Operation of gprs
GPRS can operate in three different modes. These are: initialization / idle, standby, and ready.
INITIALISATION / IDLE:
- When the mobile is turned on it must register with the network and update the location register.
- It is at registration that the network detects that the mobile has a GPRS capability.
- The SGSN also maintains a record of the location of the mobile so that data can be sent there is required.
- When the mobile performs its location update the network also performs an authentication to ensure that it is allowed to access the network.
- In this mode the mobile periodically updates its position as required.
- It monitors the MNC of the base station to ensure that it has not changed base stations and also looks for stronger base station control channels.
- The mobile also monitors the PPCH in case of an incoming alert indicating that data is ready to be sent.
- As for GSM, most base stations set up a schedule for paging alerts based on the last figures of the mobile number. In this way it does not have to monitor all the available alert slots and can instead only monitor a reduced number where it knows alerts can be sent for it.
- Thus a receiver can be turned off for longer and battery life can be extended.
- In this mode mobile is attached to the system and a virtual connection is made with the SGSN and GGSN.
- By making this connection the network knows where to route the packets when they are sent and received.
- Here, mobile is likely to use the PTCCH to ensure that its timing is correctly set so that it is ready for a data transfer should one be needed.
When Mobile is attached to the network: It is prepared for a call or data transfer.
- To transmit data the mobile attempts a Packet Channel Request using the PRACH uplink channel.
- Mobile monitors the PCCCH which contains a status bit.
- Mobile sees this status bit indicates if the receiver is idle, then it sends its packet channel request message. If accepted the base station will respond by sending an assignment message on the PAGCH on the downlink.
- This will indicate which channel the mobile is to use for its packet data transfer as well as other details required for the data transfer.
When data is transfer: This is controlled by the action of the MAC layer. it will operate in an acknowledge mode whereby the base station acknowledges each block of data. Data packets being sent in the downlink or the base station may send data packets down purely to acknowledge the data.
When disconnecting the mobile sends a packet temporary block flow message, and this is acknowledged. Once this has taken place the USF assigned to the mobile becomes redundant and can be assigned to another mobile wanting access. With this the mobile effectively becomes disconnected and although still attached to the network no more data transfer takes place unless it is re-initiated. Separate messages are needed to detach the mobile from the network.