How does number portability affect how calls are routed?
Posted: Thu May 22, 2025 9:40 am
Number portability (NP), particularly Mobile Number Portability (MNP), fundamentally alters how phone calls are routed by decoupling a phone number from its original network or geographic location. Before NP, a phone number's prefix (the first few digits, like NPA-NXX in North America) inherently identified the carrier and often the geographic central office where the number was originally assigned. This made routing straightforward: a call to a specific prefix always went to the corresponding carrier's network.
However, with NP, a subscriber can keep their existing phone number even if they switch to a different service provider. This means the traditional assumption that a number's prefix indicates its serving network is no longer valid. This capability introduces a critical challenge for routing, as the originating network needs to determine the actual current network serving the dialed number, not just its original assignee.
Here's how number portability affects call routing, primarily using the widely adopted Location Routing Number (LRN) method, which is common in many countries including Bangladesh:
1. The Need for a Central Database:
The core of number portability routing is a central database, often managed by a neutral third party (like Infozillion Teletech BD in Bangladesh for MNP). This database contains records of all ported numbers and their associated Location Routing Numbers (LRNs). An LRN is a unique 10-digit number (often resembling a standard phone number) assigned to a specific network switch or point of interconnection where calls for a ported number should be delivered. Essentially, it acts as the new "network address" for a ported number.
2. The "LRN Dip" or "All Call Query" Process:
When a call is placed to a phone number that might be ported:
Originating Network Query: The originating mobile carrier's switch (e.g., a Gateway MSC or Softswitch) first performs a real-time query (often called an "LRN dip" or "All Call Query") to the central number portability database. This query uses the dialed phone number as the key.
Database Response:
If the number has NOT been ported: The database informs the originating switch that the number is still served by its original (default) carrier. The call is then routed normally based on the original network prefix.
If the number HAS been ported: The database returns the LRN romania phone number list associated with that specific ported number. This LRN points to the current network and specific switch where the subscriber's number now resides.
3. Direct Routing to the Current Carrier:
Rerouting the Call: Instead of routing the call to the original (donor) carrier identified by the dialed number's prefix, the originating switch uses the obtained LRN to directly route the call to the new (recipient) carrier's network. This is known as Direct Routing.
Interconnection: The call then travels through the established interconnection points (e.g., Interconnection Exchanges or ICXs in Bangladesh, or direct peering links in other architectures) between the originating carrier and the recipient carrier.
Call Completion: Once the call reaches the recipient carrier's network, that network uses its internal systems (like its HLR and VLR) to locate the specific subscriber and complete the call.
4. Impact and Advantages:
Increased Complexity: Number portability adds a mandatory lookup step to almost every call routing decision, increasing the signaling traffic and potentially adding a tiny fraction of a second to call setup times (though typically imperceptible to users).
Enhanced Competition: For consumers, NP empowers them to switch providers without the inconvenience of changing their number, fostering competition among carriers who must now compete more vigorously on service quality and pricing.
Efficiency of Direct Routing: Direct routing (using LRNs) is generally preferred over "indirect routing" (where the call first goes to the original carrier and then is forwarded), as it avoids unnecessary call "tromboning" (where a call goes out to one network only to be sent back to another close to the origin), reduces latency, and optimizes network resource usage.
SMS Routing Challenges: While voice calls typically flow through ICXs or direct peering which support LRN lookups, SMS delivery can face challenges. As reported in Bangladesh, some users who port numbers may find they cannot receive texts from all operators, indicating that the SMS routing infrastructure might not always perform the same LRN lookup as voice calls, leading to delivery failures if not properly configured across all networks.
In Bangladesh, the BTRC has implemented MNP, with Infozillion Teletech BD managing the central database. While the system is designed to facilitate seamless number portability, the success and efficiency of call routing to ported numbers depend heavily on all involved mobile network operators consistently querying the central database and correctly implementing the routing updates.
However, with NP, a subscriber can keep their existing phone number even if they switch to a different service provider. This means the traditional assumption that a number's prefix indicates its serving network is no longer valid. This capability introduces a critical challenge for routing, as the originating network needs to determine the actual current network serving the dialed number, not just its original assignee.
Here's how number portability affects call routing, primarily using the widely adopted Location Routing Number (LRN) method, which is common in many countries including Bangladesh:
1. The Need for a Central Database:
The core of number portability routing is a central database, often managed by a neutral third party (like Infozillion Teletech BD in Bangladesh for MNP). This database contains records of all ported numbers and their associated Location Routing Numbers (LRNs). An LRN is a unique 10-digit number (often resembling a standard phone number) assigned to a specific network switch or point of interconnection where calls for a ported number should be delivered. Essentially, it acts as the new "network address" for a ported number.
2. The "LRN Dip" or "All Call Query" Process:
When a call is placed to a phone number that might be ported:
Originating Network Query: The originating mobile carrier's switch (e.g., a Gateway MSC or Softswitch) first performs a real-time query (often called an "LRN dip" or "All Call Query") to the central number portability database. This query uses the dialed phone number as the key.
Database Response:
If the number has NOT been ported: The database informs the originating switch that the number is still served by its original (default) carrier. The call is then routed normally based on the original network prefix.
If the number HAS been ported: The database returns the LRN romania phone number list associated with that specific ported number. This LRN points to the current network and specific switch where the subscriber's number now resides.
3. Direct Routing to the Current Carrier:
Rerouting the Call: Instead of routing the call to the original (donor) carrier identified by the dialed number's prefix, the originating switch uses the obtained LRN to directly route the call to the new (recipient) carrier's network. This is known as Direct Routing.
Interconnection: The call then travels through the established interconnection points (e.g., Interconnection Exchanges or ICXs in Bangladesh, or direct peering links in other architectures) between the originating carrier and the recipient carrier.
Call Completion: Once the call reaches the recipient carrier's network, that network uses its internal systems (like its HLR and VLR) to locate the specific subscriber and complete the call.
4. Impact and Advantages:
Increased Complexity: Number portability adds a mandatory lookup step to almost every call routing decision, increasing the signaling traffic and potentially adding a tiny fraction of a second to call setup times (though typically imperceptible to users).
Enhanced Competition: For consumers, NP empowers them to switch providers without the inconvenience of changing their number, fostering competition among carriers who must now compete more vigorously on service quality and pricing.
Efficiency of Direct Routing: Direct routing (using LRNs) is generally preferred over "indirect routing" (where the call first goes to the original carrier and then is forwarded), as it avoids unnecessary call "tromboning" (where a call goes out to one network only to be sent back to another close to the origin), reduces latency, and optimizes network resource usage.
SMS Routing Challenges: While voice calls typically flow through ICXs or direct peering which support LRN lookups, SMS delivery can face challenges. As reported in Bangladesh, some users who port numbers may find they cannot receive texts from all operators, indicating that the SMS routing infrastructure might not always perform the same LRN lookup as voice calls, leading to delivery failures if not properly configured across all networks.
In Bangladesh, the BTRC has implemented MNP, with Infozillion Teletech BD managing the central database. While the system is designed to facilitate seamless number portability, the success and efficiency of call routing to ported numbers depend heavily on all involved mobile network operators consistently querying the central database and correctly implementing the routing updates.