The Concatenation/De-concatenation blocks of the eCPRI IP implements concatenation logic of the eCPRI messages into single Ethernet frame or single IP/UDP packet. The sink_concatenation
sideband signal identifies packets that required concatenation. The below diagrams illustrates the eCPRI messages with and without concatenation.
When multiple eCPRI messages are concatenated together, 0 to 3 "zero" padding bytes are added if the following message does not start at a 4 byte boundary. The payload size specified in the eCPRI common header does not include this extra zero padding bytes.
The above waveform shows two incoming eCPRI messages entering to eCPRI IP, first eCPRI message with concatenation sideband interface signal = 1 and the second eCPRI message with concatenation sideband interface signal = 0. These two eCPRI messages are combined and send through single transport network layer protocol. On the receiving end, the combined eCPRI message will then de-concatenate into 2 eCPRI messages and output to Avalon® streaming interface
There is a timeout counter used to detect the end of the concatenation message. If the counter overflows and no message with C=0 is detected, an error will be logged and the message with C=1 will be converted to message with C=0 and send to MAC.
The message type allowed for concatenation is restricted to message type 0,1,2,3 and 6. The de-concatenation is supported on all message type except message type 5.
There are 2 different eCPRI packet concatenation scenarios which trigger error and it is shown in below diagram.
In the first scenario, there are three incoming Avalon® streaming interface
packets payload size of 500 bytes, 600 bytes and 700 bytes. The total payload size after concatenation is 1800 bytes which is bigger than maximum eCPRI IP supported maximum transmission unit (MTU) size of 1500 bytes. In this case, error will be logged in the eCPRI TX error message register and payload 0 and 1 will be sent as concatenated packets while payload 2 will be sent by itself.
In the second scenario, the first packet payload size is more than 1500 bytes. In this case, all the packets drop and error logged in eCPRI TX error message register.