Description
The Preferred Roaming List (PRL) is a database or configuration file used in mobile devices, particularly in CDMA and later multi-mode (e.g., CDMA/LTE) networks, to manage network selection during roaming. It contains information about preferred service providers, radio access technologies (RATs), and frequency bands that the device should use when outside its home network coverage. The PRL is typically provisioned by the home network operator and stored on the device's USIM (Universal Subscriber Identity Module) or in non-volatile memory, enabling the device to make intelligent decisions when scanning for available networks.
Architecturally, the PRL consists of entries that map geographic regions (via system identifiers, SIDs, and network identifiers, NIDs) to preferred networks, along with associated priorities and acquisition parameters. When a device loses connection to its home network, it consults the PRL to determine which networks to attempt registration with, in what order. The list may include both home network affiliates (for seamless roaming) and partner networks, while explicitly excluding forbidden or undesirable networks. Key components include the PRL's table structure, which defines roaming preferences per RAT (e.g., CDMA, LTE, NR), and the update mechanisms, which allow operators to remotely refresh the list via over-the-air (OTA) provisioning to adapt to changing roaming agreements.
How it works: Upon power-up or when out of coverage, the device scans for available networks and compares them against the PRL entries. It prioritizes networks based on the PRL's ranking, attempting to camp on the highest-priority available network. For example, in a multi-mode device, the PRL might prioritize LTE networks over CDMA in certain regions to leverage better data services. The PRL also includes acquisition parameters like frequency channels or band classes, speeding up the search process. This system ensures that users experience minimal service disruption while roaming, and operators can steer traffic to cost-effective or high-quality partner networks, managing roaming expenses and quality of service.
Purpose & Motivation
PRL was introduced in 3GPP Release 7, building on earlier CDMA standards, to address the challenges of network selection in roaming scenarios, especially as mobile devices began supporting multiple RATs and global mobility increased. Prior to PRL, devices might randomly select available networks or rely on basic priority lists, leading to suboptimal connections, higher roaming costs, or service denial. The motivation was to give operators control over the roaming experience, ensuring devices connect to preferred partners that offer favorable rates, reliable service, or regulatory compliance.
The creation of PRL solved problems like 'rogue roaming,' where devices might attach to unauthorized networks causing security risks or billing issues. By providing a curated list, operators could direct traffic to trusted partners, optimize network resources, and enhance user satisfaction through seamless handoffs. In regions with complex regulatory environments, PRL helps enforce national roaming policies or avoid restricted networks.
Historically, PRL evolved from CDMA's need for efficient roaming management, as CDMA networks lacked SIM-based global standards like GSM. With the convergence towards LTE and 5G, PRL's role expanded to include multi-RAT prioritization, supporting smooth transitions between technologies. It remains relevant for managing legacy system interoperability and optimizing connectivity in heterogeneous networks, though in pure 5G SA deployments, network selection may rely more on USIM-based priorities (e.g., PLMN selector lists). PRL exemplifies how operational control can be embedded in devices to balance user mobility with operator business and technical objectives.
Key Features
- Stores prioritized list of networks and RATs for roaming selection
- Configurable via OTA updates by the home operator
- Includes geographic mapping (SID/NID) and frequency parameters
- Supports multi-mode devices (e.g., CDMA, LTE, NR)
- Enables steering to cost-effective or high-quality partner networks
- Reduces network scan time by providing acquisition data
Evolution Across Releases
Introduced PRL into 3GPP specifications, adapting it from CDMA standards for use in multi-RAT environments. Defined the structure for storing preferred networks, priorities, and acquisition parameters on devices, enabling controlled roaming and network selection across evolving technologies like HSPA and early LTE.
Defining Specifications
| Specification | Title |
|---|---|
| TS 22.936 | 3GPP TS 22.936 |