LTE Roaming Architecture - LTE

What is LTE Roaming Architecture?

Public Land Mobile Network (PLMN) is a network run by one operator in one country. When a user connects to PLMN then it is said Home-PLMN but while roaming the user moves out from the home network and uses the resources of other operator’s network, this network is called as Visited-PLMN.

In general, the user connects to E-UTRAN, MME and S-GW while roaming. P-GW of either the visited or the home network is used allowed by LTE/SAE. Shown is figure below –

LTE Roaming Architecture

Users are allowed to access home network’s services even in a visited network by P-GW.

Concept of S5/S8 interface comes into picture when the two devices want to communicate. It is an interface between the serving and PDN gateways. S5 is implemented when devices are in same network and S8, if in different networks. Interface will not come into picture when mobiles are not roaming i.e., the serving and PDN gateways are integrated into a single device.

LTE Roaming Charging

There are a lot of complexities in the charging mechanisms required to support 4G roaming. The pre-paid and post-paid charging for LTE roaming is defined below-

  • Prepaid Charging – Local visited network can’t handle the customer information so they are routed back to the home network as the CAMEL standard that enables prepaid services in 3G is not supported in LTE. Because of this, mobile operators depends on the new accounting flows for accessing customer information like P-Gateways in both IMS and non-IMS environments or via CSCF in an IMS environment.
  • Postpaid Charging – As it done in 3G, the same mechanism is used in LTE as well for postpaid data-usage charging using TAP 3.11 or 3.12 versions. TAP 3.12 is needed with local breakout of IMS services.

Operators can’t track the activities of the subscriber as they are permitted to do in home-routing scenarios because the data sessions are kept within the visited network. If the operator wants to record the real-time information of both pre and postpaid customers, a Diameter interface must be established between charging systems and the visited network’s P-Gateway.

A TAP 3.12 mobile session is needed for the service usage. Visited network creates call detail records (CDRs) in case of local breakout of IMS services from the S-Gateway but these do not have all the required information to create a session. So operators correlate the CDRs with the IMS CDRs to create TAP records.

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