Location in an IMS Network


The IP Multimedia Subsystem (IMS) suite of standards is defined within 3GPP and is based around a services core network that operates over IP providing the benefits of a consistent set of services that are managed and offered in the same way over a range of different access technologies. The IMS suite of specifications offers little mention of location determination and acquisition beyond cursory commentary, such as stated assumptions that the UE will provide it and, more recently, in the IMS emergency sessions specification (see reference 8) where a Location Retrieval Function (LRF) and Route Determination Function (RDF) have been described.

The basic IMS architecture assumes that the core and the access network have an association which may be in the form of ownership, a roaming agreement, or some other kind of access arrangement (see Figure 8.12). The core network is made up of a Call Session Control Function (CSCF) and a range of ancillary functions, such as: the Home Subscriber Server (HSS), which operates in a similar manner to the HLR in cellular mobile networks; the Media Gateway Control Function (MGCF) and media gateways that provide access to things such as the PSTN; the Border Gateway Control Function (BGCF) and border gateways that provide access to other IP-related services; Application Servers (ASs); and streaming media services.

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Figure 8.12: The basic IMS architecture.

When a host attaches to the network, it discovers the Proxy CSCF (P-CSCF) in the access network and establishes a session with it. The P-CSCF contacts the serving CSCF (S-CSCF) in the core network, which authenticates the user and determines the services that a user is entitled to use. The P-CSCF is then responsible for ensuring that the correct QoS is provided to the user's service as required. P-CSCF in this model therefore provides a bridge from the local access network to the core network services.

In the case of emergency calling, it is the responsibility of the P-CSCF to detect the condition and to direct the call to an emergency control function and subsequently route the call to the serving PSAP. The IMS emergency sessions specification (see Reference 8) indicates that the User Equipment (UE) should provide location information in-band with the signaling, or that P-CSCF should obtain location information and possibly routing information from a Location Retrieval Function (LRF) before passing the call on to an Emergency CSCF (E-CSCF), which is located in the same network as the P-CSCF. This all implies that the LRF also resides in the same network as the P-CSCF.

Since the LRF provides location information for IMS networks and the access network could be any one of a range of access types, it makes sense to support a common interface that can provide location information in the form most applicable to the access type; this may be a civic location for a residential DSL network or geodetic information for a wireless 3G network.

Figure 8.13 shows the advantage of using a LIS with a HELD interface as the Location Retrieval Function (LRF) in an access network that can accommodate IMS. The key advantage comes from the fact that all requests for location arrive in the same manner, regardless of whether they are from the core IMS network, the UE/UA/MS, a visited network LIS, or a third party. The LIS determines the access type most appropriate for calculating the location of the end-point. Whereas the LIS supports multiple access types-DSL, WiFi, and cellular-the end-point can be associated with more than one network. Because of this the LIS has the option of choosing the network technology best suited for providing location at that point in time. So while the IMS provides integrated services across a range of access technologies, the LIS provides a common way to determine and acquire location across a range of access technologies.

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Figure 8.13: An integrated location solution.

Indeed, if IMS services are not to remain confined to a "walled garden" where subscribers can only use those services from a limited number of operator-provided access networks, then a LIS/HELD implementation becomes an imperative. Otherwise, the IMS applications will not know how to provide location-enhanced service to subscribers when they are on a third-party access network. Similarly, third-party applications will not be able to obtain location information from the operator network if the device cannot provide a location URI that refers back to a LIS in that network. A walled-garden model may be preferable to operators that want to maintain a full services model as seen in conventional cellular networks. However, it is counter to the Internet services model and it can be expected that users will consider it too inflexible in the long term.



IP Location
IP Location
ISBN: 0072263776
EAN: 2147483647
Year: 2004
Pages: 129

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