Cisco Certified ExpertITU Point Code Format
EMP Protocol
The ITU point code format is a little different than that of ANSI. The ITU point code is organized into three fields, just as the ANSI point code, but the ITU point code differs in how many bits are in each field. The ITU format calls for a 3-bit field, followed by an 8-bit field, and then by another 3-bit field. This gives the ITU point code a final size of 14 bits.
The first octet indicates the zone identifier (zone ID) of the point code. The zone ID is the geographical zone that the point code is a part of. The zone IDs are listed in Table 2-1.
Table 2-1 ITU Zone IDs
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2 |
Europe |
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3 |
Greenland, North America, the Caribbean, and Mexico |
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4 |
Middle East and Asia |
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5 |
South Asia, Australia, and New Zealand |
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6 |
Africa |
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7 |
South America |
For more information on signaling area network code (SANC) designations, refer to Appendix B "Signaling Area Network Code (SANC) Designations."
The second octet indicates the area or network identifier (network ID). The network ID subdivides each zone ID into smaller areas, such as countries. This can also subdivide large countries into smaller, more manageable sections. Together, the zone ID and the network ID are known as the SANC.
The last octet indicates the signaling point identifier (SPID), not to be confused with the service profile identifier (SPID) used in ISDN. The SPID indicates the node residing in a specific SANC. Figure 2-15 shows the ITU point code format with the SANC highlighted in gray.
Figure 2-15 ITU Point Code Structure
Signaling Point ID
3 Bits
This format fits the international community well because there aren't typically as many network devices requiring point codes as there are in the U.S. Even the U.S. does not use the full range of numbers allotted in each octet.
Figure 2-15 ITU Point Code Structure
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1 Zone ID |
id |
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3 Bits 8 Bits SANC 3 Bits 8 BitsA couple of locations throughout the world differ from what has been discussed. China uses a 24-bit format that is similar to the ANSI point code structure. Japan has adopted a 16-bit point code format for use in the Japanese national network. Now that the logical addressing is in place, you are ready to learn about some of the protocols that make SS7 function. Many protocols that are associated with SS7, but this chapter focuses on only a couple of them. These protocols give you a good high-level understanding of how the SS7 network operates and a foundation to work from for the rest of the book. The protocols that this chapter examines are MTP Layers 1, 2, and 3; Telephone User Part (TUP); Data User Part (DUP); and ISDN User Part (ISUP). This chapter also includes a discussion on some of the services that are offered by INs. SS7 operates at different defined levels, called layers, and each layer has a specific function. For those of you familiar with the Open System Interconnection (OSI) model, the SS7 protocol stack has a layered model that matches up with the layers displayed for OSI, as seen in Figure 2-16. Technically, the SS7 model is a four-layer stack, with MTP at Layers 1 through 3 and the various user parts at Layer 4. The protocol stacks do not match up 100 percent because the SS7 stack was designed prior to 1984, when the International Organization for Standardization (ISO) developed the OSI model. Figure 2-16 The OSI Model and the SS7 Protocol Stack Application Presentation Session Transport Network Physical
OSI Model SS7 Protocol Stack A couple of exceptions to this rule are the signaling connection control part (SCCP), which operates at Layer 3 of the SS7 protocol stack, and TCAP, which operates at a functional level that is considered Layer 7 (Application) of the OSI model. SCCP and MTP together form the Network Service Part (NSP). SCCP, which is discussed in a later section, adds both connectionless and connection-oriented functionality to MTP3. These additions to MTP3 allow for the definitive correspondence between the SS7 protocol stack and the OSI model's network layer. The purpose of MTP is to serve as a set of transport layers for the various user parts and their services. It is also responsible for the detection and correction of network failures and errors in a timely manner. MTP is broken down into three defined layers: • MTP1—Message Transfer Part Layer 1 • MTP2—Message Transfer Part Layer 2 • MTP3—Message Transfer Part Layer 3 |
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