Wireless Signaling and Intelligent Networking

Transcription

1 3 Wireless Signaling and Intelligent Networking The first two chapters provided an introduction to the history of mobile communications, its evolution, and the wireless industry standards process. With this background in mind, the focus of this chapter will be to provide an introduction to the protocol standard and industry reference models that enable wireless intelligent networking. Specifically, we will cover in some detail the Signaling System 7 (SS7) protocol and the implementation of this protocol in a wireless environment to support mobility management and wireless intelligent networking. SS7 is an out-of-band signaling protocol that provides the foundation for numerous advanced telecommunications service offerings, both wireless and landline. Out-of-band signaling is information about the call sent separately from the actual communication (bearer) channel. The signaling channel can be used to deliver far richer information about a call, thereby enabling a far richer set of features. Common examples of features that are enabled by SS7 include incoming caller identification features (Caller ID), Custom Local Area Signaling Services (CLASS) such as automatic callback, wireless seamless roaming via automatic call delivery, and as will be discussed in much greater detail later in this book, wireless intelligent network services such as network-based prepaid and short messaging services. We will begin with an overview of the purpose and history of SS7 and move into a description of the physical SS7 network and the functions of the various network elements. We will then introduce the SS7 protocol stack and its relation to the Open Systems Interconnection (OSI) reference model, and 49

2 50 Wireless Intelligent Networking finally we will describe the additional protocols used within SS7, which support the communication between SS7 network nodes. Our discussion of the SS7 protocol to this point will include example implementations from both landline and wireless networks. Since landline networks deployed SS7 before wireless networks, there are more ubiquitous applications to use as reference when explaining the SS7 protocol and its implementation in public networks. Furthermore, given the trends of convergence and potential wireless displacement of wireline (as will be discussed in Chapter 8), coverage of wireline applications is important. Having covered SS7 in detail, the rest of the chapter will focus on the role of signaling in wireless networking and mobile communications, where we will cover the wireless network reference models, intelligent networking concepts, and call modeling. This chapter will provide an introduction to signaling protocols and concepts, but one chapter is not sufficient to develop a thorough understanding. The reader is encouraged to study the standards, industry specifications, and sources referenced throughout to delve deeper into any one topic. 3.1 Overview of SS7 Network Signaling Signaling System 7 is a series of network signaling protocol standards that were developed by the International Telecommunications Union to support the increasingly complex worldwide telecommunications network. A variant of this protocol was standardized in North America by ANSI, and it is referred to simply as SS7. As telecommunications networks became more sophisticated, they required more efficient operations and network management to address the increased call volumes and the number of network interconnections. Furthermore, consumer demands and the advent of competition created a need to introduce advanced services more rapidly. These requirements necessitated an improved network signaling system architecture What Is Signaling? The term signaling in this context simply means the process of sending control information between network elements. The signaling protocol defines the structure of how this information must be communicated and what the network elements should do with this information. A good example of net-

3 Wireless Signaling and Intelligent Networking 51 Access Signaling Access Signaling Network Signaling Calling Party Switch Switch Voice Trunks Called Party All Signaling takes place on the same facility as the voice path Figure 3.1 Network signaling. work signaling is the process of two switches establishing a trunk between them to route a voice call. The first switch must inform the second switch that it has a call that it needs to deliver to the second switch. The second switch must inform the first switch that it can support the call, and they must agree on which facility they will use to deliver the call. There must also be a defined process to tear down the trunk between the two switches when the call is complete. Before the introduction of SS7, this signaling was performed within the same facilities in which the voice call would be routed. Thus a trunk between the switches was seized by the first switch to inform the second switch that a call must be set up. The signaling information was passed using the same frequency band as the voice call. This type of signaling is called inband signaling, depicted in its simplest form in Figure 3.1. This information was usually conveyed using multifrequency (MF) tones. This signaling is similar to the dual-tone multifrequency (DTMF) tones used when a landline telephone subscriber goes off-hook and dials a telephone number to make a phone call. The main difference here is that the subscriber is signaling to the network to place a call, referred to as access signaling, whereas two network elements communicating is termed network signaling Common Channel Signaling When the signaling information related to voice or data traffic is communicated via a separate network, it is referred to as common channel signaling (CCS). The first implementation of common channel signaling in the United States occurred in the 1960s. That system was called Common Channel Interoffice Signaling System #6 (CCIS#6), and its primary application was for establishing and tearing down interoffice toll trunks. SS7 is

4 52 Wireless Intelligent Networking Common Channel Signaling Network Signaling Links Switch Switch Voice Trunks Calling Party Called Party Network Signaling uses separate network from voice path Figure 3.2 Signaling with common channel signaling. derived from SS6, but it is a much more robust protocol, addressing key capabilities such as signaling to databases for applications like alternate billing number validation or intelligent call routing. Figure 3.2 depicts the network signaling scenario discussed earlier using common channel signaling. In addition to supporting call set up and tear down, SS7 supports communication between network switches and intelligent network databases. Intelligent network databases contain information like call routing data or subscriber-specific data. The network switches suspend call processing to retrieve information from one of these databases in order to process the call correctly. In fact, the first implementations of SS7 in landline networks in the late 1980s supported database access rather than call set up and tear down. The main driver for implementing SS7 in wireless networks in the early 1990s was to support communication between databases of the visited market and the home market to support roaming and prevent unauthorized or fraudulent use of the wireless network Signaling Services The following applications are examples of SS7-based services that have been introduced over the past ten years. Line Information Database (LIDB). Since 1991, the LIDB database has been used to validate telephone numbers and calling card PINs

5 Wireless Signaling and Intelligent Networking 53 for alternately billed telephone calls. For example, operator-assisted calls that require billing to a third-party telephone number must verify that the third-party telephone number is valid and will accept such charges. The operator center must launch a query to an LIDB database to get this information. In 2000, 11 LIDB databases existed, one for each of the original baby Bell local exchange companies, plus LIDBs operated by Sprint/United, GTE, Southern New England Telephone, and Illuminet. 800 Database. The FCC mandated that 800 numbers must be portable by portability means that a 10 digit 800 number could belong to any long distance carrier, so a business could change the underlying carrier used to route calls to its 800 number without having to change the number. Previous to 800 portability, the local exchange access tandem switches determined the carrier identification by examining the second three digits, or NPA, of the dialed number, which were assigned to carriers. With portability, these switches could no longer determine who the carrier was for a given NPA, so they were required to suspend call processing and launch an 800 query message to obtain the carrier ID associated with a particular 800 number. Trunk Signaling (call set up and tear down). SS7 was used to establish and tear down calls between switches using a protocol called Integrated Services Digital Network (ISDN) User Part, or ISUP. Landline networks implemented this capability by the early 1990s. Wireless carriers began implementation of ISUP in the mid to late 1990s. ISUP will be covered in greater detail later in this chapter. Caller ID. Caller ID is a feature that allows a subscriber to display the number of an incoming caller. This service is very popular in the United States, with local exchange companies charging customers as much as $8 per month for the service. Caller ID helped stimulate conversion to SS7 ISUP signaling in the United States. Penetration rates in some areas exceeded 50% by the late 1990s. The calling party identification information is contained in a field of the SS7 message used to establish call set up. The terminating switch requires software to retrieve the calling party number and deliver it to the subscriber s terminal. The ability to deliver the calling party number throughout the network enabled a number of other services that could be based on the identity of the incoming caller.

6 54 Wireless Intelligent Networking Calling Name Delivery (CNAM). This service is an extension of Caller ID with deployment beginning in the United States around The terminating switch launches an SS7 message to a database to retrieve the name associated with the incoming calling number. Wireless implementations of Calling Name Services require additional capabilities, which will be covered in much greater detail in Chapter 8. Seamless Roaming. In the early 1990s, wireless service providers needed to implement a more secure system that would support intersystem roaming. SS7 was the recommended transport protocol for intersystem messaging required to support services such as precall validation, automatic roaming, and call delivery. This service used SS7 and protocols such as ANSI-41 and GSM-MAP to allow visiting and home market networks to exchange information such as subscriber profiles. These protocols will be discussed in greater detail later in this chapter. Local Number Portability (LNP). To stimulate local service provider competition in the United States, the FCC mandated that telephone numbers in the top 100 Metropolitan Service Areas (MSAs) be portable starting in October Portability here means subscribers can change local service providers, but retain their telephone numbers. Wireless Number Portability, the ability to change service providers but retain a wireless number is not mandated until However, wireless service providers that deliver calls to landline exchanges with ported numbers were required to support call delivery to these ported numbers, hence many wireless service providers implemented the first phase of portability by LNP is heavily dependent on SS7, as switches must query a database to determine which service provider serves a particular telephone number. 3.2 Physical SS7 Network The SS7 network is separate from the voice network that it supports. It consists of nodes, or signaling points, that provide specific functions. There are three main types of nodes in a signaling network: the Service Switching Point (SSP), Signal Transfer Point (STP), and the Signal Control Point (SCP). These nodes normally interconnect via point-to-point 56-kbps circuits. Data is switched through the network using packet-switching technology.

7 5 Wireless Intelligent Networking Capabilities This chapter provides an introduction and overview of the standards for mobile network intelligence. While there are many means of providing nonstandard, prestandard, or proprietary-based intelligent network controls and call processing, this chapter will focus on methods that provide standardized intelligence. Remaining true to the focus of this book, we will not go into great detail regarding the standards themselves. Instead, we provide an overview of the main concepts and capabilities. We recommend that the reader refer Appendix A for information regarding the design concepts behind these standards. We also recommend the reader review the actual standards themselves for a more in-depth knowledge and for actual network planning and/ or application design work. 5.1 Intelligence in Telecommunications Networks Before moving into a discussion of the specific standards for improved wireless network intelligence, we will discuss the landscape and driving forces behind the development of these standards. As we discussed in Chapter 4, intelligent network standards originated on wireline networks and ultimately migrated to mobile networks. It is important to note that this head start has enabled fixed networks to capture a lead in terms of network intelligence exploitation. However, the unique advantages of wireless communications have stimulated evolution in mobile network standards that will ultimately lead to even more robust capabilities for mobile networks. 109

8 110 Wireless Intelligent Networking Fixed Network Intelligence In fixed networks, it is easier to apply network control and service logic as the position of the user is static. The wireline intelligent network standards, Advanced Intelligent Network (AIN) and Intelligent Network Application Part (INAP), serve fixed networks well, but the are inadequate for mobile networks, where service mobility is a fundamental requirement. Mobile industry practitioners realized that AIN and INAP would require modifications to handle essential mobile network requirements such as roaming, or new intelligent network standards would be required Mobile Network Intelligence As we discussed in Chapter 3, the two primary means of intersystem communication for mobile networks are ANSI-41 and GSM MAP. These standards serve the industry well in terms of basic mobile operations such as support of roaming operations and seamless support of most basic services. However, these standards do not by themselves provide capabilities necessary for many advanced features found on the wireline networks such as calling name delivery and FreePhone (toll-free service). Advanced capabilities were needed to support mobile networks. However, mobile network operators did not want to give up roaming. Somehow, roaming would have to be preserved while adding new service capabilities. Modifying existing wireline standards would therefore not suffice. Rather than attempting to modify AIN and INAP, it was decided to create new standards for mobility that could easily interoperate with ANSI-41 and GSM MAP. The answer was Wireless Intelligent Network (WIN) and Customized Applications for Mobile Enhanced Logic (CAMEL) Drivers for Improved Mobile Network Intelligence The need for improved network intelligence lead to WIN and CAMEL. These standards, however, were driven by the need to separate service intelligence from the call switching functions, which allows the switching or call processing to be maintained at a lower level and the intelligence at a higher level. This architecture allows mobile communications infrastructure vendors the flexibility to develop new and customized services that can be quickly deployed by service provider on a network-wide basis. This also enables the migration from point solutions to network-based solutions discussed in Chapter 4.

9 Wireless Intelligent Networking Capabilities 111 While the initial drivers for the standards that became WIN and CAMEL were improved service capabilities, it is important to note that the driver for improved mobile network intelligence was standardized capabilities, not services. These standardized capabilities could be used for a variety of difference services and features. The goal of improved network intelligence was therefore not simply emulation of wireline services but creation of capabilities to support the unique needs of mobile communications. As we will discuss in Chapter 8, these capabilities would eventually transcend AIN and INAP to enable services unique to mobile communications. 5.2 Standardized Intelligence for Mobile Networks: WIN and CAMEL Perhaps the most important aspect of WIN and CAMEL is that they are optimized for mobility. The unique nature of mobile communications is such that a mobile user can be at most any place at most any time. To provide services and features ubiquitously, seamlessly, and consistently to the mobile user, there is a need for capabilities to support standardized methods for service identification, invocation, processing, and delivery. These are the requirements of standardized intelligence for mobile networks and the foundation of WIN and CAMEL Enabling Architecture and Standardized Capabilities As discussed in Appendix A, WIN and CAMEL are both based on the Intelligent Network Conceptual Model (INCM). The INCM represents an architectural framework and certain capabilities, not services. Similarly, WIN and CAMEL call models represent high-level models of call control functionality that define capabilities but not services. The call model makes information concerning the call state and associated data visible to external intelligent network elements such as the SCP and HLR so they can use their logic to process the call. Because the service logic and call switching functionality are separated, external intelligent network elements can control services Phased Development of Standards WIN and CAMEL are both developing in phases, with each phase building upon the capabilities developed in the previous phase. As WIN standards are

10 112 Wireless Intelligent Networking conceived, they are assigned a project number (such as PN for prepaid charging). Once a WIN standard is adopted by the TIA, it becomes an interim standard (such as IS-771 for WIN phase I). When the industry adopts the capabilities as a standard, the interim standard shall become part of ANSI-41. For example, IS-771 is targeted to become part of ANSI-41 version E (ANSI-41E). CAMEL, however, relates differently to GSM MAP. While related to and supports GSM MAP, CAMEL is an independent standard, never joining GSM MAP as WIN joins ANSI Wireless Intelligent Network The purpose of WIN is to provide seamless terminal services, personal mobility services, and advanced network services in the ANSI-41 mobile environment. Terminal services are those services that are associated with and dependent on the capabilities of the terminal and subscription selections irrespective of the terminal user. Personal mobility services are those services that meet the needs of the user irrespective of the terminal or network. The goal of advanced network services is to identify the capabilities of the serving network, provide services based on the terminal and network abilities, and provide seamless service between wireline and wireless networks. These three goals relate to the desire for network capabilities that provide seamless service across networks while allowing flexibility in service delivery options. It is useful to point out that WIN (see Figure 5.1) builds on the network reference modeled discussed in Chapter 3 for ANSI-41. Note that in Figure 5.1 the WIN architecture introduces new network elements that were not defined by ANSI-41 alone. Perhaps most notable is the introduction of intelligent network nodes such as the Service Control Point (SCP), Service Node (SN), and Intelligent Peripheral (IP). Introduction of these nodes includes new interfaces for communication between them and other nodes. Figure 5.2 depicts how WIN functional elements [1] (FE) map to these network elements (NE). We will illustrate the use of these network elements as we discuss various service message flow diagrams throughout this chapter. 1. PN-4287 is now known as IS-826.