Base station system GPRS protocol (BSSGP)

The base station system GPRS protocol (BSSGP) resides above the frame relay network and is used to transport both control and user data over the Gb interface. The primary function of this layer is to introduce and provide the required QoS for the user as well as routing information between the BSS and the SGSN. On the uplink, the BSC will take RLC/MAC frames from the mobile device and reassemble a complete LLC packet from these to be passed within a single BSSGP packet to the SGSN. On the downlink the BSC will extract the LLC frame from the BSSGP packet and segment it into the required number of RLC/MAC frames to be transported to the mobile device. To complete this task, the BSC makes use of the TLLI which is provided by the SGSN and is carried within the BSSGP packet header. It uses this to identify the correct resources provided to the RLC/MAC that this particular packet corresponds to. Each RLC/MAC–BSSGP association is linked via the TLLI. As well as the TLLI, the SGSN provides further information to the BSSGP protocol for the specific mobile device. This information includes:

• The radio capability of the mobile device indicating the simultaneous number of time slots the device is capable of handling.

• The QoS profile which defines the peak bit rate, whether the BSSGP packet is Layer 3 signalling or data (signalling may be transferred with higher protection), whether the LLC frame being carried is ACK/SACK or not (it may be transferred with higher priority if it is ACK/SACK), the precedence class as well as the transmission mode (RLC/MAC acknowledged mode using ARQ or unacknowledged transfer) to be used when transmitting the LLC frame between the BSC and the mobile device.

• A time period for which the packet is valid within the BSS. Any packets held up for longer than this period are to be discarded locally.

The precedence class, lifetime and peak bit rate may be incorporated into the BSC radio resource scheduling algorithm for efficient transfer of LLC frames. In periods of conges- tion the BSS may initiate a network controlled cell reselection for a particular mobile device to ensure efficiency and maintain the maximum number of service requests. If such an event occurs, the BSS will update any internal references to the location of the mobile device and inform the SGSN. It is, however, the responsibility of the SGSN to cope with any LLC packets that have been discarded.

Figure 4.27, shows the format of the BSSGP frames; the various fields are explained below, and follow the format shown in the bottom of the figure.

TLLI PDU type

TLLI QoS Profile PDU Lifetime

Options LLC-PDU

32 1

MS Radio Access Capability MS Radio Access Capability Options

Alignment Down Link

TLLI PDU type

TLLI QoS Profile PDU Lifetime Cell ID LLC-PDU Cell ID Cell ID Options Options Alignment Up Link 1 32

Information element ID (IEI) Length indicator Information element value

byte 1 byte 2 byte 3-n 8 1 T L V

Information Element Coding

Table 4.10 Examples of BSSGP PDU types Value (hex) PDU type x00 DL unit data x01 UL unit data x02 RA capability x03 DPTM unit data x05 Paging packet switched x06 Paging circuit switched

x0b Suspend

x0c Suspend ack x28 Flow control MS x29 Flow control MS ack

• PDU type: identifies the type of PDU and thus the frame format to follow. Table 4.10 is a sample list of assigned PDU types.

• QoS profile: defines the peak bit rate, whether the SDU is signalling or data, the type of LLC frame (ACK/SACK or not), the precedence class and the transmission mode to use over the air.

• MS radio access capability: defines the radio capability of the mobile device. This field is optional and only present if the SGSN is aware of the mobile device capability.

• PDU lifetime: defines the time period that the PDU is considered valid within the BSS. This period is set by upper layers in the SGSN.

• Cell identifier: to support location-based services, the uplink PDU includes the cell identity where the LLC was received.

• Localized service area (LSA): this is an optional field as it is an operator-defined group of cells for which specific access conditions apply. This may, for example, be used for negotiating cell reselection.

Unlike the RLC/MAC between the mobile station and the BSC, the BSSGP does not provide error correction, and if a retransmission is required this is performed between the mobile station and the SGSN at the LLC layer. This is because, like frame relay, it assumes that the link is reliable.

There is a one-to-one mapping of the BSSGP protocol between an SGSN and a partic- ular BSS. If the SGSN controls more than one BSS, then there will be additional separate mappings to each of these BSSs from the SGSN. The BSSGP virtual connection (BVC) is carried over a single NS-VC group, which is a collection of frame relay links between an SGSN and a particular BSS; the NS-VC group can carry more than one BVC. Each cell supporting GPRS is allocated and identified by a BVCI. The BVCI and the NSEI are used within the SGSN to identify the cell where a mobile device inready mode resides. The SGSN does not need to know the BVCI of a mobile device instandby mode, it simply needs to know which NSEIs relate to the routing area that the mobile device is in for paging purposes. If a mobile device requires to send data or is paged for downlink data then the SGSN will be informed of the cell (BVCI) where the mobile device is located.

BSC 1 BSC 2 Gb NSEI-1 NSEI-2 BTS BVCI-1 BVCI-4 BVCI-2 BVCI-3 BTS BTS BTS SGSN 1

Figure 4.28 Example use of BVCI and NSEI identifiers

Figure 4.28 shows how the BVCI and NSEI can be used to successfully transport data to and from a cell where a mobile device resides. As discussed in Section 4.7.2, the NSEI identifies a group of frame relay permanent virtual circuits. The BVCI is used to identify the particular cell. It should be noted that the BVCI has relevance only over the Gb interface and that it will be mapped to the correct RLC/MAC identifier across the Abis interface. As well as being used to identify a cell, at least one BVC is also required for signalling purposes such as paging.

The BSSGP protocol actually provides a connectionless link between the SGSN and BSS. The flow control mechanism based on the ‘leaky bucket’ introduces QoS profiles, and a cell (BVC) may have a queue and context in the SGSN and BSS. This queue may be further subdivided for particular subscribers (identified by the TLLI). There are three types of context predefined: best effort, SMS and signalling, where data can be transported over either the best effort or the SMS context. If it is transported over the SMS context, then it will be transported with the QoS profile which has been established for SMS. The flow control mechanism is only required in the downlink direction, since buffers and link capacity in the uplink should be over-dimensioned to avoid loss of data in this direction.

Figure 4.29 shows an example trace from a GPRS network. In this particular trace a mobile device is requesting attachment to the network. Both the uplinkattach request and the downlinkattach accept are shown. It can be seen that the mobile device provides a great deal of information, including the QoS required and the encryption algorithms that can be supported. It can also be seen that theattach requestmessage sent with RLC/MAC ARQ functionality (acknowledged mode) whereas the downlinkattach accept message is using the unacknowledged UNITDATA transfer over the RLC/MAC.

The trace in Figure 4.30 is again taken across the Gb interface and is a session man- agementactivate PDP context request. Again, it can be seen that in the uplink over the RLC/MAC ARQ functionality is used whereas the downlinkpdp context accept message is using the UNITDATA transfer. Also transferred are the NSAPI and SAPI identifiers as well as the requested QoS and the access point to which this request is for.

Conn:1 Line:1 TS:1 Hyperch:1984 kb/s 113 13:42:59.502 UL-UNITDATA TLLI - Random TLLI - Value: 52 67 34 DE QoS Profile

- bit rate : best effort - precedence: Reserved/Unknown - Radio Interface uses RLC/MAC ARQ functionality

- The SDU contains signalling - LLC ACK/SAC present Cell Identifier - MCC : 211 - MNC : 77 - LAC : 29 (1Dh) - RAC : 161 (A1h) - cell id value: 2 (2h) Alignment Octets : FF FF FF LLC-PDU ATTACH REQUEST (GPRS MM) MS Network Capability

- Encryption algorithm GEA/1 available - SM capabilities via dedicated channels supported

- SM capabilities via GPRS channels supported

- Use of default alphabet over UCS2 prefered

- Default value of phase 1 - The ME does not support SoLSA - Mobile station supporting earlier versions of the protocol Attach Type

- GPRS attach

- No follow-on request pending GPRS Ciphering Key Seq. Nr - value: 7 (07h) DRX Parameter - no DRX used by the MS - DRX cycle length coefficient not specified by the MS

- Split pg cycle on CCCH not supported - no non-DRX mode after transfer state Mobile identity

- length: 8 (08h) - IMSI: 211772000012345 Routing Area Id. - Mobile Country Code: 000 - Mobile Network Code: 000 - Location Area Code: 0 (0h) - Routing Area Code: 0 (00h) MS Radio Access Capability GSM 900-P Access Technology Type - Length: 33 bits

- RF Power class: 1

- Encrypt. algorithm A5/1 available - Encrypt. algorithm A5/2 available - Encrypt. algorithm A5/3 not available - Encrypt. algorithm A5/4 not available - Encrypt. algorithm A5/5 not available - Encrypt. algorithm A5/6 not available - Encrypt. algorithm A5/7 not available - Controlled early Classmark Sending not implemented

- PS capability not present

- VGCS capability and notifications not wanted

- VBS capability and notifications not wanted

- GPRS Multislot Class: 6

- GPRS Ext. Dynamic Alloc. Capability not implemented

- SMS Value: 5/4 timeslot - SM Value: 2/4 timeslot - Spare ok

UPLINK

Conn:1 Line:1 TS:1 Hyperch:1984 kb/s 114 13:42:59.679 DL-UNITDATA TLLI - Random TLLI - Value: 52 67 34 DE QoS Profile

- bit rate : best effort

- precedence: Reserved(Low/Priority 3) - Radio Interface uses RLC/MAC-UNITDATA functionality

- The SDU contains data - LLC ACK/SACK not present PDU Lifetime

- 8.00 sec

MS Radio Access Capab. GSM 900-P Access Technology Type - Length: 33 bits

- RF Power class: 1

- Encrypt. algorithm A5/1 available - Encrypt. algorithm A5/2 available - Encrypt. algorithm A5/3 not available - Encrypt. algorithm A5/4 not available - Encrypt. algorithm A5/5 not available - Encrypt. algorithm A5/6 not available - Encrypt. algorithm A5/7 not available - Controlled early Classmark Sending not implemented

- PS capability not present

- VGCS capability and notifications not wanted

- VBS capability and notifications not wanted

- GPRS Multislot Class: 6

- GPRS Ext. Dynamic Alloc. Capability not implemented - SMS Value: 5/4 timeslot - SM Value: 2/4 timeslot - Spare ok DRX Parameters - no DRX used by the MS - DRX cycle length coefficient not specified by the MS

- Split pg cycle on CCCH not supported - no non-DRX mode after transfer state TLLI - Random TLLI - Value: 52 67 34 DE LLC-PDU ATTACH ACCEPT (GPRS MM) Attach Result - GPRS only attached Force to Standby - not indicated GPRS Timer - Value: 54 min Radio Priority - Priority level 3 Routing Area Id. - Mobile Country Code: 211 - Mobile Network Code: 77 - Location Area Code: 29 (1Dh) - Routing Area Code: 161 (A1h) GPRS Timer - Value: 44 sec P-TMSI - length: 5 (05h) - TMSI/P-TMSI: c00003b6 DOWNLINK

Conn:1 Line:1 TS:1 Hyperch:1984 kb/s 121 13:43:10.119 DL-UNITDATA TLLI - Local TLLI - Value: 52 67 34 DE QoS Profile

- bit rate : best effort - precedence: Normal/Priority 2 - Radio Interface uses RLC/MAC-UNITDATA functionality

- The SDU contains data - LLC ACK/SACK not present PDU Lifetime

- 8.00 sec

MS Radio Access Capab.

GSM 900-P Access Technology Type - Length: 33 bits

- RF Power class: 1

- Encrypt. algorithm A5/1 available - Encrypt. algorithm A5/2 available - Encrypt. algorithm A5/3 not available - Encrypt. algorithm A5/4 not available - Encrypt. algorithm A5/5 not available - Encrypt. algorithm A5/6 not available - Encrypt. algorithm A5/7 not available - Controlled early Classmark Sending not implemented

- PS capability not present

- VGCS capability and notifications not wanted

- VBS capability and notifications not wanted

- GPRS Multislot Class: 6

- GPRS Ext. Dynamic Alloc. Capability not implemented - SMS Value: 5/4 timeslot - SM Value: 2/4 timeslot - Spare ok DRX Parameters - no DRX used by the MS

- DRX cycle length coefficient not specified by the MS

- Split pg cycle on CCCH not supported - no non-DRX mode after transfer state TLLI

- Random TLLI - Value: 52 67 34 DE LLC-PDU:

ACT. PDP CONTEXT ACCEPT (GPRS SM) LLC Serv. Access point Id

- SAPI 3

Quality of Service - length: 3 (03h)

- Reliab. class: Unack. GTP and LLC Ack. RLC Protected data

- Delay class: Delay class 4 (best effort)

- Precedence class: High priority - Peak throughput: Up to 2 000 octet/s - Mean throughput: 200 octet/h Radio Priority

- Priority level 3 Conn:1 Line:1 TS:1 Hyperch:1984 kb/s

120 13:43:10.092 UL-UNITDATA TLLI - Local TLLI - Value: 52 67 34 DE QoS Profile

- bit rate : best effort

- precedence: Reserved(Low/Priority 3) - Radio Interface uses RLC/MAC ARQ functionality

- The SDU contains signalling - LLC ACK/SAC present Cell Identifier - MCC : 211 - MNC : 77 - LAC : 29 (1Dh) - RAC : 161 (A1h) - cell id value: 2 (2h) Alignment Octets : FF FF FF LLC-PDU

ACT. PDP CONTEXT REQUEST (GPRS SM) Network Serv. Access Point Id - NSAPI 5

LLC Serv. Access point Id - SAPI 3

Quality of Service - length: 3 (03h)

- Reliab. class: Unack. GTP and LLC Ack. RLC Protected data

- Delay class: Delay class 1 - Precedence class: High priority - Peak throughput: Up to 2 000 octet/s

- Mean throughput: 200 octet/h Packet Data Protocol Address - Length: 6 (06h)

- PDP type organisation: IETF allocated address

- PDP type number: IPv4 - Address: 10.1.21.2 Access Point Name - length : 06 (06h)

- value (hex) : 05 47 47 53 4E 31

UPLINK DOWNLINK

Figure 4.30 PDP context request trace across Gb. Reproduced by permission of NetHawk Oyj

In document Convergence Technologies for 3G Networks pdf (Page 143-149)