Page 120 - From GMS to LTE

106 From GSM to LTE-Advanced Pro and 5G

that the network will be able to find the subscriber for incoming calls or SMS messages.
In GPRS, the same principle exists. To be more flexible, the location areas are subdi-
vided into GPRS routing areas. If a mobile device in ready or standby state crosses a
routing area border, it reports to the SGSN. This procedure is called RAU.
If the new routing area is administered by a new SGSN the process is called IRAU.
Although from the mobile device point of view there is no difference between an RAU
and an IRAU, there is quite a difference from the network point of view. This is because
the new SGSN does not yet know the subscriber. Therefore, the first task of the new
SGSN is to get the subscriber’s authentication and subscription data. As the RAU con-
tains information about the previous routing area, the SGSN can then contact the
previous SGSN and ask for this information. At the same time this procedure also
prompts the previous SGSN to forward all incoming data packets to the new SGSN in
order not to lose any user data while the procedure is ongoing. Subsequently, the GGSN
is informed about the new location of the subscriber so that, henceforth, further incom-
ing data is sent directly to the new SGSN. Finally, the HLR is also informed about the
new location of the subscriber and this information is deleted in the old SGSN. Further
information about this procedure can be found in 3GPP TS 23.060, 6.9.1.2.2 [7].

2.8.2 GPRS Session Management

To communicate with the Internet, a PDP context has to be requested for use after the
attach procedure. For the end user, this in effect means getting an IP address from the
network. As this procedure is in some ways similar to establishing a voice call, it is
sometimes also referred to as ‘establishing a packet call’.
Although there are some similarities between a circuit‐switched call and a packet‐
switched call, it is important to remember one big difference: for a circuit‐switched
voice or data call the network reserves resources on all interfaces. A timeslot is reserved
for this connection on the air interface, in the radio network and also in the core net-
work. These timeslots cannot be used by anyone else while the call is established even
if no data is transferred by the user. When a GPRS packet call is established there are no
resources dedicated to the PDP context. Resources on the various interfaces are used
only during the time that data is transmitted. Once the transmission is complete (e.g.
after a web page has been downloaded), the resources are used for other subscribers.
Therefore, the PDP context represents only a logical connection with the Internet. It
remains active even if no data is transferred for a prolonged length of time. For this
reason a packet call can remain established indefinitely without blocking resources.
This is also sometimes referred to as ‘always on’.
Figure 2.29 shows the PDP context activation procedure. Initially, the subscriber
sends a PDP Context Activation Request message to the SGSN. The most important
parameter of the message is the APN. The APN is the reference that GGSN uses as a
gateway to an external network. The network operator could have one APN to connect
to the Internet transparently, one to offer Wireless Application Protocol (WAP) ser-
vices, several other APNs to connect to corporate intranets, etc. The SGSN compares
the requested APN with the list of allowed APNs for the subscriber that has been
received from the HLR during the attach procedure. The APN is a fully qualified domain
name like internet.t‐mobile.com but simple APN names such as internet can be used as
well. The names of the APN can be chosen freely by the GPRS network operator.