Netowrk Working Group H. Lu Request for Comments: 2458 Editor Category: Informational M. Krishnaswamy Lucent Technologies L. Conroy Roke Manor Research S. Bellovin F. Burg A. DeSimone K. Tewani AT&T Labs P. Davidson Nortel H. Schulzrinne Columbia University K. Vishwanathan Isochrome November 1998 Toward the PSTN/Internet Inter-Networking --Pre-PINT Implementations Status of this Memo This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1998). All Rights Reserved. Abstract This document contains the information relevant to the development of the inter-networking interfaces underway in the Public Switched Telephone Network (PSTN)/Internet Inter-Networking (PINT) Working Group. It addresses technologies, architectures, and several (but by no means all) existing pre-PINT implementations of the arrangements through which Internet applications can request and enrich PSTN telecommunications services. The common denominator of the enriched services (a.k.a. PINT services) is that they combine the Internet and PSTN services in such a way that the Internet is used for non-voice interactions, while the voice (and fax) are carried entirely over the PSTN. One key observation is that the pre-PINT implementations, being developed independently, do not inter-operate. It is a task of the PINT Working Group to define the inter-networking interfaces that
will support inter-operation of the future implementations of PINT services. Table of Contents 1. Introduction ....................................... 3 2. Terminology ....................................... 3 3. PINT Services ....................................... 4 4. Architectural Overview ............................... 5 4.1 Public Switched Telephone Network ............... 5 4.2 Pre-PINT Systems ............................... 9 5. IN-Based Solutions ............................... 20 5.1 The Lucent System ............................... 20 5.1.1 Roles of the Web Server, Service Node, and SMS ....... 20 5.1.2 A Click-to-Dial-Back Service Scenario ............... 21 5.1.3 Web Server-Service Node Interface ............... 22 5.1.4 Web Server-SMS Interface and SNMP MIB ............... 24 5.1.5 Security Considerations ........................... 26 5.2 Siemens Web Call Center ........................... 27 5.2.1 Service Description ............................... 27 5.2.2 Implementation ................................... 29 5.2.3 Derived Requirements/Lessons ..................... 35 6. Alternative Solutions ............................... 37 6.1 The AT&T System ..................................... 37 6.1.1 High Level Architecture ............................ 38 6.1.2 IP Client to CallBroker Interface .................. 39 6.1.3 Protocol ........................................... 40 6.1.4 APIs Exposed to the IP Client ..................... 41 6.1.5 Voice-Bridge Control API ........................ 41 6.2 Simple Computer Telephony Protocol ............... 41 6.2.1 Overview ........................................... 41 6.2.2 How SCTP Fits in with the Reference PINT Services .. 42 7. Session Initiation Protocol--An Emerging Standard .. 43 7.1 Overview ....................................... 43 7.2 SIP Protocol ....................................... 44 7.3 SIP Entities ....................................... 45 7.4 Providing Call Control Functionality ............... 46 8. Overall Security Considerations ..................... 47 9. Conclusion ....................................... 48 10. Acknowledgments ................................... 48 11. Appendix ....................................... 49 11.1 PSTN/IN 101 ....................................... 49 11.1.1 Public Switched Telephone Network ............... 49 11.1.2 Intelligent Network ............................... 51 11.2 Call Center Features ............................. 54
12. References ....................................... 56 Authors' Addresses ......................................... 57 Full Copyright Statement .................................. 60 1. Introduction This document contains the information relevant to the development of the inter-networking interfaces underway in the Public Switched Telephone Network (PSTN)/Internet Inter-Networking (PINT) Working Group. It addresses technologies, architectures, and several (but by no means all) existing pre-PINT implementations of the arrangements through which Internet applications can request and enrich PSTN telecommunications services. The common denominator of the enriched services (a.k.a. PINT services) is that they combine the Internet and PSTN services in such a way that the Internet is used for non-voice interactions, while the voice (and fax) are carried entirely over the PSTN. The organization of the document is as follows. First, the basic terminology and a short "intuitive" description of the PINT services are provided. The rest of the information deals, in one way or the other, with the pre-PINT support of these services where they are used as a benchmark. Thus, an architectural overview common to all present solutions is presented. The flow of the document then divides into two streams: one is dedicated to the Intelligent Network (IN)-based solutions; the other explores alternative means (i.e., CallBroker and Computer-Telephony Integration (CTI) approach). At this point, the emerging standards are explored, in particular, the Session Initiation Protocol (SIP), which promises an elegant solution to the PINT problem. Each of the above developments is addressed in a respective section. The final sections of the document contain the overall security considerations, conclusion, acknowledgments, appendix, and a set of references. The security section summarizes the PINT security requirements derived from the pre-PINT experiences and the appendix presents a tutorial on the PSTN, IN, and Call Center functions. 2. Terminology This document uses the following terminology: Authentication -- verification of the identity of a party. Authorization -- determination of whether or not a party has the right to perform certain activities. PINT Gateway -- the PSTN node that interacts with the Internet.
User or Customer -- the person who asks for a service request to be issued. In the context of PINT Services, this person will use an Internet host to make his or her request. The term "user" is also used to describe a host originating the PINT service request on behalf of this person. 3. PINT Services This document addresses four services initially identified by the PINT Working Group and presently supported by pre-PINT implementations. These services are: click-to-dial-back, click-to- fax, click-to-fax-back and voice-access-to-content. Note that the word "click" should not be taken literally. It is rather used to point out that initiation of the related services takes place on the Internet, where point and click are the most prevalent user actions. In other words, a service request could originate from any type of IP-based platforms. There is no implication that these services must be implemented by a device within the PSTN or the Internet running a Web server. The common denominator of the PINT services is that they combine the Internet and PSTN services in such a way that the Internet is used for non-voice interactions, while the voice (and fax) are carried entirely over the PSTN. (An example of such a service is combination of a Web-based Yellow Pages service with the ability to initiate PSTN calls between customers and suppliers in a manner described in what follows.) Some of the benefits of using the PSTN are high quality of the voice, an ability to route the call to different locations depending on pre-set criteria (for example, time of the day, day of the week, and geographic location), outstanding security and reliability, and access to flexible, low cost, and secure billing and charging systems. The benefits of using the Internet are the uniform, well- defined, and widely-used interfaces available anywhere, anytime. Click-to-Dial-Back With this service, a user requests (through an IP host) that the PSTN call be established between another party and himself or herself. An important pre-requisite for using this service is that the user has simultaneous access to both the PSTN and Internet. One example of an application of this service is on-line shopping: a user browsing through an on-line catalogue, clicks a button thus inviting a call from a sales representative. Note that (as is the case with the all-PSTN Free-Phone, or "800", service) flexible
billing arrangements can be implemented here on behalf of the service provider. In addition (and also similarly to the Free-Phone/800), the PSTN could route the call depending on the time of day, day of week, availability of agents in different locations, and so on. Click-to-Fax With this service, a user at an IP host requests that a fax be sent to a particular fax number. In particular this service is especially meaningful when the fax is to be sent to someone who has only a fax machine (but no access to the Internet). Consider, as an example, a service scenario in which a Web user makes a reservation for a hotel room in Beijing from a travel service page containing hotel information of major cities around the world. Suppose a specific Beijing hotel chosen by the user does not have Internet connection but has a fax machine. The user fills out the hotel reservation form and then clicks a button sending out the form to the travel service provider, which in turn generates a fax request and sends it together with the hotel reservation form to the PSTN. Upon receiving the request and the associated data, the PSTN translates the data into the proper facsimile format and delivers it to the Beijing hotel as specified in the fax request. Click-to-Fax-Back With this service, a user at an IP host can request that a fax be sent to him or her. (Consider the user of the previous example, who now requests the confirmation from the Beijing Hotel. Another useful application of the service is when size of the information that a user intends to get is so large that downloading it to the user's PC over the Internet will require a long time and a lot of disk space.) Voice-Access-to-Content With this service, a user at an IP host requests that certain information on the Internet be accessed (and delivered) in an audio form over the PSTN, using the telephone as an informational appliance. One application of this service is to provide Web access to the blind. (This may require special resources--available in the PSTN--to convert the Web data into speech.) 4. Architectural Overview 4.1 Public Switched Telephone Network From an application perspective, Internet nodes are interconnected directly, as shown in Figure 1. When two machines are to communicate, they will have the address of the destination end system, and will
send network level datagrams, assuming that the underlying infrastructure will deliver them as required. _____ __ _____/ \_____ [__] / \ [----]-.-.-.-. Internet .-. \_____ _______/ | __ \__./ __ . [__] / [__] | [----]-. [----]-. Key: .-.-. Internet Access Link Figure 1 Where all nodes are on the same (broadcast) network, there is no need for intervening routers; they can send and deliver packets to one another directly. The Internet nodes are responsible for their own communications requests, and act as peers in the communication sessions that result. This contrasts with the situation in the PSTN. There, the end systems are configured as shown in Figure 2. The end systems tend to be specific to a particular type of traffic, so that, for example, the majority of terminals are dedicated to carrying speech traffic (telephones) or to carrying facsimile data (fax machines). The terminals all connect to Central Offices (COs) via access lines, and these COs are interconnected into a network. /--\ ()/\()__ /__\ \ ................................. \ ! ! ! /--\ __ \ [-!-] [-!-] ! ()/\() \ \ \__[CO ]=========[CO ]==\\ ! ___/__\ [Fax]________[---] [---] \\ [-!-] / __ \\=======[CO ]____/ \ \ [---]________[Fax] Key: ___ Access Lines === Trunk Links (inter-CO user data links) ... Inter-CO signaling network links CO Central Office (Telephone Exchange) Figure 2
Communications between the terminals are all "circuit switched", so a dedicated synchronous data path (or circuit) needs to be placed between the end terminals for carrying all communications. Arranging for such a circuit to be made or removed (cleared) is the responsibility of the Central Offices in the network. A user makes a request via his or her terminal, and this request is passed on to the "local" Central Office. The relationship between the terminals and the local Central Offices to which they are connected is strictly Client/Server. The COs are interconnected using two different types of connections. One of these is called a trunk connection (shown as a double line in the above figure) and is used to carry the data traffic generated by the terminals. The other connection acts as part of a separate network (and is shown as a dotted line in the above figure). This is the signaling network, and is used by the Central Offices to request a connection to be made between themselves and the destination of the required circuit. This will be carried across the trunk link to the "next" Central Office in the path. The path, once in place through the PSTN, always takes the same route. This contrasts with the Internet, where the underlying datagram nature of the infrastructure means that data packets are carried over different routes, depending on the combined traffic flows through the network at the time. The call set up process can be viewed as having two parts: one in which a request for connection is made, and the other in which the circuit is made across the PSTN and call data flows between the communicating parties. This is shown in the next pair of figures (3a and 3b). /--\ () () --____ /++\ \ /----\ \ A \ [-!-] \->[CO ] [---] Time = 13:55 Figure 3a Key: ___ Access Lines === Trunk Links (inter-CO user data links) ... Inter-CO signaling network links CO Central Office (Telephone Exchange)
/--\ () () -- ................................. / \<--- ^ ! ! /--\ /----\ \ ! v ! () () A' \ [-!-] [-!-] ! -- \__[CO ]=========[CO ]==\\ v ->-/ \ [---] [---] \\ [-!-] / /----\ \\=======[CO ]____/ B' Time = 14:00 [---] Figure 3b Figure 3 shows a particular kind of service that can be provided; call booking. With this service, a request is sent for a connection to be made between the A and B telephones at a specified time. The telephone is then replaced (the request phase is terminated). At the specified time, the CO will make a connection across the network in the normal way, but will, first, ring the "local" or A' telephone to inform the user that his or her call is now about to be made. For more complex services, the requesting telephone is often connected via its "local" CO to a Service Node (SN), where the user can be played prompts and can specify the parameters of his or her request in a more flexible manner. This is shown below, in Figures 4a and 4b. For more details of the operation of the Service Node (and other Intelligent Network units), see the Appendix. When the SN is involved in the request and in the call setup process, it appears, to the CO, to be another PSTN terminal. As such, the initial request is routed to the Service Node, which, as an end system, then makes two independent calls "out" to A' and B'. /--\ [---] () () [SN ] --___ [|--] /++\ \ | /----\ \ | \ | A \ [|-!] \->[CO ] [---] Time = 13:55 Figure 4a
Key: ___ Access Lines === Trunk Links (inter-CO user data links) ... Inter-CO signaling network links CO Central Office (Telephone Exchange) SN Service Node /--\ [---] () () [SN ] -- [|--] /--\ / \<-- | ............................... () () /----\ \ | ^ ! ! -- \ | / v v / \ A' \ [|-!] [-!-] [-!-] ->-/----\ \--[CO ] [CO ] [CO ] / [---] [---] [---]___/ B' Time = 14:00 Figure 4b Note that in both cases as shown in Figures 3 and 4 a similar service can be provided in which the B' telephone is replaced by an Intelligent Peripheral (or an Special Resource Functional entity within a Service Node), playing an announcement. This allows a "wake up" call to be requested, with the Intelligent Peripheral or Service Node Special Resource playing a suitable message to telephone A' at the specified time. Again, for more details of the operation of the Special Resources (and other Intelligent Network units), see the Appendix. 4.2 Pre-PINT Systems Although the pre-PINT systems reported here (i.e., those developed by AT&T, Lucent, Siemens and Nortel) vary in the details of their operation, they exhibit similarities in the architecture. This section highlights the common features. Specific descriptions of these systems will follow. All of the systems can be seen as being quite similar to that shown in the following diagram. In each case, the service is separated into two parts; one for the request and another for execution of the service. Figure 5 summarizes the process.
_____ __ _____/ \_____ [__] / \ [-++-]-.-.>.-. Internet .-.- \_____ _______/ . \___/ v [----] . [PINT]-.- [----] % v [---] [SN ] [|--] Figure 5a Key: CO Central Office (Telephone Exchange) SN Service Node PINT PSTN/Internet Gateway .-.-. Internet Access Link %%% Gateway/Service Node Link ___ PSTN Access Lines === PSTN Trunk Links (inter-CO user data links) ... Inter-CO signaling network links _____ __ _____/ \_____ [__] / \ [----]-.-.-.-. Internet .-.- \_____ _______/ . \___/ | [----] . [PINT]-.- [-%--] % % /--\ [-%-] () () [SN ] -- [|--] /--\ / \<-- | .................... () () /----\ \ | ^ ! ! -- \ | / v v / \ A' \ [|-!] [-!-] [-!-] ->-/----\ \--[CO ]=======[CO ]======[CO ] / [---] [---] [---]__/ B' Figure 5b
Comparing Figure 4a with Figure 5a, the differences lie in the way that the information specifying the request is delivered to the Service Node. In the PSTN/IN method shown in the earlier diagram, the user connects to the SN from the telephone labeled A, with the connection being routed via the CO. In the latter case, the request is delivered from an Internet node, via the PINT gateway, and thence to the Service Node over a "private" link. The effect is identical, in that the request for service is specified (although the actual parameters used to specify the service required may differ somewhat). The figures depicting the respective service execution phases (Figures 4b and 5b) show that the operation, from the IN/PSTN perspective, is again identical. The Service Node appears to initiate two independent calls "out" to telephones A' and B'. The alternative systems developed by AT&T and by Nortel allow another option to be used in which the PINT Gateway does not have to connect to the PSTN via a Service Node (or other Intelligent Network component), but can instead connect directly to Central Offices that support the actions requested by the gateway. In these alternatives, the commands are couched at a "lower level", specifying the call states required for the intended service connection rather than the service identifier and the addresses involved (leaving the Intelligent Network components to coordinate the details of the service call on the gateway's behalf). In this way the vocabulary of the commands is closer to that used to control Central Offices. The difference really lies in the language used for the services specification, and all systems can use the overall architecture depicted in Figure 5; the only question remains whether the Intelligent Network components are actually needed in these other approaches.
The following diagram (Figure 6) shows the interface architecture involved in providing the kind of service mentioned above. Internet __ __ Server [__] _______ [__] [W3S-]-. ___/ .-.-.-[W3C-] Internet _________________|/.-.-.-.-. \ Terminal / .. . \ | Internet / . \ | \___________ . . . / \/___|____\_________/ . . . / | \ (A) (B) (E) . . . _|_ _|_ _|_ [SN ]<-(D)--[SMS]--(H)->[SCP] [|-|] --- [-!-] / \ ! (C) (I) ...........(F)...!.(G). / \ ! ! [--|] [|-!] [-!-] [CO ] [MSC] [SSP] [---] [---] \|/ [---] /--\ | |____| | /--\ ()/\() | | ()/\() /--\___| 1 |___/--\ Fixed PSTN Terminal  Fixed PSTN Terminal Mobile Terminal Key: W3S HTTP (Web) Server W3C HTTP (Web) Client/Browser CO Central Office (Telephone Exchange) MSC Mobile Switching Center (Mobile Network Telephone Exchange) SN Service Node SSP Service Switching Point SCP Service Control Point SMS Service Management System .-.-. Internet relationship ___ PSTN Access relationship ... PSTN "core" signaling relationship Figure 6
The interfaces are: A The interface over which Internet requests for service are delivered to the Service Node B The interface over which Service Management requests are sent from the Internet to the Service Management System C The interface over which the Service Node sends call control requests to a connected Central Office D The interface over which the Service Management System manages the Service Node E The interface over which Internet requests for service are delivered to the Service Control Point F The interface over which the Service Control Point sends service call control requests to the Mobile Switching Center G The interface over which the Service Control Point sends service control requests to the Service Switching Point H The interface over which the Service Management System manages the Service Control Point I The interface over which the Service Node sends service call control requests to the Mobile Switching Center In practice, a number of the interfaces have very similar purposes to one another. The means by which these purposes are achieved differ, in that some of the interfaces (C and I) reflect access arrangements, whilst others (F and G) imply a "core" signaling relationship. However, it is possible to categorize them in terms of the "intent" of messages sent across the interfaces. For example, Interfaces A and E are similar; one of the main aims of PINT work is to ensure that they are the same. Similarly, Interfaces D and H imply similar actions and are likely to carry similar messages. Interfaces C, F, G, and I are all used to request that a call be initiated, albeit via access or core signaling relationships. The interfaces can also be viewed in terms of the kind of components that are involved and the bodies by which they are codified. Interfaces A, B, and E are all going to be realized as Internet Protocols. All of the others use existing protocols in the PSTN/IN. Traditionally, these have been codified by different groups, and this is likely to be the case in the PINT work. The general arrangements for the different systems are shown below (Figures 7, 8, 9, and 10). They differ in the details of their configurations, but the main tasks they perform are very similar, and so the overall operation is similar to the generic architecture shown in Figures 5 and 6.
Key for following diagrams: Components: W3C World Wide Web Client W3S World Wide Web Server WSA Web Server "Back End Program" Interface (CGI or Servlet interface) Srvlt Servlet "back end" program/objects FS Finger Server SCTPC Simple Computer Telephony Protocol Client SCTPS Simple Computer Telephony Protocol Server CBC CallBroker Client CBS CallBroker Server SSTPC Service Support Transport Protocol Client SSF Service Switching Function SCF Service Control Function SRF Special Resource Function CO Central Office/ Public Telephone Exchange SSP Service Switching Point SCP Service Control Point SR/I.IP Special Resource/ "Internet" Intelligent Peripheral SMS Service Management System INAPAd Intelligent Network Application Part Adaptor PktFlt Packet Filter (Firewall) SNMPAg Simple Network Management Protocol Agent Protocols: P0 HyperText Transfer Protocol P1 HTTP Server <-> "Back End Program" internal protocol P2 CallBroker Client <-> CallBroker Server protocol (AT&T system), or SCTP Client <-> Server protocol (Nortel system) P3 PINT User Agent <-> PINT Gateway protocol P4 Intra-Intelligent Network protocol (e.g., INAP) P5 Proprietary (INAP-based) Gateway-> I.IP protocol P6 Finger protocol P7 Digital Subscriber Signaling 1 protocol P8 Simple Network Management Protocol P9 SMS <-> Service Control Point/Service Node protocol
_____ _______ _____ |[W3C]|----(p0)-->| [W3S] |<--(p0)----|[W3C]| |[---]| | [WSA] | |[FS.]| |-----| | ! | |[-!-]| | (p1) | |--\--| | ! | ^ | ! | (p6) | ! | \ | (p1) | \ | ! | \ |[Srvlt]| \ |___!___| \ ! \ (p3) \ Internet ! ! .+.+.+.+.+.+.+.+.+.+.+. v .+.+.+.+.+.+.+.+.+.+.+.+.+.!.+.+.+.+.+. PSTN/IN _______________!_________________ ____!_____ __________ |I [PktFlt] I| |[PktFlt]| |[PktFlt]| |N Gateway N| | ! | | ! | |A ___________________________ A| | ! | | ! | |P | | P| | ! | |[SNMPAg]| -(p4)-- |A | <-(p4)-> [SCP] <-(p4)-> | A|-(p5)->|[SR/IIP]| | [SMS] | \ |d | [-^-] | d| |[------]| | [-^-] | \ |__| ! |__| |________| |___!____| \ ! ! [-v-] !-----------------(p9)-----------------! [SSP] [---] ___| |______ | | | /--\ | /--\ | ()/\() | ()/\() |__/__\ |____/__\ Figure 7: The Siemens Web Call Center
_____ _______ |[W3C]|----(p0)-->| [W3S] | |[---]| | [WSA] | |-----| | ! | | (p1) | | ! | | ! | | ! | | (p1) | | ! | |[SSTPC]|-<---------------------------------- |___!___| ! ! (p8) (p3) ! Internet ! v .+.+.+.+.+.+.+.+.+.+.+. v .+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+. ! .+.+ PSTN/IN _______________!__________________________________ ____!_____ | [PktFlt] Service [PktFlt]| |[PktFlt]| | ! Node | | ! | | [SCF Adaptor] | | ! | | ! | |[SNMPAg]| |[SSF]<-(p4)->[SCF] <-------(p4)--------> [SRF] | | [SMS] | |[|--] [-^-] [---] | | [-^-] | |_|_____________!________________________________| |___!____| | ! ! [-v-] (p7) !-----------------(p9)-----------------! [CO.]____| [---] ___| |_______ | | | /--\ | /--\ | ()/\() | ()/\() |__/__\ |____/__\ Figure 8: The Lucent System
_____ ________ |[W3C]|----(p0)-->| [W3S] | |[---]| | [WSA] | |-----| | ! | | (p1) | | ! | |[WS/CBS]| |[Adaptr]| |___!____| ^ (p2) _____ ___v____ |[CBC]| | [CBS] | |[---]|<---(p2)-->| [---] |-<--------------------------------- |-----| |___!____| ! ! (p8) (p3) ! Internet ! v .+.+.+.+.+.+.+.+.+.+.+. v .+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+. ! .+.+ PSTN/IN _______________!__________________________________ ____!_____ | [PktFlt] Service [PktFlt]| |[PktFlt]| | ! Node | | ! | | [SCF Adaptor] | | ! | | ! | |[SNMPAg]| |[SSF]<-(p4)->[SCF] <-------(p4)--------> [SRF] | | [SMS] | |[|--] [-^-] [---] | | [-^-] | |_|_____________!________________________________| |___!____| | ! ! [---] (p7) !-----------------(p9)-----------------! [CO.]____| [---] ___| |_______ | | | /--\ | /--\ | ()/\() | ()/\() |__/__\ |____/__\ Figure 9: The AT&T System
_____ ________ |[W3C]|----(p0)-->| [W3S] | |[---]| | [WSA] | |-----| | ! | | (p1) | | ! | |[WS/ ]| |[ SCTPS]| |[Adaptr]| |___!____| ^ (p2) _______ ___v___ |[SCTPC]| |[SCTPS]| |[-----]| <-(p2)--> |[-----]|-<---------------------------------- |-------| |___!___| ! ! (p8) (p3) ! Internet ! v .+.+.+.+.+.+.+.+.+.+.+. v .+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+. ! .+.+. PSTN/IN _______________!__________________________________ ____!_____ | [PktFlt] Service [PktFlt]| |[PktFlt]| | ! Node | | ! | | [SCF Adaptor] | | ! | | ! | |[SNMPAg]| |[SSF]<-(p4)->[SCF] <-------(p4)--------> [SRF] | | [SMS] | |[|--] [-^-] [---] | | [-^-] | |_|_____________!________________________________| |___!____| | ! ! [---] (p7) !-----------------(p9)-----------------! [CO.]____| [---] ___| |_______ | | | /--\ | /--\ | ()/\() | ()/\() |__/__\ |____/__\ Figure 10: The Nortel System As these are independent systems developed by different groups, the names of the components, unsurprisingly, don't match. Some features are offered by one of the systems, while they aren't by others. However, there are a number of common features. All of the systems provide a Web-based interface (at least as an option), using "back end" programs to construct protocols to pass onwards to the Intelligent Network system.
Several Intelligent Network Functional Entities are combined into a Service Node in the Lucent, AT&T , and Nortel systems, while in the Siemens scheme they are separate units. However, this is not particularly important for the provision of the services they offer. The main difference lies in whether or not the SCF is "aware" of the Internet interface and has been modified to be "complicit" in supporting these Internet requests. The Siemens approach was to re- use an existing SCP, providing a gateway function to translate as needed. The Lucent system used a "lighter weight" SCF adapter to terminate the Internet protocols, as the SCF was modified to support the Internet interface directly. The AT&T CallBroker and Nortel SCTP Servers introduce an intermediate protocol (labeled p2) that allows an alternative to the Web based interface supported by the others. This protocol matches the "CallBroker Client API", or the "SCTP Client API". These options provide for a bi-directional protocol, with indications sent from the Call Broker or SCTP Server to the Client as needed. This is not easily possible using an HTTP-based scheme (and in the Siemens case, a dedicated Finger client/server pair was used to emulate such an interface) The protocol between the Internet server and the Intelligent Network (labeled p3 in the above diagrams) differs in each of the systems. One of the main aims of future work will be to develop a common protocol that will support the services offered, so that the p3 interface will allow different implementations to inter-operate. In the Lucent, Siemens, and Nortel systems, this was an "internal" protocol, as it was carried between entities within the Service Node or Gateway. Other contrasts between the systems lie in the support for Internet access to Service Management, and access to the Internet by Special Resources. Internet Management access was most developed in the Lucent system, in which a Simple Network Management Protocol (SNMP) agent was provided to allow inter-operation with the SMS controlling the Service Node. In the Siemens scheme, the SMS had no direct Internet access; any management actions were carried out within the normal PSTN management activities. As for Internet access to special resources, this was only required by the Siemens system as part of its support for Call Center agent notification. Equivalent functionality would be provided in the AT&T and Nortel systems as mentioned above, and this would in turn be associated with event notifications being sent as part of their (p3) Internet/IN protocol. These differences reflect the different emphases in the products as they were developed; again, future work will have to ensure that common protocols can be used to support the chosen services fully.