What is transcoding
There are different kinds of SIP codecs. Transcoding means translating the telecommunication signal of one technology or SIP codec into another. Depending on the SIP codec involved, more or less CPU of the gateway will be used. On beroNet VoIP Gateways, we talk about channels in order to give a better idea of the number of calls we can do depending on the codec used.
| Gateway model | Nr. of simple channel | Number of complex channels |
|---|---|---|
| BF400box | 16 | 4 |
| BF1600box | 64 | 16 |
| BF6400box | 128 | 64 (only 32 for T.38) |
According the this information we can say that 1 complex channel = 4 simple channels
Our gateways are compatible with lots of different SIP codecs. Depending on the codec, a simple channel or a complex one will be used on the gateway when it transcodes.
| Codec | Type of channel needed |
|---|---|
| Clear Channel | simple |
| G.711 (with DTMF & echo cancellation) | simple |
| G.723 | complex |
| G.729 | complex |
| T.38 | complex |
| SRTP | complex |
PSTN to SIP transcoding
beroNet VoIP Gateways are often used to send calls from an analog, ISDN or GSM technology to SIP. Depending one the gateway model used and the SIP codec involved, the gateway will be able to process more or less calls. If the gateway transcodes the signal from PSTN to a SIP codec using simple channels, the number of possible calls amounts to the number of simple channels available on the gateway. The same applies for complex channel SIP codecs and the number of complex channels available in the gateway.
Example:
| Gateway model | FROM | TO | Nr. of possible calls |
|---|---|---|---|
| BF400box | BRI | G.711 | 16 |
| BF400box | BRI | G.729 | 4 |
| BF1600box | PRI | Clear channel | 64 |
| BF1600box | PRI | T.38 | 16 |
SIP-to-SIP transcoding
When a beroNet VoIP Gateway is used to send calls from one SIP peer to another two calls are established in the gateway:
- SIP to Gateway = 1 call with x number of channels used
- Gateway to SIP = 1 call with x number of channels used
In order to know how many channels are used, thus how many calls the gateway can handle, we need to sum up the channels used in both calls.
| From | To | Nr. of simple channels used for one SIP to SIP call | calls bf400 | bf1600 | bf6400 |
|---|---|---|---|---|---|
| G.711 | G.711 | 2 | 8 | 32 | 64 |
| G.711 | G.729 | 5 | 3 | 12 | 42 |
| G.729 | G.729 | 8 | 2 | 8 | 32 |
| G.711 | T.38 | 5 | 3 | 12 | 25 |
SIP transcoding can requires more than one channel for specific codecs. G.711 is the least resource intensive code. G.729 and T.38 transcoding require more than on channel on our gateway.