Basics of Cellular Number Portability

by C3llph

With the possibility of number portability on the horizon yet again, I thought I'd start with explaining some of the basics and then move onto some of the more technical issues.

What number portability will allow the public to do with their preexisting cell phone numbers is similar to what you can already do, and probably get charged for on a monthly basis, with your home phone.

On your land-line bill you should see a number portability surcharge.  Cellular number portability will allow you to take your existing cell phone number with you to whichever cell phone provider you choose.  Your land-line provider, as do cable and power companies, use public utility lines to provide you with service.  This just means that no single company "owns" the actual cable or other devices that are used to provide you with their service.

In some small and large towns there are some privately-owned telephone companies but this still applies to them.  Nothing is stopping you from either changing your service to another provider, unless no other exists, because other companies have the right to use the same cables to provide you with service.

So with cell phone number portability if you get fed up with say Provider A, you can now change your service to Provider B and not have to give all your friends, family, or business contacts a new phone number to try and reach you at.  This was suggested to the government by businesses based upon how it costs them money to have new business cards reprinted, etc. in the event that they change service providers.

Currently in cell phones old and new your Mobile Directory Number (MDN) is programmed into the phone or SIM card (GAIT or GSM) to match the other equipment so your provider can identify your phone on their network for placing or receiving calls.

I'll use TDMA as an example.

When a handset is activated either Over-the-Air Programming (OAP) or manual programming is used to put the correct information into the handset.  This includes the following codes: Start of Cell (SOC), System Identification (SID), and MDN.  The MDN is your dialable 10-digit mobile number, also known as your wireless number, reach number, or Customer Telephone Number (CTN).

Currently on the newer TDMA handsets there is also a programmable MIN/MSIN, for example on the Sony Ericsson T61d, Motorola v120t, C331t, v60t/i, Nokia 8265i, 1261, 3360, and probably 3560, I can't remember offhand.

The Mobile Identification Number (MIN) or Mobile Subscriber Identification Number (MSIN) is a non-dialable 10-digit number that the customer will not know about.

If you have an older handset or a new handset that doesn't support the programming of the MIN it will be done in the provider's network anyway.  This MIN is what will allow a specific provider to identify your phone to their network.  They will use the MIN to keep their side of billing and provisioning of service unique to your phone.

Before number portability goes into effect, if you phone has the capability for both MIN and MDN they will be programmed to the same number.  After number portability begins later this year if you do decide to change providers the new provider will keep your MDN the same and issue a new MIN for their network from their specific number pools.  Again, you will not know what this new MIN is and it is non-dialable anyway.

For internal billing identification and identification on their systems the provider will use the MIN to keep track of your account while your bill will show your MDN.

The process for a call being routed to you is what will change.  When someone dials you number, it will get routed to the exchange and will first be tagged to your MDN.  Then when the call reaches your provider's network it will be call forwarded to your phone using the MIN.  This should be an insignificant delay.

Dialing out from your phone will be basically the reverse.  You place a call from your phone, the MIN will be used to identify you on your provider's network.  Your provider will then in turn route your call to the destination, but will substitute your MDN for the MIN so the receiver's system would show your MDN as the number of the incoming call, hopefully.

So, number portability is a good thing if you need to keep your existing number and change to a different provider.  To think that cell phone companies will not have any issues while getting their network to implement this without any flaws would be very presumptuous on a consumer's part.

Specifically, problems will arise if someone were to try and activate an existing number with a new provider before canceling the service with their existing provider.

Hopefully this has shed some light on what will be happening behind the scenes of number portability.

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