Page 115 - From GMS to LTE

General Packet Radio Service (GPRS) and EDGE 101

Figure 2.25 The Gr interface. Gr
MAP
TCAP
SCCP
MTP 1-3
E1

SGSN HLR

The Gr Interface
This interface connects the SGSN with the Home Location Register (HLR), which contains
information about all subscribers on the network (Figure 2.25). It was enhanced with a
software upgrade to also act as a central database for GPRS subscriber data. The following
list shows some examples:
GPRS service admission on a per‐user (International Mobile Subscriber Identity,
●
IMSI) basis;
GPRS services that a user is allowed to use (Access Point Names, APNs);
●
GPRS international roaming permissions and restrictions.
●
As described in Chapter 1, the HLR is an SS7 service control point (SCP). Therefore,
the Gr interface was initially based on E1 trunks, SS7 on layer 3 and Mobile Application
Part (MAP) on the application layer, as shown in Figure 2.25. Today, the lower layers
have been replaced by IP connectivity. The MAP protocol was also extended to be able
to exchange GPRS‐specific information. The following list shows some of the messages
that are exchanged between SGSN and HLR:

Send Authentication Information: This message is sent from the SGSN to the HLR
●
when a subscriber attaches to the network for which the SGSN does not yet have
authentication information.
Update Location: The SGSN informs the HLR that the subscriber has roamed into
●
its area.
Cancel Location: When the HLR receives an Update Location message from an SGSN,
●
it sends this message to the SGSN to which the subscriber was previously attached.
Insert Subscriber Data: As a result of the Update Location message sent by the
●
SGSN, the HLR will forward the subscriber data to the SGSN.

The Gc Interface
This interface connects the GGSN with the HLR. It is optional and is not widely used in
networks today.

The Gp Interface
This interface is described in 3GPP TS 29.060 [5] and connects GPRS networks of dif-
ferent countries or different operators with each other for GTP traffic as shown in
Figure 2.26. It enables a subscriber to roam outside the coverage area of the home