Architecture for PSTNOriginated Crossover Services

PSTN-originated crossover services originate in the PSTN, but at a later time, they cross over into the Internet for subsequent service fulfillment.

In such services, both networks — PSTN and Internet — are involved as follows: An Internet host informs the PSTN that it is interested in the occurrence of certain events, for instance, the event might be an attempt to call a certain PSTN number. When the said event occurs, the PSTN takes a snapshot of the state of the call and transfers this to the Internet host. The latter entity can execute arbitrary services upon the receipt of the notification. Thus, the state of the service is distributed across the two domains and some form of synchronization and a protocol are required to transfer the state of the service from the PSTN to the Internet for execution.

There are three conditions for a service to be considered a PSTN-originated crossover service:

1. Subscription: An Internet host subscribes to an event of interest in the PSTN

2. Action: The PSTN, during its normal course of operations, undertakes certain actions that lead to the occurrence of the event

3. Notification: The PSTN notifies the Internet host of the event and the service itself is executed on the Internet. Depending on the taxonomy of the service, it may be completely executed on the Internet, or the service execution may be shared between the two networks, as was the case with ICW.

A target architecture must thus support Internet hosts subscribing to events of interest occurring in the PSTN and the subsequent notification of the concerned Internet host about the said event of interest by the PSTN.

Given the background, we now propose our architecture for realizing PSTN-originated crossover services that meet the three conditions outlined above. The architecture is deceptively simple, and in keeping with the Internet tradition, it distributes the intelligence to the edges. In fact, the entire PSTN is simply viewed as an Internet user agent (UA) to provide crossover services. Figure 6.2 depicts the architecture.

The architecture is based on separating the network on which the service executes from the one that provides events required for service execution. The service itself is executed entirely on the Internet, but the events that lead to the execution of the service occur on the PSTN. Wireline and cellular telephone networks present a rich palette of events upon which Internet services can be built: registration, mobility, and text messaging are some of the events beyond normal call control that can influence Internet services.

Our architecture, as depicted in Figure 6.2, uses the publish/subscribe mechanism that has proved to be well suited for an event-based mobile communication model [CUG02, MEI02]. User agents (software programs)

Cellular Network

Wireline Network

User Agents

Legend:

HLR: Home Location Register. Primary Database Repository of Subscriber Information.

VLR: Visitor Location Register. Maintains Temporary Records of Subscribers.

MSC: Mobile Switching Center. Provides Cellular Radio Telephony Switching Functions.

SCP: Service Control Point. Stores program Logic for a Service Subscribed to by a Cellular Subscriber.

SMS-C: Short Message Service Center. Provides SMS Service to Subscribers.

Cellular Network

Wireline Network

SCP

Wireline Switch

Other Intelligent Network Entities

-PSTN-Specific Protocol

---Internet-Specific Protocol

Functional Interface or Private Protocol

User Agents

Legend:

HLR: Home Location Register. Primary Database Repository of Subscriber Information.

VLR: Visitor Location Register. Maintains Temporary Records of Subscribers.

MSC: Mobile Switching Center. Provides Cellular Radio Telephony Switching Functions.

SCP: Service Control Point. Stores program Logic for a Service Subscribed to by a Cellular Subscriber.

SMS-C: Short Message Service Center. Provides SMS Service to Subscribers.

Figure 6.2 PSTN-originated crossover services architecture.

on the Internet subscribe to events on the PSTN. When the event occurs, the PSTN notifies the UA that executes the desired service. The centerpiece of the architecture is the Event Manager (EM), which straddles both networks. It insulates the PSTN entities from Internet protocols and vice versa. It is also responsible for maintaining the subscription state so it can transmit notifications when an event subscribed to transpires.

Figure 6.2 depicts the EM as a stand-alone entity; however, in reality, it may be physically co-resident on the Service Control Point (SCP) or a switch. Our architecture does not limit where the EM is actually located. The only aspect our architecture requires is that the EM has a communication path to the entities in the network that will be generating events. Thus, Figure 6.2 depicts the EM connected to the various entities using dotted lines; the dotted lines represent a functional interface if the EM is co-resident on a certain entity; otherwise, they represent some message passing protocol, the details of which are immaterial to the architecture. The EM should also be able to set dynamic detection points in the SCP (see discussion in Section 3.1.5).

Figure 6.2 shows the PSTN domain on the left-hand side of the diagram and the Internet domain on the right-hand side. The PSTN domain consists of both cellular and wireline networks. Entities on these networks generate events during normal operations; it is these events that need to be captured and transported to the Internet for service execution. The service will execute on the Internet user agents.

Although the architecture appears simple enough, there are research issues that must be addressed. These are cataloged next, along with means to combat them.

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