1.10 PSTN Requirements

This topic describes PSTN requirements.

Countries have varied requirements for PSTN calls:

  • Dial rules for the called-party number on outbound PSTN calls
    1. PSTN access code
    2. National access code
    3. International access code
  • Presentation of called- and calling-party numbers on inbound and outbound calls
    1. Length of number and its components
    2. ISDN number types
    3. Overlap send and overlap receive
    4. + prefix on E.164 numbers
  • Emergency dialing

Various countries can have varied PSTN requirements. This fact makes it difficult to implement dial plans in international deployments.

One of the issues in international deployments is the variation of PSTN dial rules. For example, in the United States, the PSTN access code is 9, while in most countries in Europe, 0 is used as the PSTN code. The national access code in the United States is 1, while 0 is commonly used in Europe. The international access code is 011 in the United States, while 00 is used in many European countries. Some PSTN provider networks require the use of the ISDN TON, while others do not support it. Some networks allow national or international access codes to be combined with the ISDN TON. Others require you to send the actual number only (that is, without any access codes) when setting the ISDN TON.

The same principle applies to the calling-party number. As mentioned earlier, in variable-length numbering plans, the TON cannot be detected by its length. Therefore, the only way to determine whether the received call is a local, national, or international call is by relying on the availability of the TON information in the received signaling message.

Some countries that have variable-length numbering plans use overlap send and overlap receive. With overlap send, a number that is dialed by an end user is passed on to the PSTN digit by digit. The PSTN will indicate when it has received sufficient numbers to route the call. Overlap receive describes the same concept in the opposite direction: when a call is received from the PSTN in overlap mode, the dialed number is delivered digit by digit, and not en bloc. Some providers that use overlap send toward their customers do not send the prefix that is configured for the customer trunk, but only the additional digits that are dialed by the user who initiates the call.

When dialing PSTN numbers in E.164 format (that is, numbers that start with the country code), the plus (+) sign is commonly prefixed to indicate that the number is in E.164 format. The advantage of using the + sign as a prefix for international numbers is that it is commonly known as a + sign around the world. In contrast, PSTN access codes such as 011 (used in the NANP) or 00 (often used in Europe) are known only in the respective countries.

Finally, emergency dialing can be an issue in international deployments. As various countries have various emergency numbers and various ways to place emergency calls, users are not sure how to dial the emergency number when roaming to other countries. An international deployment should allow roaming users to use their home dialing rules when placing emergency calls. The system should then modify the called number as required at the respective site.

Issues Caused by Different PSTN Dialing

Different local PSTN dial rules can cause several issues, especially in international deployments.

How do you store PSTN contacts so that they can be used from any site?

  • Different ways to store or configure PSTN destinations:
    1. Speed dials
    2. Fast dials
    3. Address book entries
    4. Call lists
    5. AAR targets
    6. Call Forward destinations
  • Stored numbers can be used at multiple sites (countries) because of roaming users using local PSTN gateways.
    1. Cisco Extension Mobility
    2. Cisco Device Mobility
    3. PSTN backup
    4. TEHO and LCR

The main problem that must be solved in international environments is the determination of how to store telephone numbers of contacts. Address book entries, speed and fast dials, call list entries, and other numbers should be in a format that allows them to be used at any site, regardless of the local dial rules that apply to the site where the user is currently located.

The same principle applies to numbers that are configured by the administrator—for example, the target PSTN number for AAR targets. Call-forwarding destinations should also be in a universal format that allows the configured number to be used at any site. The main reason for a universal format is that a multisite deployment has several features that make it difficult to predict which gateway will be used for the call. For example, a roaming user may use Cisco Extension Mobility or Device Mobility. Both features allow an end user to utilize local PSTN gateways while roaming. If no universal format is used to store speed dials or address book entries, it will be difficult for the end user to place a PSTN call to a number that was stored according to the NANP dial rules while in countries that require different dial rules. Even when not roaming, the end user can use TEHO or LCR, so that calls break out to the PSTN at a remote gateway, not at the local gateway. If the IP WAN link to the remote gateway is down, the local gateway is usually used as a backup. How should the number that is used for call routing look in such an environment? It is clearly entered by the end user according to local dial rules, but, ideally, it is changed to a universal format before call routing is performed. After the call is routed and the egress gateway is selected, the number can then be changed as required by the egress gateway.