Page 122 - From GMS to LTE

108 From GSM to LTE-Advanced Pro and 5G

Abis Gb Gn Gi
MS PCU SGSN GGSN Internet

TFI TID
P-TMSI IP-address

Figure 2.30 Identification of user data packets on different GPRS interfaces.

to have more than one active PDP context at a time. This is quite useful, for example, to
separate Internet access from network operator internal services such as MMS.
If the GGSN grants access to the external network (e.g. the Internet) it will assign an
IP address out of an address pool for the subscriber. For special purposes it is also pos-
sible to assign a fixed IP address for a subscriber. Subsequently, the GGSN responds to
the SGSN with a PDP Context Activation Response message that contains the IP address
of the subscriber. Furthermore, the GGSN will store the TID and the subscriber’s IP
address in its PDP context database. This information is needed later on to forward
packets between the subscriber and the Internet and, of course, for billing purposes.
Once the SGSN receives the PDP Context Activation Response message from the
GGSN, it also stores the context information in its database and forwards the result to
the subscriber. The subscriber then uses the IP address to communicate with the external
network.
Different IDs are used for packets of a certain user on each network interface due to the
different nature of the protocols and due to the different packet sizes. On the GPRS air
interface, with its small data frames of only 456 bits or 57 bytes, which even includes the
overhead for error detection and correction, the three‐bit TFI is used to route the
frame to the correct mobile device. In the radio network the P‐TMSI/TLLI is used to
identify the packets of a user. Finally, in the core network, the GPRS TID is used as
identification. Figure 2.30 shows the different interfaces and IDs used on them at a glance.

Questions

1. What are the differences between circuit‐switched and packet‐switched data
transmission?

2. What are the advantages of data transmission over GPRS compared to GSM?

3. Why are different modulation and coding schemes used?

4. What is the difference between the GPRS ready state and the GPRS standby state?

5. Does the GPRS network perform a handover if a cell change is required while data
is being transferred?

6. Which are the new network elements that have been introduced with GPRS and
what are their responsibilities?