N3101 & N3103 are both used at the network side and are used as indicator for lost contact with MS.
nw assigns USF allowing the MS to transfer the data in the assigned radio block. if nw does not receive data in the block it will increment counter N3101. whenever on assignment of USF, valid data block is received from MS, it is reset.
when all ul blocks have been transferred by MS, nw acknowledges the reception by sending Final UL AN message with FAI=1 and waits for the MS acknowledgement so that tbf could be released normally, if the ms doesnot respond with packet control acknowledgement NW increments this counter. if valid packet control acknowledgement is received from MS, nw resets this counter.
in the condition when either of N3101 or N3103 reaches their maximum value(s), NW starts a timer T3169. if this timer expires the TBF is abnormally released* with the flag “No response from MS”.
* Many vendors like ericsson and nokia does not consider tbf release due to N3103 as abnormal.
hope its clear now 🙂