Local Number Portability and How to Troubleshoot
Ever wonder how businesses can keep the same number despite moving locations? Local Number Portability (LNP) enables end users to keep their telephone number when switching from one telecommunications service provider to another.
The Telecommunications Act of 1996 gave telephone subscribers the ability to keep the same phone number even if they moved across the country. Prior to this, users had to get new numbers whenever they changed service providers. For businesses, the Telecommunications Act of 1996 was crucial, as it kept customers connected to companies without a company having to change white paper listings and alert all their customers of phone number changes.
How Does Local Number Portability Work?
Typically, when a call is made through a telephony switch, the switch determines whether it can route that call based on the called digits directly from the routing table in that local switch. For example, using the first 6 digits (NPA-NXX) of the 10 digits of the standard ANSI dial plan. In other words, if the destination Number, typically a local call, is in the routing table of that switch’s database, the call can be routed directly to the destination. However, this is rarely the case.
Calls not immediately routable activate a trigger and a switch routes the call to their Access Tandem partner who will “dip” — or access a database hosted on the SCP (Service Control Point). This distributed database is supported nationally throughout North America by tier one carriers or the access tandem switch network provider.
The specific database query made by the local originating switch uses a part of the SS7 protocol suite known as TCAP (Transaction Capability Access Part). This is a simple query response protocol. The application layer protocol supporting Local Number Portability in North America is IN/AIN, a Bellcore standard protocol. The IN/AIN protocol is encapsulated by the SCCP protocol transported which rides over TCAP protocol.
The database will specify the switch on which that CallED number is now hosted. This destination switch is referred to by its Local Routing Number [LRN].
The TCAP response will carry the original called PartyID number itself if the number is not ported.
The IN/AIN application layer protocol is also designed to support other specific database queries for example to resolve 1-800 or 1-8YY numbers or a query for calling name/CNAM. Troubleshooting these subject areas will be covered in a later article.
Commonly Occurring Problems
Often the responsibility to query the LNP database falls between the cracks. The local originating carrier may fail to do it and the immediate northbound carrier [N-1] may also drop the task.
Although it is rare, it’s possible that the SCP and its backup are offline at the same time due to scheduled maintenance. In this case a message would come back stating “Subsystem Unavailable”. More frequent is a slow response from the N-1 carrier, triggering a timeout on the local originating switch. Hopefully, this would be caught by alerts generated to the support desk /NOC.
Sometimes an incorrect LRN value can be returned by the TCAP query. Basically, the LNP database has been given incorrect information. You know the old adage “garbage in, garbage out”. It starts with manual human data entry following a rather complex administrative process where errors can occur along the way.
Businesses need to show a stable, consistent, physical presence with geographic roots, wherever they are, even in these virtual times and individuals seek to be reachable by friends, colleagues and associates for a lifetime wherever they are located.
Quick and Easy Troubleshooting
What information do we need from our network monitoring tools in order to ensure the process is working correctly and to be able to troubleshoot when it is not?
Other useful information you should see in your SS7 monitoring tool is the SubSystem Number. This was always a little confusing to me. The SubSystem is not a system in the traditional sense, for example, the media transcoding subsystem or the signaling engine within your switch. it refers to an application, such as the Local Number Portability application residing in the SCP. Other examples are:
SSN or SCCP Application | SSN Value |
---|---|
CNAM (Calling Name) | 232 |
Local Number Portability (LNP) | 247 |
248 | 800 number translation AIN 1.0 |
254 | 800 number translation TCAP |
In addition to the called PartyNumber and the Calling Party Number, the Calling Carrier will also need to provide following parameters in the AIN/TCAP query in order to get a response from the LNP database/SCP:
Mandatory Parameters | Explanation |
---|---|
userID | carrierID CIC (Carrier Identifier Code): e.g. 0222 MCI, 0288 ATT, etc. |
bearerCapability | Telephony bandwidth e.g. POTS “f31kHzaudio” = 3.1kHz |
calledPartyID | Dialed number |
triggerCriteriaType | e.g. LN |
chargeNumber | callingParty/ANI |
Initial Request to the SCP LNP Database
Below is shown the AIN protocol header with parameters contained within the SS7 TCAP message requesting the up-to-date destination of the ported number
Response to the LNP Query above from the SCP Database
The number of the LRN switch to which the call should be routed is highlighted in the diagram below.
Illustrating Important Information from Your Monitoring Tool
The most important parameter derived from the AIN/TCAP database query is the Generic Address Part/GAP. This parameter appears in your ISUP/SS7 call control messages and is populated from the AIN/TCAP query. the GAP shows the original number dialed. The callED number has now been replaced with the LRN (local routing number). So your switch will route the call using point codes to that switch using this LRN which will have in its local database how to reach the ported end subscriber.
The new destination for the call on the subscriber’s home switch is now given in the ISUP/SS7 Called Party field i.e. the LRN. The original dialed digits is useful information for reference when troubleshooting and is now contained in the Generic Address Part number of your ISUP Initial Address Message (IAM) as shown below.
If your support and engineering staff have strong IP networking skills but need help with VoIP, Telephony and SS7 protocols and technologies to speed troubleshooting and service assurance, speak to the experts at Teraquant, the worldwide experts in SIP & SS7 monitoring with 13 years experience with OCOM, the longest of any company.
For more information, view the OCOM information on:
SIP Monitoring in Real-Time