DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS The general structure in terms of signalling nodes and signalling interfaces of a GSM PLMN has already been described above with reference to FIGS.
1 and 2.
FIG.
3 shows various nodes of the GSM PLMN where it is assumed, for the purpose of this discussion, that each of these nodes comprises an application part (e.
g.
MAP, INAP) having a conventional composition such that it can be supported by SS7 for the sending of signalling information to a peer application part at some other node.
Considering in particular the MSCs 3a,3b and the GMSC 1, these nodes are connected to a conventional SS7 network 11 for the purpose of communicating signalling information.
Intelligent network and mobile network specific nodes, including the IN node 6, VLR 7, HLR 8, AUC 9, and EIR 10, each have application parts (and in particular MAP and INAP) similarly constructed for peer-to-peer communication via SS7.
However, these nodes each comprise an additional interface which enables them to communicate signalling information via an IP network 12.
In order to enable interworking of the SS7 connected nodes and the IP network connected nodes, the two signalling networks 11,12 are connected by a gateway node 13.
FIG.
4 shows schematically the protocol layers present at a node connected to the IP network 12 (Node 1), at the gateway node 13 (Node 2), and at a node connected to the SS7 network 11 (Node 3).
Of the layers shown, the INAP/MAP, TCAP, SCCP, TCP/UDP, IP, physical layer, and MTP are substantially conventional in structure and therefore will not be described in detail.
Rather, the reader should make reference to the literature including "Understanding Telecommunications", vols.
1 & 2, Studentlitteratur, Lund, Sweden (ISBN 91-44-00214-9), and the ITU-T (International Telecommunications Union --Technical) recommendations starting with Q.
700 which define Signalling System Number 7 (SS7).
The present discussion is concerned in the main with the Adaptation layer which enables the TCP/IP and physical layers to replace the MTP, such that peer SCCPs can communicate over the IP network 12
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Multiplexer-related 
