To date, IP-to-IP transcoding from one vocoder to another on carrier premises was not common practice.
The main reason being that the majority of calls traversing between carriers used TDM peering, hence in most cases media gateways performing the role of interworking between IP-to-TDM were the only points where transcoding took place. Likewise, since TDM uses G.711, many carriers enforced this coder throughout their network, thus limiting the transcoding provided by media gateways for G.711 only.

These days this paradigm is beginning to shift as can be seen from a growing stream of carrier RFPs which search for an IP-to-IP transcoding solution.

As a result of its cost benefits, IP Peering is gaining momentum and many carriers, particularly CLECS are setting up IP-SIP trunks for inter-carrier communication.
The use of IP Peering gives rise to an All-IP environment on the carrier premises which on one hand reduces the need for media gateways, but on the other hand enhances the need for IP-to-IP transcoding.

A carrier that supports IP Peering is likely to need an element capable of IP-to-IP transcoding for the following reasons:
 

• The terminating network to which this carrier is connected enforces the use of a single vocoder, hence any outgoing calls to this network may require transcoding
• The two end terminals don’t share a common vocoder among their vocoder sets. You might wonder why G.711 is not supported by both end terminals. One reason is that this is a cellular handset and G.711 bandwidth resources prohibit its use
• G.711 optimized equipment. There are many VoIP devices available that either support only G.711 or the upgrading process and allowing support for other vocoders is costly. Examples of such devices include media gateways, media servers etc.
• Vendor specific vocoders. In some cases vendors develop a successful VoIP service which uses their own proprietary vocoder. In this case any calls between this service and a party outside of this service requires transcoding. Such examples include Microsoft Open Communications Server RTA vocoder and Skype’s SILK
• Bandwidth savings. A carrier may peer to other carriers or users by utilizing an expensive transport medium such as satellite communications. In this case the carrier prefers all calls going through this pricey transport pipe to be transcoded to a low bit rate vocoder in order to reduce the bandwidth consumption

Apart from these considerable reasons mentioned above, some other facts and trends further intensify the spread of IP-to-IP transcoding elements. IMS is becoming more prevalent on a daily basis. One of the key capabilities of IMS is being able to converge multiple access networks into one common core network. An example of this is a typical IMS core network which can interface cellular, cable and wireline networks. Each of these access networks is likely to use different vocoders (stemming from both functional and historical reasons), and these different vocoders converge at one point, known as the IMS core network. To be able to carry out calls between subscribers belonging to the different access networks the use of an IP-to-IP transcoding element is required. The IMS has defined the Media Resource Function Processor (MRFP) as the element responsible for IP-to-IP transcoding.

Another accelerator for IP-to-IP transcoding is vocoder licensing costs. Many low bit rate vocoders require a usage fee by their patent owners. In a world where fierce competition between handset manufacturers exists, it is apparent that a handset may only support only a few vocoders to minimize its cost. This increases the chances of vocoder mismatch between different handsets.

Lastly, the new trend of wideband vocoders is another accelerator. Wideband vocoders facilitate improved voice quality for voice calls. The introduction of these vocoders extends the overall number of vocoders in the market and thus the need for transcoding. Furthermore a new breed of IP-to-IP transcoding is necessary to enjoy the all encompassing benefits of these vocoders. An IP-to-IP transcoding device which interworks between two wideband vocoders must maintain the wide band quality of the speech.

IP-to-IP transcoding devices are not just about transcoding from one vocoder to another. Anything that has to do with RTP media manipulation can take place on the IP-to-IP transcoding device. Examples include securing voice by interworking RTP into SRTP and Voice Quality Enhancement features such as automatic gain control, de-jittering, and acoustic echo cancelation. Finally monitoring of voice quality can also be handled by these devices.

The most natural network element to evolve into an IP-to-IP transcoding device is the media gateway. This is because the media gateway is already doing transcoding from IP-to-TDM so it has both the packet and the DSP capabilities for the job.

Adapting media gateways and enabling them to become IP-to-IP transcoding elements also offers investment protection to carriers who are transitioning from TDM peering-to-IP peering. In many cases these carriers only need a software upgrade to utilize the same media gateways for the purpose of IP-to-IP transcoding and simultaneously IP to TDM transcoding.

AudioCodes’ Mediant family of gateways currently supports IP-to-IP transcoding in addition to their “legacy” role of IP-to-TDM interworking. By leveraging its extensive heritage of DSP expertise, AudioCodes’ Mediant gateways support one of the richest set of vocoders in the market, which includes wideband transcoders (a part of the High Definition VoIP program offered by AudioCodes). The Mediant is also one of the few IP-to-IP transcoders capable of transcoding from one wideband vocoder to another while maintaining the High Definition voice quality. Apart from its variety of vocoders, the Mediant also offers one of the highest density IP-to-IP transcoding capacities per rack in the industry. Lastly, the self developed, field-proven SIP stack assures that the Mediant IP-to-IP transcoding device will fit into any carrier environment.

In conclusion, IP-to-IP transcoding is set to become a key component on VoIP service providers. Media gateways are likely to play a major role in this new transcoding market place.