A GPRS Support Node (GSN) contains functionality required to support GPRS functionality for GERAN and/or UTRAN. In one PLMN, there may be more than one GSN.
The SGSN and GGSN functionalities may be combined in the same physical node, or they may reside in different physical nodes. The SGSN and the GGSN contain IP or other (operator's selection, e.g. ATM-SVC) routeing functionality, and they may be interconnected with IP routers.
The Gateway GPRS Support Node (GGSN) is the node that is accessed by the packet data network due to evaluation of the PDP address. It contains routeing information for PS-attached users. The routeing information is used to tunnel N PDUs to the MS's current point of attachment, i.e. the Serving GPRS Support Node. The GGSN may request location information from the HLR via the optional Gc interface. The GGSN is the first point of PDN interconnection with a PLMN supporting GPRS (i.e. the Gi reference point is supported by the GGSN). GGSN functionality is common for all types of RANs.
For emergency bearer service, the GGSN shall block any traffic that is not from/to addresses of network entities (e.g. P-CSCF) providing emergency service. The list of allowed addresses may be configured by the operator.
The Serving GPRS Support Node (SGSN) is the node that is serving the MS. The SGSN supports GPRS for A/Gb mode (i.e. the Gb interface is supported by the SGSN) and/or Iu-mode (i.e. the Iu interface is supported by the SGSN). At PS attach, the SGSN establishes a mobility management context containing information pertaining to e.g. mobility and security for the MS. At PDP Context Activation, the SGSN establishes a PDP context, to be used for routeing purposes, with the GGSN that the subscriber will be using.
In Iu mode, the SGSN and RNC may be interconnected with one or more IP routers.
In Gn/Gp mode and when the SGSN and the GGSN are in different PLMNs, they are interconnected via the Gp interface. The Gp interface provides the functionality of the Gn interface, plus security functionality required for inter-PLMN communication. The security functionality is based on mutual agreements between operators.
In Gn/Gp mode, the SGSN interworks signalling on the Gn/Gp interface with Iu/Gb interface signalling. In S4 mode, the SGSN interworks signalling on the S4 interface with Iu/Gb interface signalling. One SGSN may have some MSs using Gn/Gp mode and other MSs using S4 mode.
The SGSN may send location information to the MSC/VLR via the optional Gs interface. The SGSN may receive paging requests from the MSC/VLR via the Gs interface.
The SGSN interfaces with the GSM SCF for optional CAMEL control using Ge reference point. Depending on the result from the CAMEL interaction, the session and packet data transfer may proceed normally. Otherwise, interaction with the GSM SCF continues as described in TS 23.078
. Only the GSM SCF interworking points are indicated in the signalling procedures in this specification.
If there is already an emergency bearer activated, the SGSN shall reject any additional PDP context activation request by the MS for emergency services.
The functionality of the Serving Gateway is defined in TS 23.401
with the following additions and exceptions:
The Serving Gateway:
terminates the user plane interface towards the UTRAN when the Direct Tunnel feature is in use;
is the local Mobility Anchor point for SRNS relocation when the Direct Tunnel feature is in use;
is the local Mobility Anchor for inter-SGSN routeing area update;
if E-UTRAN is not in use in that PLMN, need not support functionality related to inter eNodeB mobility.
The functionality of the PDN Gateway is defined in TS 23.401