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
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` 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
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` 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.
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` 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
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` 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
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` 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
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` 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)
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` /--\
` () ()
` -- .................................
` / \<--- ^ ! ! /--\
` /----\ \ ! 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
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` 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.
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` _____
` __ _____/ \_____
` [__] / \
` [-++-]-.-.>.-. 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
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` 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.
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` 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
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` 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.
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