terleaving) Bits (BER) Increased Granularity Rx Lev Rayleigh Fading Fading Environment (I.e. TU3nFH) Average Rx Lev Figure 2.87 Impact of fading on BER, BLER and FER Radio Network Optimisation "Full Speed" TBF Ending "Slow down" Gb SGSN Gb SGSN (LLC) Buffer (LLC) Buffer (LLC -> RLC/MAC) (LLC -> RLC/MAC) Air Interface Air Interface (RLC/MAC) (RLC/MAC) MS Re-assembly MS Re-assembly (RLC/MAC -> LLC) (RLC/MAC -> LLC) Application Application Figure 2.88 Data flow (segmentation and reassembly) Changing modulation from GMSK to 8-PSK increases the path loss SO a reliable GMSK link may break down if too aggressive towards higher 8-PSK coding schemes. The FEC is more robust at MCS-5 than MCS-4 SO some coding gain can be realised that compensates for the reduced Eb/ No in 8-PSK. Actual results show that the MCS-3 performance is better than MCS-5 for short TBFs. With the increased dynamic range of bandwidth that EGPRS offers, it is very important to maintain smooth end-to-end flow of data SO that the application layer does not experience buffer overflow or underflow. Figure 2.88 illustrates two examples of how application layer datagrams flow from the packet core to the mobile subscriber. The figure on the left shows how the slow/fragmented arrival of data packets results in fragmentation and short TBFs while the right figure shows a steady (even congested) flow of data that results in steady (maximum link adapted) TBFs. The significance of this on EGPRS is that suboptimum optimisation of transmission and radio link adaptation results in stalling and buffering issues. Either the PCU or mobile LLC segmentation/reassembly falls behind or ahead of the application layer and results in unsteady performance. An important concept to consider in this discussion on EGPRS throughput is the actual desired profile of throughput and latency. The average and instantaneous throughput varies depending on the window size defined for calculating user throughput. With this in mind, it may be considered more important to ensure robust forward e