6.10 Tunnelling of non-GSM Signalling Messages Function (A/Gb mode)

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

Tunnelling of Messages (TOM) is an optional protocol layer that uses the LLC unacknowledged mode procedures to tunnel messages between the MS and the SGSN (see TS 44.064 [15]). TOM uses two LLC SAPs for communication between the MS and the SGSN; one for high-priority messages and one for low-priority messages. A network that supports TIA/EIA‑136 [49] shall support the TOM protocol and the Gs interface.

Upon receiving a non-GSM signalling message from an MS via the TOM protocol, the SGSN forwards the message to a non-GSM MSC/VLR using the BSSAP+ protocol (see GSM 09.18). The specific Gs interface used by the SGSN is determined by the:

– RAI associated with the current location of the MS when the SGSN does not provide functionality for the Intra Domain Connection of RAN Nodes to Multiple CN Nodes. When the SGSN provides functionality for Intra Domain Connection of RAN Nodes to Multiple CN Nodes, the SGSN uses the RAI and a hash value from the IMSI to determine the Gs interface; and

– information in the TOM protocol header.

Upon receiving a non-GSM signalling message from a non-GSM MSC/VLR via the BSSAP+ protocol, the SGSN forwards the message to a specific MS using the TOM protocol. The specific MS is determined by the SGSN based on the content of the BSSAP+ header.

The control plane between an MS and a non-GSM MSC/VLR that uses tunnelling procedures for non-GSM signalling is shown in Figure 45.

Figure 45: Control Plane MS – Non-GSM MSC/VLR

6.10.1 Uplink Tunnelling of non-GSM Signalling Messages Procedure

The Uplink Tunnelling of non-GSM Signalling Messages procedure is illustrated in Figure 46.

Figure 46: Uplink Tunnelling of non-GSM Signalling Messages Procedure

1) The MS sends a TOM Protocol Envelope (Non-GSM Signalling Message) to the SGSN either in ciphered or clear mode. The TOM protocol header contains information about the application using the TOM facility and any other TOM Protocol Discriminator-specific information. The TOM Protocol Envelope is received on one of the two LLC SAPs used for tunnelling of messages.

2) The SGSN identifies the non-GSM MSC/VLR to which to forward the non-GSM signalling message. It then sends a BSSAP+ Uplink Tunnel Request (IMSI, SGSN Address, TOM Priority, Cipher, Non-GSM Signalling Message) message to the identified non-GSM MSC/VLR. The Cipher parameter is set to cipher if the TOM Protocol Envelope was received in ciphered form by the LLC layer. Otherwise, it is set to not cipher. TOM Priority is set to high priority if the TOM Protocol Envelope was received on the high-priority LLC SAP, Otherwise, it is set to low priority.

6.10.2 Downlink Tunnelling of non-GSM Signalling Messages Procedure

The Downlink Tunnelling of non-GSM Signalling Messages procedure is illustrated in Figure 47.

Figure 47: Downlink Tunnelling of non-GSM Signalling Messages Procedure

1) The non-GSM MSC/VLR sends a BSSAP+ Downlink Tunnel Request (IMSI, VLR Address, TOM Priority, Cipher, Non-GSM Signalling Message) message to the SGSN associated with the MS. TOM Priority indicates whether the SGSN shall select the high-priority or low-priority LLC SAP when forwarding the non-GSM signalling message to the MS. Cipher indicates whether or not the SGSN shall cipher the non-GSM signalling message before forwarding it to the MS.

2) The SGSN sends a TOM Protocol Envelope (Non-GSM Signalling Message) to the MS using the selected LLC SAP.