I’m curious in knowing or rather understanding as to why 3GPP decided to have two related flags, i.e., RF (Re-segmentation Flag) and FI (Frame Indicator) … when both of them indicate the same thing [according to me]?
Can any one of you please help to understand it from a 3GPP design perspective?
I think RF may be used for the purpose of RLC layer re-transmission in TM acknowledge mode.
FI = indicates whether a RLC SDU is segmented at the beginning and/or at the end of the Data field.
RF = indicates whether a RLC PDU carries re-segmented data.
@Wendy You mean AM mode, don’t you?
FI also indicates whether the first and last part of Data section contains SDU segment or not? So, why RF is required?
@marcengo yes, but isn’t the purpose of FI to indicate whether first and last part of Data section contain segment or not. That itself should be enough, right? What benefit does the RLC receiver get by reading this extra RF field?
Imagine in the first transmission your data is split into four parts.
Part1: FI = 01, RF = 0.
Part2: FI = 11, RF = 0.
Part3: FI = 11, RF = 0.
Part4: FI = 10, RF = 0
Now imagine you failed the Part2 and need to retransmit it, but your radio conditions deteriorated and now you have a lower PHY throughput, which ultimately will result in RLC re-segementing your Part2.
Now you have
Part2.A: FI = 01, RF = 1.
Part2.B: FI = 10, RF =1.
FI only indicates which part of the original PDCP PDU you are working with, RF indicates if you had to resegment an original packet, either was it already segmented or not.
Yes, I mean AM mode. The reason why extra RF field is needed in the example of @marcengo. Besides that, in each new transmission, MAC request a specific payload size, and RLC layer should fulfill this size requirement to maximize the throughput. Thus SDU from PDCP layer may need re-segmented. RF field is used to carry this info to receiver side for re-assembling of PDCP’s SDU.
@marcengo @Wendy I understood now. FI only indicates if the first and last portion in RLC PDU are PDU segments or not. These PDU segments can be original PDU segments [as part of original first time transmission] or re-segmented PDU [originally it was sent as PDU but later on during re-transmission it had to be segmented to not enough grants]. In the latter case, RF will be set.
Is this understanding correct?
Yes you did!