Transcoding is the process of converting one protocol to another. In the IP communications world, this might entail converting H.323 to SIP. H.323 was the first widely used VoIP protocol and many older systems have not been updated to speak SIP. Through transcoding, these older systems can still participate in a SIP communications cloud.
Transcoding also might mean converting one codec to another. For example, an enterprise may wish to bring all calls into an Interactive Voice Response (IVR) unit as G.711. G.711 is a good codec for DTMF transportation and detection as well as speech recognition. However, G.711 is a fairly high bandwidth consumption codec and you might want to reduce the network impact of those calls by converting them to low bandwidth code like G.729 after the IVR has completed its task and transfers the call to a contact center agent.
One of the most common uses of transcoding is when a SIP application like a voicemail system can only speak one codec (e.g. G.711) and your enterprise has standardized on a different codec (e.g. G.729).
In the SIP world, a common place to perform transcoding is the Session Border Controller (SBC). However, not all SBCs support transcoding, so it’s important to pay attention to which ones do and which ones don’t. The following is a short list of SBCs that will transcode from one codec to another.
Sonus SBC 1000
Sonus SBC 2000
Sonus 5100
Sonus 5200
Acme 3820
Acme 4500
AudioCodes Mediant 1000 E-SBC
AudioCodes Mediant 3000 E-SBC
AudioCodes Mediant 4000 E-SBC
Genband Quantix Q20
Absent from my list are the Genband Quantix Q10, the Avaya, and the Acme Session Director SBCs. If transcoding is required then it must be done elsewhere.
Finally, it’s important to consider what you want to transcode to and from. There are a plethora of codecs out there (G.711, G.722, G.723.1, G.726, iLBC, AMR-wb, etc.) and not every SBC will transcode to and from every codec. It’s not enough to ask if an SBC supports transcoding. You need to determine the list of codecs it works with.
For deeper dives into SBCs, please refer to my blogs, Choosing the Right SBC – It All Depends and Andrew’s Session Border Controller Checklist.
Tao, Zen, and Tomorrow 
I don’t think the word “transcoding” as signaling protocol convert like signaling gateway of SIP to H.323 and vice versa. Usually the word is accepted in the meaning of CODEC conversion between G.711u or a, and G.729, 723 for compression at the border between public and enterprise, or G.711u/1 and amr etc. at the border between public and mobile…
In my meaning of transcoding/transcodec, acme 3xxx/4xxx has no functionality for that, although 9xxx has the functionality. (The NP of 4500 has some RTP packet header control, but not for codec change itself). For the GENBAND QUANTiX, some of new model have DSP media card within it, but most of existing series require external equipments for the CODEC change.
Thank you for your comment. The Acme 3820 and 4500 can perform codec transcoding, but you need to add a hardware module to the boxes to do so. The Genband Q20 does support transcoding, but the Q10 does not.
Hi Andrew, does Genband Q10 already support transcoding?
The last time I looked, it did not. Only the Q20 did. I found the Q10 data sheet on the Internet and it’s very vague.