Page 101 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 87

The Idle State
In this state the mobile device is not attached to the GPRS network at all. This means
that the SGSN is not aware of the user’s location, no Packet Data Protocol (PDP) context
is established and the network cannot forward any packets for the user. It is very unfortu-
nate that the standards body named this state ‘idle’ because in the GSM circuit‐switched
‘idle mode’ the mobile device is attached to the circuit‐switched side of the network and
is reachable by the network. Therefore, great care has to be taken not to mix up the
packet‐switched idle state with the GSM circuit‐switched idle mode.

The Ready State
When the user wants to attach to the GPRS network, the mobile device enters the ready
state as soon as the first packet is sent. While in ready state, the mobile device has to
report every cell reselection to the network so that the SGSN can update the user’s
position in its database. This process is called ‘cell update’. It enables the network to
send any incoming data for a user directly to the mobile device instead of having to page
the mobile device first to locate the user’s serving cell. The mobile device will remain in
the ready state while signaling or user data is transferred and for a certain time after-
ward. The timer that controls how long the mobile device will remain in this state after
the last block of data is transferred is called T3314. The value of this timer is broadcast
on the BCCH or PBCCH as part of the GPRS system information. A typical value for
this timer that is used in many networks is 44 seconds. The timer is reset to its initial
value in both the mobile device and the SGSN whenever data is transmitted. When the
timer reaches 0 the logical connection between the mobile device and the network
automatically falls back into the standby state, which is further described below.
It is important to note that the ready state of a mobile device is not synonymous with
the ability of the mobile device to transfer data to and from the Internet. To transfer
user data, a so‐called PDP context is necessary, which is further described in
Section 2.8.2. Being in ready state simply means that both signaling and possibly user
data can be sent to the mobile device without prior paging by the network.
The ready state resembles in some ways the GSM dedicated mode. However, it should
be noted that in the GPRS ready state the network is not responsible for the user’s
mobility as would be the case in the GSM dedicated mode. The decision to select a new
cell for an ongoing data transfer is not made by the network (see Section 1.8.3) but by
the mobile device. When the signal quality deteriorates during an ongoing data transfer
and the mobile device sees a better cell, it will interrupt the ongoing data transfer and
change to the new cell. After reading the system information on the BCCH it reestab-
lishes the connection and informs the network of the cell change. The complete proce-
dure takes about two seconds, after which the communication resumes. Data of the
aborted connection might have to be resent if it was not acknowledged by the network
or the mobile device before the cell change.
To minimize the impact of cell changes, an optional method, requiring the support of
both the mobile device and the network, has been added to the GPRS standard, which
is referred to as Network‐Assisted Cell Change (NACC). If implemented, it is possible
for the mobile device to send a Packet Cell Change Notification message to the network
when it wants to change into a different cell. The network responds with a Packet
Neighbor Cell Data message, alongside the ongoing user data transfer, that contains all
necessary parts of the system information of the new cell to allow performance of a