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Understanding Modem Network Selection for Multi-Network IoT SIMs

How 3GPP protocol 23.122 and PLMN lists influence your IoT connectivity—and why standard modems aren't optimized for multi-network SIM performance.

Why Standard Modems Aren't Optimized for Multi-Network SIMs

When you insert a multi-network data SIM card into a router, smartphone, tablet, or dongle, it's the modem—not the SIM—that determines which mobile network to connect to. This selection process is governed by 3GPP protocol 23.122, a technical standard designed for traditional single-network SIM cards.

The problem? Standard modems were never optimized for the flexibility and performance requirements of multi-network IoT SIM cards. Understanding how this network selection algorithm works is critical to maximizing the performance of your IoT deployments.

How 3GPP Protocol 23.122 Influences Your IoT SIM

The 3GPP (3rd Generation Partnership Project) protocol 23.122 defines the technical specifications for how mobile devices and modems select and register with mobile networks. This network selection algorithm can be influenced by what's known as a PLMN list (Public Land Mobile Network list).

What is a PLMN List?

A PLMN list is a preference list of mobile networks programmed into the SIM card itself. It tells the modem which networks to prefer, typically based on commercial agreements and cost considerations rather than performance or signal quality.

Example: A SIM's PLMN list might prioritize Network A over Network B, even if Network B has stronger signal and faster speeds at your location.

Steered vs Unsteered SIM Cards: The Critical Difference

Steered SIMs (With PLMN List)

  • Forced to specific networks based on cost
  • Only switch networks if no signal on preferred network
  • Ignore better signals from alternative networks
  • Optimized for provider cost, not performance
  • Poor performance for multi-network deployments

Unsteered SIMs (No PLMN List)

  • No predefined network preferences
  • Connect to strongest available signal
  • Flexibility to switch between networks
  • Optimized for signal strength and availability
  • KeySIM's approach to multi-network performance

KeySIM: Unsteered Multi-Network SIM Technology

KeySIM SIM cards do not have a PLMN list, making them true unsteered multi-network SIMs. This means they aren't artificially restricted to specific networks based on commercial agreements—giving you genuine multi-network flexibility and the ability to manually optimize network selection for your specific deployment requirements.

The Strongest Signal Isn't Always the Fastest Connection

Even without a PLMN list, the standard 3GPP protocol has a fundamental limitation: when a SIM is first switched on, it connects to the mobile network with the strongest signal between the modem and the local cell tower.

Worst-Case Scenario

Using standard protocol, your modem could lock onto a sub-1 Mbps network with strong signal but severe congestion—grinding your IoT project to a halt despite other networks offering 50+ Mbps speeds nearby.

Why This Happens

Signal strength only measures the radio connection quality. It doesn't account for network congestion, backhaul capacity, available bandwidth, or carrier prioritization policies that dramatically affect real-world performance.

The KeySIM Solution: Manual Network Control

This performance limitation was a common and incredibly frustrating problem we encountered in the early days of multi-network IoT deployments. That's when our engineering team developed a groundbreaking solution: the KeySIM Network Steerer.

Take Control of Network Selection

With a single click, you can manually update which mobile network your KeySIM connects to—from anywhere in the world. No more relying on flawed automatic selection algorithms that prioritize signal strength over actual performance.

  • Remote Network Switching: Change networks without physical access to your device
  • Real-Time Monitoring: Watch your SIM connect to the selected network via the KeyOnline portal
  • Performance Optimization: Test different networks to find the fastest option for your location
  • Global Coverage: Works with UK networks (Vodafone, O2, EE, Three) and international roaming groups
Learn More About Network Steerer

3GPP Protocol 23.122 Network Selection Flow

The diagram below illustrates how the 3GPP protocol governs modem network selection. Understanding this process helps explain why manual network steering provides superior performance for multi-network IoT deployments.

3GPP protocol 23.122 network selection algorithm diagram showing how modems select mobile networks for IoT SIM cards

3GPP Protocol 23.122: Standard network selection algorithm used by mobile modems

Key Takeaways for IoT Deployments

Avoid Steered SIMs

Steered SIMs with PLMN lists force connections to specific networks regardless of performance. For true multi-network flexibility, choose unsteered SIMs like KeySIM.

Signal ≠ Speed

The network with the strongest signal isn't necessarily the fastest. Network congestion, backhaul capacity, and bandwidth availability all impact real-world performance.

Manual Control Matters

Don't rely on automatic network selection. Use network steering technology to manually optimize which network your SIM connects to for maximum performance.

Overcome Network Selection Limitations

Experience the difference of unsteered multi-network SIMs with manual network control. Optimize your IoT connectivity with KeySIM's Network Steerer technology.

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