The Connectivity Triangle: SIM, Modem & Network
When deploying IoT devices with cellular connectivity, many people assume that inserting a SIM card is all they need. In reality, reliable cellular connectivity depends on three components working together: the SIM card, the modem (inside your router), and the mobile network infrastructure.
The “Connectivity Triangle” — all three components must work together for reliable IoT connectivity
A roaming SIM card can solve many connectivity problems by giving your device access to multiple networks. But it’s important to understand that the SIM alone cannot detect all types of connection failures. Some problems require the modem to actively monitor the connection and take corrective action.
SIM Card Role
Stores subscriber identity (IMSI), network preferences, and authentication credentials. Multi-IMSI SIMs can switch between different operator profiles.
Modem Role
Scans for networks, handles registration, manages the radio connection, and stores the last-used network (RPLMN) in memory.
Network Role
Authenticates the SIM, assigns IP addresses, routes data traffic, and may provide steering instructions to the device.
What is a Roaming SIM Card?
A roaming SIM card (also called a multi-network SIM) is designed to connect to multiple mobile network operators rather than being locked to a single carrier. This provides several advantages for IoT deployments:
- Better coverage — If one network has poor signal, the device can switch to another
- Network redundancy — If one operator experiences an outage, alternatives are available
- Simplified logistics — One SIM can work across multiple regions or countries
- Cost optimisation — Some roaming SIMs can select the most cost-effective network
How Roaming Works
When a SIM connects to a network other than its “home” network, it’s called roaming. The home network has commercial agreements with visited networks (roaming partners) that allow subscribers to use their infrastructure. The visited network authenticates the SIM through the home network’s systems.
💡 Home Network vs Visited Network
Your SIM’s Home PLMN (HPLMN) is determined by the IMSI stored on the card. When you connect to any other network, you’re on a Visited PLMN (VPLMN). Roaming agreements determine which visited networks your SIM can access.
Types of Roaming SIM
| Type | How It Works | Best For |
|---|---|---|
| Single-IMSI Roaming | One identity, multiple roaming partners | Simple deployments, single-region use |
| Multi-IMSI | Multiple identities on one SIM, switches between them | Global deployments, cost optimisation |
| eUICC / eSIM | Profiles downloaded over-the-air, can change operator remotely | Future-proof deployments, remote management |
Multi-IMSI Technology Explained
Multi-IMSI SIMs take roaming further by storing multiple subscriber identities on a single physical SIM card. Each IMSI is essentially a different “passport” that allows the SIM to appear as a local subscriber on different networks.
How Multi-IMSI Works
- Multiple profiles stored — The SIM contains several IMSI numbers, each associated with a different operator or region
- Network detection — When powered on or when signal degrades, the modem scans for available networks
- IMSI selection — Based on location, signal strength, or pre-configured rules, the SIM selects the appropriate IMSI
- Registration — The device registers on the network using the selected identity
- Automatic switching — If registration fails or signal drops, the SIM can rotate to another IMSI
⚠️ IMSI Switching Takes Time
When a multi-IMSI SIM switches identities, the modem must re-register on the network. This process can cause a brief data interruption (typically 10-60 seconds). The SIM cannot switch identities instantly — it requires a new network attachment.
Global Coverage
Different IMSIs provide access to local networks in different countries, avoiding permanent roaming restrictions.
Cost Savings
By registering as a local subscriber rather than a roamer, data costs can be significantly lower.
Redundancy
If one IMSI’s network experiences issues, the SIM can fall back to another identity and network.
Compliance
Some countries require local operator presence — multi-IMSI can provide local IMSIs where needed.
How Network Selection Works (PLMN)
PLMN stands for Public Land Mobile Network — essentially, a mobile operator’s network in a specific country. Each PLMN is identified by a unique code combining the Mobile Country Code (MCC) and Mobile Network Code (MNC). For example, 234-15 is Vodafone UK.
When your IoT router powers on, the modem must decide which PLMN to connect to. This process follows strict rules defined by 3GPP standards, and understanding it helps explain why devices sometimes take a long time to connect or why they might stay on a poor network.
The PLMN Selection Priority
In automatic mode (the default for most IoT devices), the modem selects networks in the following priority order:
- RPLMN — Registered PLMN (the last network successfully used, stored in modem memory)
- EPLMN — Equivalent PLMNs (networks the current network says are equivalent)
- HPLMN/EHPLMN — Home PLMN derived from the IMSI on the SIM
- UPLMN — User Controlled PLMN list (preference list stored on SIM)
- OPLMN — Operator Controlled PLMN list (set by the SIM provider)
- High-quality networks — Any network with signal above -110dBm (random order)
- All other networks — Remaining networks in order of signal strength
🔴 The RPLMN Problem
Because the modem always tries the last-used network first, moving a router to a new location can cause connection delays. If the old network (RPLMN) has poor or no coverage at the new site, the modem will keep trying it before eventually scanning for alternatives. This is why a SIM that worked perfectly in one location may take much longer to connect after being moved.
First Boot vs Reboot Behaviour
| Scenario | What Happens | Typical Time |
|---|---|---|
| First Boot (new SIM) | No RPLMN stored — full network scan required, checks all bands and PLMNs | 30 sec to several minutes |
| Reboot (same location) | RPLMN found — attempts last network directly, skips scan | 5-15 seconds |
| Reboot (new location) | RPLMN attempted first but fails — then falls back to full scan | 30 sec to several minutes |
Steered vs Unsteered SIMs
When evaluating roaming SIM providers, you’ll encounter the terms “steered” and “unsteered”. Understanding the difference is important:
Unsteered SIMs have no operator preference lists (empty OPLMN). The modem selects any available network based on signal strength and 3GPP priority rules. This typically means connecting to the strongest signal, but network selection is somewhat unpredictable.
Steered SIMs have populated preference lists that guide the modem toward specific networks. The SIM provider uses these to direct devices to preferred roaming partners, avoid problematic networks, or optimise costs.
⚠️ Steering Can Cause Problems
Aggressive steering can force devices onto networks with weak signal when stronger alternatives exist. If your SIM provider’s preferred network has poor coverage at your deployment site, you may experience connectivity issues even though other networks would work fine.
| Aspect | Unsteered SIM | Steered SIM |
|---|---|---|
| Network selection | Based on signal strength | Based on preference lists |
| Predictability | Less predictable | More predictable |
| First connection time | Can be slower | Often faster |
| Best signal guaranteed? | ✓ Usually | ✗ Not always |
| Provider control | Minimal | Significant |
PLMN Network Selection & Handover
Interactive demonstration of how IoT routers select cellular networks
The Stale Connection Problem
Here's a critical point that many IoT deployers miss: a roaming SIM cannot detect all types of connection failure. One particularly problematic scenario is the "stale connection" — where the device appears connected but no data can actually flow.
What is a Stale Connection?
A stale connection occurs when:
- The SIM is registered on the network (appears "connected")
- The device has an IP address assigned
- Signal strength looks normal
- BUT — the network path to the internet is broken somewhere
This typically happens when the cell tower's backhaul connection to the operator's core network fails, or when there's a routing problem in the mobile network's infrastructure. From the device's perspective, everything looks fine — the radio link to the tower is working. But data packets sent to the internet simply disappear.
📡
Router
↔
🗼
Cell Tower
✗
🌐
Core Network
↔
☁️
Internet
Stale connection: Radio link works (green) but backhaul is broken (red) — SIM still shows "connected"
Why Can't the SIM Detect This?
The SIM card only handles authentication and identity — it has no visibility into whether data is actually flowing. The modem knows about the radio connection to the tower, but it doesn't monitor end-to-end connectivity. Neither component can detect that the path to the internet is broken.
⚠️ Switching Networks Won't Help Automatically
Even a multi-IMSI SIM with access to multiple networks won't automatically solve this problem. The SIM has no reason to switch — as far as it knows, the connection is fine. The device needs an active monitoring mechanism to detect stale connections and trigger recovery.
ICMP Ping Reboot: The Solution
To detect stale connections, industrial IoT routers implement a feature called ICMP Ping Reboot (also known as "Cellular Watchdog" or "Connection Check"). This feature actively tests the connection and takes corrective action when it fails.
How Ping Reboot Works
- Periodic testing — The router sends ICMP ping packets to a reliable internet target (e.g., 8.8.8.8) at regular intervals
- Response monitoring — The router waits for ping replies within a timeout period
- Failure counting — If a ping fails, a counter is incremented
- Threshold action — After a configurable number of consecutive failures (e.g., 3), the router takes action
- Recovery — Typically, the router restarts the cellular modem or performs a full reboot
By rebooting the modem, the device forces a new network registration. This clears any stale state and often resolves the connectivity issue — especially if the problem was temporary or if a different network/cell is now available.
✅ Why This Works
ICMP ping tests end-to-end connectivity — from the router, through the cellular network, to the internet and back. If pings fail but the SIM shows "connected," you know the problem is in the network path, not the radio link. Rebooting forces a fresh connection that often routes through different infrastructure.
Configuration Best Practices
| Setting | Recommended Value | Why |
|---|---|---|
| Ping Target | 8.8.8.8 or 1.1.1.1 | Highly available DNS servers unlikely to be blocked |
| Ping Interval | 30-60 seconds | Frequent enough to detect issues, not so frequent it wastes data |
| Timeout | 5-10 seconds | Allow for cellular latency while detecting real failures |
| Failure Threshold | 3-5 failures | Avoid rebooting on transient glitches while catching real outages |
| Action | Modem restart (preferred) | Faster than full reboot, usually sufficient to recover |
ICMP Ping Reboot / Cellular Watchdog
See how routers detect and recover from stale cellular connections
Roaming SIM & PLMN Network Selection
How IoT routers select cellular networks on first boot, reboot, and live handover
RPLMN - Last Registered PLMN (stored in modem memory)
EPLMN - Equivalent PLMNs (provided by network)
HPLMN/EHPLMN - Home PLMN derived from IMSI
UPLMN - User Controlled PLMN selector list on SIM
OPLMN - Operator Controlled PLMN selector list on SIM
High Quality - Networks with RSRP ≥ -110dBm (random order)
All Others - Remaining networks by decreasing signal strength
