 TLLI. The BSS uses flow control to adjust the flow of BSSGP UNITDATA PDUs to a BVC buffer. The amount of buffered BSSGP UNITDATA PDUs in the BSS should be optimized to efficiently use the available radio resource. The volume of buffered BSSGP UNITDATA PDUs for a BVC or mobile station should be low. BSSGP UNITDATA PDUs queued within the BSS that are not transferred across the radio interface before the PDU life- time expires are locally deleted from the BSS. The local deletion of BSSGP UNITDATA PDUs in the BSS is signaled to the SGSN by the transmission of an LLC-DISCARDED PDU. For each FLOW-CONTROL PDU received by an SGSN, a confirmation is always sent across the Gb interface by the SGSN. The confirmation uses the tag that was received in the FLOW-CONTROL PDU, which was set by the BSS to associate the response with the request. When receiving no confir- mation to a FLOW-CONTROL PDU, the reasons that gave rise to the trig- gering of a flow control message may trigger another message, or, if the condition disappears, it may not. For the repetition of nonconfirmed FLOW- CONTROL PDUs, the maximum repetition rate still applies in the BSS. Control of the Downlink Throughput by the SGSN The principle of the BSSGP flow control procedures is that the BSS sends flow control parameters that enable the SGSN to locally control its trans- mission output in the SGSN to BSS direction. The SGSN performs flow Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) Chapter 9 control on each BVC and on each mobile station. The flow control is per- formed on each LLC-PDU first by the mobile station flow control mecha- nism and then by the BVC flow control mechanism. If the LLC-PDU is passed by the individual mobile station flow control, the SGSN then applies the BVC flow control to the LLC-PDU. If an LLC-PDU is passed by both flow control m