GPRS System Survey

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GPRS Protocols

GPRS System Survey

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Multiframe structure

GPRS System Survey

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GPRS System information

• BCCH– GPRS supported or not on the cell.

– GPRS Radio Resources allocated or not.

– The position of the PDCH carrying PBCCH and PCCCH.

• PBCCH (PSI1, 2, 3, 3bis are used by Ericsson)– GPRS BA list, Frequency Hopping information.

– BSIC.– Cell Selection parameters.

– MS Power Control parameters.

– Discontinuous Reception (DRX) parameters.

GPRS System Survey

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Logical Channels in GPRS

• Packet Data Common Control Channels– PRACH: Packet Data Random Access Channel (uplink)

– PPCH: Packet Paging Channel (downlink)– PAGCH: Packet Access Grant Channel (downlink)

– PTCCH: Packet Timing Advance Control Channel (downlink)

• Broadcast channel– PBCCH: Packet Broadcast Channel

• Packet Traffic Channels– PDTCH: Packet Data Traffic Channel

– PACCH: Packet Associated Channel

GPRS System Survey

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Radio Resource ManagementCommon resource pool

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GPRS System Survey

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Channel Reservation Strategy

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GPRS System Survey

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Multiframe Structure for PDCH

• PDCH with PCCCH– Down-link

• B0 may be used as PBCCH and B1-B11 may contain PBCCH, PAGCH,PPCH, PNCH, PTCCH, PDTCH or PACCH.

– Up-link• B0-B11 may contain PRACH, PTCCH, PDTCH or PACCH.

• PDCH without PCCCH– All blocks may be used as PTCCH, PDTCH or PACCH.

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GPRS System Survey

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GPRS PDU Structure

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GPRS System Survey

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RLC/MAC Block

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GPRS System Survey

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RLC/MAC Data Blockdownlink uplink

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GPRS System Survey

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RLC/MAC Control Block

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GPRS System Survey

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Radio Block StructureHeader

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GPRS System Survey

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Mobility Management Procedures

• Attach/Detach

• Security procedures

• Authentication and ciphering• P-TMSI allocation• Identity check

• Location management procedures• Cell update• Routing area updating• Periodic updating

• Attach/Detach

• Security procedures

• Authentication and ciphering• P-TMSI allocation• Identity check

• Location management procedures• Cell update• Routing area updating• Periodic updating

GPRS System Survey

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Mobility Management State Diagram

STANDBY

IDLE

READY

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Ready Timer ExpiryorForce to STANDBYorAbnormal RLC condition

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STANDBY timer expiry orCancel Location

STANDBYtimer expiry

Ready Timer ExpiryorForce to STANDBY

PDU receptionPDU transmission

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Mobility Management states(kept in SGSN and MS)

• IDLE state– Entered when mobile is switched on

– No MM context is established between the mobile and SGSN– PLMN selection and GPRS cell selection are performed by the mobile

– The mobile may receive PTM-M transmissions

– PTP or PTM-G as well as paging is not possible– Mobile may initiate the GPRS Attach procedure

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Mobility Management states(kept in SGSN and MS)

• READY state– Entered when mobile makes a GPRS attach

– SGSN knows the Cell Id of the mobile– Mobile may send/receive PTP data

– Mobile may receive PTM-M and PTM-G data

– Mobile may initiate the GPRS Detach procedure

GPRS System Survey

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Mobility Management states(kept in SGSN and MS)

• STANDBY state– Entered upon Ready state timer expiry

– SGSN knows the Routing Area of the mobile (but not its Cell Id)– Mobile may receive PTM-M data, but not PTP or PTM-G data

– Mobile may receive pages for PTP or PTM-G

– Mobile may receive circuit switched pages via SGSN

GPRS System Survey

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Connection between MM states and RR modes

MM states RR modes

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GPRS System Survey

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Radio Resource (RR) Management modes

(kept in BSS and the mobile)

• Packet idle mode– No TBF exists.

– Mobile monitors PPCH if it exists, otherwise PCH.– Moves to packet transfer mode when transfer of LL-PDU is

needed.

• Packet transfer mode– TBF exists together with one or several PDCHs.– Continuous transfer of LL-PDUs.

– Moves to packet idle mode when no more LL-PDUs are buffered.Radio Resource (RR) Management modes

GPRS System Survey

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A Temporary block flow TBF

• Every data packet sent between MS and PCU belongs to a TBF

• A TBF is identified by a TFI (Temporary Flow Identity) value, value range 0 -31 per cell

• Before any data is exchanged between MS and PCU the mobile is given aTFI which shall be used in all exchange of data packets.

• Uplink packet data and downlink packet data are handled independently anda TBF is treated as a single direction flow

• The TBF is released as soon as the data flow ends

• If the flow is uplink, the MS will be given a USF id for each uplink channel it isgoing to use, value range 0-5 per channel.

• When a MS recognises its own USF in a downlink message it means that it isgiven permission to send one RLC-block uplink.

GPRS System Survey

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GPRS RLC Connection Control

• The function takes care of all requests to send data from MS or SGSN

• The function assigns the TFI to the MS and if it is an uplink request it alsoassigns a USF to the mobile.

• The function will get channels reserved for the MS from the function GPRSChannel Handling.

• The function will do the initial handshaking (PACKET UPLINK/DOWNLINKASSIGNMENT) with the MS.

• The function releases the TBF (TFI and USF) when the MAC or RLC protocolindicates “no more data” or “too many retransmissions”

• The function communicates with MS at release of PDCHs (downgrading)

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Downlink Packet Transfer

• Initiated by LL-PDU received from SGSN to a mobile in MMREADY and in packet transfer mode. PDU received in DL-UNITDATA message.

• PCU actions:– Segment LL-PDU into RL-PDUs, add RLC Header (containing TFI) and

store the RLC Data Block in MS buffer.

– Immediately before transmission over the air, add the MAC Headercontaining USF for an uplink scheduled mobile.

– Regularly ask for acknowledgements of sent RLC/MAC Blocks.

– Retransmit missing RLC/MAC Blocks.

GPRS System Survey

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Downlink Packet Transfer

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GPRS System Survey

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Downlink RLC Connection Release

• PCU Actions:– Send last RLC/MAC Block with Final Bit Indicator set to one.

– When all downlink blocks have been acknowledged by the mobile,delete all PDCH reservations associated with the connection and updateRLC context accordingly.

– Start timer. If timer expires before a new RLC Connection is needed tothe same mobile, delete RLC context.

GPRS System Survey

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Uplink RLC Connection establishment (1)

• Initiated by the mobile sending the message: 3$&.(7&+$11(/�5(48(67– The message is received on

• PRACH if GPRS is set up in the cell.

• RACH if GPRS is not set up in the cell.

• PCU actions:– Create RLC context (MS Id = random number).

– Reserve PDCH(s) with their corresponding USF(s).– Send a 3$&.(7�83/,1.�$66,*10(17 message

• on PAGCH if GPRS is set up in the cell.

• on AGCH (via the BSC) if GPRS is not set up in the cell– Start uplink scheduling of the mobile.

GPRS System Survey

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Uplink RLC Connection establishment (2a)One phase access

• PCU actions:– Contention resolution:

• Mobile sends the complete MS Id (TLLI) in the first three RLCHeaders.

• PCU returns the TLLI in a 3$&.(7�83/,1.�$&.�1$&.message and enters packet transfer mode.

– Update RLC context with MS Id = TLLI.

GPRS System Survey

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Uplink RLC Connection establishment (2b)Two phase access

• PCU actions:– Detect the 3$&.(7�5(6285&(�5(48(67 message on the

reserved PDTCH. The message contains the TLLI for contentionresolution.

– Reserve PDCH(s) and their corresponding USF(s).– Update RLC context.

– Send a 3$&.(7�83/,1.�$66,*10(17 message on PACCH.The message contains the TLLI for contention resolution.

GPRS System Survey

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Uplink RLC connection establishment

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GPRS System Survey

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Uplink Packet Transfer

• PCU actions:– Schedule USF for uplink RLC/MAC Block.

– When receiving an RLC/MAC Block, evaluate the parameters inthe MAC Header.

– For RLC/MAC Control Blocks take action depending on thecontent.

– For RLC Data Blocks extract the RLC Header and reassembleinto LL-PDUs.

– Regularly send a PACKET UPLINK ACK/NACK message to notifythe mobile of which RLC/MAC Blocks to retransmit.

– Forward LL-PDUs to SGSN in UL-UNITDATA messages.

GPRS System Survey

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Uplink RLC Connection Release

• PCU Actions:– When last RLC/MAC Block has been correctly received (Countdown

Value = 0), acknowledge all outstanding RLC/MAC Blocks.

– When receiving a PACKET CONTROL ACKNOWLEDGEMENTmessage from the mobile, delete all PDCH reservations associated withthe connection and update RLC context.

– Start timer. If timer expires before a new RLC Connection is needed tothe same mobile, delete RLC context.

GPRS System Survey

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Uplink Packet Transfer

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GPRS System Survey

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Uplink multiplexing of radio blocks on PDCH

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GPRS System Survey

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Retransmission of Radio Blocks

Automatic Repeat Request (ARQ)

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A number of Radio Blocks are transmitted within a sendwindow. The blocks are then acknowledged with a “bit map”and erroneous blocks are retransmitted.

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GPRS System Survey

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Handover

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GPRS System Survey

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MS moving to other cell

• MS gets silent in old cell

• MS sends a location update to SGSN in a new cell

• SGSN sends a FLUSH REQUEST to old cell (to RLC-protocol)

• RLC protocol will send unacknowledged messages to new cell if new cell isincluded in the message, otherwise RLC protocol will discard all buffers forMS.

GPRS System Survey

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Cell change procedure

• Mobile sends any LLC PDU to PCU.• PCU adds BSSGP header and sends PDU containing cell ID

to SGSN.• The cell change is detected by SGSN.• SGSN sends “Flush Request“ to PCU.• If new cell is handled by the old PCU, outstanding

RL-PDUs are moved to new cell’s buffer.Otherwise, old cell’s buffer is cleared and the last non-acknowledged LLC PDU is retransmitted to the new PCU (cellbuffer) from SGSN.

• PCU sends “Flush Completed” to SGSN.