SIGNALLING SYSTEM 7 (SS 7) EXPLAINED

Signaling System No. 7 (SS7) is a set of telephony signaling protocols which are used to set up most of the world's public switched telephone network telephone calls. The main purpose is to set up andtear down telephone calls. Other uses include number translation, prepaid billing mechanisms, short message service (SMS), and a variety of other mass market services.

It is usually referenced as Signaling System No. 7 or Signaling System #7, or simply abbreviated to SS7. In North America it is often referred to as CCSS7, an acronym for Common Channel Signaling System 7. In some European countries, specifically the United Kingdom, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7 (Common Channel Interoffice Signaling 7).

There is only one international SS7 protocol defined by ITU-T in its Q.700-series recommendations. There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI and ETSI, which are in turn based on the international protocol defined by ITU-T. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC) and Japan (TTC) national variants.

The Internet Engineering Task Force (IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7:

§  MTP2 (M2UA and M2PA)

§  MTP3 (M3UA)

§  Signalling Connection Control Part (SCCP) (SUA)

but use a Stream Control Transmission Protocol (SCTP) transport mechanism. This suite of protocols is called SIGTRAN

There are two essential components to all telephone calls. The first, and most obvious, is the actual content—our voices, faxes, modem data, etc. The second is the information that instructs telephone exchanges to establish connections and route the “content” to an appropriate destination. Telephony signaling is concerned with the creation of standards for the latter to achieve the former. These standards are known as protocols. SS7 or Signaling System Number 7 is simply another set of protocols that describe a means of communication between telephone switches in public telephone networks. They have been created and controlled by various bodies around the world, which leads to some specific local variations, but the principal organization with responsibility for their administration is the International Telecommunications Union or ITU-T. 

Signalling System Number 7 (SS#7 or C7) is the protocol used by the telephone companies for interoffice signalling. In the past, in-band signalling techniques were used on interoffice trunks. This method of signalling used the same physical path for both the call-control signalling and the actual connected call. This method of signalling is inefficient and is rapidly being replaced by out-of-band or common-channel signalling techniques. 

To understand SS7 we must first understand something of the basic inefficiency of previous signaling methods utilized in the Public Switched Telephone Network (PSTN). Until relatively recently, all telephone connections were managed by a variety of techniques centered on “in band” signaling. 

A network utilizing common-channel signalling is actually two networks in one: 

   1. First there is the circuit-switched "user" network which actually carries the user voice and data traffic. It provides a physical path between the source and destination. 

   2. The second is the signalling network which carries the call control traffic. It is a packet-switched network using a common channel switching protocol.

SS7 PROTOCOL SUIT

The SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions. The SS7 protocol is designed to both facilitate these functions and to maintain the network over which they are provided. Like most modern protocols, the SS7 protocol is layered.

Physical Layer (MTP-1)  This defines the physical and electrical characteristics of the signaling links of the SS7 network. Signaling links utilize DS–0 channels and carry raw signaling data at a rate of 56 kbps or 64 kbps (56 kbps is the more common implementation).  Message Transfer Part—Level 2 (MTP-2) The level 2 portion of the message transfer part (MTP Level 2) provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling messages. It incorporates such capabilities as error checking, flow control, and sequence checking.  Message Transfer Part—Level 3 (MTP-3)  The level 3 portion of the message transfer part (MTP Level 3) extends the functionality provided by MTP level 2 to provide network layer functionality. It ensures that messages can be delivered between signaling points across the SS7 network regardless of whether they are directly connected. It includes such capabilities as node addressing, routing, alternate routing, and congestion control.

Signaling Connection Control Part (SCCP)

The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.

While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.

ISDN User Part (ISUP)

ISUP user part defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched network (PSN), and to manage the trunk network on which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes.

Transaction Capabilities Application Part (TCAP)

TCAP defines the messages and protocol used to communicate between applications (deployed as subsystems) in nodes. It is used for database services such as calling card, 800, and AIN as well as switch-to-switch services including repeat dialing and call return. Because TCAP messages must be delivered to individual applications within the nodes they address, they use the SCCP for transport.

Operations, Maintenance, and Administration Part (OMAP)

OMAP defines messages and protocol designed to assist administrators of the SS7 network. To date, the most fully developed and deployed of these capabilities are procedures for validating network routing tables and for diagnosing link troubles. OMAP includes messages that use both the MTP and SCCP for routing.

 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.

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, 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  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.