GPRS Network Nodes

One can see a number of new network elements and interfaces.
In particular, we find the Packet Control Unit (PCU), the Serving GPRS Support Node (SGSN), the Gateway GPRS Support Node (GGSN),
and the Charging Gateway Function (CGF).

The PCU is a logical network element that is responsible for a number of
GPRS-related functions such as the air interface access control, packet
scheduling on the air interface, and packet assembly and re-assembly.
Strictly speaking, the PCU can be placed at the BTS, at the BSC, or at the SGSN. Logically, the PCU is considered a part of the BSC and in real implementations,
one finds the PCU physically integrated with the BSC.

The SGSN is analogous to the Mobile Switching Center (MSC)/Visitor
Location Register (VLR) in the circuit-switched domain. Just as the
MSC/VLR performs a range of functions in the circuit-switched domain, the
SGSN performs the equivalent functions in the packet-switched domain.
These include mobility management, security, and access control functions.
The service area of an SGSN is divided into routing areas (RAs), which
are analogous to location areas in the circuit-switched domain. When a
GPRS MS moves from one RA to another, it performs a routing area update,
which is similar to a location update in the circuit-switched domain. One
difference, however, is that an MS may perform a routing area update during
an ongoing data session, which in GPRS terms is known as a Packet
Data Protocol (PDP) context. In contrast, for an MS involved in a circuitswitched
call, a change of location area does not cause a location update
until after the call is finished.

A given SGSN may serve multiple BSCs, whereas a given BSC interfaces
with only one SGSN. The interface between the SGSN and the BSC (in fact,
the PCU within the BSC) is the Gb interface. This is a Frame Relay-based
interface, which uses the BSS GPRS protocol (BSSGP). See Figure 5-3. The
Gb interface is used to pass signaling and control information as well user
data traffic to or from the SGSN.

The SGSN also interfaces to a Home Location Register (HLR) via the Gr
interface. This is an SS7-based interface and it uses MAP, which has been
enhanced for support of GPRS. The Gr interface is the GPRS equivalent of
the D interface between a VLR and HLR. The Gr interface is used by the
SGSN to provide location updates to the HLR for GPRS subscribers and to
retrieve GPRS-related subscription information for any GPRS subscriber
that is located in the service area of the SGSN.
An SGSN may optionally interface with an MSC via the Gs interface.
This is a SS7-based interface that uses the Signaling Connection Control
Part (SCCP). Above SCCP is a protocol known as BSSAP+, which is a modified
version of the Base Station Subsystem Application Part (BSSAP), as
used between an MSC and a BSC in standard GSM. The purpose of the Gs
interface is to enable coordination between an MSC/VLR and a SGSN for
those subscribers that support both circuit-switched services controlled by
the MSC/VLR (such as voice) and packet data services controlled by the
SGSN. For example, if a given subscriber supports both voice and data services,
and is attached to an SGSN, then it is possible for an MSC to page the
subscriber for a voice call via the SGSN by using the Gs interface.

The SGSN interfaces with a Short Message Service Center (SMSC) via
the Gd interface. This enables GPRS subscribers to send and receive short
messages over the GPRS network (including the GPRS air interface). The
Gd interface is an SS7-based interface using MAP.
A GGSN is the point of interface with external packet data networks
(such as the Internet). Thus, the user data enters and leaves the Public
Land Mobile Network (PLMN) via a GGSN. A given SGSN may interface
with one or more GGSNs and the interface between an SGSN and GGSN is
known as the Gn interface. This is an IP-based interface used to carry signaling
and user data. The Gn interface uses the GPRS Tunneling Protocol
(GTP), which tunnels user data through the IP backbone network between
the SGSN and GGSN.
A GGSN may optionally use the Gc interface to an HLR. This interface
uses MAP over SS7. This interface would be used when a GGSN needs to
determine the SGSN currently serving a subscriber, similar to the manner
in which a Gateway MSC (GMSC) queries an HLR for routing information
for a mobile-terminated voice call. One difference between the scenarios,
however, is the fact that a given data session is usually established by the
MS rather than by an external network. If the MS establishes the session,
then the GGSN knows which SGSN is serving the MS because the path
from the MS to the GGSN passes via the serving SGSN. Therefore, in such
situations, the GGSN does not need to query the HLR. A GGSN will query
the HLR when the session is initiated by an external data network. This is
an optional capability and a given network operator may choose not to support
that capability. In many networks, the capability is not implemented as
it requires that the MS has a fixed packet protocol address (an IP address).
Given that address space is often limited (at least for IP version 4), a fixed
address for each MS is not often possible.
An SGSN may interface with other SGSNs on the network. This inter-
SGSN interface is also termed the Gn interface and also uses GTP. The primary
function of this interface is to enable the tunneling of packets from an
old SGSN to a new SGSN when a routing area update takes place during
an ongoing PDP context. Note that such forwarding of packets from one
SGSN to another occurs only briefly—just as long as it takes for the new
SGSN and the GGSN to establish the PDP context directly between them,
at which point the old SGSN is removed from the path. This is different
from, for example, an inter-MSC handover for a circuit-switched call, where
the first MSC remains as an anchor until the call is finished.