asterisk users - Feb 2007 - Maximum Number of Calls Asterisk Can Handle

Can anyone share their experience on the maximum number of calls a given 
asterisk box/asterisk software can handle?
I see the asterisk business edition can handle up to 240 simultaneously 
with appropriate licensing, but that doesn't seem to be many at all.

For now, I plan to use the stable open source versions - would it be 
reasonable to say that it is more of hardware limitation on the number 
of calls that can be made simultaneously, or are there only so many 
calls the asterisk software programming is equipped to handle 
simultaneously?

Also is the asterisk software written to effectively take advantage of 
multiple processors?

The systems I plan to use for asterisk have the following specs:

dual 2.8GHZ+ Pentium's
2GB RAM+
Gigabit interfaces

In my situation, I have no plans to run anything other than G.711/SIP, 
so my transcoding need is probably only limited to the playback of 
pre-recorded messages as well as any processing involved in leaving 
voicemail.

in theory, a gigabit interface can move 1048576Kbit/sec - now if i 
generously allocate 96Kbit/sec for every G.711 call, the network 
transport can handle, again in theory, 10922 simultaneous calls. would 
it be wrong to expect performance near this mark for the asterisk software?

Feedback appreciated,

-- 
-------------------------------------------
John C. Wolosuk Jr.
Unix/Linux Systems Administrator
Academic Computing & Communications Center
University of Illinois @ Chicago

E-Mail: jwolosuk at uic dot edu
-------------------------------------------

John C. Wolosuk Jr. wrote:
> in theory, a gigabit interface can move 1048576Kbit/sec - now if i 
> generously allocate 96Kbit/sec for every G.711 call, the network 
> transport can handle, again in theory, 10922 simultaneous calls. would 
> it be wrong to expect performance near this mark for the asterisk 
> software?
>
> Feedback appreciated,
Yes, it would be wrong to expect performance near that mark.  Most 
systems cannot handle the TCP processing load generated by a gigabit 
ethernet interface, let alone process everything that goes along with 
calls associated with that traffic.  A TCP offloader engine will help, 
but the limitation is still within Asterisk itself.  There is a lot that 
goes into processing everything related to a call.  Now, if you can get 
the media to be re-invited to a media gateway, then you can handle 
significantly more calls.

My experience is that when you are running media through an Asterisk 
server, 240 calls is an average maximum for a typical server.

> in theory, a gigabit interface can move 1048576Kbit/sec - now if i
> generously allocate 96Kbit/sec for every G.711 call, the network
> transport can handle, again in theory, 10922 simultaneous calls. would
> it be wrong to expect performance near this mark for the asterisk software?
10922 on any currently available PC architecture? Nope. It's closer to
160 kpbs per call (two legs, 80 kbps each) in either direction. With
20 ms packet size, for 10922 calls you'd be looking at 2184400
packets/sec processed by Asterisk... I don't think so.

Plus with 10922 calls and an average of 2 mins/call, you're looking at
about 90 call setups/tear downs a second.

I don't think even without running the RTP through Asterisk this box
could handle 10922 concurrent calls.

--Luki

John C. Wolosuk Jr. wrote:
> Can anyone share their experience on the maximum number of calls a given 
> asterisk box/asterisk software can handle?
> I see the asterisk business edition can handle up to 240 simultaneously 
> with appropriate licensing, but that doesn't seem to be many at all.
> 
> For now, I plan to use the stable open source versions - would it be 
> reasonable to say that it is more of hardware limitation on the number 
> of calls that can be made simultaneously, or are there only so many 
> calls the asterisk software programming is equipped to handle 
> simultaneously?
Hardware plays a part as does what you are doing. The execution of 
different things for different tasks can also yield different scaling 
issues.

First example:

Simple channel comes in channel goes out with no media scenario. 
Asterisk is obviously going to scale better since it doesn't have to do 
as much. It's basically handling signalling and call setup/teardown.

Second example:

Simple channel comes in channel goes out with media.
Media is going to be moved from kernel space to user space, sent to the 
other channel in an Asterisk frame (which may or may not allocate 
memory, depending on if you are using 1.4 and caching), turned into a 
proper frame to be sent out to network (in the case of RTP it gets RTP 
headers attached), moved to kernel space, and sent. This can be a very 
intensive process and doesn't scale as well as above obviously.
> Also is the asterisk software written to effectively take advantage of 
> multiple processors?
Yes, it is multithreaded and can take advantage of multiple processors.
> The systems I plan to use for asterisk have the following specs:
> 
> dual 2.8GHZ+ Pentium's
> 2GB RAM+
> Gigabit interfaces
> 
> In my situation, I have no plans to run anything other than G.711/SIP, 
> so my transcoding need is probably only limited to the playback of 
> pre-recorded messages as well as any processing involved in leaving 
> voicemail.
Your issue is probably going to be hard disk access. For example: With 
minimal tweaking on my development machine I can get 330 channels up 
with full RTP in both directions playing back audio from a hard disk. If 
I move to a ramdisk based solution this goes up to 550. That's a 220 
channel increase. Pushing the channels past this yields degraded audio 
quality. (AMD Athlon64 X2 4200+ with 1GB of RAM, 80GB SATA hard drive 
for those who are curious).

The easiest way to know though is to setup your system and test it using 
something like sipp. Identify where your bottlenecks are and see what 
you can do to alleviate them. In the above scenario hard disk access was 
a bottleneck so I took it out of the picture and look what happened.
> in theory, a gigabit interface can move 1048576Kbit/sec - now if i 
> generously allocate 96Kbit/sec for every G.711 call, the network 
> transport can handle, again in theory, 10922 simultaneous calls. would 
> it be wrong to expect performance near this mark for the asterisk software?
In a perfect world maybe that would happen but this is a simple PBX 
running on Linux.

Joshua Colp
Software Developer
Digium, Inc.