Address Translation

A subject that is often confused with gateway discovery is the translation of a telephone number into an IP address. Among the services that need such translation are phone-to-PC calls made from a PSTN telephone to a PC (or to any telephony-capable IP appliance) on the IP network. A way to accommodate such a call from the PSTN is to assign the PC a telephone number. This not only allows a PSTN operator to leverage its existing PSTN infrastructure to offer IP telephony services, but also makes it easy for the telephone user to place a call to a PC. Of course, telephone numbers for this purpose should adhere to a special numbering plan that is distinct from the ones used in traditional telephony services. Depending on its geographical scope, this special numbering plan is under the administration of ITU-T or the national (or regional) telephone numbering authority.

Another service in question is Internet Call-Waiting, with which an end user can be notified of an incoming call while using the telephone line for a dial-up connection to the Internet. Upon receipt of the notification, the user then has the option to reject or accept the call. Either way, there are further details on the disposition of the call. What is relevant here is that the notification is delivered to the called party (identified by a telephone number) over the Internet. Ensuring instant notification of a call in waiting typically requires the knowledge of the IP address of the PC connecting to the Internet via the telephone line.

In general, any directory-like technology can support the type of translation in question. An example is a domain name system, which is best known for mapping a domain name (for example, to an IP address. Another example is LDAP, which is used to look up information (for example, John’s e-mail address) in a directory that is organized in a special tree structure. A caveat is that whatever existing directory technology is used must be adapted to satisfy the special requirements posed by IP telephony. One such requirement is that the directory used must allow for frequent updates of its entries. This arises where an IP endpoint is assigned an IP address dynamically, as is often the case in dial-up connections. In contrast to the relatively static telephone number given to the IP endpoint, the IP address changes at each connection. Another special requirement has to do with the real-time performance need of IP telephony. The additional step for directory lookup must not introduce significant delay in the IP telephony setup procedure.

An immediate benefit of using the general directory technology for telephone-number-to-IP-address translation is that other attributes associated with the endpoint can also be obtained with the operation. Consider as an example Mary, who wishes to receive calls at her PC over the IP network only from her daughter between 5:00 and 6:00 p.m. every day. Such information can be stored in the directory. If so, a call to Mary could trigger a directory query whose response includes the IP address of Mary’s PC as well as her preference for receiving calls. The additional information can then be used to process all calls to Mary. For instance, a call from a friend at 5:15 p.m. will result in no attempt at call setup inside the network. Instead, the call will be redirected to Mary’s voice mail. If you find this scenario familiar, you are right. It is similar to an IN-supported service where the user’s policy (or service logic) plays a part in the overall call processing and the policy is stored somewhere inside the network. This similarity again suggests that a networked repository for policies and dynamic information (for example, IP addresses) with simultaneous access from the PSTN and IP network is an effective device for supporting integrated services.

The subject of telephone number translation has been addressed in the IETF Telephone Number Mapping (enum) working group ( ), which has been chartered to “define a DNS-based architecture and protocols for mapping a telephone number to a set of attributes (for example, URLs) which can be used to contact a resource associated with that number.”

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