SIM Card Traffic Assignment

TL;DR

Yes, a mobile operator can assign traffic to every SIM card in their network, but it’s complex. It relies on core network elements like the HLR/HSS, MSC/VLR, and SGSN/MME, combined with subscriber data and routing policies. The process involves identifying the SIM, locating its current serving node, and then directing traffic accordingly.

How Mobile Operators Assign Traffic to SIM Cards

1. Subscriber Data Storage (HLR/HSS): The Home Location Register (HLR) or Home Subscriber Server (HSS) is the central database. It holds information about every subscriber, including their IMSI (International Mobile Subscriber Identity), MSISDN (Mobile Station International Subscriber Directory Number – phone number), and current serving network node.
   • IMSI: Unique identifier for the SIM card.
   • MSISDN: The phone number associated with the SIM.
2. Locating the Subscriber (MSC/VLR & SGSN/MME): When a subscriber makes a call or uses data, the network needs to know where they are currently located.
   • 2G/3G Networks: The Mobile Switching Center (MSC) and Visitor Location Register (VLR) track subscribers roaming in its area.
   • 4G/5G Networks: The Serving Gateway (SGSN) or Mobility Management Entity (MME) performs this role.
3. Traffic Routing: Once the subscriber’s location is known, traffic can be routed.
   • Voice Calls (2G/3G): The MSC routes calls based on the destination number and subscriber’s profile. Routing tables are used to determine the best path.
   • Data Sessions (4G/5G): The MME/SGSN directs data traffic through the Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW).
4. Diameter Signaling: The Diameter protocol is commonly used for communication between network elements to exchange subscriber information and routing details. For example, a request from an application server might trigger a query to the HSS to get the subscriber’s profile.

   # Example Diameter message (simplified)

5. Policy Control: Operators use Policy and Charging Rules Function (PCRF) to control how traffic is handled for each SIM. This includes QoS (Quality of Service), data limits, and access restrictions.
   • The PCRF interacts with the HSS to get subscriber-specific policies.
6. Assigning Traffic – Practical Steps:
   1. Incoming Call: When a call arrives at the network, the destination MSISDN is used to query the HLR/HSS for the subscriber’s current location.
   2. Data Session Request: When a SIM initiates a data session, the MME/SGSN identifies the subscriber and requests their profile from the HSS.
   3. Routing Update: If the subscriber moves to a new cell, the network updates its location information in the HLR/HSS.

Challenges

• Network Complexity: Modern mobile networks are incredibly complex with many interacting components.
• Scalability: Handling millions of SIM cards requires a highly scalable infrastructure.
• Security: Ensuring the security of subscriber data and preventing unauthorized access is crucial for cyber security.
• Roaming Agreements: When subscribers roam on other networks, traffic routing becomes more complicated due to agreements with partner operators.