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Chapter 9

Chapter 9. Common Channel Signaling No. 7 (CCS7). 1 Common Channel Signaling. In CCS , a Common Signaling Link ( SL ) carries Signaling Messages for a number of trunks .

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Chapter 9

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  1. Chapter 9 Common Channel Signaling No. 7 (CCS7) Bahman R. Alyaei

  2. 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-generationSPC exchanges that were introduced in the 1960s such as 4ESS, 5ESS, and DMS-100. • First-generationCCS 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. Bahman R. Alyaei

  3. Continue… Fully disassociated channel signaling Bahman R. Alyaei

  4. 2 Signaling System No. 7 • SS7 is a CCS system that defines the Procedures and Protocol by which Network Elements (Signaling Points) in the PSTNExchange Information over a Digital Signaling Network to facilitate Wired and Wireless (Cellular) Call Setup, Routing and Control. Bahman R. Alyaei

  5. Continue… • The SS7 Network and Protocol are used for: • Basic call setup, management, and tear down. • Wireless services such as wireless roaming and mobile subscriber authentication. • Toll free (800/888) and toll (900) wireline service. • Enhanced call features such as call forwarding, calling party name/number display, and three-way calling. Bahman R. Alyaei

  6. Continue… • In an SS7 Network, Control Messages (Packets) are routed through the network to perform: • Call management (setup, maintenance, and termination) and, • Network management functions. • Therefore, the CCS7network is a packet-switched network, that is been controlled bycircuit-switched network. Bahman R. Alyaei

  7. 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 CCSlinks are attached. Bahman R. Alyaei

  8. Continue… SS7 sample network Bahman R. Alyaei

  9. Continue… • For example, an exchangethat 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 bidirectional56 kbps or 64 kbpstransport facility for CCSmessages between two SPs. • A Signaling Relation exists between any pair of SPs that need to communicate by CCS. Bahman R. Alyaei

  10. 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. Bahman R. Alyaei

  11. 3 Signaling Points • An SS7 Network is comprised of NetworkElements (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. Bahman R. Alyaei

  12. 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 (Signaling Points): • Service Switching Point (SSP). • Signal Transfer Point (STP). • Service Control Point (SCP). Bahman R. Alyaei

  13. STP STP SSP SSP Service Switching Point STP Signal Transfer Point STP Service Control Point Continue… Signaling Network Elements Bahman R. Alyaei

  14. 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: • The combination of a Voice Switch and an SS7 Switch. • Or an Adjunct Computer connected to a Voice Switch. Bahman R. Alyaei

  15. 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. Bahman R. Alyaei

  16. 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. Bahman R. Alyaei

  17. 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. Bahman R. Alyaei

  18. Continue… • This SS7 Message contain: • The calling and called telephone numbers, • The Information about the type of decoding used in the voice transmission, • Type of the call. • The adjacent Switch GrantsPermission to connect this Trunk by sending back an Acknowledgment to the Originating Switch. Bahman R. Alyaei

  19. 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. Bahman R. Alyaei

  20. 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. Bahman R. Alyaei

  21. 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. Bahman R. Alyaei

  22. Continue… • Both STPs in a pair, process traffic using Load Sharing. • An STPdoes notoriginate Messages. • The STPonly processes the Transport Layers (Message Transfer Part (MTP) or TCP/IP-based protocols). • The STPRoutes SS7 Messages as received from the various SSPs throughout the Network to their appropriate destinations. Bahman R. Alyaei

  23. Continue… • The STP can be an Adjunct to a Voice Switch or a Standalone Packet Switch. • Many Tandem Switches provide both Switching Functions and STPFunctions (through the use of an adjunct computer). • Although several manufacturers provide STP equipment, very few provide a StandaloneSTP. Bahman R. Alyaei

  24. 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. Bahman R. Alyaei

  25. 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 LocalSTP with the destination address of the STP. Bahman R. Alyaei

  26. Continue… • The STP looks at the dialed digits in the Signaling Connection Control Part (SCCP) portion of theMessage 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. Bahman R. Alyaei

  27. 3.3 Service Control Point (SCP) • The SCP serves as an Interface to Telephone Company Databases. • These Databases provide: • The storage of information about subscribers’ services. • The routing of special service numbers such as 800 and 900numbers, and calling-card validation. Bahman R. Alyaei

  28. 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. Bahman R. Alyaei

  29. 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 MinicomputerSystem that is used for the actual Database. • These ComputerSystems usually are linked to the SCP through X.25 or IP links. • The SCP communicates with the DatabaseApplication through the use of Primitives. Bahman R. Alyaei

  30. Continue… • 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. Bahman R. Alyaei

  31. Continue… • The Databases used most commonly within either of these networks are as follows: • Call Management Services Database (CMSDB): • It includes certain network management functions used to prevent congestion on the network. • It also provides billing information. • Local Number Portability (LNP): • It enable subscribers to change telephone companies without having to change their telephone numbers. Bahman R. Alyaei

  32. Continue… • Line Information Database (LIDB): • provides information regarding subscribers, such as calling-card service, third-party billing instructions. • Calling Name (CNAM): • The user’s Personal Identification Number (PIN) is stored in this Database for comparison when a user places a call. • This Database provides the name of the calling party. Bahman R. Alyaei

  33. Continue… • 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. • Visitor Location Register (VLR): is used for determining the location of a wireless subscriber in real time. Bahman R. Alyaei

  34. Continue… • A Visitor Location Register (VLR) is a Database that contains information about the Subscribers Roaming within a Mobile Switching Center’s (MSC) Location Area. • The primary role of the VLR is to minimize the number of queries that MSCs have to make to the Home Location Register (HLR), which holds permanent data regarding the cellular network’s subscribers. • Ideally, there should be only one visitor location register per MSC, but it is also possible for a single VLR to serve multiple MSCs. Bahman R. Alyaei

  35. Continue… • 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 DatabaseEntity. Bahman R. Alyaei

  36. 4 Basic Signaling Architecture • Figure on the next slide shows a small example of how basic Network Elements (NE) of an SS7 Network are deployed to form two InterconnectedNetworks. • The following points should be noted: • STPsW and X, perform identical functions, they are redundant, and referred to as mated pairsof STPs. Similarly are Y and Z. Bahman R. Alyaei

  37. Continue… SS7 sample network Bahman R. Alyaei

  38. Continue… • Each SSP has two links, one to each STP of a mated pair. All SS7 Signaling to the rest of the world is sent out over these links. Messages sent over either link will be treated equivalently. • The STPs of a mated pairare joined by a link or set of links. Bahman R. Alyaei

  39. Continue… • Two mated pairs of STPs are interconnected by four links which are referred to as quad. • SCPs are usually (not always) deployed in pairs which are referred to as mated pair of SCPs. They are not directly joined by a pair of links. Bahman R. Alyaei

  40. 4 Signaling Links • All signaling points are interconnected via signaling links. • These links are 56 or 64-kbps. • Links areBidirectional, using transmit and receive pairs for simultaneous transmission in both directions. • Links always should be Terrestrial, although Satellite Links are also supported. Bahman R. Alyaei

  41. Continue… • Signaling Links (SL) are labeled according to their relationship in the network. • There is no technical difference between the various Links. • They are only different in the way the links are used during message transfer and how network management interacts with the links. Bahman R. Alyaei

  42. SS7 Link Types Bahman R. Alyaei

  43. Continue… • Links are placed into groups called Linksets. • All the links in a linkset must have the same adjacent node. • The Switching Equipment alternates transmission across all the links in a linkset to ensure equal use of all facilities. • A Signaling Point (SP) must define Routes. Bahman R. Alyaei

  44. Continue… • A Route: is a collection of Linksets used to reach a particular destination. • A Linkset can belong to more than one route. • Routeset: is a collection of routes. • A Routeset provides alternate routes to the same destination in the event that any one route becomes unavailable. Bahman R. Alyaei

  45. Continue… • Six different types of links are used in SS7: • Access Links (A-Links): • Are used between the SSP and the STP or the SCP and the STP. • These links provide access into the network and to Databases through the STP. • The maximum number of A-links to any one STP from SSP is 16, usually, one A-link for 9600 subscribers served by SSP. Bahman R. Alyaei

  46. A-links connect end-signaling points to the SS7 network Bahman R. Alyaei

  47. Continue… • Bridge Links (B-Links): • Are deployed in a quad fashion and are used to connect mated STPs to other mated STPs at the same hierarchical level. • A maximum of eight B-links can be deployed between mated STPs. Bahman R. Alyaei

  48. B-links connect a mated pair of STPs to another mated pair of STPs Bahman R. Alyaei

  49. A typical SS7 network with multiple B-links and A-links Bahman R. Alyaei

  50. Continue… • Cross Links (C-Links): • Connect an STP to its mate STP. • They are always deployed in pairs to maintain redundancy on the network. • NormalSS7 traffic is not routed over these links, except in congestion conditions. • The only messages to travel between mated STPs during normal conditions are Network Management Messages. • A maximum of eight C-links can be deployed between STP pairs. Bahman R. Alyaei

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