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RFC 2458


Toward the PSTN/Internet Inter-Networking--Pre-PINT Implementations

Part 2 of 3, p. 20 to 43
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5. IN-Based Solutions

5.1 The Lucent System

   Figure 11 depicts the overall interconnection architecture of the
   Lucent prototype in support of the four PINT services. The IN-based
   architecture utilizes the Service Node and Service Management System
   in addition to the Web server, which enables Web-based access to the
   PINT services. This section summarizes the roles of these elements
   (complemented by a click-to-dial-back service scenario), outlines the
   interfaces of Web Server-Service Node and Web Server-Service
   Management System (i.e., the interfaces A & B), and addresses the
   common security concerns.

5.1.1 Roles of the Web Server, Service Node, and Service Management

   Web Server

   The Web Server stores the profiles of content providers as well as
   pre-registered users. The content provider profile contains
   information such as content provider ID, telephone number, and fax
   number. In addition, the profile may also include service logic that
   specifies, for example, the telephone (or fax) number to be reached
   based on time of the day, day of the week, or geographical location
   of the user, and the conditions to accept the charge of the calls.

   Similar to the content provider profile, the pre-registered user
   profile contains information such as user name, password, telephone
   number, and fax number. The last two pieces of information can also
   be linked to time of the day and day of the week so the user can be
   reached at the appropriate telephone (or fax) number accordingly.

   Service Node

   Situated in the PSTN, the SN, like the SCP, performs the service
   control function [1, 2, 3]. It executes service logic and instructs
   switches on how to complete a call. The SN also performs certain
   switching functions (like bridging of calls) as well as a set of
   specialized functions (like playing announcements, voice recognition
   and text-to-speech conversion).

   Service Management System

   The SMS performs administration and management of service logic and
   customer-related data on the SN. It is responsible for the
   replication of content provider profiles and provision of these data
   on the SN. These functions are non-real time.

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    Web Users
    O --------------------------  | Internet |-------------------
                                  ------------                  |
   ----------------            --------------               ------------
   | Service Node |     D      | Service    |       B       |Web Server|
   |     (SN)     |------------| Management |---------------|          |
   |              |            |System (SMS)|               |          |
   |              |      A     --------------               |          |
   |              |-----------------------------------------|          |
   ----------------                                         ------------
      |         |
      | I       | C
      |         |
   ----------- ---------
   |Mobile   | |Central|
   |Switching| |Office |
   | Center  | ---------
   -----------     |
        |          |
        |          |
        O          O

       Mobile      Wireline PSTN
       Users       Users

   Figure 11: Overall Interconnection Architecture of the Lucent System

5.1.2 A Click-to-Dial-Back Service Scenario

   A Web user, who has simultaneous access to the Web and telephone
   services (this can be achieved, for example, by having an ISDN
   connection), is browsing through a sales catalogue and deciding to
   speak to a sales representative.

   When the Web user clicks a button inviting a telephone call from the
   sales office, the Web Server sends a message to the SN over the A
   interface, thus crossing the Internet-to-PSTN boundary. By matching
   the information received from the Web Server with the content
   provider profile that had been previously loaded and activated by the
   SMS over the D interface, the SN recognizes the signal.

   At this point, the SN calls the Web user. The user answers the call,
   hears an announcement, e.g., "Please wait, while we are connecting
   you to the sale agent", and is waiting to be connected to the sale
   agent. Then the SN invokes service logic as indicated in the profile.

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   The execution of this logic selects an appropriate sales agent to
   call based on the time of the day. It is 8 P.M.  in New York where
   the Web user is located, and the New York sales office has closed.
   The San Francisco office, however, is still open, and so the SN makes
   a call to an agent in that office. Finally, the SN bridges the two
   calls and establishes a two-party call between the sales agent and
   the Web user.

5.1.3 Web Server-Service Node Interface

   Lucent developed the Service Support Transfer Protocol (SSTP) for
   communications between the SN and Web Server. SSTP is of a
   request/response type running on top of a reliable transport layer,
   such as TCP. The Web Server sends a request to the SN to invoke a
   service and the SN responds with a message indicating either success
   or failure. Note that SSTP engages only the service control function
   [1, 2, 3] of the SN. Web Server to Service Node

   In this direction, three kinds of messages may be sent: the
   Transaction Initiator message, the Data Message, and the End of Data

   The latter two messages are needed if the service to be invoked
   involves data (such as the case in click-to-fax, click-to-fax-back
   and voice-access-to-content). This was so designed to handle the
   varying size of data and to ensure that the size of each stream is
   within the allowable size of the underlying transport packet data
   unit (imposed by some implementations of TCP/IP).

   a. Transaction Initiator

   This message provides all the necessary information but data for
   invoking a service. It includes the following information elements:

   + Transaction ID, which uniquely specifies a service request. The
   same transaction ID should be used for all the accompanying data-
   related messages, if the service request involves data. One way for
   generating unique transaction IDs is to concatenate the information:
   date, time, Web Server ID (uniquely assigned for each one connected
   to the SN), and transaction sequence number (a cyclic counter
   incremented for each service request).

   + Service ID, which specifies the service to be invoked. The service
   may be click-to-dial-back, click-to-fax, click-to-fax-back or voice-

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   + Content Provider ID, which uniquely represents the content
   provider. This information is the key to accessing the content
   provider's service logic and data on the SN.

   + Content Provider Directory Number, which is the telephone or fax
   number of the content provider to be called through the PSTN.

   + User Directory Number, which is the telephone or fax number of the
   user requesting the service.

   + Billed Party, which specifies the party (either the user or content
   provider), to be billed.

   In addition, optional parameters may be sent from the Web Server to
   the SN. For example, a retry parameter may be sent to specify the
   number of times the SN will attempt to complete a service request
   upon failure before the transport connection times out.

   b. Data Message

   This message provides the (encapsulated) user data part of a service
   request. For example, in the case of click-to-fax-back such data are
   the content to be faxed to the user. Each message is composed of the
   transaction ID and a data segment. The transaction ID must be the
   same as that of the transaction initiator part first invoking the

   c. End of Data Message

   This message contains the transaction ID and the end of data
   delimiter. The transaction ID is the same as that of the relevant
   transaction initiator message. Service Node to Web Server

   The SN must respond to a service request from the Web Server. The
   response message consists of the information elements:

   transaction ID, service type, result, time, and error code.

   + Transaction ID, which is the same as that of the original service

   + Service Type, which is the same as that of the original service

   + Result, which is either success or failure.

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   + Time, which indicates the time of the day completing the request.

   + Error Code, which gives the reason for failure. Possible reasons
   for failure are content provider telephone (or fax) busy, content
   provider telephone (or fax) no answer, user telephone busy, user
   refusal to complete, user no answer, nuisance control limit reached,
   and content provider telephone (or fax) not in the SN database. Usage Scenarios: Click-to-Fax and Click-to-Fax-Back

   For the click-to-fax and click-to-fax-back services, the Lucent
   system implemented only the case where the data to be sent as
   facsimile reside in the Web server. There are at least three messages
   that need to be sent from the Web server to the Service Node for
   these services.

   The first message is the Transaction Initiator that identifies the
   service type as well as a unique Transaction ID. It also includes the
   sender/receiver fax number.

   The next is one or more messages of the data to be faxed. Each
   message carries the same unique Transaction ID as the above.

   Last comes the end of message. It consists of the Transaction ID
   (again, the same as that of the messages preceding it) and the end of
   data delimiter.

   Upon receiving these messages, the Service Node, equipped with the
   special resource of a fax card, converts the data into the G3 format,
   calls the receiver fax, and sends back the result to the Web server
   immediately.  Note that the receiver fax busy or no answer is
   interpreted as failure. Further, while the receiver fax answering the
   call is interpreted as success, it does not necessarily mean that the
   fax would go through successfully.

5.1.4 Web Server-SMS Interface and SNMP MIB

   This interface is responsible for uploading the content provider
   profile from the Web Server to the SMS and for managing the
   information against any possible corruption. The SN verifies the
   Content Provider ID and the Content Provider Directory Number sent by
   the Web Server with the content provider profile pre-loaded from the

   The content provider profile was based on ASN.1 [4] structure and
   SNMP [5] was used to set/get the object identifiers in the SMS

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   Following is an example of the simple MIB available on the SMS.

   inwebContProviderTable OBJECT-TYPE
           SYNTAX          SEQUENCE OF InwebContProviderEntry
           MAX-ACCESS      not-accessible
           STATUS          current
                   " A table containing Content Provider profiles "
           := { inweb 1}

   inwebContProviderEntry OBJECT-TYPE
           SYNTAX          InwebContProviderEntry
           MAX-ACCESS      not-accessible
           STATUS          current
                   " A conceptual row of the inweb. Each row
                           contains profile of one Content Provider"
           INDEX   { inwebSmsNumber }
           := { inwebContProviderTable 1 }

   InwebContProviderEntry := SEQUENCE {
           inwebSmsNumber                  Integer32,
           inwebContentProviderId          Integer32,
           inwebContentProviderPhoneNumber Integer32,
           inwebContentProviderFaxNumber   Integer32

   inwebSmsNumber OBJECT-TYPE
           SYNTAX          Integer32
           MAX-ACCESS      read-only
           STATUS          current
                   " Serial number of the SMS - used for SNMP indexing "
           := { inwebContProviderEntry 1 }

   inwebContentProviderId OBJECT-TYPE
           SYNTAX          Integer32
           MAX-ACCESS      read-create
           STATUS          current
                   " A number that uniquely identifies each Content
   Provider "
           := { inwebContProviderEntry 2 }

   inwebContentProviderPhoneNumber OBJECT-TYPE
           SYNTAX          Integer32
           MAX-ACCESS      read-create
           STATUS          current

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                   " Content Provider's Phone Number "
           := { inwebContProviderEntry 3 }

   inwebContentProviderFaxNumber OBJECT-TYPE
           SYNTAX          Integer32
           MAX-ACCESS      read-create
           STATUS          current
                   " Content Provider's Fax Number "
           := { inwebContProviderEntry 4 }

5.1.5 Security Considerations

   The Lucent prototype addressed the security issues concerning the
   interface between the Web Server and the SN. Those concerning the
   interface between the Web Server and SMS, which was based in SNMP,
   were handled by the built-in security features of SNMP.

   + Secure Communication Links

   If the Network Operator (PSTN provider) is also the Web Service
   provider, the Web Server and SN/SMS will communicate over a corporate
   intranet. This network is almost always protected by the
   corporation's firewall and so can be deemed secure. This was the case
   handled by the Lucent prototype.

   Nevertheless, if different corporations serve as the Network Operator
   and the Web Service Provider, then it is likely that there may not
   exist a dedicated secure communication link between the Web Server
   and SN/SMS. This raises serious security considerations. One possible
   solution is to use Virtual Private Networks (VPN). VPN features
   support authentication of the calling and called parties and
   encryption of the messages sent over insecure links (such as those on
   the Internet).

   + Non-Repudiation

   All transactions were logged on both the Web Server and the Service
   Node to account for all operations in case of doubt or dispute. The
   log information on the SN may also be used to generate bills.

   + Malicious Requests of Users

   A user may make repeated requests to a content provider directory
   number maliciously. This scenario was handled by setting a Nuisance
   Control Limit (NCL) on either the SN or the Web Server or both. The
   NCL has two parameters: one defining the number of requests from a

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   user and the other the period over which these requests takes place.

   A user may also attempt to request a call from a directory number
   other than that of a content provider. This scenario was handled by
   verifying the directory number (and the content provider ID) against
   the database on the SN containing all the content provider
   information. If the directory number (or the content provider ID) was
   not in the database, the request would be rejected.

5.2 Siemens Web Call Center

5.2.1 Service Description

   The Web Call Center is an Intelligent Network System that accepts
   requests from Internet nodes for services to be provided on the PSTN.
   As the name suggests, it was designed to support a cluster of
   services that, taken together, provide a subset of the features of a
   Call Center, with almost all user interactions provided via World
   Wide Web requests and responses. See the appendix for a background
   description of Call Center Features.

   From an Intelligent Network perspective, there are a number of
   services that, when combined, provide the Call Center features. The
   Call Center features as implemented supported the scenario in which a
   customer makes a request to be called back by an agent at a time of
   the customer's choosing to discuss an item of interest to him or her.
   The agent will be selected based on his or her availability and
   expertise in this topic; the agent will be told whom he or she is
   calling and the topic of interest, and then the agent will be
   connected to the customer.

   In addition, the individual services that were deployed to support
   this scenario provided support for management of the list of
   available agents as well. This involved allowing the agent to "log
   into" and "out of" the system and to indicate whether the agent was
   then ready to handle calls to the customer. The list of services, as
   seen from a user perspective, follows.

   The services support:

   i)  Customer Request service - the customer explores a corporate Web
   site, selects a link that offers to request an agent to call the
   customer back and then is redirected to the Web Call Center server.
   This presents customer with a form asking for name, the telephone
   number at which he or she wishes to be called, and the time at which
   the call is to be made. Note will also be made of the page to which
   the customer was referred to when he or she was redirected. Once the
   form has been returned, the customer receives an acknowledgment page

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   listing the parameters he or she has entered.

   ii)  Agent Registration/Logon - An agent requests a "login" page on
   the Web Call Center server. The service checks whether it has a
   record of an agent present at the Internet node from which th call is
   made. If not, then the caller will be sent a form allowing him or her
   to enter the service identity, the company's agent identifier and
   password. On return, the service identity and company agent
   identifier will be checked against a list of known identities. If
   found, the password will be checked, and if this matches the record
   held by the service then a new session record is made of this
   identity and the Internet node from which the call has been made.

   NB: This is very similar to the Universal Personal Telecommunications
   (UPT) service feature "register for incoming calls". It implies that
   the identified person has exclusive use of the Internet node from
   that point onwards, so messages for them can be directed there.

   iii)  Agent Ready - an agent who has already logged on can indicate
   that he or she is ready by requesting an appropriate "ready" page on
   the Web Call Center Server. The service will match the agent by the
   Internet node Identifier and Agent Identity passed along with the Web
   request against its list of "active" agents. It will mark them as
   being ready to handle calls in its list of available agents (with
   their pre-defined skill set).

   iv)  Agent Not Ready - an agent can request an appropriate "ready"
   page on the Web Call Center Server to indicate that he or she is
   temporarily not ready to handle calls.

   v)  Agent Logoff - an agent can request an appropriate "Logout" page
   on the Web Call Center Server to indicate that he or she is no longer
   associated with a particular Internet node. The service will match
   the agent by the Internet Node Identifier and Agent Identity passed
   along with the Web request against its list of "active" agents. Once
   found, the session record for that agent is removed and the caller is
   notified of this with an acknowledgment page.

   NB: This is very similar to the UPT "unregister" service feature.

   vi)  Call Center Agent Selection and Notification - When the time
   that the customer selected has arrived and an available agent with
   the right skills has been selected from the appropriate list, this
   service will send a notification to the Internet node associated with
   that agent. A dedicated server is assumed to be running on the
   agent's machine that, on receiving the notification, triggers the
   agent's browser into requesting a "Agent Call In" page from the Web
   Call Center Server. Once the agent's machine has made this request,

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   he or she will be told that there is a customer to call.

   NB: This is similar to a "Message Waiting" or "Wake Up Call" service.

   Note: As implemented, the agent is led automatically into the
   following service (the returned Web page includes an automatic reload

   vii)  Agent Instruction - a selected agent makes a request of the
   "Customer Processing" page on the Web Call Center Server. The
   Internet node Identifier and Agent Identity the agent uses will be
   matched against a list of agents expected to handle calls, and the
   instructions for the calls will be returned to the agent.

   NB: This is similar to a "Voice Mail Replay" message service, but in
   this case the message is automatically generated; there is no
   associated voice mail record feature accessible.

   Note: As implemented, the instructions page will include a number of
   buttons, allowing the agent to view the page the customer was looking
   at when he or she made the request, and to trigger the customer
   callback (as described next).

   ix)  Agent/Customer Telephony Callback -  the agent will make a
   request of a "dial-back" page on the Web Call Center Server. The
   Internet node Identifier and Agent Identity he or she uses will be
   matched against a list of agents expected to handle calls, and, when
   the appropriate records have been found, the service will make the
   telephone call through to the customer and then connect the agent to
   this telephone call (using the telephone number registered in the
   respective Call Center service record).

5.2.2 Implementation Introduction

   The Siemens Web Call Center used an existing IN system and service
   logic that supported Call Center features. The scenario it supports
   is very similar to the Siemens IN-based Call Center on which it was
   based; one of the goals was to minimize changes to the service
   offered. It is also virtually identical to the service "Internet
   Requested Telephony Dial-back" provided by the Lucent system.

   As provided via the Internet, the services involved are mostly the
   same as those provided via the PSTN and IN alone. The main
   differences lie in the use of the World Wide Web as an interface to
   the services rather than a telephone, SSP, and Intelligent
   Peripheral. Also, the feature by which a telephone call is made

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   between the agent and the customer is implemented within the IN
   system in a different way; this is the only element in which the PSTN
   is involved. Web Call Center Configuration

   The general arrangement for the Web Call Center system is shown in
   Figure 7.  The components that were added to an existing IN system to
   deal with the Internet interface are described next.

   In addition to the SCP, SSP and SMS that were part of the original
   IN-based system, another unit was included to send notification
   messages to agents; in the IN system the agents were sent "wake up"
   telephone calls when they were required to handle their next
   customers' call back. This unit is called the "Internet Intelligent
   Peripheral", and its use is described later under "Non-World Wide Web

   As there was a need to re-use as many of the existing IN components
   unchanged, a Gateway unit to deal with the interface between the
   Internet and the SCP was provided. This injected INAP (Intelligent
   Network Application Protocol) messages into the SCP, making it think
   that it had received an Initial DP trigger from an SSP. It also
   intercepted the "Connect To Resource" and "Prompt and Collect" INAP
   messages sent from the SCP, acting on these to return the parameters
   generated by the Internet users when they filled in the forms that
   triggered the service transaction. It also translated the "Play
   Announcement" message sent to the Intelligent Peripheral into a form
   that it could use.  Finally, it passed on the INAP message used by
   the SCP to trigger SSP into making the telephone call back. User Interaction

   In the IN/PSTN-based system, the services have contact with the
   customers and agents via their telephones, SSPs, and Intelligent
   Peripherals programmed to play announcements to them and to capture
   their responses. These responses are indicated by DTMF tones sent by
   pressing keys on the telephones.

   In this case, almost all interactions are provided via World Wide Web
   requests and responses. The sequence of announcements and responses
   for each service are "collapsed" into individual form filling
   transactions, and the requests are not limited to digits (or "star"
   and "hash"). The implications of the use of forms on service
   operation are covered in more detail later (under HTTP/IN Service

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   When provided via the IN/PSTN-based system, the services are passed
   the Calling Line Identity (CLI) of the caller and the number the
   caller dials (the DN). The CLI value is used extensively to identify
   the caller and (in the case of the agent) to index into service data
   tables to decide what to do next.  While an equivalent value to the
   DN is passed to the Web-based transactions as the requested Universal
   Resource Locator (URL), the CLI cannot be given reliably. The nearest
   equivalent caller identifier is the IP Address of the customer or
   agent's machine. However, the use of HTTP proxies means that this
   "original" Internet node Address may not be available; if a proxy is
   used then its IP Address will be associated with the request.

   In providing these Call Center features the customer only has one
   Web-based transaction; that of providing the initial request for a
   PSTN telephone callback. To do so he or she will have to fill in a
   form so as to specify not only the time to be called back, but also
   the telephone number to be reached.  These values can be used if
   needed to identify the customer, and so the problem of originating
   Internet Node ambiguity is not relevant.

   With the agents, however, there are sequences of coupled
   transactions, and the particular sequence must be identified. There
   will be a number of such transactions being carried out at once, and
   there needs to be some identifier to show which agent is being
   handled in each case.

   Such an identifier is not part of a sequence of basic Web
   transactions. In a Web transaction, the HTTP Client/Web Browser makes
   a request, and the HTTP Server will respond to this, normally
   including some content in its reply message that will be processed by
   the browser, after which it closes the TCP connection. That's the end
   of the transaction; the HTTP client and server cannot normally
   maintain state information beyond this point. Any sequence is reduced
   to a set of unrelated transactions.

   A result of this simple pattern is that any state information
   reflecting longer or more complex interactions must be stored (at
   least partially) in the client system. One approach is the use of
   cookies [6]. These can be set by HTTP servers as part of their
   response to a request, and will be sent back with all subsequent
   requests for appropriate URLs as extra HTTP headers. These cookies
   allow the HTTP server to identify the client in the following
   requests, so that it can continue an extended session with the

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   Cookies are used in providing the Internet Call Center. Persistent
   cookies are installed into the Web Browser on machines that are to be
   used by call center agents as a service management (pre-service)
   task. The cookie value is unique to the machine and is used to index
   into a list of machine IP addresses that is stored as part of the
   service data.

   Also, a session cookie is stored onto the agent's machine when the
   agent registers, and is cleared when he or she de-registers. This is
   used to identify the agent and so the IP address of the node with
   which the agent is associated (and from which the agent's subsequent
   requests should originate).  The services that interact with Call
   Center agents use the agent session cookie value as an identifier; in
   principle this is unnecessary but it does simplify the session data
   lookup procedure. The rest of the services use the persistent machine
   identifier in place of the CLI, indexing into their service data
   using it. Both cookies are sent with each agent request; if they are
   not present, then the request is redirected to other services (for
   example to the agent Logon service). Mapping from HTTP Transactions to IN-Based Service Features

   All of the client-initiated services require user interaction. With
   the IN/PSTN-based system, the majority of the services are typified
   by the callers being connected to an announcement unit that plays
   them a list of choices and captures their selection. The caller can
   pre-dial the digits needed; in this case the prompts are not needed
   and are not made.

   The pattern of operation is somewhat different in the Internet case,
   as the initial HTTP request returns a response, after which the Web
   transaction has ended. Where that initial response returns a form to
   be filled in by the caller, subsequently submitting the form
   initiates a new HTTP transaction.  This is all part of one instance
   of service, however. The service consists of two request/response
   pairs in tandem.

   Although it is possible to design a service to handle this pair of
   Web transactions as a single unit, it may be better to reconfigure
   it. The design of a service that deals with two Web exchanges as a
   single extended transaction is quite complex. It must maintain state
   across the pair of Web exchanges, and it has to handle a number of
   failure cases including dealing with time-outs and "out of time"
   submission of forms. The alternative is to split the service into two
   sub-features. The first of these reflects the initial request and
   delivery of the form by return, with the second one dealing with
   processing of the submitted form and returning any confirmation by

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   The services offered don't all require form-filling, and so can be
   treated as a single IN feature. There are two cases where forms are
   required. The first of these is the Customer Request service, while
   the other one is the "Agent Registration" service. In both cases the
   initial Web transaction (by which the form is requested and returned
   to the client) need not involve specific service logic processing;
   the initial delivery of the form to a customer or agent can be
   handled by a "normal" Web Server. In both cases the service logic is
   only triggered when the form is submitted; this means that, again,
   each of the services can be treated as a single IN feature.

   The IN service logic that deals with these requests has a general
   pattern of action. An HTTP request is received, and this triggers the
   IN service logic into action. The service logic "sees" this as an
   Initial DP message and starts its processing as if it had been sent
   from an SSF. The SCF uses what appears to it to be an Intelligent
   Peripheral to collect the parameters of the request, and then to send
   back final announcements to the requesting entity.

   The main difference, from the perspective of the IN service logic
   running on the SCF, is that the service does not need to instruct the
   SSF to make a temporary connection to the Intelligent Peripheral. It
   is as if this connection had already been made. Similarly, there is
   no need to close the service transaction by sending an explicit
   "Continue Execution" message to the SSF.

   The sequence of "prompt/collect" instructions used to collect service
   parameters from a caller in an IN service maps quite well to a
   sequence of requests to extract a data value from the HTTP request,
   based on a tag. This is a fairly standard feature of Web Server CGI
   or Servlet processing. Using this mapping minimizes the changes to
   the service design, in that the service logic "sees" an Intelligent
   Peripheral to which it sends normal "Request Report Prompt & Collect"
   messages, and from which it receives data values in response.

   All services have to fit in with the underlying HTTP interaction
   pattern, and so will be expected to send a final "Announce"
   instruction to the Intelligent Peripheral at the end of the service;
   this is done in many IN services anyway and in all of the service
   features described here. These announcements form the content
   returned to the Web Client.

Top      Up      ToC       Page 34 Non-World Wide Web Interactions

   There are two exceptions to the sole use of the World Wide Web for
   interaction. The first one occurs in the "Message Waiting"/"Wake Up
   Call" service by which the selected agent is informed of a callback
   request. World Wide Web transactions are very simple; the client
   browser makes a request for content associated with a particular HTTP
   URL, and the server sends a response, marking the end of the
   transaction. The server cannot make a spontaneous association with a
   client; it must be initiated by the client request.

   While it would be possible for the server to defer closing an earlier
   transaction (by not sending back all of the content specified and
   leaving the TCP connection open) it was decided that an alternative
   scheme would be more convenient. The "wake up call" was arranged by
   an "Internet Intelligent Peripheral" sending a request to a daemon
   process running on the selected agent's machine, using the Finger
   protocol [7]. The daemon sent back a standard response, but in
   addition the Web Browser on the agent's machine was triggered into
   making a further HTTP request of the server. In this way the "Agent
   Instruction" transaction is started automatically, while still
   allowing it to use a normal HTTP request/response pattern.

   The second exception occurs in the final "Agent/Customer Telephony
   Callback" service. While this transaction is initiated by the agent
   selecting a link on the "call instructions page" returned to them,
   and includes a "confirmation" page being sent back to them in an HTTP
   response, the purpose of this service is to make a telephone
   connection via the PSTN between the agent's telephone and the
   customer's telephone. It is the only service element that involves
   the PSTN directly. From an IN/PSTN perspective, the resulting
   telephone connection is different from that provided in the scheme
   using the IN and PSTN alone. In this case, a PSTN call is made out to
   the agent's telephone, another call is made out to the customer's
   telephone, and these calls are bridged. This differs from the earlier
   scheme, in which the agent originated a call to the voice mail replay
   system, and this call was redirected to a new destination (the
   customer's telephone). As this feature differs in purpose from the
   other services, and it requires a different implementation within the
   IN and PSTN system, it was organized as a separate service in this

Top      Up      ToC       Page 35 Security Considerations

   In the case of this system, assumptions were made that the interface
   presented to requesting agents and customers was provided via a fire
   wall to deal with most attacks on the IN components. The interface
   appeared as a Web Server, and there was no direct access to the HTTP
   documents served, nor to the servlets providing the service logic.

   The Callback service was deemed to have simpler security requirements
   than other IN services as it was akin to a free phone "1-800" service
   access number; the agents work for the service subscriber and are not
   charged directly. Similarly, the requesting customer is not charged
   for his or her request, nor for the resulting call back. Service
   subscribers would be willing to pay the costs of telephone calls
   generated as a result of this cluster of services, and the costs of
   running the agent services could be charged directly to them. As such
   the authorization for service is defined by the contract between the
   service subscriber and the service provider.

   Authentication of agents was seen as a problem. As an interim
   measure, cookies were used, but this scheme delivers the cookie data
   as a plain text item (a header of the Web request). Secure Socket
   Layer connections were required for communication with the agent
   services, and this had an impact on the performance of the IN system.

5.2.3 Derived Requirements/Lessons

   Security is seen as a major issue. A firewall was used to control
   access to the IN Components. Similarly, SSL was used for
   communication with the Agents, so as to protect the cookie values
   that they were sending with their requests.

   For other services, it is likely that the entity from which requests
   appear to originate will be charged for the service to be rendered.
   This has implications in terms of authentication and authorization of
   service provision at the time of the request. It is necessary for the
   service to be authorized in such a way that non-repudiation is
   ensured; this is likely to mean that a certificate of identity be
   provided from the person making the request, and that this can be
   tied in with a financial account that that person has with the
   service provider. The certificate can then be stored as part of the
   billing record.  While the process of electronic commerce is outside
   of the scope of this work, the mechanism by which a request for
   confirmation of identity is passed out to the requesting user and is
   delivered back to the service logic must be considered.

Top      Up      ToC       Page 36 
   When changing from a "pure" IN/PSTN system to one supporting requests
   via the Internet, the differences in the way that clients interacted
   with the services meant that the service logic had to be redesigned.
   It was realized that maintaining the state of a service during its
   processing was going to be a problem; this problem was side-stepped
   by re-engineering the services as form processors, allowing them to
   deal with fully specified requests as a single (Web) transaction. In
   addition, a "normal" Web Server was used to deliver the forms to the
   users. This is a change from the IN system, where the equivalent of
   the form (the prompts) were sent in sequence as part of the same
   service process.

   The Call Center features provided suited this change. However, this
   may not be the case for other IN services. It is quite common for
   services to be designed such that the user is prompted for a
   response, and the service continues dependent on this response. The
   Web form presents all of the options at once, so this kind of variant
   prompt/collect sequence is not possible. From this, it is difficult
   to see how an IN service could be reused without some degree of

   An intermediate "gateway" system was provided to "cocoon" the service
   logic as far as possible from the details of the components with
   which it was working.  Where needed, this unit translated calls from
   the service logic into commands that operated with the Internet (and
   the Web Server that acted as the interface). Our experience was that
   an SCP could be "spoofed" into thinking that it was operating with
   other IN components in the normal way. Within the limits of the
   service used, this proved simpler than was originally expected.

   Selecting this simple approach still allows a considerable range of
   services to be provided while maintaining any investment in existing
   IN systems.  Modification of existing IN service logic was also
   easier than feared. All of the services examined provided
   announcements at the end of the service transaction, and this could
   be used to trigger a Web response to be sent back to the requesting
   Internet user. The changes to the Call Center service logic turned
   out to be minor; it took as long to analyze the service and see how
   it could be arranged as a sequence of "form processing" transactions
   as it did to make the changes to the service logic.

   In the Siemens Web Call Center, the "Internet Intelligent Peripheral"
   with which the service logic communicated was running as a separate
   program on the same node. Where more complex behavior is required of
   it (such as conversion of text to speech data and interface with the
   PSTN) then it would almost certainly be on a separate node. If data
   is transferred from the Internet in such a scheme, any intermediate
   gateway would be involved in relaying the data to this node.

Top      Up      ToC       Page 37 
6. Alternative Solutions

6.1 The AT&T System

   AT&T developed a framework for controlling voice and voice-band data
   (e.g., fax) and for providing PINT services. Key to the framework is
   CallBroker, a logical entity that acts on behalf of a user to set up
   sessions and make requests for PSTN resources. The sessions typically
   include initiation of calls between two or more end points specified
   by the user. In addition to its interactions with the PSTN for call
   setup, the CallBroker is responsible for other functions, when
   necessary, such as authentication and usage recording.

   This section briefly discusses the protocol at the two interfaces
   that need to be defined and the corresponding APIs to provide the
   above services. The two interfaces are (1) the one between the
   CallBroker (or Web Server) and the Service Control Function in the
   Service Node in the PSTN and (2) the one between the IP client and
   the CallBroker. The latter interface, in particular, will enable
   service providers to extend the architecture defined here to serve as
   a platform for other advanced/value-added services (to be identified
   later). In addition, the view taken here is that the IP client is
   more general, and implements a protocol for communication with the
   CallBroker that allows full two-way communications. For example, this
   is required for the cases where a called party hangs up and an
   indication may be necessary to be given to the IP Client about this
   status/progress. This is also necessary when conferencing to give an
   indication/status of various parties joining the call.

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6.1.1 High Level Architecture

   A high level architecture depicting various logical entities and the
   Interfaces among these logical Entities and the IP Client is shown in
   Figure 12.

             1        _____   / 2   _____
    /|________________|    |________|    |   PSTN
                      |____|  \     |____|
                      Call     \    / SCF\
                      Broker    \  /  SN  \
                                /          \
                               /            \
                              /              \
                             __              __
                             /\              /\

                           Calling       Participant
                            Party      (Called Party)

                Figure 12:  The CallBroker Architecture

   The CallBroker, in addition to the initiation and control of calls on
   behalf of the user, performs additional functions. These functions
   include authenticating the IP Client, usage recording, and management
   of the session for the IP Client for the telephony call. The notion
   of the session requires that a client state machine be maintained in
   the CallBroker. This also helps in notifying the IP Client about the
   status/progress of the requests generated from the IP Client.

   From the perspective of the IP Client, the logical entities needed
   for the above functions are within the CallBroker and are as shown in
   Figure 13 below.  These correspond to the functions already
   discussed: Usage Recording Function, Session Management Function,
   Voice Bridge, and the Authentication Function.  The fact that some of
   these functions may be physically separate from the CallBroker (such
   as the Voice Bridge being in the PSTN) is not inconsistent with the
   general view adopted here. Thus, the CallBroker Model mediates
   requests for network services and enables us to define various value
   added services in the future.

Top      Up      ToC       Page 39 
   l              l
   l Call Broker  l                  Authentication
   l  Server      l                  Function
   l  ______      l    Interface 2a   ______
   l |      |x x xlx x x x x x x x x  |    |
   l |______|x    l                   |____|
   l       x    x  l
   l        x    xl  Interface 2b
   lSession State lx
   l   Mnmgt. x   l  x            Usage Recording
   l  Function    l     x             Function
   l _______    x l       x          ______
   l |     |      l          x  x x  |    |
   l |_____|     xl                  |____|
                   x Interface 2c
                 |     |


           Figure 13: Functional Entities in the Call Broker

   Various interfaces (i.e., 2a, 2b, 2c in Figure 13) between different
   functional entities in the CallBroker may also be standardized. The
   Session State Management Function may be physically realized as part
   of the CallBroker Server.

6.1.2 IP Client to CallBroker Interface

   Communication on the IP Client to CallBroker Interface (Interface 1
   in Figure 12) is a simple ASCII based protocol running directly on
   TCP. The messages on this interface are primarily requests from the
   client to the CallBroker, responses from the CallBroker to the IP
   client responding to the requests and unsolicited events from the
   CallBroker to the IP client. Since the communication is not strictly
   transaction oriented, traditional encapsulation protocols like HTTP
   cannot be used. There has been some ongoing work attempting to use
   multiple concurrent HTTP POST requests to support event delivery but,
   without too much difficulty, the ASCII protocol specified here can
   easily be mapped to the POST payload of the HTTP protocol.

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6.1.3 Protocol

   Basic Format

   The basic format of the protocol is as follows:


   The header and body of the protocol are separated by 2 line feed
   characters.  The format of the header and the body is described
   below. Line feed characters in the header or body will be escaped
   using simple URL encoding.


   [session-id | 0]<<LF>

   All CallBroker transactions are identified by sessions. A session
   does not necessarily correspond one-to-one to a TCP session. If the
   IP client is attempting to initiate a new session with the CallBroker
   the session-id field is populated with '0' to indicate session
   creation request. Every session request needs to be accompanied by
   sufficient information regarding authentication for the CallBroker to
   create the session.

   Message-id represents the operation of the message.

   Version-info contains optional version information of the protocol.
   This is to aid possible version mismatch detection and graceful error


   The body of the protocol messages consists of name value pairs. These
   name-value pairs are interpreted with reference to the message-id
   which signifies the operation to be performed by the CallBroker.

Top      Up      ToC       Page 41 
6.1.4 APIs Exposed to the IP Client

   The APIs of the CallBroker exposed to the IP client are distinct and
   different from the APIs that the CallBroker uses from the different
   supporting subsystems including the authentication subsystem and the
   usage recording subsystem. The IP client APIs enable clients to
   effectively control voice conferencing.

6.1.5 Voice-Bridge Control API

   The Voice Bridge Control API is used by CallBroker applications to
   access voice bridging functionality. The API distinguishes between
   sessions and calls. Calls represent actual voice calls placed from/to
   the voice bridge.  These calls can be grouped together in sessions.
   All the calls that belong to a session are bridged. Calls have a
   significance outside the scope of sessions. Every call can be
   associated with multiple sessions with different weights at the same
   time. The advantage of this approach is the ability to support
   concepts like whispering in a conference call. Calls can also be
   dropped from a conference session and bridged together in a new
   session to give the notion of a sub-conference. These calls can later
   be re-added to the main conference session.

6.2  Simple Computer Telephony Protocol

6.2.1 Overview

   The Simple Computer Telephony Protocol (SCTP) is a third party call
   control protocol and as such does not comply with the PINT charter.
   SCTP is described in this section to show how PINT services could be
   implemented using SCTP, and where SCTP fits into the PINT

   In addition to third party call control, SCTP also provides
   subscriber (i.e., user) feature management (e.g., allows a user to
   set do not disturb, call forwarding parameters), and subscriber
   monitoring of terminal, line and address status. SCTP is strictly
   client/server-based. It has no provisions for peer to peer
   communications. SCTP runs as a TCP application protocol. It is
   ASCII-based and uses sockets. The SCTP Server is usually connected to
   a switch via a CTI (Computer-Telephony Integration) connection.
   Because of this, feature interactions are limited to those within the
   context of a single call, and not between PSTN services. The SCTP
   Server within a PINT Gateway could also be connected to an SN, or an
   SCP. See figures below. SCTP does NOT carry media.

Top      Up      ToC       Page 42 
6.2.2 How SCTP Fits in with the Reference PINT Services

   SCTP Client as Part of a Web Server

   +------+    +--------+       +--------+    +------+
   |      |    |        | SCTP  |        |    |      |
   |      |----|        |-------|        |----|      |
   |      |    |        |       |        |    |      |
   +------+    +--------+       +--------+    +------+
   User's PC   Web Server/      PINT Gateway  SN/SCP/Switch

             Figure 14: SCTP Client as Part of a Web Server

   In this architecture, the SCTP Client is embedded in the Web Server.
   It is there for the specific purpose of initiating calls to the PSTN
   based on user requests. The SCTP Server is within the PINT Gateway.
   We go through the classic PINT examples:

   Click-to-dial-back: The SCTP Client issues an SCTP MakeCall to the
   SCTP Server with the calling number supplied by Web page, and called
   number supplied by the user.

   Click-to-fax-back: SCTP Client issues an SCTP MakeCall to the SCTP
   Server with called number set to user's fax machine, and calling
   number set to Web Server's fax machine, and treatment set to the URI
   for the file to be faxed.  The SCTP Server takes the file and feeds
   it into the call just as a fax machine would.

   Click-to-fax: SCTP Client issues an SCTP MakeCall with calling number
   set to user's fax machine, and called number set to Web Server's fax
   machine. How the file is supplied to the user's fax machine is
   outside the scope of SCTP.

   Voice-access-to-content: SCTP Client issues an SCTP MakeCall with
   called number set to user's telephone number, and calling number set
   to Web Server and treatment set to a URI for the file of the
   particular Web page to be read to the called number. The SCTP Server
   takes care of the file to voice conversion and this is fed into the
   call as if it were voice.

   In all of the above cases, the SCTP Client can generate a variety of
   different Web pages to send to the Web Server via CGI (Common Gateway
   Interface). The content of these pages is based on the call
   completion status of the CallMake SCTP action.

Top      Up      ToC       Page 43 
   SCTP Client Running on the User's PC

                 HTML        |      |  INTERNET
   +-----+    /--------------|      |
   |     |---/               +------+
   |     |                   Web Server
   |     |---\
   +-----+    \
   User's PC   \ SCTP        +------+       +------+
                \------------|      |-------|      | PSTN
                             |      |       |      |
                             +------+       +------+
                             PINT Gateway   SN/SCP/Switch

            Figure 15: SCTP Client Running on the User's PC

   In this architecture, the user has an SCTP Client co-located with it.
   If the user is using the telephone line for connection to a Web
   Server and there is an incoming call, then the SCTP Server in the
   PINT Gateway will post this event to the SCTP Client. A window will
   pop up on the user's screen with options available to the user for
   handling of the incoming call. The user can choose to take the call,
   send it to voice mail, or send it to another number.

   For the Fax back service, for example, if the user had a separate fax
   machine from his or her PC, then the SCTP Server would tell the SCTP
   Client there is an incoming fax. The user would end or suspend his or
   her Internet connection, the fax would come in, and the user could
   then resume the Internet connection.

(page 43 continued on part 3)

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