SS7 Protocol Overview

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:

• Message Transfer Parts (MTP 1, 2, and 3)

• Signaling Connection Control Part (SCCP)

• Transaction Capabilities Application Part (TCAP)

• Telephony User Part (TUP)

• 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:

• 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.

• 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.

SS7 Network Architecture

SS7 can employ different types of signaling network structures. The choice between these different structures can be influenced by factors such as administrative aspects and the structure of the telecommunication network to be served by the signaling system.

The worldwide signaling network has two functionally independent levels:

• International

• National

This structure makes possible a clear division of responsibility for signaling network management. It also lets numbering plans of SS7 nodes belonging to the international network and the different national networks be independent of one another.

SS7 network nodes are called signaling points (SPs). Each SP is addressed by an integer called a point code (PC). The international network uses a 14-bit PC. The national networks also use a 14-bit PC—except North America and China, which use an incompatible 24-bit PC, and Japan, which uses a 16-bit PC. The national PC is unique only within a particular operator’s national network. International PCs are unique only within the international network. Other operator networks (if they exist) within a country also could have the same PC and also might share the same PC as that used on the international network. Therefore, additional routing information is provided so that the PC can be interpreted correctly—that is, as an international network, as its own national network, or as another operator’s national network. The structure of point codes is described in Chapter 7, “Message Transfer Part 3 (MTP3).”

Signaling Links and Linksets

SPs are connected to each other by signaling links over which signaling takes place. The bandwidth of a signaling link is normally 64 kilobits per second (kbps). Because of legacy reasons, however, some links in North America might have an effective rate of 56 kbps. In recent years, high-speed links have been introduced that use an entire 1.544 Mbps T1 carrier for signaling. Links are typically engineered to carry only 25 to 40 percent of their capacity so that in case of a failure, one link can carry the load of two.

To provide more bandwidth and/or for redundancy, up to 16 links between two SPs can be used. Links between two SPs are logically grouped for administrative and load-sharing reasons. A logical group of links between two SP is called a linkset. Figure 4-2 shows four links in a linkset.

A number of linksets that may be used to reach a particular destination can be grouped logically to form a combined linkset. For each combined linkset that an individual linkset is a member of, it may be assigned different priority levels relative to other linksets in each combined linkset.

A group of links within a linkset that have the same characteristics (data rate, terrestrial/satellite, and so on) are called a link group. Normally the links in a linkset have the same characteristics, so the term link group can be synonymous with linkset.

Routes and Routesets

SS7 routes are statically provisioned at each SP. There are no mechanisms for route discovery. A route is defined as a preprovisioned path between source and destination for a particular relation. Figure 4-3 shows a route from SP A to SP C.

History of SS7

The first specification (called a recommendation by the CCITT/ITU-T) of CCITT Signaling System No. 7 was published in 1980 in the form of the CCITT yellow book recommendations. After the yellow book recommendations, CCITT recommendations were approved at the end of a four-year study period. They were published in a colored book representing that study period.

Under the CCITT publishing mechanism, the color referred to a published set of recommendations—that is, all protocols were published at the same time. The printed matter had the appropriate colored cover, and the published title contained the color name. When the ITU-T took over from the CCITT, it produced single booklets for each protocol instead of producing en bloc publications as had been the case under the supervision of the CCITT. Under the new mechanism, the color scheme was dropped. As a result, the ITU-T publications came to be known as “White Book” editions, because no color was specified, and the resulting publications had white covers. Because these publications do not refer to a color, you have to qualify the term “White Book” with the year of publication.

As Table 4-1 shows, when SS7 was first published, the protocol stack consisted of only the Message Transfer Part 2 (MTP2), Message Transfer Part 3 (MTP3), and Telephony User Part (TUP) protocols. On first publication, these were still somewhat immature. It was not until the later Red and Blue book editions that the protocol was considered mature. Since then, the SS7 protocols have been enhanced, and new protocols have been added as required.

Figure 4-1 shows how many pages the ITU-T SS7 specifications contained in each year. In 1980, there were a total of 320 pages, in 1984 a total of 641 pages, in 1988 a total of 1900 pages, and in 1999 approximately 9000 pages.