Roaming management covers the tools and processes used to control the roaming behavior for the subscribers in a mobile telecommunications network. When a subscriber leaves his home network and receives service from another network, he is said to be roaming.
A subscriber can roam to networks with which the operator of the subscriber's home network has a roaming agreement. Operators tend to have roaming agreements with as many other network operators as possible. However, the case is often that in any given roaming situation, there is one network that is preferred. This could be a network with which the operator has a better agreement and thus pays a lower price for its usage. It could also be a network that belongs to the same operator group as the home operator. The earnings for the subscribers' network usage would thus stay in the same company group if the subscriber could use the preferred networks as much as possible. If the roaming behavior could be efficiently controlled and the subscribers be made to roam into the most preferred network at any given time, large savings could be achieved for the operator. It shall, however, be noted that the opinion on which is the most preferred network may change over time.
Roaming management is thus an important area for the operators to improve the revenue stream. Roaming management gives the operator improved control of what networks its subscribers roam into when the home network can not be reached. Controlling this behavior becomes increasingly important as alliances are formed and the operator needs to manage this behavior on a continuous basis.
Roaming behavior is defined in the telecom standards and is controlled by data that is stored on the SIM (Subscriber Identity Module) card. The handset will modify its roaming behavior based on the contents of the roaming control files on the SIM card.
One such standard, in which the mechanisms that control roaming behavior in the GSM/3G network are defined is TS 23.122 in release 99 version. According to that standard, the roaming behavior is to a large extent controlled by two network selector files on the SIM card of the mobile phone. One of the files is the so-called subscriber-defined PLMN (Public Land Mobile Network) list and the other one is the operator-defined PLMN-list. Older versions of the standard define only one list.
When the phone shall select a network, it first looks for networks defined in the subscriber-defined list. Each network indicated by the subscriber-defined network selector file on the SIM is tried according to the priority order given in that file. If none of the networks listed in that file are possible to select, the phone tries the networks that are listed in the operator controlled network selector file. If still none of these are possible to select, the phone shall randomly chose a network whose signal strength exceeds a threshold value. In reality, this random selection often selects the strongest network. If none of the above methods have succeeded, the phone tries all other networks in order of decreasing signal strength. If the phone receives the information that a network is not allowed for roaming, the phone adds the network to the list of forbidden networks and will not access this network while the phone remains in automatic network selection mode.
However, once the phone is roamed into a network, it will stay on that network. According to the above standard, another file on the SIM shall control how often the phone searches for a higher preference network.
The phone will periodically search for the home network as well as a higher preference non-home network to roam into. Since this periodic network re-selection attempts only consider network of the same country as the network to which the phone is currently registered, the home network will only be attempted when the subscriber is nationally roamed.
The network selection functionality described above is the one defined in TS 23.122 in release 99 version. The functionality is, however, not yet fully implemented in most of the existing phones. This pertains especially to the periodic network re-selection attempts. It is anticipated that future phones will start to support these periodically performed attempts to find a higher preference network.
The fact that most phones do not yet support the periodic network re-selection means that once a phone registers to a non-preferred network, it has a tendency to remain there until something extra-ordinary, like coverage loss, takes place.
Even if the phone is turned off and on, the phone will remember the last network it was roamed onto and will try to go back to that network again by reading the information from the (Location Information) LOCI file on the SIM card. Thus the subscriber remains even longer than needed on the wrong network.
Only if coverage is lost, the phone will search for and switch to another network, which might be a preferred network if that network has coverage.
If no network has coverage, the phone will not be able to register to any network. If the coverage then returns, at the same time, for the previous network and a preferred network, the phone will still tend to register back onto the previous network.
There are a number of mechanisms that are employed today in order to control the roaming behavior as described above. Some of these are described in the following and the weaknesses or short-comings of these are touched upon. All descriptions given assume that the phone performs network selection without user interaction. When network selection takes places without user interaction, the phone is said to be operating in automatic network selection mode. If network selection is performed by the user, the phone is said to be operating in manual network selection mode. In manual network selection mode, it is in fact the subscriber that manually controls the roaming behavior. Consequences of being in manual network selection mode will be discussed further down in the document.
The most basic form of Roaming Management is to define the contents of the roaming control files, i.e. the PLMN list or lists and the network search period, on the SIM cards at the time of issuance. This becomes a static definition of preferred roaming networks.
An improvement to the static model is defined in the GSM 03.48 standard, in which the roaming control files are made updatable over the air. This enables remote update of the roaming control files when price models, agreements and operator constellations change. It is also possible to update the whole subscriber base in this manner. If the operator so chooses, it is possible to limit the group to subscribers that are likely to be roamers.
Due to the reasons described above, the handling of roaming control files as described above Is referred to as statistical roaming management since the statistical chance that a subscriber will register with a preferred network is greatly improved if the roaming control files are kept updated. The statistical roaming management thus controls roaming by updating the roaming control files on the SIM. Correctly used, the files can greatly improve the ratio of subscribers that roam into the preferred networks. However, there is never any guarantee that a subscriber will roam onto the preferred network.
The behavior of the statistical roaming management is non-intrusive for the subscriber. The effects are only that a preferred network is chosen initially at network selection or after the defined time period as described above. If no preferred network is present, the subscriber stays on the current network.
If the update of the roaming control files is made before the subscriber enters a roaming area, the phone will select the preferred network if it is present in the area. If the files are updated after the subscriber enters the roaming area, and the subscriber is in the wrong network, the subscriber is connected onto a preferred network only if a loss of coverage of the current network takes place and the preferred network is present.
Due to the above limitations, to the statistical roaming management, solutions for so-called dynamic roaming management exist. In some of these solutions, Dynamic Roaming Management uses active knowledge regarding a subscriber's roaming state and tries to achieve a change in the current roaming situation. In dynamic roaming management, the operator of a subscriber's home network knows when the subscriber roams into a new network. If that is an undesired network, a trigger is generated to a roaming server. The roaming server takes update measures towards the subscriber's SIM to make the phone select a better network. The dynamic roaming management is intrusive to the extent that since it attempts to perform an active task of moving the operator from one network to another.
In the dynamic roaming management scenario, a special roaming management application (RMA) monitors roaming events in the network, for example by interfacing to the HLR (Home Location Register). When the subscriber roams into a foreign network, the RMA is notified. If the subscriber roamed into a non-preferred network, attempts are made to dynamically move him to a preferred network. This behavior tends to lower the perceived quality of service for the subscriber and might therefore be undesirable from that aspect. Since the operator has a possibility of making money, a dynamic roaming management solution might be employed anyway.
Finally, it shall also be noted that the size of the roaming control file is always going to be limited. That means that it will never be possible to list all the preferred networks in this file.