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Figure 5.10. SS7 reference model.
The following list provides a brief description of each SS7 protocol and its relation to
the OSI-RM:
• Message Transfer Part (MTP)—MTP consists of three protocols (MTP Levels 1,
2, and 3) that provide physical, data link, and connectionless network
functions. MTP level 1 defines the physical transport and interfaces used with
SS7. Defined interfaces include T1, E1, DSO (64Kbps), and 56Kbps
dedicated. MTP Level 2 defines the SS7 message format and provides
end-to-end message delivery and error recovery. MTP level 3 provides
message routing and (internal) network management functions. The MTP
levels operate at the OSI-RM Layers 1 and 2.
• Signaling Connection Control Part (SCCP)—This element provides message
delivery services for SS7 ULPs, except for ISDN-UP. SCCP provides four
classes of network delivery service:
o Class 0, connectionless datagram service
o Class 1, datagram service with sequencing
o Class 2, temporary and permanent connection service; supports
message segmentation and reassembly
o Class 3, flow control connection-oriented service, along with OSI
addressing capabilities