Axon PCM Packetization
May 20, 2021
A video that discusses how the Axon handles PCM packetization, references difference with KAM-500.
Video transcript for the hearing and visually impaired
So an ethernet to PCM bridge is a way of getting data from multiple distributed ethernet DAUs transported across an FTI network, selected in real time and placed in a coherent single PCM output. In KAM-500 this would correspond to data from KAM _CHS1 and data from KAM _CHS1 both aggregating into an FTI ethernet switch and sent over a gigabit link to an EBM module in KAM _CHS3 where it's selected and placed in a PCM transmitter.
How this is configured in DAS Studio - in DAS Studio I have a 6U KAM chassis with an ADC in slot 6. I have a second 6U KAM chassis with an ADC in slot 4 and they are both connected to a SWI/101/C. The first chassis is connected to port 1, the second chassis is connected to port 2. The data is aggregated out to port 8 of the NET/SWI/101. The output of the NET/SWI/101 goes to an EBM/102/B where the data is selected out of the ethernet transport packets and sent to the PCM encoder. In the PCM frame I have data from ADC/2 in the second KAM chassis and ADC/1 in the first KAM chassis at a mixture of 2048Hz and 1024Hz placed in PCM.
At compile time, DAS Studio builds transport packets to get the data across the network into the PCM stream in a coherent manner.
So you can see on the output of the first chassis I have two transport packets, one with parameters of 2048Hz, another one with parameter 1024Hz built out of that chassis. On the second KAM chassis I have again two packets of 2048Hz samples and 1024Hz samples. They are both aggregated on the ethernet switch, to output port 8 and sent to the EBM in slot 3 of the 3U chassis at the end.
So what's actually happening inside slot 3? In KAM chassis 3 you have the KAD/BCU with its 100Base-T link for PTP link to the switch. You have the KAD/EBM in the chassis taking the iNET-X transport packets on the network and you have the KAD/ENC/106 doing the PCM encoding. The EBM takes the extracted samples out of the packets and sends them over the backplane to the encoder, which transmits them in PCM.
These next couple of slides explain how this works in Axon. In Axon, because it's designed to work seamlessly with the KAM-500 system, you can have the KAM chassis connected to the same switch and an Axon chassis connected to the same switch and there's a gigabit link then to Axon chassis 2 with the PCM out of that. You will notice there is only one link, there is no EBM required in the Axon chassis 2.
So just to show how this is configured in Das Studio. My Axon CHS1 has an AXN/BCU/402/B and an ADC/401 in slot 2. My KAM chassis has a BCU/140/D and an ADC/109/C/S1 in slot 4. Again, they are both connected to the ethernet switch, the NET/SWI/101 and aggregated out to port 8 which connects to the BCU in the Axon chassis.
So what's actually happening on the Axon chassis to get the data across to the ENC/401 PCM encoder? In Axon chassis 2 there's the gigabit link to the BCU. That has the BCU PTP synchronisation packets to the switch and the iNET-X transport packets come in on that link.
They are captured by the AXN/BCU and sent over the Axon backplane to the AXN/ENC/401. The EBM block in the IP of the AXN/ENC/401 does the data extraction onboard. There is no need for a separate EBM. The data is extracted in the EBM block and placed in the transmitter. So in Axon you do not need the EBM module to do the ethernet to PCM bridge.
Again just to show the PCM frame out of the Axon, I have data from the AXN/ADC at 2048Hz and 1024Hz and data from the KAM/ADC at 2048Hz and 1024Hz. The same way that it works in KAM-500, transport packets are built to get the data across the network at the required rate so I have my 2048Hz parameters and my 1024Hz parameters in separate packets and then in KAM-500 I have my 2048Hz and 1024Hz packets out of the KAM-500. They are again aggregated in the switch and sent to the Axon controller which sends them over the backplane to the ENC/401. So when doing an ethernet to PCM bridge in Axon there is less wiring because you need one less port on your ethernet switch. The same wires, because it's a gigabit link, to the BCU can be used to aggregate the data down to the ENC/401.