Definition
Roaming refers to the ability of a terminal to access voice, data, and other services through a visited operator's (VPMN) network when outside the coverage of its home operator (HPMN). Roaming involves HLR/HSS user data queries, access authentication (based on triplets/quintuplets), billing data collection, and inter-operator settlement. IoT SIM cards require roamer agreements (such as IR.21) between operators to support domestic cross-province roaming, inter-operator roaming, and international roaming, which is a fundamental capability for cross-border connected vehicles, logistics tracking, and cross-border IoT devices.
Types and Processes of Roaming
Domestic Roaming
After leaving their home area, terminals access the network of their operator in another province or connect through roaming on a different network operator. The visited MSC/MME initiates a location update and authentication request to the home HLR/HSS to obtain the user's subscription data, and billing is usually based on the visited network tariff or settled with the home network.
International Roaming
After leaving the country, terminals access foreign operators' networks. Bilateral roaming agreements between operators are required, supporting GRX/IPX interconnection. Authentication and billing are handled by the home network, and data traffic can be forwarded via Home-routed (back to home network) or Local Breakout.
Data Roaming and Billing
In roaming scenarios, the CDR (Call Detail Record) is collected by the visited network and transmitted to the home network, with inter-operator settlement based on the billing price. In IoT scenarios, data pools, monthly data plans, or pay-as-you-go billing are commonly used, and it is necessary to clarify whether roaming data counts toward the data plan and the applicable tariff rules.
Considerations for IoT Roaming
Scenarios such as connected vehicles, logistics tracking, and cross-border devices are sensitive to roaming stability, latency, and tariffs. Attention should be paid to operators' roaming coverage, APN roaming support, the impact of location update frequency on power consumption, and the availability of private network access (e.g., VPDN) in roaming scenarios.
SimtoLink provides IoT SIM cards that support both domestic and international roaming for connected vehicle, cross-border logistics, and other customers. Roaming capabilities depend on cooperation agreements with operators. SimtoLink configures roaming permissions according to the customer's business region when issuing SIM cards and assists customers in understanding roaming tariffs and data attribution. For cross-border devices, SimtoLink can provide multi-operator, multi-region SIM pool solutions to reduce the risk of relying on a single roaming network.
Authentication Goes Home, the User Plane Does Not Have To
While roaming, the device’s radio attaches to a base station and a mobility node of the visited network. Whether that identity is allowed to use the service is checked back at the home network. The visited network sends the identity request to the home HSS or UDM. Authentication vectors and the subscription are produced on the home side, and the visited side runs the challenge. Charging records are generated on the visited side and then sent home for settlement. The data path has two forms. Home routing pulls the user plane back to the home PGW or UPF, and only then out to the public internet or the private network. Local breakout exits nearby in the visited place. Delay is shorter, but the enterprise private network, the fixed address, and the access policy may no longer be there. Both paths can succeed at authentication and still differ completely in whether the service can be reached.
A trip across provinces inside one country usually stays on different regions of the same network, and policy inheritance is more complete than in international roaming. International roaming also needs interconnection between operators and an APN that has been tested. A country with no agreement is not “weak signal.” The subscription simply does not allow registration.
Permission, Interconnection, and the APN Have to Be Open Together
Roaming is usable when the SIM subscription allows the target network, the interconnection between visited and home networks is available, the device supports the local bands, and the APN the service needs has a route on the visited network. Satisfy only the first two, and a call or public-internet data may work, while an APN that points at the enterprise computer room fails when the session is set up. Local breakout also requires the visited exit to meet data-residency rules. A device that must enter the enterprise firewall should stay on home routing and accept the longer round trip. In-vehicle and logistics devices register back and forth at a border. If the tracking area list is cut very fine across operators, location updates become very frequent, and both power and signaling rise.
IoT SIMs are often shipped with international roaming, or roaming in some provinces, switched off by default. A lab test that passes at home does not prove the SIM works outside the province. Before go-live, use an attach record from the target region to see which operator the device registered on, whether the APN received an address, and whether the user plane returned to the expected anchor. Voice, SMS, and data roaming switches are sometimes separate. Being able to receive an SMS does not mean the data APN is usable.
Attach Succeeds, and There Is No Service Access Point
A common mistake is to treat “registered” as roaming complete. Registration only means the mobility node accepted this SIM. The session and the APN are the next step. If the APN is unknown on the visited network, the private-network tunnel does not extend across networks, or local breakout is forbidden by home policy, the failure is in session setup, not in radio coverage. Another case is a device locked to the home carrier or PLMN. Abroad it has signal and still does not scan the visited network, which looks like roaming has stopped.
The charging mistake is to assume roaming traffic automatically counts against the local bundle. Whether it counts, and at which side’s unit price, depends on the subscription, not on whether the technology can connect. When the private network is unavailable in the roaming region, someone temporarily switches to a public APN to get by. The device is then exposed directly, which is the opposite of the original isolation assumption. Record the visited network that was registered, the reject cause, the address obtained, and the anchor location together. When the cause says the APN is unknown, comparing RSRP further has no point.
Voice, SMS, and data roaming permission sometimes sit on different switches. An SMS that arrives only shows that signaling reached the home network. It does not show that the service access point has a route on the visited network. Confirm the anchor location for local breakout and for the home-routed user plane when the session is established. Do not guess from how the delay feels. A device that registers back and forth at a border, with a list that switches between two networks, updates far more often than ordinary movement. Record the visited-network identity, the reject cause, and whether the address obtained belongs to the expected pool.