Page 100 - From GMS to LTE

86 From GSM to LTE-Advanced Pro and 5G

The Random Access Channel (RACH): When the mobile device wants to transmit
●
data blocks to the network, it has to request uplink resources. This is done in the
same way as already described in Chapter 1 for voice calls. The only difference is in
the content of the Channel Request message. Instead of asking for a circuit‐switched
resource, the message asks for packet resources on the air interface.
The Access Grant Channel (AGCH): The network will answer to a channel request
●
on the RACH with an Immediate Packet Assignment message on the AGCH that con-
tains information about the PDTCH timeslot the mobile device is allowed to use in the
uplink. As the network is not aware at this stage of the identity of the device, the first
uplink transmissions have to contain the temporary logical link identifier (TLLI, also
known as the packet‐temporary mobile subscriber identity, P‐TMSI) the mobile device
was assigned when it attached to the network. All further GPRS signaling messages are
then transmitted over the PACCH, which shares the dedicated GPRS timeslots with
the PDTCH. Once data is available for the mobile device in the downlink direction, the
network needs to assign timeslots in the downlink direction. This is done by transmit-
ting a Packet Timeslot Reconfiguration message with information about which times-
lots the mobile device can use in the uplink and downlink directions.
The Paging Channel (PCH): In case the mobile device is in standby state, only
●
the location area of a subscriber is known. As the cell itself is not known, resources
cannot be assigned right away and the subscriber has to be paged first. GPRS uses
the GSM PCH to do this.
The Broadcast Common Control Channel (BCCH): A new system information mes-
●
sage (SYS_INFO 13) has been defined on the BCCH to inform mobile devices about
GPRS parameters of the cell. This is necessary to let mobile devices know, for example,
if GPRS is available in a cell, which NOM is used, if EDGE is available, and so on.

2.4 The GPRS State Model

When the mobile device is attached to the GSM network, it can be either in ‘idle’ mode
as long as there is no connection, or in ‘dedicated’ mode during a voice call or exchange
of signaling information. Figure 2.13 shows the state model introduced to address the
needs of a packet‐switched connection for GPRS.

Idle

Standby timer GPRS attach GPRS detach
expired
Ready
Timer Transfer of a
expired data frame
Standby

Figure 2.13 The GPRS state model.