What is PDCP sequence number in LTE?

i) Data coming into PDCP first go through “Sequence Numbering” Procedure. It means that PDCP add “Sequence Number” to each of incoming data block . Once it add ‘Sequence Number’, it has to manage the number.

LTE Protocol Stack Layers

The value of the PDCP sequence number ranges from 0 to 65535 . The PDCP SN window size indicates the maximum number of PDCP SDUS, not confirmed to have been successfully transmitted to the peer entity by lower layer, that can be numbered at any given time. The PDCP SN window size is configured by upper layers.