8 Basic packet mode service


8.1 General

Basic packet mode services are the services offered to the mobile users, as described in GSM 02.02, 3x‑series. Offering the basic packet mode services requires that GSM PLMNs support the demand access to the PSPDN port in the PSPDN according to CCITT X.32 or to a packet handler of the ISDN (X.31 Case A). See note. This may be either in the network associated with the home PLMN of the respective mobile subscriber or in any other network to which the subscriber is entitled to establish calls.

NOTE: The Case B is currently not feasible within GSM PLMNs because this case neither allows for addressing of PHs of different providers of packet switched data transmission services nor enables the handling of connections for roaming subscribers. The missing prerequisites for accomplishing this functionality in X.31 Case B are:

‑ for calls from roaming foreign subscribers to the home PH ‑‑> support of the according addresses; and

‑ for calls to roaming foreign subscribers via the PH, ‑‑> the PSPDN to provide redirection functionality as the gateway of home PLMNs does for routing circuit switched calls for their roaming subscribers.

The scenarios described in clause 7 shall apply.

The mobile user may have to subscribe separately to the services offered by the PSPDN port of a PSPDN or by the PH of the ISDN or PSPDN and/or home PLMN, which may depend on the policy or agreements reached between GSM PLMN operator and ISDN or PSPDN operators.

The GSM PLMN networks shall provide only the circuit connection to/from the PH or the PSPDN port. The routing is based on the E.164/E.163 numbering plan because the location of both PH and PSPDN port is identified by E.164/E.163 numbers. Once the connection to the PH or PSPDN port has been established the GSM PLMN is transparent for the user information and in‑band control information.

The mobile user shall set in the SETUP message the GSM BC‑IE, LLC‑IE, and HLC‑IE according to GSM 07.01 for 30‑series type of services and the applicable variant and shall insert as the called address the E.164/E.163 number associated with the PH or the PSPDN port. That kind of access allows for mobile users to use customized and standard profiles according to CCITT X.32. These profiles are invisible to the GSM PLMNs. The characteristics of the service are those offered via the packet handler interface for X.31 Case A.

The address of the called subscriber for mobile originated calls is sent to the PH or PSPDN port in the "called address" field of the X.25 CALL REQUEST packet and is a number in the format as required by the PH or the PSPDN port. On the basis of this number the PH or the PSPDN port is able to initiate the call establishment to the called subscriber.

8.2 Service provided by the GSM PLMN network

Within the GSM PLMN a subscription to the bearer services BS 32, 33 and/or 34 is required. In order to select the basic packet mode service the mobile subscriber indicates during the call setup the GSM bearer capability IE according to GSM 07.01 "Bearer service 3x; X.32 case" and as the called address the E.164/E.163 address of the PH or the PSPDN port.

The MSC‑A shall insert for this type of call the calling party information element containing the Basic MSisdn of the calling mobile subscriber. This information shall be free for being used in conjunction with the calling line identification presentation supplementary service to be performed by the PH, unless the respective GSM PLMN supports the supplementary service Calling Line Identification Restriction and the mobile subscriber wants to apply Calling Line Identification Restriction also to the basic packet mode service. In this case only packet services for unidentified subscribers (see ETS 300 099) would be available to the mobile subscriber.

For roaming subscribers using the basic packet mode services the connection shall be established by the visited PLMN to the PSPDN port or to the PH either in the home country of the respective mobile subscriber or in any other country to which the subscriber is entitled to establish calls, i.e. via an international ISDN/PSTN link.

8.3 Mobile originated calls

8.3.1 Subscriber on home PLMN

This case illustrates a mobile subscriber being in its own GSM PLMN area. The subscriber requires the packet services to be supported by the GSM PLMN network he is subscribed to.

The subscriber requests the appropriate 3x‑series service stated in GSM 02.02 by using the BC_IE setting according to GSM 07.01 (restrictions see subclause 8.7). The SETUP message shall contain the E.164/E.163 address of the PH or PSPDN port.

The MSC‑A shall select the appropriate IWF and shall establish a circuit connection to the addressed PH or to the PSPDN port.

"In case where the PH only supports X.31 case B, the PLMN shall take care of the X.31 case A/B conversion to proceed the call."

8.3.2 Subscriber on visited PLMN

In this case the MSC‑A of the VPLMN shall allocate the appropriate interworking functions within the VPLMN, identified from the mobile call set‑up request. If the subscriber is accordingly entitled to, the call shall be routed via the visited PLMN’s national transit ISDN or PSTN to the PH or PSPDN port identified by the E.164/E.163 ("Dial‑in"‑case of figure 4 and 5/GSM 09.06). The location of this PH or PSPDN port can be the ISDN or PSTN of the home PLMN or the home PLMN itself or any other ISDN or PSTN. The routing is exactly as for normal data or telephony call addressed to any other subscriber.

"In case where the PH only supports X.31 case B, the PLMN shall take care of the X.31 case A/B conversion to proceed the call."

In this case the visited PLMN may not have any knowledge that the mobile subscriber requests a packet service and no additional functions are required, to those necessary for a corresponding "data circuit duplex synchronous" (3x‑series) service.

Figure 4/GSM 09.06: VPLMN and ISDN/PSTN transit network; 3,1 kHz audio ex PLMN

NOTE: In case of a PH only supporting case B, the case A/B conversion is necessary in the HPLMN. This is indicated by the mobile using a service number (see clause 3).

Figure 5/GSM 09.06: VPLMN and ISDN transit network; unrestricted digital information

8.4 Mobile terminated calls

8.4.1 Subscriber on home PLMN

The scenarios applicable are those given in figures 1 to 3/GSM 09.06. The PH or PSPDN port translates, if required, the called address received in the X.25 INCOMING CALL packet to the E.164 mobile subscriber number and initiates a circuit connection to be routed firstly through to the GMSC of the home PLMN.

From there on, the call shall be managed in the same manner as any other data call (see GSM 09.07 and GSM 07.03).

"In case of more than one subscribed basic packet mode service in the HLR, the highest subscribed user rate is used. If in the Visited PLMN this user rate is not supported, the call will fail."

8.4.2 Subscriber on visited PLMN

"The scenarios applicable are those given in figure 1 to 3. In this case the call will be handled as for any other data call for MS in VPLMN (see "Dial out"‑case of figures 4 and 5/GSM 09.06)."

8.5 Mobile originated mobile terminated calls

The call will be handled for the mobile originated part according to subclause 8.3 and for the mobile terminated part according to subclause 8.4. The X.25 packet switching function will be performed in the PSPDN.

8.6 Subscriber profiles

All in‑band access identification procedures between the DTE associated with the MS and the PSPDN port or the PH, if applicable, shall be passed "transparently" through the GSM PLMN.

The service profile selection at the PSPDN port or the PH shall be established on the basis of those identification procedures. The subscriber may have a "default", a "standard" or a "customized service profile" according to ETS 300 099/X.32.

8.7 GSM bearer services quality of service

GSM 03.10 identifies the protocol layer structures for both the transparent and the non‑transparent mode.

Both transparent and non‑transparent mode of operation are applicable to the PSPDN Port scenario identified in figure 1. For the PH scenario shown in figure 2 only the non‑transparent mode is possible. The scenario shown in figure 2 assumes a 64 kbit/s service provided via the ISDN and supported by the PH using "X.31 flag stuffing" capabilities. This form of service is not supportable by means of the transparent mode because of the inherent higher throughput rate required, i.e. 64 kbit/s. The non‑transparent mode can support this form of service because it is able to remove/add flags and bring the actual "real" throughput rate down/up to one, which GSM and the PH, respectively, can support.

For the transparent mode the GSM PLMN shall provide a synchronous data service with no provision for higher layer capabilities over and above layer 1 (see GSM 09.07 and GSM 07.03).

In case of the non‑transparent mode, the call set‑up procedures are comparable to those identified above with the exception that the "connection element" in the call set‑up message indicates "non‑transparent". The receipt of this attribute value causes the MT and the IWF to select the L2R/RLP capabilities (see GSM 07.03 and GSM 09.07). The X.25 data link layer is terminated by the L2R at the MT towards the terminal and reconstituted at the IWF towards the transit network. In case of access to packet handler flag stuffing type of rate adaptation is used between the IWF and the PH. The data link layer parameters to be used at the R and S reference points, respectively, are listed in table 3/GSM 09.06.

The following is applicable for mobile terminated calls:

The IWF will not initiate the establishment of the LAPB link to the PH/PSPDN port, but will wait for an SABM, DM, or DISC, to be able to determine the link layer address to be used. The IWF will use this address until the circuit switched connection to the PH/PSPDN port is released.

XID information received from the MS prior to the IWF’s LAPB link establishment must either be discarded by the IWF or stored for later transmission if no DM, DISC or XID frames have been received from the fixed network. The first XID frame received (if any) from the PSPDN port prior to LAPB link establishment will be interpreted as a command. The link layer address derived from this first XID will be used for XID information exchange until an SABM, DM or DISC is received.

NOTE: This use of LAPB address allows calls from a PH using any of the two alternative address assignment mechanisms defined in CCITT Rec.X.32 and applicable for X.31 case A. In addition, direct interworking of X.25 DTEs (DTE‑DTE operation) between an X.25 DTE on a PLMN and an X.25 DTE on a PLMN or ISDN is possible. In this case the DTE address is used instead of the PH address.

The X.25 packet layer information shall be passed transparently from the terminal to the PSPDN.