System Information and Main Flows - GPRS Basic Principles

Chapter 10 GPRS Radio Network Planning and Optimization

10.1 GPRS Basic Principles

10.1.5 System Information and Main Flows

I. Packet system information

The packet system information (PSI) is used to broadcast the parameters needed by the MS when the MS intends to access the network. If the cell supports GPRS service, the SI13 must be added to the BCCH. The SI13 includes the information on GPRS cell access control, GPRS cell selection and reselection, and GPRS power control. The PBCCH can be either configured or not configured in a cell. The SI13 will tell the MS whether the PBCCH is present in the cell or not. On the PBCCH mainly the GPRS dedicates PSI is broadcast, including PSI1, PSI2, PSI3, PSI3bis, PSI4, PSI5, and PSI13. The following lists them respectively.

z PSI1 contains the information on cell selection, PRACH control, control channel

description, and power control parameters.

z PSI2 contains the information on frequency list, cell allocation table, GPRS mobile

allocation table, and PCCCH description.

z PSI3 contains the information on neighbor cell BCCH allocation table and the

service cell/non-service cell selection parameters.

z PSI3bis contains the information on neighbor cell BCCH allocation table and

non-service cell selection parameters.

z PSI4 contains the PDCH lists used by the MS to make measurement in service

cells.

z PSI5 contains the information on measurement report and network control cell

reselection.

z PSI13 is the same as the SI13 broadcast on the BCCH. It contains the information

relative to the access of the specific GPRS cells.

The PSI1, PSI2, PSI3, and PSI4 can be broadcast either on the PBCCH or PACCH. The PSI5 can only be broadcast on PBCCH. The PBCCH can only be broadcast on PACCH. If there is a PBCCH in the cell, it does not send the PSI13 and the PSI1 is broadcast periodically on the PACCH. If there is no PBCCH in a cell, the PSI13 is broadcast periodically on the PACCH.

The GPRS cell reselection is independent of the GSM cell reselection. Handover is not used in the GPRS system. In either packet transfer mode or packet idle mode, the service cell is selected according to cell reselection program in the GPRS system. In the GPRS system, the cell reselection can be performed by the MS automatically or controlled by the network side.

1) MS automatically-started cell reselection

The MS always monitors the PBCCH/BCCH carrier of the neighbor cells. It chooses to attach the best cell according to the signal strength and the base station color code (BCC) of the carriers. Meanwhile, it starts the RA update procedure to notify its RA to the network.

If no PBCCH is present in service cell where the MS is in, the MS will monitor the SI broadcast on the BCCH and reselects the cell using the C1/C2 rules that are used in the GSM.

For the parameter CRH, however, it is processed according to different situations. When the MS is in packet idle mode, if the RA of the service cell is different from that of the reselected new cell, the CRH must be used. When the MS is in ready state, if the MS reselects a new cell, the CRH must be used.

If the PBCCH is configured with the service cell where the MS is in, the MS needs to monitor the SI broadcast on the PBCCH only. In this case, the MS can reselect the cell using the C31/C32 rules used in the GPRS. (For details, see protocols GSM05.08.) Generally, no PBCCH is configured with the current GPRS system, so the MS reselect the cell using the C1/C2 rules only.

2) Network-controlled cell reselection

The network side can also control the cell reselection. In this case, the MS will submit the measurement report to the network side periodically according to the parameters broadcast in the SI. The network will send the packet cell change order to assign the cell that the MS must attach according to the factors, such as the measurement report from the MS and the load of the neighbor cells. Upon receiving the packet cell change order, the MS will stop transmitting the data on the uplink immediately and decoding the data on the downlink. Meanwhile, the MS starts the timer T3174 and begins to enter the reselected cell. This process is controlled by the parameter NETWORK_CONTROL-ORDER, which is a network control mode parameter.

There three types of network control modes, including NC0, NC1, and NC2. They are defined as follows.

b) NC1

In this mode, the MS will send the measurement report to the network side while reselect the cell automatically.

c) NC2

This is the network control mode. In this mode, the MS will send the measurement report to the network side but not reselect the cell automatically. Instead, it accepts the network-controlled cell reselection.

The NC1 and NC2 are applicable to the MSs in ready state only. The NC0 is applicable to the MSs both in ready state and standby state. That is, the MSs in standby state can only use the NC0 mode.

In the current GPRS phase, only the NC0 mode is used for cell reselection. That is, the network-controlled cell reselection has not been used.

III. Uplink TBF establishment

When there is data to be transmitted on the upper layer of the MS, the RLC/MAC layer of the MS will initiate packet access procedure. The packet access of the MS can be divided into short access, one phase access, two phase access, paging response, cell update, and mobility management. They are detailed as follows:

z If the data blocks to be transmitted are less than 8 RLC blocks, the channel

request type of the MS is short access. The number of data packets is calculated according to CS-1.

z If the data blocks to be transmitted are more than 8 TLC blocks and the RLC mode

is required to be the acknowledged mode, the channel request of the MS is one phase access or two phase access.

z If the measurement report of the MS is to be transmitted, the channel request type

of the MS is the first phase of the two phase access.

z If the paging response, cell update, or mobility management works as the channel

request type, it is generally be treated as one phase access or two phase access. The same procedure is used for short access or one phase access. That is, the radio resource (such as the TFI and dynamically-allocated USF, or the radio block bit table) is allocated to the MS once, and then the MS begins to transmit the data.

For the two phase access channel request, the network side allocates one radio block to the MS only for the first time. The MS sends the Packet Resource Request message on the single allocated radio block, and then the network side allocates the radio

A packet channel request is an 8-bit or 11-bit access burst, which carries only a little information, but a packet resource request is a RLC/MAC control block coded according to CS-1, which carries more information, including the TLLI of the MS, the multisolt capability of the MS, and the radio priority class), so this is helpful for the network to allocate more suitable resource to the MS.

In the current GPRS system, most carriers adopt the two phase access, so the following mainly introduces the two phase access procedure.

Figure 10-10 shows the uplink two phase access (acknowledged mode) flow on CCCH.

Figure 10-10 Uplink two phase access (acknowledged mode) flow on CCCH The following details the flow:

The MS sends a Channel Request message to the network side on RACH.

a) Upon receiving the Channel Request message, the network side will send an Immediate Assignment message, which carries an assigned uplink block, on the corresponding AGCH.

b) The MS sends a Packet Resource Request message, which carries TLLI, in the assigned uplink block.

c) The network side allocates the uplink channel to the MS according to the packet resource request and receives the Packet Uplink Assignment message, which

d) For dynamic allocation, the network side sets the USF on the assigned uplink packet channel, and the MS sends the RLC data blocks, which carries no TLLI, on the corresponding uplink blocks. To acknowledge that the RLC data blocks have been received, the network side sends back a Packet Uplink Ack/Nack message to the MS after receiving multiple uplink RLC data blocks according to the range and the data transmission delay of the sliding window. In addition, the network side sends back the Packet Uplink Ack/Nack message to the MS upon receiving an uplink RLC data block whose SI field is 1.

e) When the data is being transmitted, the network will acknowledge the data. After that, the MS will consider retransmitting the data according to the acknowledgement.

f) When the data is transmitted to the last BS_CV_MAX (it is a parameter broadcast in the SI) data block, the MS will start the countdown flow. In the last uplink RLC data block, the MS will se the CV field to 0.

g) Upon receiving the data block whose CV field is 0, the network side will send a Packet Uplink Ack/Nack message to the MS. If the network side has received all the RLC data blocks, it will set the FAI field of the Packet Uplink Ack message to 1. If the network has not received all the network RLC data block, it will set the FAI to 0. In this case, the MS must retransmit the unacknowledged data block. h) Upon receiving a Packet Uplink Ack/Nack message whose FAI is 1, the MS will

send a packet control ack message to the network side, and then release the TBF.

The data transmission flow in the uplink unacknowledged mode is almost the same as that in the uplink acknowledged mode except that the data do not have to be retransmitted in the former mode.

IV. Downlink TBF establishment

If the network side needs to send data to the MS in packet idle mode, it must establish the downlink TBF through packet paging.

1) Paging coordination

In the GPRS system, the network side can page the circuit service through the paging coordination as well as packet paging. For the GPRS-attached and IMSI-attached MS, the MSC/VLR can page the circuit service when the network works in network of operation mode I (NMOI). In this case, the connection between the SGSN and MSC

2/27/2009 All rights reserved Page48 of 94 must support the Gs interface. If the MS is in standby state, the page is performed in the RA. If the MS is in ready state, the page is performed in the cell.

Under NMOI, if a PDCH is allocated to the MS, the system sends the paging message on the PACCH. If no PDCH is allocated but a PCCCH is configured with the system, the system sends the paging message on the PPCH. If the PCCCH is not configured with the system, the system sends the paging message on the PCH through Pb interface. If no Gs interface is provided between the SGSN and MSC, the GPRS/GSM system can works in NMOII and NMOIII. In this case, the system can send the Packet Paging message on the CCCH only. Upon receiving the Packet Paging message, the MS accesses the RACH and then starts to connect the circuit.

If the MS is performing the GPRS service, the MS will starts the GPRS-suspended procedure to suspend the GPRS service unless the circuit connection is released. After that, the network side starts the recovery procedure to recover the GPRS service. For the coordination between GPRS NMO and paging, see Table 10-16.

Table 10-16 Coordination between GPRS NMO and paging NMO Channel for _{circuit paging} Channel for _{GPRS paging} Paging coordination

PPCH PPCH

PCH PCH I

PACCH –

The paging coordination function is available and the Gs interface should be selected. For the GPRS-attached MSs, the packet paging channel assigned by the network functions the same as the circuit paging channel, so the MSs have to monitor one type of the channels only. If having allocated a PDCH to the MS, the network delivers the circuit paging information to the MS on the PDCH.

II PCH PCH

The paging coordination function is not available. All paging messages are delivered on PCH and the MS has to monitor the PCH only, even if it has been assigned the PDCH.

PCH PPCH III

PCH PCH

The paging coordination function is not available. The network delivers the Circuit Paging message on the PCH. If the cell is configured with the PCCCH, the network delivers the Packet Paging messages on the PPCH; otherwise it is sent on the PCH.

2) Downlink TBF establishment on CCCH

The following detail the flow:

i) Where there is downlink data to be transmitted to the MS, the SGSN must send a Packet Paging message.

j) The SGSN sends the Packet Paging message to the PCU through Gb interface, and then the PCU converts the message into a Packet Paging Request message at the Um interface and sends it to the MS. If the BSS system is configured with the PCCCH, this message is sent on the PPCH. If no PCCCH is configured for the BSS system, the PCU delivers this message to the BSC through Pb interface, and the BSC sends it on the PCH.

k) Upon receiving the Packet Paging message, the MS will initiate the TBF establishment flow, and then send the message in the form of RLC data block to the PCU at the Um interface through packet response packets. The RLC data block carries the TLLI.

l) The PCU sends the paging response data block in the form of uplink PDU to the SGSN.

in the form of downlink PDU.

n) Upon receiving the downlink PDU from the SGSN, the PCU delivers an Immediate Assignment message on the CCCH through Pb interface. The Immediate Assignment message carries the assigned downlink packet channels and the starting time of TFI and TBF.

o) During the downlink TBF establishment, to enable the MS to obtain the valid TA, the network side can deliver the Packet Polling Request message with TFI as the identification on the downlink PACCH.

p) Upon receiving the Packet Polling Request message, the MS will responds a Packet Control Acknowledgement message whose type is access burst (AB) on an immediate assignment channel.

q) The network side extracts the TA from the AB, and then notifies the TA to the MS through a Packet Power Control/Timing Advance message. After that, the network assigns the downlink channel to the MS by delivering a Packet Downlink Assignment message.

r) The network side fragments the PDU into several downlink RLC data blocks and sends them to the MS through the downlink packet channel. According to the range of the sliding window, the network side sets the RRBP field in the sent RLC data blocks. According to the indication of the RRBP, the network side acknowledges the received the RLC data block by sending the Packet Downlink Ack/Nack message on the corresponding PACCH.

s) The network side will set the FBI to 1 in the last downlink data block. If the MS has receives all the RLC data blocks, it will send the packet downlink ack/nack whose FAI is 1; otherwise it will send the packet downlink ack/nack whose FAI is 0 to request the network to resend the data blocks that it has not received. t) Upon receiving the Packet Downlink Ack/Nack message whose FAI is 1, the

network side will release the TBF. V. Mobility management

The GPRS mobility management includes many aspects; including GPRS attach/detach and cell/RA update, and coordinated RA/LA update. When the MS completes the GPRS attach, the SGSN will establish mobility management for the MS, stores the current location and state of the subscriber. After that, when the MS roams between cells or RAs, it will start the cell/RA update automatically. The SGSN will monitor the MS and store the latest location of the MS if the MS performs the cell/RA update successfully. When the MS performs the coordinated GPRS attach/IMSI attach

The following introduces the GPRS attach, cell update, location area (LA) update, and GPRS detach respectively.

1) GPRS attach

The GPRS attach is divided into normal GPRS attach and coordinated GPRS attach. The normal GPRS attach attaches the IMSI of the MS to the GPRS service and the coordinated GPRS attach attaches the IMSI of the MS to both the GPRS service and non-GPRS service. The GPRS attach is applied to most of the current networks. Figure 10-12 shows the coordinated GPRS attach.

7d. Cancel Location Ack 7c. Cancel Location 7b. Update Location

7g. Update Location Ack 7e. Insert Subscriber Data 7f. Insert Subscriber Data Ack 6d. Insert Subscriber Data

6c. Cancel Location Ack 6b. Cancel Location 3. Identity Response 2. Identification Response 2. Identification Request 1. Attach Request 5. IMEI Check 3. Identity Request 4. Authentication

6a. Update Location

7a. Location Update Request

7h. Location Update Accept 6f. Update Location Ack 6e. Insert Subscriber Data Ack

MS BSS new SGSN old SGSN GGSN EIR HLR

old MSC/VLR new MSC/VLR 9. Attach Complete 8. Attach Accept

10. TMSI Reallocation Complete

Figure 10-12 Coordinated GPRS attach The following details the flow:

2/27/2009 All rights reserved Page52 of 94 a) The MS sends an Attach Request message to the SGSN. The message carries the IMSI (or P-TMSI and the old RAI), MS classmark, attach type, DRX parameter, and the old P-TMSI signature (it is used when the P-TMSI is used). Whether to use the IMSI or not depends on whether the current P-TMSI is valid or not. The P-TMSI and the old RAI must appear at the same time. The attach type indicates the current attach types, including GPRS attach, IMSI attach, and coordinated GPRS/IMSI attach.

b) If the MS is identified by the P-TMSI and the it has moved to another SGSN service area when it is in detach state, the SGSN obtains the IMSI of the MS

In document Huawei GPRS Network Planning & Optimization.pdf (Page 43-63)