. Looking at the uplink TBF retainability trend again, there is a clear change in the middle of the time period where one network improves while the other degrades. The improvement was caused by decreasing TBF release delay parameters. This resulted in more rapidly releasing TBFs (typically second) after the RLC flow timed out. The degradation on the other network was caused by an adjust- ment to the link adaptation algorithm that biased the MCS to 5 instead of 3. This change effectively increases the vulnerability of many short uplink TBFs since they became exposed to 8-PSK instead of GMSK and the forward error correction of MCS-5 could not compensate for the reduced signal level. TBF Blocking When packet data channels are overloaded, TBF blocking occurs and causes interruptions in the data service. The solution is to dimension the voice and data TRX capacity or groom the traffic such that data have enough room to allocate timeslots without blocking. On the one hand, the introduction of EGPRS does help alleviate this problem by increasing the transfer rate and reducing the probability of simultaneous access. However, on the other hand (and actual reality), EGPRS has made this 'problem' more prevalent (a good thing for growth!) due to the increased data demand on the network. Figure 2.93 outlines several examples of networks with high and nominal TBF blocking. In this example, both the uplink and downlink TBF blocking ranges from a residual < 0.5 to bursts of well over %. It is clear that the network with the high uplink and downlink TBF blocking requires special attention in re-balancing the traffic and is likely to involve expanding TRX capacity on select busy cells to permit additional allocation of packet traffic channels. EGPRS Optimisation in the Field This section briefly introduces some key concepts and knowledge that can be applied for field optimisation of EGPRS networks. One of the key aspects of field optimisation is to understand first the bottlenecks and performance in a sample test area in 'ideal' 