SS7 Network Architecture






SS7 Network Architecture

The SS7 network is used to switch information messages to set up, manage, and release telephone calls as well as to maintain the signaling network. SS7 network nodes are equipped with SS7 functionality and features to become SS7 signaling points or elements. SS7 is a common-channel signaling network, in that all signaling information is carried on a common signaling plane. The signaling planes and the voice circuit planes are logically separated.

SS7 networks consist of three signaling elementsService Switching Point (SSP), Signal Transfer Point (STP), and Service Control Point (SCP)and several link types, as illustrated in Figure. This section covers the signaling elements and signaling links in more detail.

SS7 Network Architecture


Signaling Elements

Signaling elementswhich also are referred to as signaling points, endpoints, exchanges, switches, or nodes separate the voice network from the signaling network. All signaling elements are identified by a numerical point code, which acts as a routing address in SS7. Each signaling message contains the source and destination point code address. Signaling elements route signaling messages and provide access to the SS7 network and to databases.

Figure shows the three types of signaling elements in the SS7 network.

Signaling Elements/Endpoints


  • SSPs are end office or tandem switches that connect voice circuits and perform the necessary signaling functions to originate and terminate calls.

  • The STP routes all the signaling messages in the SS7 network.

  • The SCP provides access to databases for additional routing information used in call processing. Also, the SCP is the key element for delivering IN applications on the telephony network.

The following sections explore the three signaling elements of the SS7 network in more detail.

SSP

SSPs are telephone switches that are provisioned with SS7 capabilities. End office SSPs originate and terminate calls, and core network switches (STPs) provide tandem or transit calls. The SSP provides circuit-based signaling messages to other SSPs for the purposes of connecting, disconnecting, and managing voice calls. Non-circuit based messages are used to query databases when the dialed number is insufficient to complete the call.

End office SSPs connect directly to users on their subscriber interfaces. The protocols used can vary from analog to digital and can be based on ISDN Primary Rate Interface (PRI) or channel-associated switching (CAS). The end office is in charge of translating subscriber protocol requests into SS7 messages to establish calls.

The SSP uses the dialed number to complete the call, unless, for example, it is an 800, 8xx, 9xx, or Local Number Portability exchange (or is ported NXX). In the latter case, a query is sent to an SCP requesting the routing information (number) necessary to complete the call.

The following steps help explain the functions an SSP uses to complete a call. In this case, assume that the originating and destination SSPs are directly attached, as illustrated in Figure:

1.
The SSP uses the called number from the calling party or routing number from the database query to begin circuit connection signaling messages.

2.
Then the SSP uses its routing table to determine the trunk group and circuit needed to connect the call.

3.
At this point, a signaling setup message is sent to the destination SSP requesting a connection on the circuit specified by the originating SSP.

4.
The destination SSP responds with an acknowledgment granting permission to connect to the specified trunk and proceeds to connect the call to the final destination.

STP

STPs, as illustrated in Figure, are an integral part of the SS7 architecture providing access to the network. STPs route or switch all the signaling messages in the network based on the routing information and destination point code address contained in the message.

The STP provides the logical connectivity between SSPs without requiring direct SSP-to-SSP links. STPs are configured in pairs and are mated to provide redundancy and higher availability. These mated STPs perform identical functions and are considered the home STPs for the directly connected SSP or SCP. The STP also is capable of performing global title translation, which is discussed later in this section.

Circuit-based messages are created on the SSP. Then, they are packetized in SS7 packets and sent from the SSP. Usually they contain requests to connect or disconnect a call. These packets are forwarded to the destination SSP where the call is terminated. It is the STP network's job to properly route such packets to the destination.

Non-circuit based messages that originate from an SSP are database queries requesting additional information needed to complete the call. It is the STP network's job to properly route packets between the SSP and the database interface known as the SCP. These packets are routed to the destination SCP and are addressed to the appropriate subsystem database. The SCP is the interface to the database that provides the routing number required to complete the call.

STPs also measure traffic and usage. Traffic measurements provide statistics such as network events and message types, and usage measurements provide statistics on the access and number of messages per message type. This information is used by the carrier's network planning teams to look at overall system capacity and future planning.

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.

SCP

The SCP, as shown in Figure, 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 DatabaseProvides the routing information for special numbers, such as 800, 877, 888, 900, and 976 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.

Signaling Links

All signaling points in the SS7 network are connected by signaling links. These full-duplex links simultaneously transmit and receive SS7 messages over the network link. The signaling links are typically 56- and 64 kbps data network facilities, either on standalone lines or extracted on channelized facilities such as structured T1/E1 trunks.

This section covers the following topics:

Signaling Modes

The SS7 network has three modes of signaling:

  • Associated Signaling

  • Nonassociated Signaling

  • Quasi-associated Signaling

Associated signaling, illustrated in Figure, is the simplest form of signaling, in that the signaling and voice paths are directly connected between the two signaling endpoints. This is not common in North America because end office switches would require direct connections to all other end office switches; however, associated signaling is common in Europe, where the signaling path is actually derived within the E1 trunk facilities.

Associated Signaling


Nonassociated signaling uses a separate logical path for signaling and voice. As illustrated in Figure, the signaling messages travel through multiple endpoints before reaching the final destination. Alternatively, the voice path can have a direct path to the destination end office switch. Nonassociated signaling is the most common form of signaling in the SS7 network.

Nonassociated Signaling


Quasi-associated signaling, shown in Figure, uses a separate logical path for signaling through the minimal number of transfer points to reach the final destination. The benefit of quasi-associated signaling is that network delay is minimized due to the low number of transfer points between the origin and destination. The quasi-associated method is more costly than the nonassociated method, however, because signaling links need to be backhauled to a small number of STPs.

Quasi-Associated Signaling


Signaling Links and Linksets

The signaling links in the SS7 network are identified by the function provided to the corresponding endpoints, as illustrated in Figure.

Signaling Links


The following list outlines the six types of links present in the SS7 network:

  • A-links are interconnects between signaling endpoints and STPs, as illustrated in Figure. The signaling endpoints are SSPs or SCPs, and each has at least two A-links that connect to the "home" STP pair. It is possible to have only one A-link to an STP; however, this is not common practice. These links provide access to the network for the purposes of transmitting and receiving signaling messages. The STP routes the A-link signaling messages received from the originating SSP or SCP toward the destination.

  • Bridge Links (B-links) are interconnects between two mated pairs of STPs, as illustrated in Figure. These mated STPs are peers operating at the same hierarchical level and are interconnected through a quad of B-links. B-links carry signaling messages from the origin to the intended destination.

  • Cross Links (C-links) interconnect an STP with its mate, as illustrated in Figure. The STP pairs perform identical functions and are mated to provide redundancy in the network. C-links are used only when failure or congestion occurs, causing these links to become the only available path to the network. Under normal conditions, these links carry only management traffic.

  • Diagonal Links (D-links) are used to interconnect mated STP pairs of one hierarchical level to mated STP pairs of another hierarchical level, as illustrated in Figure. The D-links are connected in a quad-like fashion similar to B-links. These links provide the same function as B-links; the distinction between B- and D-links is arbitrary.

    D-Links Interconnect Mated STPs on Different Hierarchical Levels

  • Extended Links (E-links) are used to interconnect an SSP to an alternate STP, as illustrated in Figure. The SSP also is connected to the home STP pair through A-links; however, if more reliability is required, you can implement E-links. This is not common practice, however, because the SSPs have dual A-links to redundant mated STP pairs. These links are used only if failure or congestion occurs in the home STPs.

  • F-links are used to directly interconnect two signaling endpoints, as illustrated in Figure. These links are used when STPs are not available or when high traffic volumes exist. This is the only link type whose signaling traffic is allowed to follow the same path as the voice circuits. The signaling messages between the two signaling endpoints are associated only with the voice circuits directly connected between the two signaling endpoints. This method is not commonly used in North America; it is common in Europe, however.

Signaling links are grouped together into linksets when the links are connected to the same endpoint. Signaling endpoints provide load sharing across all the links in a linkset. Combined linksets are used when connecting to mated STP pairs with different point code addresses. In this case, links are assigned to different linksets and are configured as a single combined linkset.

Load sharing across combined linksets occurs when signaling endpoints re-address the messages to adjacent point codes. You can configure alternate linksets to provide redundant paths, increasing reliability over other signaling links such as E- and F-links, as described later in this section.

Signaling Routes

Signaling endpoints have statically predefined routes for destination endpoints. The route is made up of linksets; linksets can be part of more than one route. Groups of routes are called routesets and are defined in routing tables to provide alternate routes when the current route is unavailable.

Signaling Link Performance

The availability of signaling in the SS7 network is critical to connect and serve telephone network users. Signaling links provide signaling transmission and access to the SS7 network and, therefore, must be available at all times. If congestion or failure occurs in the network, the links and STP pairs must handle the additional traffic. The STP mated pairs and linkset configurations provide the necessary load sharing and redundancy required to maintain SS7 network reliability.



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