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
). 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  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.
The Subscriber Management function provides a mechanism to inform the nodes about changes of the subscription data for a specific subscriber.