10 Interworking with PSDN (X.75/X.25)

09.613GPPGeneral Packet Radio Service (GPRS)Interworking between the Public Land Mobile Network (PLMN) supporting GPRS and Packet Data Networks (PDN)TS

10.1 General

GPRS shall support interworking with PSDN networks. The interworking may be either direct or through a transit network.

GPRS shall support both ITU-T Recommendation X.121 [14] and ITU-T Recommendation E.164 [11] addressing.

GPRS shall provide support for ITU-T Recommendation X.25 and ITU-T Recommendation X.75.

The GPRS TE’s shall have addresses provided , and controlled, by their GPRS operator. The PSDN TE sends data to the GPRS TE by use of that TE’s GPRS DNIC (Data Network Identification Code) or equivalent which uniquely identifies that GPRS network worldwide.

The GGSN for interworking with PSDNs is the access point of the GSM GPRS data network.

There are two models for PSDN interworking.

– X.75 over the Gi reference point.

– X.25 over the Gi reference point with the DCE located within the PSDN and the DTE located within the TE of the GPRS PLMN.

Both X.75 and X.25 access methods are supported when mobile users are resident on HPLMN or VPLMN. A roaming user may be allocated a dynamic address from the VPLMN.

10.2 PSDN Interworking Models

The two models of X.75 and X.25 represent the different scenarios for PSDN interworking with the GPRS network.

The model differences lie in the interconnection protocol over the Gi reference point.

10.2.1 X75 Interworking at the Gi Reference Point

Figure 3 represents the case where X.75 is used as the interworking protocol, as used between interconnect X.25 PSDNs currently. The GPRS network will look like any other PSDN in all respects and uses X.75 addressing. Figure 4 shows the interconnecting protocol stacks to the GPRS bearer. The GPRS bearer is described in 3GPP TS 07.60 [10], which uses the protocols described in 3GPP TS 03.60 [3].

Figure 3: PSPDN Interworking with X.75 at Gi Reference Point

Figure 4: The Protocol Stack for the X.75 Gi Reference Point Numbering and Addressing

A PLMN GPRS network requires a DNIC or PNIC.

X.121 addresses allocated to subscribers belong to the PLMN operator. Charging

Charging of X.25 packets is done at the GGSN.

10.2.2 X25 Interworking at the Gi Reference Point

Figure 5 represents the case where X.25 is used as the interconnect protocol between a DCE and a DTE.

The DTE resides within the GPRS network. The DCE resides within the PSDN.

The GPRS Network is seen as part of the PSDN, as the Gi reference point is the interconnect point between the DCE and the DTE.

The protocol stack for this model is shown in figure 6.

NOTE: The PSDN can interwork at X.75 to other PSDN’s

Figure 5: PSDN Interworking with X.25 over Gi Interface

Figure 6: The Protocol Stack for the X.25 / Gi Reference point

Figure 6 shows the transmission plane only. In this case the GGSN shall resolve the association between the MS GPRS bearer and the X.25 DCE. L1 is left to operators to determine connection to other networks.

The X.25 Relay performs the following:

– mapping of logical channel numbers. Numbering and Addressing

A fixed X.121 address for the MS maybe allocated by the PSDN operator, and is integral to the PSDN numbering plan. A dynamic X.121 address can also be used which is assigned by the GPRS network at PDP context activation. Charging

The charging information may be collected in the X.25 network, depending upon the agreement between the GPRS operator and the PSDN operator. The charging may also be collected in the GPRS network. If the VPLMN assigns the dynamic address, the charging of the GPRS and the external network shall be gathered and sent to the HPLMN.

10.3 User Facilities

The set of user facilities as defined in ITU-T Recommendation X.25 [12] may be supported.

As a minimum the following shall be supported:

– reverse charging;

– reverse charging acceptance;

– fast select restricted;

– fast select unrestricted;

– fast select acceptance.

10.4 The GPRS Interworking to PSDN Characteristics

Table 0 describes the differences in addressing, and user profile for each interconnect type. The static X.121 address in the following table indicates an address which is permanently allocated to the GPRS subscriber by the network operator. The dynamic X.121 address is assigned automatically on the PDP Context Activation procedure. The dynamic address is allocated from a free pool held in the GGSN. This is described in 3GPP TS 03.60 [3].

Table 0: PSPDN GPRS Interconnection Characteristics


X.75 – Stand Alone PSPDN
X.25 – PSPDN Sub Network

Static X.121 address

Dynamic X.121 address

X.25 profile

User determined in X.25 DCE

Only Default Profiles allowed in X.25 DCE- Selected upon PDP context activation


Address in GGSN

Address in GGSN after PDP Context Activation