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.In particular, it supports the transfer of LLC-PDUs between MS and SGSN and provides flow control for this purpose.Communication between the BSSGP entities (SGSN and BSS) takes placeon either a point-to-point (PTP) or point-to-multipoint (PTM) basis usingBSSGP virtual connections (BVC) over the network, each of which has itsown identifier (BVCI).Data transfer between the MS and SGSN is PTP,while some BSS management functions are PTM.PTM BVCI are config-ured statically and PTP dynamically at the SGSN, while at the BSS the PTPBVCI can be either static or dynamic.The SGSN provides the BSS with information about MS data transfersthat is stored in BSS contexts.Each BSS context refers to a single MS butmay contain several distinct packet flows, each identified by its own packetflow identifier (PFI).The PFI is needed if the optional packet flow context(PFC) is supported.If the PFI is not stated, then packet flow is handled on a best-efforts basis.Packet flow control is only provided on the DL.The BSS has a bufferfor each BVC and indicates to the SGSN via FLOW-CONTROL-PDUs themaximum flow it can take together with the maximum rate for a specific MSand TLLI.This is based on a leaky bucket approach, and the informationsent consists of the bucket size (BMax), bucket leak rate (R ), and bucket full ratio for the BVC or for an MS.An LLC-PDU is only sent if it is compatiblewith both of these flow control measures [i.e., if the current bucket count (B ) plus the LLC-PDU length does not exceed either BMax].Rules are providedfor calculation of the MS bucket size, and there is also a default.There is also a minimum interval for FLOW-CONTROL-PDUs in normal conditions.The peak rate quoted in the QoS profile (see Sections 5.5 and 9.2) is related to the bucket leak rate (R ).Precedence in the QoS profile is the radio priority (14) for the UL-UNITDATA and high, medium, or low (coded as 0, 1, 2)for the DL.GPRS1075.3 Layer 2 RLC and MACLayer 2 consists of the RLC and the MAC [7] beneath it, which are bothdistinct from those described in Chapter 3 for UMTS.RLC communicateswith the RRC layer and upper layer signaling and LLC user traffic, while theMAC interfaces to the physical layer and controls access to the radio inter-face.Most pure signaling traffic bypasses this sublayer.A separate protocol, BSSGP, is used at this level on the link from the BSS to the SGSN.Thestructure of the RLC/MAC layer and its position in relation to the LLC,MM, and physical layers is shown in Figure 5.6.As indicated, the control channels use the data link layer [14] (whichuses 21 or 23 octet blocks), while the packet data uses the RLC/MAC.AsMM sublayerLLC sublayerRR-SAPGMMRR-SAPGRR-SAPRR sublayerRR managementNon-RRRRUpper layers'RRPDPDUsRLC/MACPCCCHPDTCHPBCCHPACCHSAPI 0SAPI 3RACHPCH SDCCHFACCHSACCHBCCHAGCHSACCHSDCCHData link layer (signaling layer 2)PDCHPhysical link layerFigure 5.6 RLC/MAC.(Source: [7] © ETSI 2001.)108QoS in Integrated 3G Networksregards QoS, the latter is much more significant and is described in the fol-lowing sections.The permitted combinations of logical channels over thephysical channels are specified in [15].5.3.1 RLCThe prime function of the RLC is to segment and reassemble logical linkcontrol PDUs into RLC/MAC blocks [7].It has two modes of operation:AM and UM.In AM the RLC is also responsible for sequencing, acknowl-edgment, and retransmission of blocks to provide backward error correction.In EGPRS AM it also provides incremental redundancy that avoids reseg-mentation for ARQ retransmissions.Packet transfer is based on temporary block flows (TBF) that each havetwo peer RLCs as end points, each of which has a receiver and a transmit-ter for RLC/MAC blocks.Each end-point RLC has both a transmit and areceive window size (WS) for these blocks.Flow control also makes use ofa sequence number space (SNS) and a block sequence number (BSN).InRLC AM each end point maintains receive and transmit state variables V(Q)and V(S ), respectively, as well as an acknowledge state variable V(A).InRLC UM the sole use of the BSN is for reassembly of LLC PDUs fromRLC/MAC blocks.V(S ) denotes the BSN of the next block to be transmitted, and it is setto 0 at the start of a TBF and incremented by 1 with each block transmitted.It lies in the range 0 to SNS-1 and must not exceed the value of V(A) mod-ulo SNS by more than the window size WS.The receive state V(Q) must satisfy the inequality V(Q) ≤ BSN ≤V (Q) + WS.The transmitter maintains the value V(A) for the BSN modulo SNS ofthe oldest RLC/MAC block for which it has not yet received an acknowledg-ment from its peer.This value is updated on the basis of data in the received block bitmap contained in ACK/NACK messages from the peer.The ACK/NACK message is sent in both RLC AM and UM but maybe ignored in the latter.It consists of a starting sequence number (SSN) and the received block bitmap (RBB).Both the receiver and transmitter maintaincopies of the RBB.The receiver sets the SSN to the value of the currentreceive state and makes an array of WS entries in which it sets each entry to 1for a valid received block and to 0 for an invalid block
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