PCS network signaling using SS7 - IEEE Personal Communications

Supporting interconnection with the PSTN

PCS Network Signaling Using SS7

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Y I - B I N GL I N A N D S T E V E N K. DEVRIES

ersonal Communications Services (PCS) facilitates the exchange of information (voice, data, video, image, etc.) for mobile users independent of time, location, and access arrangement. To support PCS, mobile communications protocols such as E I W I A Interim Standard 41 (IS-41) 112-161 or Global System for Mobile Communications (GSM) [20] have been defined for PCS Network (PCN) inter-system operations. To support interconnection between a PCN and the Public Switched Telephone Network (PSTN), it is essential that the mobile communicationsprotocol interactswith the PSTN signaling system for mobility management and call control. This article describes the interactions between a PCN and the PSTN in four aspects: Interconnection Interfaces -What are the network interfaces between a PCN and the PSTN? Message Routing - How is the information exchanged among the PCNPSTN network elements? Mobility Management - How d0es.a PCN recognize the locations of the mobile users? Call Control - How are the calls set up between the mobile users and the wireline users? Mobility management merits further discussion. In a PCN the current location of a mobile user is usually maintained by a two-level hierarchical strategy with two types of databases. The Home Location Register (HLR) is the location register to which a mobile user identity is assigned for record purposes such as mobile user information (e.g., directory number, profile information, current location,validationperiod).The Visitor LocationRegister(VLR) is the location register other than the HLR used to retrieve information for handling of calls to or from a visiting mobile user [14]. When a mobile moves from the home PCN to a visited PCN, its location is registered at the VLR of the visited PCN. The VLR then informs the mobile’s HLR of its current location. The details of the registration processwill be discussed in asubsequent section. When a call is delivered to a mobile, the HLR is first queried to find its location (i.e., the VLR corresponding to its current location). The details of location tracking can be found in 1121, and the call setuphelease process is discussed in the section on PCN/PSTN call control using ISUP. Since it is likely that two PCNs are connected through the PSTN, it is important to understand how the PSTN is involved in PCS mobility management. To address these four aspects, we consider IS-41 as the mobile communications protocol and Signaling System No. 7 (SS7) [2-51 as the PSTN signaling protocol. The IS-41 protocol considered in

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1070-9916/95/$04.000 1995 IEEE

this article is based on EIA,TIA IS-41 Revision B. We also describe some potential extensions of the IS-41 protocol based on a draft of IS-41RevisionC. In this article, “IS-41.C”refers to EIAlTIA PN-2991,the Baseline Text Draft 3 9-2-94of IS-41 Revision C 1191.

Signaling System No. 7 ommon Channel Signaling (CCS) is a signaling method which provides control and management in the telephone network. CCS consists of supervisory functions, addressing, and providing call information. A CCS channel conveys messages to initiate and terminate calls, check on the status of some part of the network, and control the amount of traffic allowed. CCS uses a separate out-of-band signaling network to carry signaling messages. In all figures of this article, the signal links will be represented by dashed lines, and the trunks will be represented by solid links. SS7 is a CCS system developed to satisfy the telephone operating companies’ requirements for an improvement to the earlier signaling systems (which lacked the sophistication required to deliver much more than Plain Old Telephone Service or POTS). This section provides a brief introduction to the SS7 network architecture and protocol from the perspective of PCN interconnection. The reader is referred to [ 11 for a more complete SS7 tutorial.

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The SS7 Network Architecture Figure 1 illustrates an example of the SS7 signaling network. The figure only shows the parts that involve the interconnection between a PCN and the PSTN. The network consists of three distinct components. *Service Switching Point (SSP) is a telephone switch interconnectedbySS7links.TheSSPsperformcallprocessingoncalls that originate, tandem, or terminate at that node. In this article, an SSP in a PCN are called a Mobile Switching Center (MSC). *Signal Transfer Point (STP) is a switch that relays SS7 messagesbetween network switches and databases. Based on the address fields of the SS7 messages, the STPs route the messages to the correct outgoing signaling links. To meet the stringent reliability requirements, STPs are provisioned in mated pairs. Service Control Point (SCP) contains databases for providing enhanced services. An SCP accepts queries from an SSP and returns the requested information to the SSP. In mobile

IEEE Personal Communications June 1995

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applications,an SCP may contain an HLRor a VLR. There are six types of SS7 signaling links. Two types of the linksare 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 suppliers have limited the number to 16. 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 deployedin a quad arrangement with three-waypath diversity. The maximum link set size is 64.

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A-Link

A-Link

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The SS7 Protocol The basic parts of the SS7 protocol and the corresponding OS1 layers are shown in Fig. 2. In the protocol hierarchy, the Operations, Maintenance, and AdministrationPart ( O W ) and Mobile Application Part (MAP) are TCAP (i.e., Transaction Capabilities Application Part; to be defined) applications. The details of the OMAP are omitted and can be found in [7]. The MAP will be elaborated based on the IS41 protocol. The other parts of the SS7 protocol are described below. *The Message Transfer Part (MTP) consists of three levels corresponding to the OS1physical layer, data link layer, and network layer, respectively. 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 signalingpoints. The MTP Level 3 provides the functions and procedures related to message routing and network management. * T h e Signaling Connection Control Part (SCCP) provides additional functions such as Global Title Translation to the MTP to transfer non-circuit-related signaling information such as PCS registration and cancellation. *The Transaction Capabilities Application Part (TCAP) provides the ability t o exchange information between applications using non-circuit related signaling. *Integrated Services Digital Network User Part (ISUP) establishes circuit-switched network connections (e.g., call setupirelease). Pass-alongsignaling service sends the signalinginformationto each switching point involved in a call connection. The IS-41 protocol [12, 1.51 is implemented in the MAP as an application of the TCAP. The wireless call setup/release is completed by using the ISUP. The MTP and the SCCP provide routing services between a PCN and the PSTN.

lnterconnection and Message Routing between PCN and PSTN everal types of interconnections between a PCN and the PSTN have been described in [8, 171. We describe four types of SS7 interconnection between a PCN and the PSTN (specifically, the local telephone networks) using SS7:

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IEEE Personal Communications

June 1995

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W Figure 1. An example of the SS7 network architecture.

OS1 layers

The SS7 layers

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two for SS7 signaling, and two for trunk (voice circuit)connections [8].The signaling interfacesrepresentsthe physical signalinglink connectionbetween a PCN and the PSTN. The SS7 signaling interconnection methods are described as follows: A-link signaling interface involvesA-linksfrom an MSC to a PSTN STP pair (Figs. 3a and c). OD-linksignaling interface involves D-links from a PCN STP pair to a PSTN STP pair (Figs. 3b and d). The trunk interfaces represents a physical SS7supported trunkconnection between aPCN and the PSTN. The types of the SS7 trunk interconnections are described as follows: *Type 2A with SS7 trunk interface provides connection between a PCN and a PSTN tandem switch. The Type 2A with SS7 interfaces are shown in Figs. 3a and b. *Type 2B with SS7 trunk interface provides connection between a PCN and a PSTN end office

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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 RETURN RESULT(Last) CONVERSATIONWITHPERMISSION FacilitiesDirective ISUP) are not delivered with GTT. Details of ISRETURN ERROR FacilitiesDirective RESPONSE 41 message routing can be found in Appendix B. The FacilitiesDirective REJECT RESPONSE appendix describes the fields of an IS-41 message which are used for the routing purpose and how HandoffMeasurementRequest INVOKE(Last) QUERYWITHPERMISSION the routing procedure is performed. HandoffMeasurementRequest RETURN RESULT(Last) RESPONSE HandoffMeasurementRequest RETURN ERROR HandoffMeasurementReauest REJECT

IINVOKE

MobileOnChannel

RESPONSE RESPONSE

Mobility Management Using TCAP

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QualificationRequest QualificationRequest QualificationRequest QualificationRequest

INVOKE(Last) QUERYWITHPERMISSION RETURN RESULT(Last) RESPONSE RETURN ERROR RESPONSE REJECT RESPONSE

RegistrationCancellation RegistrationCancellation RegistrationCancellation RegistrattonCancellation

INVOKE(Last) QUERYWITHPERMISSION RETURN RESULT(Last) RESPONSE RESPONSE RETURN ERROR REJECT RESPONSE

RegistrationNotification RegistrationNotification RegistrationNotification RegistrationNotification

INVOKE(Last) QUERYWITHPERMISSION RElllRN RESULT(Last) RESPONSE RESPONSE RETURN ERROR REJECT RESPONSE

ServiceProfileRequest ServiceProfileRequest ServiceProfileRequest ServiceProfileRequest

INVOKE(Last) QUERYWITHPERMISSION RETURN RESULT(La5t) RESPONSE RETURN ERROR RESPONSE REJECT RESPONSE

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switch. The Type 2B with SS7 interfaces are shown in Figs. 3 c and d. Several applications that require calls to be tandemed (e.g., operator services and 800 call setup) are supported by Type 2A with SS7 interface, but are not available to Type 2B with SS7 interface. Both Types 2A and 2B were originally supported by multi-frequency (MF) signaling protocols [8]. Other interfaces supported by MF signaling are Type 1, Type 2C for 911 emergency service calls, and Type 2D for operator services. Both 911 calls (Type 2C) and operator services (Type 2D) could be handled along with other types of calls on a Type 2A with SS7 interface. The details of MF signaling for PCN can be found in [SI. We will show how signaling messages a r e deliveredwith the configurationsin Fig. 3. Basically, SS7 message routing is performed at the MTP and the SCCP of a node (SSP, STP, or SCP). At the MTP level, the signaling messages are delivered with the actual destination address. The MTP level receives messages from an adjacent node (SSP, STP, or SCP) or from the TCAP (and thus the SCCP) layer or the ISUP layer of the same node. The Destination Point Code (DPC) of the message uniquely identifies the destination node. Routing to the destination node is determined by the MTP using look-up tables. In the mobile application, every handset is assigned a Mobile Identification Number (MIN).l When an MIN is dialed, the originating node may not have enough knowledge to identify the actual address of the destination. In this case, the actual destinationaddress is translated by a technique called Global Title Translation (GTT) performed at the SCCP level of the protocol. GTT is supported in

I A mobile user may also be assigned a Universal Personal Telecommunication (UPT) number. The relationship between the UPT number and the MIN is illustrated in 1221. This article assumes that a mobile user is identified by the MIN.

In IS-41.C, 28 new operations are expected to be included. In IS-41.C, another Package Type called ConversationWithoutPermission will be

introduced. Same number of the Cvmponent Types are expected in IS-41 Revision C.

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wenty six TCAP operations2 are defined in IS-41.B [ 131 for three purposes: Inter-MSC Handoff [15], Automatic Roaming [12], and Operations, Administration, and Maintenance [16]. The details of handoff and automatic roaming (mobility management) alternatives can be found in [21-271. A TCAP message consists of two portions: the Transaction Portion and the Component Portion. The Transaction Portion specifies the Package Type. Four of the seven Package Types [5, 131 are defined in IS-41.5-B.3 ~YwI~PEREIIssIoNNnitiates aTCAP transaction and informs the terminating node that it may end the TCAP transaction. RESPONSEends the TCAP transaction. CON~E;~SATION~I~~ERMISSION continues a TCAP transaction and informs the destination node that it may end the TCAP transaction. UNIDIRECTIONAL sends information in one direction only with no reply expected. The Component Portion specifies the number and the types of components (operations) to be performed. Four of the six Component Types are defined in IS-41 Revision B.4 INVOKE (Last)is used to invoke an operation (such as location registration). “Last”indicates that the operation is the last component in the Component Part. RE”RESULT ( u st ) is used to retum the results of an invoked operation. If a node receives an INVOKE and the operation is executed successfully, the node shall respond with a RETURN RESULT. RETURN ERROR is

used to report the unsuccessful completion of an invoked operation. For example, the MIN in the INVOKE message is not currently serviced by the HLR. REJECT is used to report the receipt and rejection of an incorrect Package or Component (e.g., ill-formatted). When a node receives a REJECT message, it stops its timer, exits the current task, and performs error recovery. Note that an SS7 transaction alwavs begins with a query message (QueryWithPermission in IS-41), and ends with a response message (Responsein IS-41). In IS-41,both the INVOKE and the RETURN RESULT types are “Last”which imply that every IS-41 TCAP message performs exactly one operation. In an IS-41 implementation, we may expect that operations such as authentication and registration notificationare combined into a TCAPmessage. In the remainder of this article, we omit the notation “Last.”All IS-41.B transactions are two-message (a query and a response) transactions except for two cases. *A TCAP message with the operation FlashRe