Today’s Public Switched Telephone Network (PSTN) offers many different services, each with a desirable suite of features and functionality. Service providers (SPs) can offer competitive services, differentiate themselves, and generate revenue from these offerings. Many of these services are not currently available in packet voice networks. However, much work is being done to determine the best and most scalable way to add these and many new services and functions to packet voice networks.
From an external point of view—or in terms of what the end user can use—the current generation of digital switching systems offers similar, if not identical, functionality. This makes it difficult for SPs to differentiate among the offerings. Traditionally, most of the value-added software used with advanced services is bundled with the switch itself.
This chapter explains how the PSTN offers services to consumers and business customers. It discusses ways corporations use different applications to drive their business and bottom-line profits, the main features of these applications, and how enterprise and PSTN networks work together. The following services are discussed:
Plain old telephone service (POTS)
Business services, including virtual private networks (VPNs), Centrex, and call-center services
SP services, including database and operator services
Standard telephony service is commonly referred to as plain old telephone service (POTS). This service provides a dial tone to rotary and touch-tone telephones, as well as access to national and international carriers. Standard dialing devices such as 500-type telephones (rotary), 2500-type telephones (touch-tone), facsimiles, and modems are compatible with POTS.
POTS evolved internationally to include the emergency number services and operator services. The following additional features and services are now almost universally available to POTS subscribers:
Each feature is discussed in more detail in the following sections.
The services available from custom calling features have been popular since they were introduced in the PSTN. Although you can activate and use each function individually, SPs usually combine features in a single package for simplicity and convenience.
The providers enable and control custom calling features from within end office switches directly. Signaling System 7 (SS7) messaging and service enablers are not required to operate these features. The following list describes some common custom calling features:
Call forwarding—Enables calls to follow as the subscriber moves from one location to another
Call waiting—Indicates an incoming call when the subscriber is already involved in a call
Three-way calling—Enables subscribers to conference a third party in on a conversation
Speed dialing—Provides a convenient way for subscribers to store frequently used numbers (this is often provided via the memory of the end-user phone)
Added number—Enables subscribers to add a second line that they can identify by a distinctive ring and call-waiting tones
CLASS is a popular suite of features available to subscribers. CLASS features provide subscribers with a powerful and convenient tool to control incoming and outgoing calls. Telecordia, formerly known as Bell Communications Research (Bellcore), defined the CLASS standard, which added to the custom calling feature foundation. With CLASS, users interact with the switch software from their own telephone sets and give instructions on which services they want. SS7 messages and functions are then invoked and sent within the network to perform the requested operations.
The following list describes some common CLASS features:
Customer-originated trace—Enables the subscriber to dial a code after he or she receives a harassing call, which notifies the local law enforcement agency.
Automatic callback—Used when the subscriber receives a busy signal. This feature notifies the subscriber when the called party line is free by placing the call. (This feature is also referred to as camp on.)
Automatic recall—Enables the subscriber to easily return a missed call.
Display features—Requires a display telephone to display the calling name and calling number.
Calling number blocking—Enables the called party to hide his or her identity when dialing subscribers who have CLASS display capabilities.
Call screening—Enables subscribers to accept, reject, or forward calls based on a list of received calling numbers.
PSTN-based voice mail enables SPs to offer an alternative to answering machines. This is attractive because subscribers do not need to purchase or operate any additional equipment. Another benefit of network-based voice mail is that voice messaging is still available even if the called party’s line is busy. Network-based voice mail also enables the subscriber to retrieve their voice mail from remote locations. The two main voice mail services available to residential and small-business users are
Voice messaging—Enables subscribers to store and play recorded greetings and receive, review, and distribute messages from outside users
Fax messaging—Enables subscribers to receive faxes and view them at a later time
Business services are important to SPs because they represent a large portion of SP’s revenue base. Corporate environments require an extensive array of communication services to support their businesses. The following services are described in this section:
Virtual Private Voice Networks cost-effectively interconnect corporate voice traffic between multiple locations over the PSTN. The alternative to Virtual Private Voice Networks is dedicated tie-lines between locations. Because multiple locations are not typically served by the same exchange, however, Virtual Private Voice Networks are a far more economical solution in such scenarios.
SPs offer cost-competitive Virtual Private Voice Network services by maximizing the private use of public infrastructure. Therefore, public network facilities are somewhat balanced by corporate use during weekdays and residential use during nights and weekends.
Virtual Private Voice Network customers access the public network by interconnecting private network facilities, such as T1 circuits. SS7 facilities, messaging, and inter-working enable VPNs across the public infrastructure. SS7 capabilities also enable corporate and PBX features to be carried transparently across the network. Another benefit of deploying a voice-capable network is to ease the process of adding new and multiple sites to an existing Virtual Private Voice Network. With Virutal Private Voice Networks, this is as easy as adding a new connection to the network and provisioning the appropriate translations and dialing plans. With tie-lines, on the other hand, new end-to-end connections are required between the new location and each existing location, resulting in higher costs to the customer.
Public switching systems identify, process, and route calls based on different protocols and dialing plans. The identity of each Virtual Private Voice Network is made possible by customer-group ID numbers that are maintained and transmitted across the public network through SS7. You use this capability to distinguish and route public calls from private intra-network calls. The customer group identity and other information is inserted in ISDN User Part (ISUP) messages for transmission across the public network.
Dialing plans ensure proper handling of full North American Numbering Plan (NANP) 10-digit station routing and 7-digit on-net to on-net, on-net to off-net, and off-net to on-net routing. The call processing, routing, and dialing capabilities provide uniform dialing plans and access to users at remote locations.
Note
Dedicated Access Lines (DALs) connect to the public network. DALs provide both public and private routing of calls and connect through various signaling protocols. These include ISDN, channel-associated signaling (CAS), QSIG (Q Signaling), and Digital Private Network Signaling System (DPNSS), which are detailed in Chapter 3.
Centrex enables SPs to offer smaller businesses voice and data services similar to those found in larger and more costly private solutions. Centrex services can be delivered by public switching infrastructure and do not require costly customer premise equipment.
The Centrex software loaded in the switch can create a virtual private business network. Centrex services are comparable to on-premise systems and provide call handling, distribution, accounting, and data networking between sites. You can access Centrex services in the following ways:
POTS—You can designate and use these lines as Centrex lines.
Feature lines—Equipped with fully featured telephones, these lines can provide additional features and functionality over standard POTS lines, ISDN circuits, or Switched 56/64 circuits.
Centrex offers many features to subscribers. The following list explains some of Centrex’s services and capabilities:
Call handling—Includes call waiting, call forwarding, call park, hunt grouping, and voice mail
Convenience features—Include automatic dial, speed dialing, ring again, and calling line identification (caller-ID)
Custom dialing plans—Provide customized plans for each customer group and enable abbreviated dialing for internally placed calls
Management—Can track and control various aspects of a business’ service
Security—Includes line restrictions, employee authorization codes, virtual access to private networks, and detailed call records to track unusual activity
Call centers have large volumes of incoming calls and need to be able to efficiently handle and distribute these calls among multiple agents. The distribution method needs to be intelligent enough to route calls to answering positions that are occupied with the appropriate personnel. Reservation centers, courier companies, and government agencies all need call center-type services. The following implementations are the most commonly used in call centers:
Automatic call distribution (ACD)— Efficiently routes incoming calls to multiple answering stations. ACDs also enable call centers to track usage patterns as well as traffic and agent performance. ACD systems provide many specific features, such as queuing of customer calls, answer in order of arrival, and agent routing based on most idle. ACD systems are located in the central office or at the customer site. In central offices, SPs deliver the service over public lines such as Primary Rate Interface (PRI). If the ACD is located at the customer site, it is typically connected to the public network using trunks.
Switch-Computer Applications Interface (SCAI)—Enables SP switches to communicate to the call center computer for appropriate call routing and handling. In the SCAI method, call center computers store and coordinate incoming and outgoing calls and provide the switch with appropriate routing information. SCAI services include providing information on the calling party as the call is passed to the agent. These services can also interconnect calling parties to an Interactive Voice Response (IVR) system to collect more information before routing the call to the appropriate answering position.
Service provider (SP) services are internal functions provided in the background to support PSTN users. They include number translation, routing, calling services, and special assistance. The following SP services are discussed in this section:
Databases enable SPs to maintain, access, and translate information used to support special service and access numbers. These information databases are centrally located and accessible to all end offices. Databases hold subscriber calling and called information that is used in inter-exchange call processing. Databases in the United States frequently provide the following services:
800 number services—Enable SPs to offer the called party, typically businesses, bulk usage rates for high levels of incoming calls. Translation from these service numbers to the actual number is provided by SP 800-database access.
900 number services—Typically used for providing information, contest call-in numbers, and public opinion polls. The dialed 900 number is translated into the actual number by accessing the database. Unlike 800 services, 900 services imply that the calling party pays to access the service.
Calling card services—Enable subscribers to access long-distance services from almost any type of PSTN access. Databases provide access and billing by authenticating subscriber account codes and personal identification numbers (PINs), as well as recognize dialed access codes for routing to the long-distance service or requested carrier.
Authorization services—Establish access to VPN services and provide security against fraud. Activation and validation are enabled through five- to seven-digit authorization codes.
Operator services have changed tremendously in the past few years. The main driver of this change was the advancement of technology in automated systems, speech recognition and recording, online information databases, and SS7 service-invoking capabilities.
Today, central office switching systems can handle most of the calls that once required operator intervention. This enables operators to focus on special-need and value-added types of revenue-generating services. The typical operator services available today are as follows:
Toll and assistance—Even in today’s PSTN, operator intervention is required to complete some long-distance (toll) calls and provide general assistance. The reasons for using toll operators include accounting for coin usage in public payphones, alternative billing needs such as called party charging, and controlling guest bills for hotels. By simply dialing 0, hotel guests can reach a hotel operator who can provide room rate information, can accurately transfer calls, and can provide valuable emergency assistance.
Directory assistance—Directory services are available by dialing a three-digit national code or an area-specific code. Directory assistance operators can search for telephone numbers based on directory listings. After the operator finds a match, he or she transfers the call to an audio response unit that quotes the number to the customer. The database search engines available to SPs are extensive and provide effective and timely responses for requesting subscribers.
Billing services—Operator intervention is still required for about 20 percent of long-distance calls in the U.S. The bulk of this intervention is for collect calls, third-party billing, calling cards, and credit card services. The remaining 80 percent of long-distance calls are handled by automated systems, speech recognition, recording technology, and databases all linked together through an underlying SS7 network.
Plain old telephone service will not be plain for much longer. With broadband (DSL, cable, wireless) access to the home, voice will simply be another application in everyone’s home. The available features listed in the previous list are just the tip of the iceberg. This chapter discussed enhanced services, where the PSTN operators make a hefty portion of their revenue.
The PSTN offers many valuable services to subscribers and is critical to the operation of small, medium, and large businesses. These subscribers and businesses are, however, increasingly relying on the power and value of data networking and the Internet. To this end, the PSTN services discussed in this chapter as well as new voice services will, over time, be delivered over data networks and the Internet.