ilities should not be underestimated for subscribers in mobile or static (deep fading) environments. The measurement speed and accuracy of mobiles will continue to play a significant role in future NACC and NCCR capable mobiles and will have an even bigger impact to the EGPRS throughput and latency performance for data subscribers. The Anatomy of an EGPRS Cell Reselection EGPRS performance is also affected by the inherent delays in idle mode cell reselection. Figure 2.85 shows the typical downlink RLC/LLC throughput response for an EGPRS cell reselection. Note how the downlink throughput degrades after the cell reselections. The key to understanding why this is the case is that the EGPRS mobile is no longer 'listening' to the previous cell SO it must re-tune Radio Network Planning and Optimisation Figure 2.85 EGPRS cell reselection throughput response to the new cell and then decode the system information messages (1 to 4 and 13). Following this, the EGPRS mobile will perform a RACH request to identify that it is on the new cell. This RACH is followed by a dummy packet control to alert the SGSN of the new base station virtual circuit identifier (BVCI) to route the LLC packets. Finally, when the packets are re-routed, the downlink sends a packet downlink assignment to re-establish full downlink data transfer on DL timeslots 5, 6 and 7. Figure 2.86 highlights the Layer 3 messages that make up the EGPRS cell reselection procedure. In reviewing the procedure, it is noted that (in this cell reselection) the EGPRS mobile took about Source Direction Message Message Details Downlink Synch Channel Information Uplink Packet Downlink Ack/Nack Downlink Packet Uplink Assignment Downlink Packet Uplink Ack/Nack System Information Type 4 Uplink Packet Control Acknowledgement Time: 20:26:01.95 Downlink System Information Type 4 Downlink System Information Type 1 Message Details Downlink System Information Type 2 Downlink System Information Type 3 Downlink System Information Type 3 Downlink System Information Type 13 Channel Reque