Signaling System No. 7 (SS7/C7): Protocol, Architecture, and Services

Signaling System No. 7

By Lee Dryburgh and Jeff Hewett

Point Codes

As discussed in Chapter 4, "SS7 Network Architecture and Protocols Introduction," each node is uniquely identified by a Point Code. A national Point Code identifies a node within a national network, and an International Signaling Point Code (ISPC) identifies a node within the international network. An International Switching Center (ISC) is identified by both a national and international Point Code. All nodes that are part of the international signaling network use the ITU-T ISPC globally. However, national point codes are based on either the ITU national format or the ANSI format (North America). The structure for international and national Point Codes is discussed in the sections on ITU-T and ANSI, later in this chapter.

Each MSU contains both an Originating Point Code (OPC) and a Destination Point Code (DPC). The DPC is used for identifying the message's destination, and the OPC is used for identifying which node originated the message. As mentioned in the previous section and further discussed in the section "Signaling Message Handling," the DPC is the key entity for routing messages within a network. The OPC identifies which node originated the message.

The identity of the originator is needed for the message to be processed for the correct node. The received OPC can also be used to determine how to populate the DPC when formulating response messages. Because Point Codes are an integral part of MTP3, this chapter discusses them in various contexts, such as network hierarchy, message format, and Signaling Message Handling.

ITU-T International and National Point Codes

ITU-T defines Point Codes for both national and international networks. The international Point Code is based on a hierarchical structure that contains the following three fields:

  • Zone
  • Area/Network
  • Signaling Point

As shown in Figure 7-1, the ITU-T has defined six major geographical zones that represent the major areas of the world. A Zone number that forms the first part of the Point Code represents each geographical zone.


Figure 7-1 ITU-T World Zone Map

Each zone is further divided into an Area or Network based on a specific geographical area within the zone, or as designated by a particular network within the zone. Together, the Zone and Area/Network form the Signaling Area/Network Code (SANC). ITU-T Q.708 lists the SANC codes for each geographical zone. For example, Figure 7-2 shows the SANC designations for the United Kingdom area. The SANC codes are administered by the ITU. ITU operational bulletins publish updates to the numbering assignments after the publication of Q.708.


Figure 7-2 UK Network/Area Point Code Numbers

The Signaling Point identifies the individual signaling node represented by the Point Code.

ITU-T National Point Codes do not have a standardized scheme for defining hierarchy. Each Point Code is a single identifier that designates a specific node.

ANSI National Point Codes

For national Point Codes, ANSI uses a hierarchical scheme similar to that defined by the ITU-T for international signaling. The ANSI Point Code is comprised of three identifiers:

  • Network
  • Cluster
  • Member

The Network identifier represents the highest layer of the SS7 signaling hierarchy and is allocated to telecommunications companies that have large networks.

For example, each of the major operating companies in the U.S. (Verizon, Southwestern Bell, Bellsouth, and Qwest) is allocated one or more Network identifier codes, which identify all messages associated with their network. Smaller, independent operating companies share Network Identifiers, in which case they must use the remaining octets of the Point Code to discriminate between them. Within a network, the Cluster is used to group nodes in a meaningful way for the network operator. If an operating company owns a Network Identifier, it can administer the Cluster assignments in any manner of its choice. Clusters are often used to identify a geographical region within the operator's network; the Member identifies the individual signaling node within a cluster. Figure 7-3 shows the address hierarchy of ANSI networks.


Figure 7-3 Address Hierarchy of ANSI Networks

For the purpose of Point Code allocation, networks are divided into three categories:

  • Large Networks
  • Small Networks
  • CCS Groups

Assignable Point Code Network IDs are numbered 1–254. Network ID 0 is not used, and Network ID 255 is reserved for future use. Point Codes for large networks are assigned in descending order, beginning with Network ID 254.

Point Codes for small networks are assigned in ascending order from the point codes within the Network ID range of 1–4. Each small network is assigned a cluster ID, along with all of the Point Code members within that cluster. A small network operator may be assigned multiple clusters if the network is large enough to warrant the number of Point Codes.

Network ID 5 is used for CCS groups. These groups are blocks of Point Codes belonging to a set of signaling points that are commonly owned but do not have any STPs in the network. These are the smallest category of networks. Point Codes within a cluster may be shared by several different networks depending on the size of the CCS groups. Telcordia administers ANSI Point Codes.

Network ID 6 is reserved for use in ANSI-41 (Mobile Networks) and CCS groups outside of North America.

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