The number of possible protocol stack combinations is growing. It depends on whether SS7 is used for cellular-specific services or intelligent network services, whether transportation is over IP or is controlling broadband ATM networks instead of time-division multiplexing (TDM) networks, and so forth. This requires coining a new term—traditional SS7—to refer to a stack consisting of the protocols widely deployed from the 1980s to the present:
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Message Transfer Parts (MTP 1, 2, and 3)
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Signaling Connection Control Part (SCCP)
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Transaction Capabilities Application Part (TCAP)
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Telephony User Part (TUP)
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ISDN User Part (ISUP)
Such a stack uses TDM for transport. This book focuses on traditional SS7 because that is what is implemented. Newer implementations are beginning to appear that use different transport means such as IP and that have associated new protocols to deal with the revised transport.
The SS7 physical layer is called MTP level 1 (MTP1), the data link layer is called MTP level 2 (MTP2), and the network layer is called MTP level 3 (MTP3). Collectively they are called the Message Transfer Part (MTP). The MTP protocol is SS7’s native means of packet transport. In recent years there has been an interest in the facility to transport SS7 signaling over IP instead of using SS7’s native MTP. This effort has largely been carried out by the Internet Engineering Task Force (IETF) SigTran (Signaling Transport) working group. The protocols derived by the SigTran working group so far are outside the scope of this introductory chapter on SS7. However, full details of SigTran can be found in Chapter 14, “SS7 in the Converged World.”
TUP and ISUP both perform the signaling required to set up and tear down telephone calls. As such, both are circuit-related signaling protocols. TUP was the first call control protocol specified. It could support only plain old telephone service (POTS) calls. Most countries are replacing TUP with ISUP. Both North America and Japan bypassed TUP and went straight from earlier signaling systems to ISUP. ISUP supports both POTS and ISDN calls. It also has more flexibility and features than TUP.
With reference to the Open System Interconnection (OSI) seven-layer reference model, SS7 uses a four-level protocol stack. OSI Layer 1 through 3 services are provided by the MTP together with the SCCP. The SS7 architecture currently has no protocols that map into OSI Layers 4 through 6. TUP, ISUP, and TCAP are considered as corresponding to OSI Layer 7 [111]. SS7 and the OSI model were created at about the same time. For this reason, they use some differing terminology.
SS7 uses the term levels when referring to its architecture. The term levels should not be confused with OSI layers, because they do not directly correspond to each other. Levels was a term introduced to help in the discussion and presentation of the SS7 protocol stack. Levels 1, 2, and 3 correspond to MTP 1, 2, and 3, respectively. Level 4 refers to an MTP user. The term user refers to any protocol that directly uses the transport capability provided by the MTP—namely, TUP, ISUP, and SCCP in traditional SS7. The four-level terminology originated back when SS7 had only a call control protocol (TUP) and the MTP, before SCCP and TCAP were added.
The following sections provide a brief outline of protocols found in the introductory SS7 protocol stack, as illustrated in Figure 4-18.
MTP
MTP levels 1 through 3 are collectively referred to as the MTP. The MTP comprises the functions to transport information from one SP to another.
The MTP transfers the signaling message, in the correct sequence, without loss or duplication, between the SPs that make up the SS7 network. The MTP provides reliable transfer and delivery of signaling messages. The MTP was originally designed to transfer circuit-related signaling because no noncircuit-related protocol was defined at the time.
The recommendations refer to MTP1, MTP2, and MTP3 as the physical layer, data link layer, and network layer, respectively. The following sections discuss MTP2 and MTP3. (MTP1 isn’t discussed because it refers to the physical network.) For information on the physical aspects of the Public Switched Telephone Network (PSTN), see Chapter 5, “The Public Switched Telephone Network (PSTN).”
MTP2
Signaling links are provided by the combination of MTP1 and MTP2. MTP2 ensures reliable transfer of signaling messages. It encapsulates signaling messages into variable-length SS7 packets. SS7 packets are called signal units (SUs). MTP2 provides delineation of SUs, alignment of SUs, signaling link error monitoring, error correction by retransmission, and flow control. The MTP2 protocol is specific to narrowband links (56 or 64 kbps).
MTP3
MTP3 performs two functions:
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Signaling Message Handling (SMH)— Delivers incoming messages to their intended User Part and routes outgoing messages toward their destination. MTP3 uses the PC to identify the correct node for message delivery. Each message has both an Origination Point Code (OPC) and a DPC. The OPC is inserted into messages at the MTP3 level to identify the SP that originated the message. The DPC is inserted to identify the address of the destination SP. Routing tables within an SS7 node are used to route messages.
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Signaling Network Management (SNM)— Monitors linksets and routesets, providing status to network nodes so that traffic can be rerouted when necessary. SNM also provides procedures to take corrective action when failures occur, providing a self-healing mechanism for the SS7 network.
