The effectiveness of IAX versus SIP

The effectiveness of IAX (Inter Asterisk Exchange) versus SIP (Session Initiated Protocol)

Ready. Fight! We placed two signalling technologies in the ring to battle-it-out for best transmission all-rounder. To help us, we invited the minds behind one of South Africa’s leading VoIP and Internet Service providers, BitCo.

In one corner we have Session Initiated Protocol also known as SIP. In the other, Inter Asterisk Exchange, commonly referred to IAX. The judges, BitCo’s very own technical top dogs: Brian Wolfaardt (Technical and Operations Manager), Brandon Williams (Regional Manager for the North West Province) James Farnell (Interconnect Manager) and presiding judge, Kobus Mathee (Voice Director)
Round 1. It all started out rather amicably (and boring to say the least). Our judges were polite. Happy to praise each contender’s positives.

Session Initiated Protocol
SIP is versatile. Partial to Port numbers 5060/1, it is widely entertained by most Private Branch Exchanges (PBX). SIP is well organised, preferring the use of packet markers to determine what is Real Time traffic or HTTP data and prioritising them accordingly. SIP’s popularity was abundantly clear. Consumer SIP enabled devices such as SIP terminal adapters; gateways and trunking service continue to grow. All are suitable replacements for ISDN telephone lines.

Inter Asterisk Exchange
IAX had nothing to worry about after SIP’s rave review. IAX’s stability held it in good stead. This method rarely suffers from a shaky voice. Being less bandwidth hungry than its counterpart, its efficiency is also noteworthy. Simplicity came out tops. IAX only uses one port for signalling and audio (fool proof, if you please). It is thus also easier to configure the firewall.

Round 2. We asked the judges:
In which instances would SIP or IAX be used and why.
Technical and operations manager for BitCo, Brian Woolfaardt, dished out a heavy blow: “SIP is best used when IAX is not an option. In instances where clients seek to change to VoIP without changing their legacy PBX, SIP is the obvious choice. SIP can be used across a wide range of products that are SIP enabled. IAX however uses less bandwidth and generally more stable than SIP, hence why it is the preferred trunk to use when transmitting Voice traffic.”

Brandon Williams, the regional manager for BitCo’s North-West office, concurred that IAX has numerous advantages over SIP. He tipped the scales though when he conceded that one of IAX’s strengths could also be its weakness, “The main disadvantage of IAX is that it is susceptible to denial of service attacks because it only uses a single port.”
SIP however was not yet in the clear. James Farnell, BitCo’s Interconnect Manager awarded IAX a swift left hook with a few indisputable facts.

In a no-nonsense tone Farnell begins, “IAX is much more efficient in terms of data use than SIP and can triple the amount of calls per megabit when using the G.729 codec. This is because IAX is “information-element” based compared to SIP which is ASCII encoded. This is an older technology and therefore a heavier bandwidth user. IAX has a clear definition between layer 2 and 3. What this means is that the signalling and audio have clearly defined states, adding to the ease and reliability of IAX.”
As he continues the knock-out is delivered, “SIP uses port 5060/1 for signalling. SIP audio generally uses between UDP ports 16384-32767. This range is not registered (being so broad). Thus, users can generally choose their own 10000-20000 for RTP streams. This can pose as a problem when dealing with NAT’ing and multiple firewalls. IAX uses a single port, 4569, for both signalling and audio, multiplexing them over a single UDP. This adds to heightened security where by firewall administrators only need open 1 port compared to 10002 ports for SIP.”

Kobus Mathee, Voice Director for BitCo, qualifies James’ points. “Generally, SIP and NAT’ing are not even used in the same sentence. Whilst SIP signalling ports are easily NAT’able, the problem sits with the RTP (especially RTP return) packets. This means that you will more likely than not experience 1 way audio should you implement NAT’ing along with SIP. As with anything in IT, there are a few ways around it, none of them without a downside however.”
SIP was out cold, however, it remains the belt holder. Despite the trend seen with more hardware manufacturers and PBX software developers alike begin implementing IAX, SIP is still the preferred method for transporting voice traffic over a data network. Its legacy and compatibility make it a timeless choice.

Post-match verdict.
Unable to be resuscitated in time in this match the judges caution spectators around the country. Judging panel Spokesperson, Kobus Mathee concludes , “Although IAX is more efficient than SIP, it is still largely limited for use in Asterisk based solutions only. SIP is still the industry standard, and this is why basically all IP-enabled telephony solutions support this. You are likely to find SIP is at the forefront of most of the IP enabled telecom network interconnects in South Africa as well”

IAX is humble in its acceptance speech, thanking and acknowledging SIP as the enabler for the VoIP industry.
With an attitude like that we can see IAX fighting its way to the top and log and remaining there for many years.


Find out more about BitCo’s Voice Solutions.



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