Global Title Translation

In addition to performing basic SS7 packet routing, STPs are capable of performing gateway services such as global title translation. This function is used to centralize the SCP and database selection versus distributing all possible destination selections to hundreds or thousands of distributed switches. If the SSP is unaware of the destination SCP address, it can send the database query to its local STP. The STP then performs global title translation and re-addresses the destination of the database query to the appropriate SCP.

Global title translation centralizes the selection of the correct database by enabling queries to be addressed directly to the STP. SSPs, therefore, do not have the burden of maintaining every potential destination database address. The term global title translation is taken from the term global title digits, which is another term for dialed digits.

The STP looks at the global dialed digits and through its own translation table to resolve the following:

• The point code address of the appropriate SCP for the database

• The subsystem number of the database

The STP also can perform an intermediate global title translation by using its translation table to find another STP. The intermediate STP then routes the message to the other STP to perform the final global title translation.

STP Hierarchy

STP hierarchy defines network interconnection and separates capabilities into specific areas of functionality. STP implementation can occur in multiple levels, such as:

• Local Signal Transfer Point

• Regional Signal Transfer Point

• National Signal Transfer Point

• International Signal Transfer Point

• Gateway Signal Transfer Point

The local, regional, and national STPs transfer standards-based SS7 messages within the same network. These STPs usually are not capable of converting or handling messages in different formats or versions.

International STPs provide international connectivity where the same International Telecommunication Union (ITU) standards are deployed in both networks.

Gateway STPs can provide the following:

• Protocol conversion from national versions to the ITU standard

• Network-to-network interconnection points

• Network security features such as screening, which is used to examine all incoming and outgoing messages to ensure authorization

You can deploy and install STP functions on separate dedicated devices or incorporate them with other SSP functions onto a single end office or tandem switch. Integrating SSP and STP functions is particularly common in Europe and Australia. This is why fully associated SS7 or CCS7 (CCS7 is the ITU-T version of SS7) networks are prevalent in those areas. Fully associated SS7 occurs when the same transmission channel carries the bearer's information and the signaling information.

The SCP, as shown in Figure 4-2, provides the interface to the database where additional routing information is stored for non-circuit based messages. Service-provider SCPs do not house the required information; they do, however, provide the interface to the system's database. The interface between the SCP and the database system is accomplished by a standard protocol, which is typically X.25. The SCP provides the conversion between the SS7 and the X.25 protocol. If X.25 is not the database access protocol, the SCP still provides the capability for communication through the use of primitives.

The database stores information related to its application and is addressed by a subsystem number, which is unique for each database. The subsystem number is known at the SSP level; the request originated within the

PSTN contains that identifier. The subsystem number identifies the database where the information is stored and is used by the SCP to respond to the request.

The following databases are the most common in the SS7 network:

• 800 Database—Provides the routing information for special numbers, such as 800, 888, and 900 numbers. The 800 database responds to the special number queries with the corresponding routing number. In the case of 800, 888, and 900 numbers, the routing number is the actual telephone number at the terminating end.

• Line Information Database (LIDB)—Provides subscriber or user information such as screening and barring, calling-card services including card validation and personal identification number (PIN) authentication, and billing. The billing features of this database determine ways you can bill collect calls, calling-card calls, and third-party services.

• Local Number Portability Database (LNPDB)—Provides the 10-digit Location Routing Number (LRN) of the switch that serves the dialed-party number. The LRN is used to route the call through the network, and the dialed-party number is used to complete the call at the terminating SSP.

• Home Location Register (HLR)—Used in cellular networks to store information such as current cellular phone location, billing, and cellular subscriber information.

• Visitor Location Register (VLR)—Used in cellular networks to store information on subscribers roaming outside the home network. The VLR uses this information to communicate to the HLR database to identify the subscriber's location when roaming.

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