Supporting interconnection with the PSTN PCS Network Signaling Using SS7 YI-BINGLIN AND STEVEN K. DEVRIES ersonal Communications Services this article is based on EIA,TIA IS-41 Revision B. We also describe , (PCS) facilitates the exchange of information (voice, data, video, some potential extensions of the IS-41 protocol based on a image, etc.) for mobile users independent of time, location, draft of IS-41Revision C. In this article, “IS-41.C”refers to EIAlTIA -and access arrangement. Tosupport PCS, mobile communications PN-2991, the Baseline Text Draft 3 9-2-94of IS-41 Revision C 1191. protocols such as EIWIAInterim Standard 41 (IS-41) 112-161 or Global System for Mobile Communications (GSM) [20] have been defined for PCS Network (PCN) inter-system oper- Signaling System No. 7 ations. To support interconnection between a PCN and the Public Switched Telephone Network (PSTN), it is essential ommon Channel Signaling (CCS) is a signaling method that the mobile communicationsprotocol interactswith the PSTN which provides control and management in the telephone signaling system for mobility management and call control. Cnetwork. CCS consists of supervisory functions, address- This article describes the interactions between a PCN and ing, and providing call information. A CCS channel conveys the PSTN in four aspects: messages to initiate and terminate calls, check on the status of some Interconnection Interfaces -What are the network inter- part of the network, and control the amount of traffic allowed. faces between a PCN and the PSTN? CCS uses a separate out-of-band signaling network to carry Message Routing - How is the information exchanged signaling messages. In all figures of this article, the signal links among the PCNPSTN network elements? will be represented by dashed lines, and the trunks will be Mobility Management - How d0es.a PCN recognize the represented by solid links. SS7 is a CCS system developed to locations of the mobile users? satisfy the telephone operating companies’ requirements for an Call Control - How are the calls set up between the mobile improvement to the earlier signaling systems (which lacked the users and the wireline users? sophistication required to deliver much more than Plain Old Mobility management merits further discussion. In a PCN Telephone Service or POTS). the current location of a mobile user is usually maintained by a This section provides a brief introduction to the SS7 network two-level hierarchical strategy with two types of databases. The architecture and protocol from the perspective of PCN inter- Home Location Register (HLR) is the location register to which connection. The reader is referred to [ 11 for a more complete a mobile user identity is assigned for record purposes such as mobile SS7 tutorial. user information (e.g., directory number, profile information, cur- rent location,validationperiod). The Visitor Location Register (VLR) The SS7 Network Architecture is the location register other than the HLR used to retrieve infor- Figure 1 illustrates an example of the SS7 signaling network. mation for handling of calls to or from a visiting mobile user [14]. The figure only shows the parts that involve the interconnec- When a mobile moves from the home PCN to a visited PCN, its tion between a PCN and the PSTN. The network consists of location is registered at the VLR of the visited PCN. The VLR then three distinct components. informs the mobile’s HLR of its current location. The details of the *Service Switching Point (SSP) is a telephone switch inter- registration processwill be discussed in asubsequent section. When connectedbySS7links.TheSSPsperformcallprocessingoncallsthat a call is delivered to a mobile, the HLR is first queried to find originate, tandem, or terminate at that node. In this article, an SSP its location (i.e., the VLR corresponding to its current location). in a PCN are called a Mobile Switching Center (MSC). The details of location tracking can be found in 1121, and the call *Signal Transfer Point (STP) is a switch that relays SS7 mes- setuphelease process is discussed in the section on PCN/PSTN sages between network switches and databases. Based on the address call control using ISUP. Since it is likely that two PCNs are fields of the SS7 messages, the STPs route the messages to the connected through the PSTN, it is important to understand correct outgoing signaling links. To meet the stringent reliability how the PSTN is involved in PCS mobility management. requirements, STPs are provisioned in mated pairs. To address these four aspects, we consider IS-41 as the mobile Service Control Point (SCP) contains databases for pro- communications protocol and Signaling System No. 7 (SS7) [2-51 viding enhanced services. An SCP accepts queries from an SSP as the PSTN signaling protocol. The IS-41 protocol considered in and returns the requested information to the SSP. In mobile 44 1070-9916/95/$04.000 1995 IEEE IEEE Personal Communications June 1995 1, applications,an SCP may contain an HLRor a VLR. There are six types of SS7 signaling links. Two types of the links are introduced in this article. Other types of links can be found in [7]. Each SSP and SCP will have a minimum of one signal link to each STP pair. The signal link is referred to as the Access Link (A-link). The number of A-links between an SSP and an STP pair can be up to 128 though most switch suppli- A-Link A-Link ers have limited the number to 16. I Signaling links that connect STPs of different networks (e.g., PCN and PSTN in our example) are called Diagonal Links (D-links). D-links are deployed in a quad arrangement with three-waypath diversity. The maximum link set size is 64. I 1 The SS7 Protocol ~~------ ~- ~~ The basic parts of the SS7 protocol and the cor- W Figure 1. An example of the SS7 network architecture. responding OS1 layers are shown in Fig. 2. In the protocol hierarchy, the Operations, Maintenance, and AdministrationPart (OW)and Mobile Appli- cation Part (MAP) are TCAP (i.e., Transaction OS1 layers The SS7 layers Capabilities Application Part; to be defined) applications. The details of the OMAP are omit- -/I ted and can be found in [7]. Application The MAP will be elaborated based on the IS- *I I 41 protocol. The other parts of the SS7 protocol are described below. ...__________-.--..-.~-~~ ISDN-UP *The Message Transfer Part (MTP) consists of Presentation three levels corresponding to the OS1 physical layer, session data link layer, and network layer, respectively. transport The MTP Level 1 defines the physical, electrical, and functional characteristics of the signaling links connecting SS7 components. The MTP Level 2 provides reliable transfer of signaling messages between two directly connected signaling points. The MTP Level 3 provides the functions and procedures related to message routing and network manage- ment. I MTP level 3 I *The Signaling Connection Control Part (SCCP) provides additional functions such as Data link MTP level 2 Global Title Translation to the MTP to transfer I I non-circuit-related signaling information such as PCS registration and cancellation. Physical MTP level 1 *The Transaction Capabilities Application I I Part (TCAP) provides the ability to exchange information between applications using non-circuit .~ _.~ related signaling. W Figure 2. The SS7signalingprotocol.~ ~ *Integrated Services Digital Network User Part (ISUP) establishes circuit-switched network connections (e.g., call setupirelease). Pass-alongsig- two for SS7 signaling, and two for trunk (voice naling service sends the signaling information to each circuit) connections [8]. The signaling interfaces rep- switching point involved in a call connection. resents the physical signalinglink connectionbetween The IS-41 protocol [12, 1.51 is implemented in a PCN and the PSTN. The SS7 signaling inter- the MAP as an application of the TCAP. The connection methods are described as follows: wireless call setup/release is completed by using A-link signaling interface involves A-links from the ISUP. The MTP and the SCCP provide rout- an MSC to a PSTN STP pair (Figs. 3a and c). ing services between a PCN and the PSTN. OD-linksignaling interface involves D-links from a PCN STP pair to a PSTN STP pair (Figs. 3b and d). lnterconnection and Message The trunk interfaces represents a physical SS7- supported trunkconnection between aPCN and the Routing between PSTN. The types of the SS7 trunk interconnections PCN and PSTN are described as follows: *Type 2A with SS7 trunk interface provides everal types of interconnections between a connection between a PCN and a PSTN tandem PCN and the PSTN have been described in switch. The Type 2A with SS7 interfaces are shown S [8, 171. We describe four types of SS7 inter- in Figs. 3a and b. connection between a PCN and the PSTN (specif- *Type 2B with SS7 trunk interface provides ically, the local telephone networks) using SS7: connection between a PCN and a PSTN end office IEEE Personal Communications June 1995 45 IS-41 Revisions B and C, but not in IS-41 Revision A. The ISUP messages for wireless call control FacilitiesDirective INVOKE(Last) QUERYWITHPERMISSION (see the section on PCN/PSTN call control using FacilitiesDirective RETURN RESULT(Last) CONVERSATIONWITHPERMISSIONISUP) are not delivered with GTT. Details of IS- FacilitiesDirective RETURN ERROR RESPONSE 41 message routing can be found in Appendix B. The FacilitiesDirective REJECT RESPONSE - appendix describes the fields of an IS-41 message HandoffMeasurementRequest INVOKE(Last) QUERYWITHPERMISSION which are used for the routing purpose and how HandoffMeasurementRequest RETURN RESULT(Last) RESPONSE the routing procedure is performed. HandoffMeasurementRequest RETURN ERROR RESPONSE HandoffMeasurementReauest REJECT RESPONSE Mobility Management MobileOnChannel I INVOKE ]RESPONSE ~ Using TCAP QualificationRequest INVOKE(Last) QUERYWITHPERMISSION QualificationRequest RETURN RESULT(Last) RESPONSE wenty six TCAP operations2 are defined in QualificationRequest RETURN ERROR RESPONSE IS-41.B [ 131 for three purposes: Inter-MSC QualificationRequest REJECT RESPONSE T Handoff [15], Automatic Roaming [12], and Operations, Administration, and Maintenance RegistrationCancellation INVOKE(Last) QUERYWITHPERMISSION [16].