08.183GPPBase Station System (BSS) - Serving GPRS Support Node (SGSN)BSS GPRS ProtocolGeneral Packet Radio Service (GPRS)Release 1999TS
6.1 Downlink UNITDATA procedure
On the downlink, a DL-UNITDATA PDU shall contain information elements to be used by the RLC/MAC function and an LLC-PDU. There shall be only one LLC-PDU per DL-UNITDATA PDU. The LLC-PDU shall always be the last information element in the DL-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.
An SGSN provides the BSSGP with a current TLLI, identifying the MS. If an SGSN provides a second TLLI, indicating that an MS has recently changed its TLLI, this shall be considered as the "old" TLLI. A BSS uses the "old" TLLI to locate an MS’s existing context. Subsequent uplink data transfers for this MS shall reference the current TLLI, and not the old TLLI.
The SGSN shall include the IMSI in the PDU. As an exception, the SGSN may omit the IMSI in the PDU if the mobile station identified by the TLLI is in MM non-DRX mode period (i.e. during a GMM procedure for GPRS attach or routing area updating defined in GSM 24.008 ) and the SGSN does not have a valid IMSI.
The SGSN may include the Service UTRAN CCO (Cell Change Order) information element in the PDU (relevant if the network initiated cell change order to UTRAN procedure is used). If this information element is received in both the DL-UNITDATA PDU and the CREATE-BSS-PFC PDU, the information element received in the DL-UNITDATA PDU shall take precedence.
If the SGSN has valid DRX Parameters for a TLLI, then the SGSN shall include them in the PDU. Nevertheless, the SGSN can omit the DRX Parameters if the MS identified with the TLLI is in MM non-DRX mode period to speed up the transmission of the LLC-PDU on the radio interface. The SGSN shall not send a DL-UNITDATA PDU without the DRX Parameters IE if the MS identified with the TLLI is not in MM non-DRX mode period.
An SGSN provides the BSSGP with MS specific information, enabling the RLC/MAC entity in a BSS to transmit an LLC-PDU to the MS in a user specific manner. The information made available to the radio interface includes:
– MS Radio Access Capability. This defines the radio capabilities of the ME. If there is valid MS Radio Access Capability information known by the SGSN for the associated MS, the SGSN shall include it in the DL‑UNITDATA PDU. Otherwise, MS Radio Access Capability shall not be present;
– Packet Flow Identifier. This identifies the packet flow context associated with the LLC PDU and is included by the SGSN if the packet flow context feature is negotiated. If the mobile station does not support the PFC feature or if the PFI is not known (e.g. the new SGSN did not get the PFI from the old SGSN during a RAU) then the SGSN shall use the pre-defined PFI to indicate best-effort QoS;
– QoS Profile. This defines the (peak) bit rate, the type of BSSGP’s SDU (signalling or data), the type of LLC frame (ACK, SACK, or not), the precedence class, and the transmission mode to be used when transmitting the LLC-PDU across the radio interface. If the PFI is included then the maximum bit rate for downlink specified in the PFC ABQP shall supersede the peak bit rate specified in the QoS Profile IE;
– PDU Lifetime. This defines the remaining time period that the PDU is considered as valid within the BSS. If the PDU is held for a period exceeding the "PDU Lifetime" time period, the PDU shall be locally discarded. The PDU Lifetime is set within the SGSN by the upper layers.
A BSS may incorporate the PDU Lifetime, the Precedence and the (peak) bit rate into its radio resource scheduler. If the PFI is present then the BSS may incorporate the information from the associated ABQP into its radio resource scheduler. The algorithm to do this is out of scope of the present document.
Two types of BSSGP SDU are distinguished within the QoS Profile: layer 3 signalling and data. Layer 3 signalling may be transmitted over the Um interface with higher protection. If the MS has an RR connection to the network (see GSM TS 04.18), Layer 3 signalling may be transmitted over the Um interface on the main signalling link of the RR connection, provided that the LLC PDU meets length restrictions imposed by the BSS. In this case, the BSS shall include the LLC PDU contained in the BSSGP PDU in the correspondent Layer 3 Um interface message (see GSM TS 04.18).
The type of LLC frame indicates if the LLC frame type is an ACK or SACK command/response, or not (see 3GPP TS 04.64). An ACK or SACK command/response frame type may be transmitted over the Um interface with higher protection.
Two transmission modes across the radio interface are possible: acknowledged (using RLC/MAC ARQ functionality) and unacknowledged (using RLC/MAC unitdata functionality). These transmission modes do not apply when the MS has an RR connection to the network and BSS uses the main signalling link of the RR connection, in which case the acknowledged transmission mode is used.
If Priority is present, only the priority-level field shall be regarded. The management of priority levels is implementation dependent and under operator control. The preemption capability indicator, the queuing allowed indicator and preemption vulnerability indicator shall be ignored.
In addition to constructing the DL-UNITDATA, the SGSN supplies the LSP, the BVCI and the NSEI associated with the MS to the lower layer network service, enabling network service routeing to the peer entity. These parameters are not transmitted as part of the BSSGP across the Gb-interface.
In the case where localised service area is supported the SGSN may inform the BSS as to which LSA identities that the mobile has preferences by sending the LSA INFORMATION element. The BSS stores this information and uses it e.g. for network controlled cell re-selection when determining specific cell selection parameters for the mobile. The algorithm for determining specific cell selection parameters for the mobile is not defined further in this technical specification.
6.1.1 Abnormal conditions
The following actions are defined in periods of congestion.
To satisfy the maximum number of service requests, the BSS may redistribute MSs among cells (i.e. network controlled cell reselection is initiated). If this occurs, the BSS may inform the SGSN through the RADIO STATUS PDU (Radio Cause value: cell reselection ordered). The BSS shall update any internal references that indicate the location of the MS. The BSS may attempt to internally re-route queued LLC frames to an MS that has been moved to a new cell. If this functionality is not supported, or if it is not possible to internally re-route LLC frames, the LLC frame shall be discarded.
It is the responsibility of the higher layer protocols in the SGSN to cope with discarded LLC frames.
6.2 Uplink UNITDATA procedure
On the uplink, a UL-UNITDATA PDU shall contain information elements derived from the RLC/MAC function (except when GTTP is used in the Um interface, see GSM TS 04.18), meaningful to higher-layer protocols in an SGSN, and an LLC-PDU. There shall be only one LLC-PDU per UL‑UNITDATA PDU. The LLC-PDU shall always be the last information element in the UL-UNITDATA PDU, and shall be aligned on a 32 bit boundary for efficient processing.
The BSS shall provide the TLLI, received from the MS, to the SGSN.
The BSS shall provide a BVCI and an NSEI indicating the PTP functional entity (i.e. the cell) upon which the LLC‑PDU was received. The SGSN shall obtain the BVCI and the NSEI from the underlying network service; the BVCI and the NSEI are not visible in the UL-UNITDATA PDU.
The BSS provides the SGSN with the QoS Profile used in the LLC-PDU’s transmission from the mobile station across the radio interface.
– QoS Profile. This reports the (peak) bit rate, the precedence used at radio access and the transmission mode used across the radio path. The type of the BSSGP SDU, layer 3 signalling or data, and the type of LLC frame, SACK, ACK, or not, are not meaningful on the uplink and shall be ignored.
– Packet Flow Identifier. This identifies the packet flow context that is obtained from the mobile. If the mobile station does not provide a PFI then the BSS shall use the pre-defined PFI to indicate best-effort QoS.
In order to support location based services, the BSS shall include the cell identifier of the cell upon which the LLC-PDU was received.
In the case where localised service area is supported, the BSS shall include the LSA identities of the cell upon which the LLC-SDU was received. The BSS may exclude LSA identities that are not included in the LSA INFORMATION element.
In addition to constructing the UL-UNITDATA, the BSS supplies the LSP, the NSEI and the BVCI associated with the MS to the lower layer network service, enabling network service routeing to the peer entity. These parameters are not transmitted as part of the BSSGP across the Gb-interface.
6.2.1 Abnormal conditions
6.3 RA-CAPABILITY procedure
The SGSN stores an MS’s current radio access capability (which may be changed by higher layer mobility management procedures). An MS’s current radio access capability, and the TLLI identifying the MS, are conveyed to a BSS in a RA-CAPABILITY PDU. The received MS’s radio access capability, if valid, shall then replace any radio access capability previously associated with the MS.
6.3.1 Abnormal conditions
If the BSS receives an unknown Access Technology Type in the MS Radio Access Capability field, it shall ignore the fields associated with that Access Technology type.
If the BSS receives unknown fields within a known Access Technology Type in the MS Radio Access Capability field, it shall ignore the unknown fields.