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Chapter 9 Common Channel Signaling No. 7 (CCS7) 19/04/2013 Bahman R. Alyaei 1 1 Common Channel Signaling • In CCS, a Common Signaling Link (SL) carries Signaling Messages for a number of trunks. • CCS was developed for the third-generation SPC exchanges that were introduced in the 1960s such as 4ESS, 5ESS, and DMS-100. • First-generation CCS was introduced in the 1970s. • Signaling System No. 7 (SS7) also known CCS7 specified by ITU-T in 1980s. • It is widely in service in the International and National Networks. 19/04/2013 Bahman R. Alyaei 2 Continue… Fully disassociated channel signaling 19/04/2013 Bahman R. Alyaei 3 2 Signaling System No. 7 • SS7 is a CCS system that defines the Procedures and Protocol by which Network Elements (Signaling Points) in the PSTN Exchange Information over a Digital Signaling Network to facilitate Wired and Wireless (Cellular) Call Setup, Routing and Control. 19/04/2013 Bahman R. Alyaei 4 Continue… • The SS7 Network and Protocol are used for: 1. Basic call setup, management, and tear down. 2. Wireless services such as wireless roaming and mobile subscriber authentication. 3. Toll free (800/888) and toll (900) wireline service. 4. Enhanced call features such as call forwarding, calling party name/number display, and threeway calling. 19/04/2013 Bahman R. Alyaei 5 Continue… • In an SS7 Network, Control Messages (Packets) are routed through the network to perform: 1. Call management (setup, maintenance, and termination) and, 2. Network management functions. • Therefore, the CCS7 network is a packetswitched network, that controls circuitswitched network. 19/04/2013 Bahman R. Alyaei 6 2.1 Signaling Network • Telecommunication networks that employ CCS require, in addition to the network of Trunks and Exchanges, a Signaling Network. • This network consists of Signaling Points (SPs), interconnected by Signaling Links (SLs). • SP: is an entity in the network to which CCS links are attached. 19/04/2013 Bahman R. Alyaei 7 Continue… SS7 sample network 19/04/2013 Bahman R. Alyaei 8 Continue… • For example, an exchange that serves CCS Trunk Groups has CCS links and is therefore a SP. • Likewise, a Network Database that is accessed via CCS links is a SP. • SL: is a bidirectional 56 kbps or 64 kbps transport facility for CCS messages between two SPs. • A Signaling Relation exists between any pair of SPs that need to communicate by CCS. 19/04/2013 Bahman R. Alyaei 9 Continue… • For example, when two exchanges, say, A and B, are interconnected by a group of CCS trunks, there is a signaling relation between these SPs. • Signaling Route: is a predetermined path for the CCS Messages of a particular relation. • Usually, there is a signaling route set, consisting of several routes, for each signaling relation. 19/04/2013 Bahman R. Alyaei 10 3 Signaling Points • An SS7 Network is comprised of Network Elements (NEs) connected together using Signaling Links (SLs). • Such a NE that is capable of handling SS7 Control Messages is called a SP. • All SPs in an SS7 Network are identified by a unique address known as a point code that defines the Signaling Relation. 19/04/2013 Bahman R. Alyaei 11 Continue… • In SS7 Messaging, both the origination point code and the destination point code are provided. • There are three different basic types of Network Elements: 1. Service Switching Point (SSP). 2. Signal Transfer Point (STP). 3. Service Control Point (SCP). 19/04/2013 Bahman R. Alyaei 12 Continue… SSP STP STP SSP STP STP Service Switching Point Signal Transfer Point Service Control Point Signaling Network Elements 19/04/2013 Bahman R. Alyaei 13 3.1 Service Switching Point (SSP) • SSP is the Brain of the SS7 Network and is located in the Switch. • The SSP is the Local Exchange in the Telephone Network. • An SSP can be: 1. The combination of a Voice Switch and an SS7 Switch. 2. Or an Adjunct Computer connected to a Voice Switch. 19/04/2013 Bahman R. Alyaei 14 Continue… • The SSP must convert signaling from the Voice Switch into SS7 Signaling Messages, which then can be sent to other Switches in the Network. • The Switch typically will send messages related to its voice circuits to the switches with a direct voice trunk connection to it. 19/04/2013 Bahman R. Alyaei 15 Continue… • In the case of database access, the SSP will send database queries through the SS7 Network to Computer Systems located centrally to the network (or regionally). • The SSP function uses the information provided by the calling party (such as dialed digits) to determine how to connect a call. 19/04/2013 Bahman R. Alyaei 16 Continue… • A Routing Table in the Switch itself will identify which Trunk Circuit to use to connect the call and at which Switch this Trunk terminates. • An SS7 Message must be sent to this adjacent Switch requesting a Circuit Connection on the specified Trunk. 19/04/2013 Bahman R. Alyaei 17 Continue… • This SS7 Message contain: 1. The calling and called telephone numbers, 2. The Information about the type of decoding used in the voice transmission, 3. Type of the call. • The adjacent Switch Grants Permission to connect this Trunk by sending back an Acknowledgment to the Originating Switch. 19/04/2013 Bahman R. Alyaei 18 Continue… • Using the Called Party Information in the Setup Message, the adjacent Switch then can determine how to connect the call to its final destination. • The same process is followed using a Setup Message to any adjacent Switches and Circuits connecting those Switches. • The entire call may require several connections between several switches. 19/04/2013 Bahman R. Alyaei 19 Continue… • The SSP function in each Switch, manages these connections but really has no knowledge of the status of remote connections (nonadjacent connections). • The SSP only has visibility of its own connections and does not maintain the status of all the connections needed to connect and maintain a call. 19/04/2013 Bahman R. Alyaei 20 3.2 Signal Transfer Point (STP) • All SS7 Packets travel from one SSP to another through the services of an STP. • The STP serves as a Router in the SS7 Network. • To maintain redundancy and diversity in the network, STPs are always deployed in pairs. • Should one STP node fail, the other node assumes all the traffic. 19/04/2013 Bahman R. Alyaei 21 Continue… • Both STPs in a pair process traffic using Load sharing. • An STP does not originate messages. • The STP only processes the Transport Layers (Message Transfer Part (MTP) or TCP/IP-based protocols). • The STP Routes SS7 Messages as received from the various SSPs throughout the Network to their appropriate destinations. 19/04/2013 Bahman R. Alyaei 22 Continue… • The STP can be an Adjunct to a Voice Switch or a Standalone Packet Switch. • Many Tandem Switches provide both Switching Functions and STP Functions (through the use of an adjunct computer). • Although several manufacturers provide STP equipment, very few provide a Standalone STP. 19/04/2013 Bahman R. Alyaei 23 Continue… • Routing to Databases is another important STP function. • For example, the SCP may support calling cards, 800 routing, and calling-name display. • Each of these services in the SCP must have a unique address. 19/04/2013 Bahman R. Alyaei 24 Continue… • The STP uses a function called Global Title Translation (GTT) to determine which Database will receive the queries generated by SSPs. • By using an STP and GTT, the SSPs only need to know the address of the STP. • The SSP sends a Database Query to the local STP with the destination address of the STP.3 19/04/2013 Bahman R. Alyaei 25 Continue… • The STP looks at the dialed digits in the Signaling Connection Control Part (SCCP) portion of the Message Signal Unit (MSU) (or global title digits as they are often called) and determines the address of the Database through its own Translation Tables. • The GTT consists of the Subsystem Number (Address) of the Database and the Point Code of the SCP that interfaces with the Database. 19/04/2013 Bahman R. Alyaei 26 3.3 Service Control Point (SCP) • The SCP serves as an interface to telephone company databases. • These databases provide: 1. The storage of information about subscribers’ services, 2. The routing of special service numbers (such as 800 and 900 numbers), callingcard validation, and even Advanced Intelligent Network (AIN) services 19/04/2013 Bahman R. Alyaei 27 Continue… • The SCP is actually a function of the computer used as a front end to the database application itself. • The SCP does not necessarily have to be a standalone computer system. • Some new SCP database applications are being implemented in STPs, providing an integrated solution. • In all cases, the address of the SCP is a Point Code, whereas the address of the database is a subsystem number. 19/04/2013 Bahman R. Alyaei 28 Continue… • Entities within the network route to SCPs using the SCCP protocol. • The SCP function does not necessarily store all the data, but it is the interface to the mainframe or minicomputer system that is used for the actual database. • These computer systems usually are linked to the SCP through X.25 or IP links. • The SCP communicates with the database application through the use of primitives. 19/04/2013 Bahman R. Alyaei 29 Continue… • A primitive is an interface that provides access from one level of the protocol to another level. • In the case of the database, the database is considered an application entity, and TCAP is the protocol used to access and interface with this application entity. • The type of database depends on the network. 19/04/2013 Bahman R. Alyaei 30 Continue… • The databases used most commonly within either of these networks are as follows: 1. Call Management Services Database (CMSDB): It includes certain network management functions used to prevent congestion on the network. It also provides billing information. 2. Local Number Portability (LNP): It enable subscribers to change telephone companies without having to change their telephone numbers. 19/04/2013 Bahman R. Alyaei 31 Continue… 3. Line Information Database (LIDB): provides information regarding subscribers, such as calling-card service, third-party billing instructions. The user’s personal identification number (PIN) is stored in this database for comparison when a user places a call. 4. Calling Name (CNAM): This database provides the name of the calling party. 19/04/2013 Bahman R. Alyaei 32 Continue… 5. Home Location Register (HLR): is found in wireless networks and is used to store information regarding a wireless subscriber. Billing and feature information is stored in this database along with location information. 6. Visitor Location Register (VLR): is used for determining the location of a wireless subscriber in real time. 19/04/2013 Bahman R. Alyaei 33 • Each database is given a unique address called a subsystem number . • The subsystem number is used to route queries from SSPs through the SS7 network to the actual database entity. 19/04/2013 Bahman R. Alyaei 34