steaks you intend on cutting.