The LIS: A Location Server for Access Networks

There are lines in Figure 1.11 that identify some very important points of demarcation of responsibility in the context of the i2 architecture. The emergency network is responsible for dealing with the call and the dispatch of the correct first-responder resources. The VoIP provider is responsible for delivering the call to the correct PSAP based on the location of the caller and for ensuring that the location information is also delivered to the PSAP. In terms of the determination of the location information, however, the responsibility is shown as being with the access network provider.

In this architecture, a network entity called a location information server (LIS) is defined to exist within the access network. The primary function of the LIS is to provide location information to IP devices operating within the access network or to properly authorized applications requesting location through a reference mechanism. In terms of the question of trust previously discussed, the LIS should also support a mechanism that securely identifies the source of the location information. That is, the location information-if it is in a literal form-should be coded such that it securely identifies the source and time of applicability. Typically, this will be achieved by digital security techniques that provide a signature on the location information. The signature can be used by the recipient to determine the source of the information, the entity that signed the information, and to detect whether the information has been tampered with (for instance, changed since the signature was created).

As a case study, the emergency services application highlighted a number of requirements that an LIS should meet. It should be emphasized that an LIS and the location information it deals with are useful in far more ways than just emergency services and, similarly, the requirements are applicable to more applications than just emergency services. Let's list them.

• The LIS shall be able to determine the location of a client device based on its IP address.

• The LIS shall be able to deliver location information applicable to a device on request from that device.

• The device shall be able to ask for a literal location or for a unique reference that can be used by a third-party application and the LIS shall be able to respond accordingly.

• The LIS shall be able to ensure that it only delivers a device's literal location to authorized third-party applications.

• The LIS shall be able to provide location information in such a fashion that a third-party consumer of that location information can identify a responsible source of the location information.

At this stage, it should be pointed out that the preceding requirements do not in any way explain how an LIS actually operates or goes about meeting the requirements. Be patient, this information is on its way now that we are nearly finished establishing some context. Before getting into the nitty gritty of just how an LIS can work and how devices and applications talk to it, we shall revisit some of the lessons from the cellular network world once more and generalize them so that they are applicable regardless of the nature of the access network.