Setting the Context for Evolution and Convergence of Networks

Transcription

1 Chapter 1 Setting the Context for Evolution and Convergence of Networks What do network, evolution and convergence signify in the title of this chapter? Network has one of its common meanings, namely, the total set of facilities that an operator needs to provide a telecommunications or information service. Facilities are the hardware and software elements and systems that are necessary to provide a service. A service at its most general denotes an offering by a provider to customers or end-users. A service may also be offered by one set of facilities to another. Evolution captures the continual development of enabling technologies and service offerings that compete for acceptance in the market. As in biological evolution, some technologies are successful because they are adapted to the prevailing conditions while other are not and die out. Similarly, evolution indicates incremental development building on existing successes. Convergence identifies a general pattern in the evolutionary process, namely the tendency to bring entities together, for example the coming together of classical telecommunications, the Internet, information technology and broadcasting, the ability to offer multiple services on a single network or the ability to offer the same service via more than one medium. The starting point of our analysis of evolution and convergence of networks and services is an appreciation of the status of the networks of today, together with the services they offer. Section 1.1 sketches the development of telecommunications both circuit and packet switched and the Internet since the 1970s, leading to current networks and services. Section 1.2 identifies and describes six principal present-day networks with distinctive service offerings: switched circuit networks, both fixed and mobile; the Internet; enterprise networks; packet-switched interconnection networks and leased line services. The capabilities and evolutionary limitations of present networks are analysed. Section 1.3 then introduces convergence as a theme in seeking to overcome the limitations of present networks and to facilitate new service offerings. Specific instances of convergence are identified from various fields: fixed and mobile networks; telecommunications and the Internet; telecom, information, entertainment and broadcasting services, applications and business models. Convergence is not however a simple phenomenon, but spans technologies, networks, services and business models. A number of general characteristics of convergence are identified. Convergence is a process rather than an event and we need a concept to describe its desired end point. To this end we introduce the next generation network (NGN) in COPYRIGHTED MATERIAL Network Convergence: Services, Applications, Transport, and Operations Support Hu Hanrahan c 2007 John Wiley & Sons, Ltd

2 2 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE Environment Monopolistic Deregulation Competition Technology Packet Switching Circuit Switching Transmission Mobile Networks Value-added Services VLSI and Microprocessor ARPANET Electromechanical Switching PDH on Radio and Cable X.25 TCP /IP Digital Switching and Control Optic Fibre Moore's Law and Software Engineering ATM Frame Relay ISDN SDH /Sonet First generation Mobile Supplementary Services IP Dominance Dense Wavedivision Multiplex Second Generation Mobile IN-based Valueadded Services Figure 1.1. Historical developments leading to present state of ICT. Section 1.4 to represent possible integrated networks that support multiple service offerings. There are many possible NGNs and most must interwork with legacy networks. We therefore use the ITU-T concept of the NGN as embracing the collective improvements to the service provision infrastructures from the base of traditional networks. 1.1 HISTORICAL BACKGROUND TO PRESENT NETWORKS Telecommunications has a long history. The computer age is also half a century old and has now become the Internet age. While their development was largely independent at first, computing and communications have become inextricably bound and mutually dependent. We seek a unified understanding of convergent information and telecommunications services and the underlying network and software technologies. To visualize the future, we need to understand the historical development leading to present day and emerging technologies. Figure 1.1 identifies several historical threads leading to the present networks. A useful starting point is the mid- 1970s. By that stage, large-scale integrated circuits and the microprocessor allowed a change from analogue to digital transmission and switching of information. The decade saw several important developments. The already established principles of packet switching led to the development of the X.25 standards. By the end of this period, TCP/IP had been adopted as the basis of the ARPANET, the predecessor of the Internet. In parallel, digital telephone switching developed in two ways: first the incorporation of processor-based control and, second, digital encoding of speech as the basis of switching. The concept of a single network providing both voice and data services, the first multiservice network, was developed into the narrowband Integrated Services Digital Network (N-ISDN) standards. In the transmission area, the first optic fibre cable was deployed. First generation (analogue) mobile networks started operation.

3 1.1. HISTORICAL BACKGROUND TO PRESENT NETWORKS 3 The next decade, , saw increasingly important developments, both technological and regulatory. The break-up of the Bell System was the first step in a worldwide trend toward deregulation and competition in telecommunications. The decade started with one thousand hosts on the Internet. Digital telephone switching penetrated into public and private networks. Packet switching standards expanded to include Asynchronous Transfer Mode (ATM) and Frame Relay. The World Wide Web was launched and the number of hosts on the Internet grew to one million by the end of the period. The GSM second generation mobile networks were standardised and successfully launched. In the PSTN world, the concept of the Intelligent Network was formulated as a means of implementing value-added services in the PSTN and the first standards were developed. In the transmission arena, the Synchronous Digital Hierarchy gave network operators the opportunity to provide readily configured and managed transmission services, both for their own needs and to customers requiring point to point connections. The recent decade, , saw the launch of commercial Internet service providers in 1995, taking over from government agencies. Web usage overtook other types of Internet services in volume of data transferred. The IN standards developed through two capability sets and became the basis for value-added services in the PSTN. Standards for telephony using Internet Protocol (IP) networks were developed and the concept of a new multiservice network was formulated as the next generation network. The first third generation (3G) mobile network licences were issued but deployment was limited by excessive licence fees and economic downturn. This decade also saw the dot.com boom with unlimited optimism about new Internetbased services; this optimism was soon followed by the March 2000 crash. Optic fibre transmission capacity increased due to both higher speeds of transmission and the use of multiple wavelengths on a single fibre. The growth of the Internet to over 100 million hosts called for rapid increases in core transmission capacity, yet many optic fibres operated at a fraction of their capacity. Internet technologies, for example the use of IP networks and browser-based applications, became the way of delivering IT applications and corporate communications. Interworking between circuit-switched and packet networks was enabled by the development of media and signalling gateways. This book attempts to create an understanding of the present and emerging network technology as well as its future trajectory. What does the decade hold? Several trends are already evident. The switched circuit network is likely to start its decline in the volume of traffic carried during the period. Packet switching for various classes of traffic in a single network will take over. The distinction between telecommunications and the Internet will be increasingly blurred. Telecommunication networks are likely to become open to control of services by applications in other service provider domains. New business models will emerge. The variety of terminals and access methods will increase and the distinction between fixed and mobile networks will become less meaningful. As these next generation networks and services develop, the need to interwork with legacy networks remains important. The state of telephone and data networks and the Internet at the end of the decade , described in Section 1.2, is the point of departure of this book.

4 4 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE 1.2 DEFINING PRESENT STATE USING REFERENCE MODELS This book is about a range of future network architectures, collectively called next generation networks and shaped by the process of convergence. We identify a number of present day networks as well as their characteristics and distinctive services that define the point of departure for considering convergence. 1. Switched circuit networks (SCN) encompass fixed-line and mobile telephone networks. The fixed line public switched telecommunications network (PSTN) and the Integrated Services Digital Network (ISDN) provide basic voice bearer services and value-added services implemented using the Intelligent Network (IN) overlay. With suitable terminals, ISDN also supports video conferencing. Despite providing digital connectivity to the customer premises, the ISDN has made little impact on data services apart from dial-up Internet access. The provider is a telecommunications company (telco). 2. Closely related to fixed networks in (1) are second generation (2G) mobile telephone networks. Such networks provide voice and data services to mobile users, together with messaging (SMS and MMS). Mobile networks differ from fixed networks in two main aspects: first, the access method is radio, and second, mobility management is required to keep track of mobile phones. Value-added services are also implemented using the IN. Mobile networks are multiservice networks, supporting voice and various data services through a single air interface. 3. The Internet, a worldwide arbitrary interconnection of autonomous networks unified by the IP, supports Internet services: World Wide Web, , file transfer, transactional services and, increasingly, peer-to-peer services such as file sharing. A user of Internet services is reliant on both an Internet access provider (IAP) for the physical means of connecting to the Internet and an Internet service provider (ISP) for logical access to the Internet, that is the ability to address other parties and be addressed. The ISP and IAP roles may be common or separate. Internet content providers are generally independent of ISPs. 4. Enterprise networking, using both telecommunications and data networking technology to create private networks, is devoted to supporting the information and communication requirements of corporations and institutions. Telcos or ISPs provide interconnection between sites and they and other providers may provide completely managed enterprise networks. 5. Telecommunications companies own data networking facilities to provide switched interconnection services to support activities such as private and virtual private networking and Internet service provision. Both layer 2 and 3 connectivity is provided, for example Frame Relay, X.25 and, in some cases ATM. Increasingly, IP switched interconnections are offered, usually with virtual private network (VPN) support.

5 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 5 Content Content Provider Content Broadcaster Content Application Service Switching Transmission Fixed Network IN Value-adding TDM Circuit Switching SDH, Optical Microwave Mobile Network IN-CAMEL Value-adding TDM Packet SDH, Optical Microwave Telco Data Managed Network FR IP ATM SDH, Optical Microwave Application AAA, DHCP, DNS IP ISP IAP RAS Application Signal Distribution Satellite, Optical Microwave Access Copper Loop DSL Base-station Subsystem Dial-up, ADSL, LAN Transmitter Stations Terminal Plain phone Mobile phone Computer Radio/TV Receivers Figure 1.2. Vertically integrated traditional telecommunications and broadcasting businesses. 6. Leased-line services provide semi-permanent, nonswitched connections between client sites at specified bit rates in standard multiplexing hierarchies. Telcos are the usual leased line providers. A seventh category, broadcasting, will become increasingly important as convergence proceeds. Section examines some current relationships between broadcasting and telecommunications SILO MODEL FOR VERTICALLY INTEGRATED NETWORKS Historically, different telecommunications services, the Internet and broadcasting have been vertically integrated both as businesses and sets of facilities. 1 The vertically integrated nature of these businesses is often depicted by a silo or stovepipe metaphor, as shown in Figure 1.2. Three present telecommunications businesses, fixed telephony, mobile telephony and switched interconnection services, theinternetand broadcasting, are shown. Within each silo, the required facilities are layered to identify commonalities. A set of layers that provide a backdrop for various networks is shown in Figure 1.2. Section reviews fixed and mobile switched-circuit networks. Commonalities between fixed and mobile telephone networks occur in the transmission, switching and service layers in Figure 1.2 and are examined in Section Different technologies are used in the terminal and access layers. 1 We use facilities to denote hardware and software required to provide services. Infrastructure is a special case of facilities that exist in areas other than the customer s or provider s premises.

6 6 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE Telco-switched interconnection services form a separate silo, marked Telco Data, also relying on common transmission facilities. Internet service provision falls into three domains. Physical access to the Internet is usually provided by a telco that acts as the Internet access provider. Internet service provision involves service and switching layer concerns. The ISP provides admission control, allocation of network addresses and a connection to the IP network. Content provision on the Internet, often linked to an application, is not necessarily linked to the ISP. Figure 1.2 shows that the transmission layer exhibits the greatest degree of commonality as all networks require high-volume short and long distance transmission. Historically, broadcasting was vertically integrated. Traditional broadcasting divides naturally into two areas, now operating as different businesses in many cases, as shown in Figure 1.2: 1. Programme and content assembly and playout of the broadcast signal into the signal distribution system. This is the broadcasting function. 2. Signal distribution, comprising the distribution of signals to transmitter stations and the transmitters, provides radio coverage of the reception area(s), analogous to the core and access regions of a telecommunications network. Contact between telecommunications and broadcasting was limited to leasing transmission links from telcos to support signal distribution. Broadcasting, for most of its history, has been a one-to-many, one-way service but is progressively becoming two-way. Initially, broadcasters used phones and the Short Message Service to allow the audience to respond and react to programme material. Interactive services became possible over broadcast networks when the downlink acquired a data transmission capability that could be complemented by an uplink, for example, by telephone. With digital broadcasting, the availability of a return channel enables interactive services. Early incarnations of such services are essentially Internet-type services, often related to the broadcaster s business. Broadcasting is also practised over a wired network, for example cable television networks and by audio streaming on the Internet. Cable TV provides a physical access network that is readily adapted to support two-way telephony [155] PRESENT STATE: FIXED AND MOBILE NETWORKS WITH IN OVERLAY Switched circuit networks are vertically integrated with fixed and mobile networks constrained to their respective silos by historic regulatory practices, despite having a degree of commonality of technology. Figure 1.3 shows the main architectural features of two circuit-switched networks. We use this reference model for current fixed and mobile networks to distinguish the two networks with respect to terminal capability, access network and mobility management. The model also shows commonality in voiceband switching, call control, signalling network and Intelligent Network overlay. PSTN users are shown connected via copper pairs with two types of interfaces in the subscriber line concentrator. Subscriber line concentrators may be remote from the exchange or co-located. Analogue subscriber loops terminate on an interface

7 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 7 Customer Premises Access Network Edge Switching Core Gateway SCP SDP NT ISDN SS7 BORSCHT End Exchange Transit PBX SRP VLR SCP SDP HLR BTS BSC BTS MSC ATM Core G-MSC Figure 1.3. Reference network architecture for switched circuit networks. that performs the BORSCHT functions. ISDN subscribers are connected via digital subscriber loops [100]. Both interfaces deliver time-division multiplexed signals to the end exchange. The end exchange is the network edge element providing the network service access point to the user. Legacy PSTNs have core switching also based on exchanges switching time-division multiplex (TDM)signals. All exchanges use the Signalling System No. 7 network for inter-exchange signalling. In the core, PSTN and ISDN services are supported by the same switches and transmission systems. The mobile network differs from the fixed line network principally in the cellular wireless access network but also in its ability to track mobile users. Mobility management is achieved by three principal means. First, the mobile station assists the network by determining the strongest signals that it receives from time to time and reporting these to the network. Second, using signal strength information, the mobile can be handed over between frequencies on a base station, between base stations or between Mobile-system Switching Centres (MSC). Third, the mobile network has two databases that support tracking the location of the mobile station. The Home Location Register (HLR) is normally associated with the Gateway MSC, that is the MSC that provides interconnection with other networks. A Visitor Location Register (VLR) is associated with every MSC. The HLR contains the permanent subscriber data (subscriber profile) as well as the identity of the VLR to which the mobile station is currently logged. The VLR contains data that supports the management of the subscribers currently in the serving area of the visited MSC. A common practice in mobile networks shown in Figure 1.3 is to use packet-mode core networks rather than TDM transit switches as in the legacy PSTN. The TDM trunks are adapted to the ATM network using either ATM Adaptation Layer 1 (AAL1)

8 8 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE Mobility Management MM MAP INAP ISUP SSF CCF Service Switching Call Control SSF CCF INAP ISUP TDM Trunks RC Resource Control Switching Matrix RC TDM Trunks PSTN Switch Mobile-system Switching Centre Figure 1.4. Comparison of the functionality of fixed line and mobile telephone network switches. performing circuit emulation or AAL2 in which 64 kbit/s speech is compressed and packetised before being adapted for transport in ATM cells. Interconnection of fixed and mobile telephony networks occurs between a transit exchange in the fixed network and a designated MSC, the Gateway MSC (G-MSC) in the mobile network. The Gateway MSC is associated with the Home Location Register and therefore has access to the subscriber profile data and temporary location information. Speech signals are transferred in 64 kbit/s time slots on a TDM link. Signalling interchange between the G-MSC and the transit exchange uses the ISDN User Part ISUP call control messages. Each network in Figure 1.3 is shown with an Intelligent Network overlay, consisting of a Service Control Point (SCP), a supporting database, the Service Data Point (SDP) and a Specialised Resource Peripheral (SRP) that allows announcements to be played to users and dialled digits to be collected. The principles underlying the Intelligent Network are reviewed in Section Most principles are common for fixed and mobile networks, except for the latter requiring location and mobile system user information to be available. Switch Functionality in Fixed and Mobile Networks Figure 1.4 identifies the essential functions in exchanges for fixed and mobile networks based on circuit switching, for example those shown in Figure 1.3 [91]. The heart of each switch is the actual switching matrix that operates on the timedivision multiplex signals. Speech signals are encoded as 64 kbit/s streams and each channel is allocated a time slot on an incoming line to the switch. The switching operation involves reallocating the data bytes representing this speech signal to an allocated (and possibly different) time slot on the desired output line from the switch. The Call Control Function (CCF) is concerned with making the end-to-end connection. Connections are set up one hop at a time. Each exchange has a first choice and alternate route toward every destination end exchange. An exchange on the path between source and destination signals to its adjacent exchange to agree on a time slot to be allocated. The first choice route is tried first and the alternate route is used if no time slot is available on the first choice. A CCF signals to the CCF of

9 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 9 the neighbouring exchange using the ISUP protocol. The Call Control Function must exhibit the external behaviour specified in the ISUP standard. Control of the switching operation in a particular exchange is essentially the allocation of a time slot on an outgoing trunk. An essential function in the exchange is therefore resource control (RC), the resource being the set of available time slots on each link or trunk group to adjacent exchanges. A standard Circuit Identification Code (CIC) is used in ISUP messages to identify the resource used but, in general, implementations of resource control are proprietary. Closely linked to the CCF is the Service Switching Function (SSF). The SSF was introduced in the development of the Intelligent Network architecture as a standard way of allowing the CCF process to invoke the assistance of service logic on an external computing platform called the Service Control Point. The SSF is based essentially on the definition of a standardised set of points, called Detection Points, in the execution of the CCF logic. Examples of such points are AddressCollected, thatis the user has dialled a number that is judged to be complete, and AddressAnalysed, when the CCF logic has completed its analysis of the dialled number to determine whether it represents a routable destination or requires special treatment. At each Detection Point, the CCF reports to the SSF. The SSF tests whether the detection point is enabled to invoke external logic (armed) and whether call-specific conditions are fulfilled that external logic should be invoked. If such conditions are fulfilled, the CCF process waits while the SSF engages in a transaction with the SCF using the Intelligent Network Application Protocol (INAP). A request, usually an INAP InitialDp operation, is sent to the SCF to invoke external logic. The external logic may invoke queries on a database called the Service Data Pointor instruct the Specialised Resource Peripheral to prompt the user and collect information. To complete execution of the external logic, the SCP logic sends one or more operations to the SSF to instruct the CCF to resume execution and how to proceed with call processing. The Mobile-system Switching Centre has an additional function, namely mobility management. This distributed function interacts with the HLR and VLR and receives information from the Base Station Subsystem. Mobility-related operations are invoked using the Mobile Applications Part (MAP) protocol in the core network. Signalling System No. 7 Switched circuit networks are robust, high-availability networks. Robustness and availability are due largely to the use of the Signalling System No. 7 (SS7) as a common channel signalling system that enables switches and nonswitching nodes such as Service Control Points to exchange service control information. Signalling System No. 7 is defined as an architecture and a number of protocols [158]. The SS7 architecture has two types of node. The Signalling Point (SP) is the point of access to the SS7 network for the user, for example a switch, SCP, HLR or VLR. The Signal Transfer Point (STP) is a high-performance packet switching node. Nodes are connected by bundles of links, allowing load sharing and a means of dealing with link failure. The principles underlying the architecture of a SS7 system are usually depicted by the diagram in Figure 1.5(a). Two interconnection patterns give the network the ability to deal with link or node failure without degrading its performance. The quad

10 10 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE A A B B C C B B A A Group of Switches C C C B B (a) = SP = STP SCP 1 (b) SCP 2 Figure 1.5. (a) Quad structure in Signalling System No. 7 architecture. (b) Typical architecture of an SS7 network. structure consists of two pairs of STPs connected by four bridge or B-links. Each Signalling Point is connected to a pair of STPs in a quad via two access or A-links. In addition, a cross or C-link joins the two STPs forming a mated pair. An actual SS7 network generally has more than four STPs and supports hundreds or thousands of SPs. Multiple quads must therefore exist. The number of switches, and hence SPs, is often at least an order of magnitude greater than the number of STPs. The number of SCPs in a network is generally small but each SCP requires significant protection from access link or STP node failure as well as failure of its own interfaces. Figure 1.5(b) shows a configuration that addresses these requirements. Groups of switches in a geographical area are connected by A-links to the STPs of a mated pair serving that area. Service Control Points are usually constructed as highavailability computers with duplication of functions. Duplication may include the use of access links to different mated pairs of STPs. Signalling System No. 7 supports two classes of protocol [189]. The first is concerned with setting up connections between exchanges. In most cases today, the connection-oriented application layer protocol supporting inter-ccf signalling is ISUP. The second class of protocol is transaction-oriented and is geared to supporting large volumes of database queries or remote operation invocations. Examples of transaction oriented application protocols are INAP and MAP used in mobile networks to query the HLR and VLR. A special application sub-layer, the Transactions Capability Application Part (TCAP), supports transaction-oriented application protocols. TCAP allows multiple applications to have multiple concurrent transactions in progress at any time. TCAP also allows related transactions to be linked. Both classes of protocol are supported by a robust, high performance protocol stack at OSI-RM layers 1, 2 and 3 called the Message Transfer Part (MTP). The data link layer, MTP Layer 2 (MTP-2), contains protection against frame loss and mis-sequencing, freeing other layers from the need to perform error recovery procedures. The network layer, MTP Layer 3 (MTP-3), is connectionless and uses absolute addresses for Signalling Points called Point Codes that are unique to the network. MTP Layer 3 network functions are supplemented by the Signal Connection and Control Part (SCCP). While SCCP provides connection-oriented services, these are

11 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 11 ISUP CAP MAP TCAP (a) SCCP MTP -3 SCCP MTP -3 (b) MTP -2 MTP -2 MTP -1 MTP -1 Figure 1.6. (a) Circuit-oriented protocol stack in Signalling System No. 7. (b) Transaction-oriented protocol stack in Signalling System No. 7. seldom used. Two valuable enhanced addressing modes are provided in SCCP. The first addresses the MTP s limited capacity for identifying upper layer users at each SP. The SCCP sublayer therefore provides a one-byte Subsystem Number allowing a number of upper layer users to be connected to a Signalling Point with a single Point Code. The second allows destination Signalling Points to be addressed by means of an E.164 number, called the Global Title. One or more of the STPs in the SS7 network must be able to translate the Global Title to a Point Code (and Subsystem Number if required). The Global Title is useful for addressing elements such as SCPs, HLRs and VLRs and in routing ISUP messages to control international calls. The protocol stacks for the connection-oriented and transaction-oriented classes of protocols are shown in Figure 1.6. The ISUP protocol is shown making use of MTP-3 directly or SCCP with Global Title addressing. Two examples of application layer protocols, namely the CAMEL Application Part (CAP) (the mobile network version of INAP) and MAP, are shown. The Classical Intelligent Network The term Intelligent Network (IN) refers specifically to a method of providing valueadded services in telephone networks according to the ITU-T s Q series of Recommendations [93, 151]. In view of the emergence of other types of intelligence in networks, the term classical Intelligent Network is used for this architecture. We review the classical Intelligent Network to elucidate its role in fixed and mobile networks and as a baseline for value-added services in next generation networks in later chapters. We have introduced the physical elements that comprise the classical IN, the SCP, SDP and the SRP, into Figure 1.3. These elements are overlaid on the PSTN or 2G mobile network, using Signalling System No. 7 for message transport. The IN standards are complex and use a framework called the Intelligent Network Conceptual Model (INCM) to provide levels of abstraction. The IN standards were originally conceived to support an evolutionary progression of services and underlying network capabilities. The IN standards are therefore based on the concept of capability sets. A capability set represents a level of functionality available in the IN overlay elements as well as in the switching network. The particular level of capability allows services in a stated range of complexity to be implemented.

12 12 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE Service Plane Benchmark Services List of target services supportable by underlying functionality Benchmark Service Features Service building blocks supportable by underlying functionality Global Functional Plane SIB 1 SIB 2... Basic Call Process... SIB n-1 SIB n Distributed Functional Plane SDL SIB Definitions Basic Call State Model Functional Entity FE 1 IF 1 Functional IF 2 Entity FE 2 Functional Entity FE 3 Physical Plane Physical Entity B Physical Entity A INAP Protocol Operations Physical Entity C Figure 1.7. The four planes of the Intelligent Network Conceptual Model. While the number of capability sets was open-ended, only two have been implemented significantly. The level of capability is indicated by a set of benchmark services, that is target services in a range of complexity that telcos may wish to offer. Listing benchmark services is not standardising services. Since the objective of classical IN is to allow telcos to compete by differentiating their services, services are not standardised. The first level of abstraction in the INCM is the Service Plane shown in Figure 1.7. The Service Plane provides exemplars or benchmarks of services that should be capable of implementation at the stated level of capability of the IN infrastructure. The benchmark services for Capability Set 1 (CS-1) [115] are characterised by only one call process (originating or terminating) interacting with the logic on the SCP and the SCP being invoked only during call setup or clear down, not in the active phase of the call. Examples of such services involve number translation and alternative billing: freephone, split charging, premium rate, abbreviated dialling, and credit card calling.

13 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 13 Voice virtual private networks based on PSTN infrastructure are implemented using the IN architecture. Capability Set 2 (CS-2) [124] benchmark services include all CS-1 services with the added ability to perform call party handling (CPH), that is to manipulate bearer connections in mid-call under user control. Such benchmark services include call waiting and conference calls. Capability Set 2 supports services that require cooperating Service Control Points, for example, in different networks in an international freephone service. The Global Functional Plane (GFP) of the INCM represents the software creation methodology based on defined reusable elements. The method used is based on Service Independent Building Blocks (SIB). The SIB is a scripting type of software methodology. Typical SIBs are User Interaction and Translate Data. The former initiates playing an announcement and optionally collecting digit(s) from the user. The latter initiates a database lookup. Service logic is created by means of a script that chains SIBs and has branching paths depending on conditions encountered during execution of SIBs. The Global Functional plane also contains an abstract description of the call process, the Basic Call Process. Service logic launches from the Basic Call Process and returns to it in standardised ways. While standards bodies defined sets of SIBs for Capability Sets 1 and 2, little benefit occurred from standardisation since each IN equipment vendor developed a proprietary set of SIBs. The Distributed Functional Plane (DFP) of the INCM contains an abstract definition of the functionality that supports the execution of the SIBs, that in turn allows services typified by the benchmark services to be implemented. The logic of each SIB is distributed across different nodes in the IN. For example logic embodied in a SIB called Translate Data starts and ends executing in the SCP but performs the database lookup in the SDP. Abstract data definitions are given and the internal and external behaviour of the SIBS are specified using the Specification and Description Language (SDL) and message sequence charts (MSC). Each description shows the distribution of the SIB logic and the communication between parts. Similar sets of functionality from different SIBs are allocated to abstract elements called functional entities. Three principal functional entities are defined. The Service Control Function (SCF) provides an implementation-independent definition of the functions that may be performed on the external service platform. The Specialised Resource Function (SRF) is an abstraction of functions for interacting with the a caller. The Service Switching Function (SSF) contains the functions needed to set and test trigger conditions and to interact with the SSF. Capabilities are therefore defined in terms of functional entities and the information flows between functional entities. The Distributed Functional Plane specification also defines the Basic Call State Model (BCSM). The BCSM defines standard interface mechanisms for invoking services hosted on a service control point. The next section examines the standardisation approach and how external logic is invoked from the call process. The Physical Plane represents possible physical realisations of the IN using a finite repertoire of physical elements, such as the SCP, SDP and SRP. The Physical Plane defines rules for allowed mappings of the abstract functional entities onto predefined types of physical nodes, for example SCF to SCP. The Physical Plane specification defines the application-layer protocol, INAP, at the various capability sets. Variants on INAP exist. ETSI has defined a reduced form of the ITU-T INAP, called the

14 14 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE SCF -1 O-BCSM ISUP Signalling T-BCSM SCF-2 O-BCSM ISUP Signalling T-BCSM Figure 1.8. Representation of a PSTN call by two half-call models with possible relationships to Service Control Functions. ETSI Core INAP [56], to ensure interoperability between different implementations. Network-specific billing formats are incorporated into implementations of chargingrelated operations in INAP. The GSM mobile network standards define a rich set of switch-based supplementary services. New supplementary services cannot be added speedily or readily differentiated to meet the needs of different operators. GSM networks therefore rely on IN-based services for new features [147]. A leading example of an IN-based service in mobile networks is prepaid calling. The ETSI standards for IN in GSM networks are known as Customised Applications for Mobile network Enhanced Logic (CAMEL) [59]. A version of INAP at CS-1 level, called the CAMEL Application Part, has been defined for supporting services in mobile networks under the CAMEL standard. CAMEL standards allow the invocation by a roaming user of SCP-based logic hosted in the home network. Call Models and Invocation of IN Services The Intelligent Network was overlayed on pre-existing digital PSTN switches. These switches conform to external signalling specifications, namely ISUP and Q.931, but implementation detail differs among vendors. The authors of the IN standards were faced with the problem of enabling the different switch vendors to expose consistent interfaces between their uniquely implemented call processes and the Service Switching Function. The call process was abstracted using two Basic Call State Models shown in Figure 1.8. The Originating Basic Call State Model (O-BCSM) encapsulates the processes associated with the originating side of the call, for example authorising

15 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 15 Detection Point x x 1 2 DPP (unarmed) p: Point in Call y q: Point in Call z 7 3 DPP (armed T) 6: Arm 8 9 DPP 10 (armed- N) 4 5 Service Logic r: Point in Call CCF SSF SCF Figure 1.9. Role of detection points, detection point processing and service logic. the user, collecting digits and routing. The Terminating Basic Call State Model (T-BCSM) abstracts the functions at the terminating side of the call including authorising and alerting the called party. The calling party signals to the O-BSCM. In general, the O-BCSM and the T-BCSM are in different switches and signal to each other through zero or more exchanges using the ISUP protocol. Interaction with a SCP containing service logic is possible from either BCSM, but normally only one at a time. Figure 1.9 illustrates several concepts in the IN standards. The BCSMs model call control functionality and are located in the CCF. A Basic Call State Model has two building blocks: detection points and points in call. A Detection Point (DP) is a stage in the call control process where external logic hosted by the SCF can be invoked if predetermined criteria, the trigger criteria, are met. For example, the number translation logic for a freephone service must be invoked if the dialled number has a specified prefix, say 080. At each detection point, the call process halts and sends a notification carrying call parameters to the Service Switching Function as shown in Figure 1.9. In the SSF, detection point processing (DPP) determines whether call parameters satisfy the trigger criteria. The first notification (1 in Figure 1.9) shows the case of a detection point with no criteria set or the trigger parameters not meeting the criteria. Execution of the call process resumes execution where processing was interrupted (2). The call process between this point and the next detection point is encapsulated in a Point in Call (PIC). The PIC abstracts this part of the vendor s implementation of the call process. A PIC can receive and emit signalling such as ISUP and Q.931 messages. The second notification (3 in Figure 1.9) encounters a case where call parameters meet criteria that have been preset by a management function for that detection point. A detection point with a permanent set of criteria is said to be of Trigger Detection Point (TDP) type. For example, in an abbreviated dialling service, three dialled

16 16 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE digits contain a hash follows by two digits. The translation between the short code and the called party s routable number is held in a database associated with the SCF. The combination of digits #nn meets the trigger conditions. A message (4) is sent to the SCF containing a Service Key generated in the DPP that identifies the service logic to be executed. When the service logic has executed, it returns a message (5) containing one or more instructions to the DPP and call process. For example, a DP elsewhere in the call process could be armed to detect some event or condition later in the call, as shown at 6. Such a temporarily armed detection point is called an Event Detection Point (EDP). The call process is instructed (7) to resume at a specified DP. For example, in an abbreviated dialling service, the called party s full number is returned and the call process must re-analyse that number. At 8, we show the call parameters being passed to the DPP for testing against criteria set at 6. If the test result is positive, two actions are possible. As shown at 9, an event report is sent to the service logic and no reply is expected. For example, the service logic may need to know whether the call is answered. The call process resumes execution at the start of the next PIC, as shown at 10. Alternatively, a request may be sent to the service logic that calls for a response. In this case, the call process is not instructed to continue until the response is received. The O-BSCM and T-BCSM for IN Capability Set 1 are shown in Figure The BCSM for second generation GSM networks is a simplified version formed by merging points in call as shown in Figure Detection points are eliminated when PICs are merged. For example, Analysed Information is not available in 2G mobile networks. Example: Call Connection and Value-added Services in the PSTN/IN This example illustrates two aspects of practice in the PSTN/IN: first, invoking external logic to enhance services (messages 4 12), and second routing connections via a first choice route (messages 13, 14) or an alternate route if no circuit is available on the first choice (messages 15 19). Three forms of signalling in use in the public switched telecommunications network are illustrated in Figure 1.11: Loop Signalling: the user and network use loop signalling, represented by pseudomessages 1 3, 17, 20 and 21. ISUP: the ISDN User Part Signalling Protocol is used for setting up and clearing down connections between switches. This message sequence chart shows some ISUP messages (13, 14 16, 18, 19, 22, 23). INAP: control logic on the external Service Control Point (SCP) provides value added services and interacts with one of the switches and the Specialised Resource using the Intelligent Network Application Protocol (4 8, 11, 12) The numbered messages perform the functions described below Using loop signalling, the caller, A, goes offhook, receives a dialtone and dials a freephone number. 4. The originating exchange (E1), on analysing the dialled digits, discovers that it cannot route the call to this number. Exchange E1 therefore sends the INAP

17 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS 17 7 O_Abandon 1: O_Null& Authorise _Attempt 6: O_Exception 1 O_Attempt_Authorised 2: Collect_Information 2 Collected_Information 3: Analyse Information 3 Analysed _Info 4: Routing & Alerting 7 O_Answer 4 Route_Select_Failure 5 O_Called_Party_Busy 6 O_No_Answer 7 5: O_Active 7 O_Mid_Call (a) O- BCSM 18 7: T_Null& Authorise _Attempt 11: T_Exception T_Abandon 12 T_Attempt_Authorised 8: Select Facility & Present 13 T_Called_Party_Busy 9: Alerting 15 T_Answer 14 T_No_Answer 17 10: T_Active 16 T_Mid_Call (b) T-BCSM Figure Basic Call State Models for Intelligent Network Capability Set 1. The shaded areas show merged points in call for GSM.

18 18 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE A E1 SCP SDP SRP 1: Offhook 2: Dialtone 3: Digits 4: Initial DP 5: Connnect To Resource 8: DisconnnectFwd Connection 11: Connect 12: FurnishCharge Info 6: PromptAndCollectUserInfo 7: PromptAndCollectResult 9: Search 10: Search Result T1 13: IAM 14: REL T2 E2 B 20: Ringtone 15: IAM 19:ACM 23:ACM 16: IAM 18:ACM 22:ANM 17: Ringing 21: Offhook Figure Example of a message sequence for PSTN call setup with Intelligent Network service support. InitialDp message to the SCP requesting it to translate the dialled number to a routable number and to determine the charging details. 5. The SCP needs further information from the user, for example a service option, and requests the switch to connect the caller to an SRP. 6, 7. The SCP instructs the SRP to prompt the caller with a set of options and asks for a digit to be entered. The entered digit is returned to the SCP. 8. The SCP instructs the switch to clear the temporary connection to the SRP. 9, 10. The SCP queries the database (SDP) for the called party routable number corresponding to the freephone number dialled and the choice digit entered by the caller.

19 1.2. DEFINING PRESENT STATE USING REFERENCE MODELS The SCP instructs the switch to complete connecting the call using the INAP Connect operation, using the routable number as the B-party number. 12. The SCP instructs the switch to mark the billing ticket for reverse charges. 13. Exchange E1 now has a routable number and continues processing the call with the new destination number. E1 sends an ISUP Initial Address Message (IAM) to T1, the transit exchange on the first choice route to the destination, requesting it to reserve a circuit and to take over routing. 14. Here we assume that T1 finds that it cannot route the call onward due to lack of circuits. T1 refuses the request by returning a Release Message (REL). 15. Exchange E1 must use the alternate route to the destination via transit exchange T2. E1 sends the IAM message to T T2 has free circuits to the terminating exchange E2 and forwards the IAM to E The called party is free and ringing current is applied to the phone. 18, 19. An indication of the ringing condition is returned using the Address Complete Message (ACM). 20. Exchange E1 plays ringing tone to the caller. 21. The called party answers; ringing current is disconnected by E2. 22, 23. The Answer Message (ANM) passes back to the originating exchange; ringing tone is removed by E1; the connection is made. Limitations of Switched Circuit Networks Public circuit-switched networks set the standard for network availability, voice quality and grade of service. Switched circuit voice networks have a significant range of valueadded services implemented by means of the Intelligent Network. Fixed line switched circuit networks are limited as multiservice networks because their analogue subscriber loops require voiceband data modems to adapt the digital user data to an analogue signal suitable for the voiceband channel. Voiceband modems are limited to speeds of 56 kbit/s. Voiceband access is also inefficient. The voiceband channel occupies a circuit through at least the end exchange before encountering the access element to a packet network. The N-ISDN, while it sets out to be a multiservice network, offers only a few more real-time services than the PSTN: videoconferencing and high-fidelity audio. The N-ISDN is inherently limited as a data network because of the use of circuit-oriented B-channels that limit dial-up user speeds to 64 or 128 kbit/s and need to be switched through the end exchange in circuit mode. The Intelligent Network approach to implementing value-added services, while enjoying widespread deployment, has several limitations. First, classical IN is strongly linked to the underlying circuit-switched voice network. IN is thus a vertically

20 20 CHAPTER 1. SETTING THE CONTEXT FOR CONVERGENCE BTS BTS BSC VLR SS 7 Circuit Switched Network HLR CSD IWF SMS SC G b WAP Gateway MMS Server SGSN GPRS Packet Switched Backbone GGSN Internet Figure Reference network architecture for data services in 2/2.5G mobile networks: GPRS, SMS and MMS. integrated solution. A single provider offers access, connectivity and the value added service. Second, there has been limited success in opening the Service Control Points in a secure manner to allow requests generated on the Internet to initiate or enhance telecommunication services. Third, the SIB approach to reuse of software suffers from both the dominance of proprietary products and not being an object-oriented paradigm PRESENT STATE: DATA SERVICE IN MOBILE NETWORKS The GSM network is an integrated services digital network since it can support multiple services (voice and various data services) over a single interface. Initially, GSM phones and the network supported only circuit-switched data and the Short Message Service (SMS). Recently, packet mode data services have been introduced in the form of the General Packet Radio Service (GPRS). The GPRS allows the enhancement of messaging in the form of the Multimedia Messaging System (MMS). Figure 1.12 provides a framework for discussing four data services available on GSM mobile networks. The principal architectural elements of the GSM network supporting circuit-switched services as well as added elements to provide the General Packet Radio Service are shown. Circuit-switched Data The mobile station and air interface support a range of synchronous and asynchronous data transfer capabilities using time slots on the air interface. Such data channels are switched through the MSC. GSM standards specify means of interworking with other ISDNs and ITU-T packet standard networks such as X.25. Interworking with the Internet requires an interworking function (IWF) for circuit-switched data (CSD). The IWF acts as the Internet service provider s point of presence (PoP). Circuitswitched data rates are generally limited to 9.6 kbit/s. Higher rates are obtained in High Speed Circuit Switched Data services by allocating multiple time slots to the