An introduction, overview or tutorial of the basics of GPRS the basic
operation of the GPRS cellular system.
When looking at the way in which GPRS operates, it can be
seen that there are three basic modes in which it operates. These are:
initialisation / idle, standby, and ready.
Initialisation / idle
When the mobile is turned on it must register with the
network and update the location register. This is very similar to that performed
with a GSM mobile, but it is referred to as a location update. It first locates
a suitable cell and transmits a radio burst on the RACH using a shortened burst
because it does not know what timing advance is required. The data contained
within this burst temporarily identifies the mobile, and indicates that the
reason for the update is to perform a location update.
When the mobile performs its location update the network also
performs an authentication to ensure that it is allowed to access the network.
As for GSM it accesses the HLR and VLR as necessary for the location update and
the AuC for authentication. 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.
Standby
The mobile then enters a standby mode, periodically updating
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 will also monitor 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. In this way the receiver can be turned off for longer and battery life
can be extended.
Ready
In the ready mode the 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. In
addition to this the 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.
With the mobile 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. As this may be busy the mobile monitors
the PCCCH which contains a status bit indicating the status of the base station
receiver, whether it is busy or idle and capable of receiving data. When the
mobile sees this status bit indicates the receiver is idle, 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.
This only sets up the packet data transfers for the uplink.
If data needs to be transferred in the downlink direction then a separate
assignment is performed for the downlink channel.
When data is transferred this is controlled by the action of
the MAC layer. In most instances it will operate in an acknowledge mode whereby
the base station acknowledges each block of data. The acknowledgement may be
contained within the 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 will send 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.
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