Supplementary Services

Supplementary services are one of the ISUP advantages noted in this chapter's introduction. ISUP provides many messages and parameters that are explicitly created for the support of supplementary services across the network. The introduction of ISUP has helped to greatly standardize widely used services, allowing them to operate across networks and between vendors more easily. Service specifications still vary between different networks based on differences in locales and market needs. ISUP provides the flexibility to accommodate these differences using a rich message set and a large set of optional parameters.

The ITU-T defines a core set of widely used ISDN services in the Q.730–Q.739 series of specifications using ISUP network signaling. The actual specification of these services at the national level can vary. In addition, national networks and private networks offer many services outside of those that are specified by the ITU-T. In the United States, Telcordia has defined a large number of services in various Generic Requirements (GR) specifications for U.S. network operators.

The list of services implemented on modern telephony switches has grown quite long. However, the purpose of this section is not to explore the services themselves, but to provide examples of how ISUP is used to support them. Two examples of common services have been chosen to discuss how ISUP provides support for them: Calling Line Identification and Call Forwarding Unconditional.

Calling Line Identification (CLI) Example

ITU Q.731 specifies Calling Line Identification (CLI). Calling party information can be used at the terminating side of a call in many different ways. Following are a few examples:

• Calling Number Delivery (CND)

• Calling Name Delivery (CNAMD)

• Incoming Call Screening

• Customer Account Information Retrieval (Screen Pops)

Being able to identify the calling party allows the called party to make decisions before answering a call. For example, an end user can use call screening to allow them to choose which calls they wish to accept. A business might use the incoming number to speed the retrieval of customer account information to call centers. If the called party subscribes to Calling Name Delivery, the CgPN is used at the terminating exchange to retrieve the name associated with the number.

CLI is specifically defined by the ITU-T as:

• Calling Line Identification Presentation (CLIP)

• Calling Line Identification Restriction (CLIR)

The ISUP CdPN parameter contains an Address Presentation Restricted indicator that specifies whether the calling party identification can be presented to the called party. The Address Presentation Restricted indicator has the following possible values:

• Presentation allowed

• Presentation restricted

• Address not available

• Reserved for restriction by the network

If the terminating party subscribes to the CLI service, the terminating exchange uses this indicator's value to determine whether the number can be delivered. The number is delivered only if the value is set to Presentation allowed. If the connection encounters non-SS7 interworking, the address information might not be available for presentation. In addition, transit network operators might not transport the information in some cases, depending on regulatory policies. While the actual display to the end-user varies depending on location, it is quite common to see restricted addresses displayed as "private" and unavailable addresses displayed as "unknown" or "out of area."

In some networks, if the CLI is not present in the IAM, it might be requested from the calling party using an Information Request (INR) message. The originating exchange delivers the requested CLI using an Information (INF) message.

Call Forwarding Example

Call Forwarding is part of a larger suite of services known as Call Diversion services. There are many variations of Call Forwarding. The ITU-T in the Q.732 specification defines the standard set of Call Forwarding variations as follows:

• Call Forward Unconditional (CFU)

• Call Forward No Reply (CFNR)

• Call Forward Busy (CFB)

Other variations of Call Forwarding exist within localized markets. For example, Call Forwarding Selective is another variation that allows forwarding for calls that originate from selective calling numbers. For this example, we have chosen Call Forward Unconditional to illustrate the use of ISUP signaling.

In Figure 8-20, the ITU-T message flow is shown for CFU at SSP B. The ANSI message flow differs slightly from that shown for ITU. A subscriber at SSP B has forwarded their calls to a number at SSP C. When SSP B attempts to terminate the call and encounters the Call Forward service, a new IAM is sent to SSP C. Keep in mind that a call might be forwarded multiple times before reaching its destination. The additional parameters included in the IAM for Call Forwarding convey information about the first and last instances of forwarding. In our example, the IAM to SSP C contains the following parameters, specific to the call redirection:

• Redirection Information (RI)

• Redirecting Number (RN)

• Original Called Number (OCN)

The inclusion of the RI parameter varies among different networks, so it might or might not be present. The RI parameter contains the following information fields:

• Redirecting Indicator— Not specified for ANSI networks. This field indicates how the call was forwarded and the presentation restriction indicators regarding the RI and RN.

• Original Redirecting Reason— Indicates why the first forwarding station forwarded the call (for example, no reply or unconditional). This field is set to unconditional in the example illustrated in Figure 8-20.

• Redirection Counter— Indicates the number of times a call has been forwarded. This counter is used to eliminate forwarding loops where a call ties up network resources because it is forwarded an excessive number of times. The ITU and ANSI standard for maximum redirections is five. In ANSI networks, the Hop Counter parameter provides this counter when RI is not included for forwarded calls. This field is set to 1 in the example illustrated in Figure 8-20.

• Redirecting Reason— Indicates the reason the call is being forwarded. In our example using CFU, the reason indicator is set to unconditional.

The OCN is the number dialed by the originator at A. The RN is the number of the station that forwarded the call. The RN is usually the same as the OCN, unless the call has been forwarded multiple times. If multiple forwardings have occurred, the RN is the number of the last station that forwarded the call. The CdPN will be set to the "forwarded to" number. Translation and routing using the new CdPN from the forwarding service at SSP B determine that the call should be directed to SSP C.

At SSP B, an ACM is returned to the originator and a new call is attempted to the forwarding destination. Note that for ANSI networks, an ACM is not returned until the ACM is received from the new destination exchange, therefore, eliminating the CPG message.