MSIDs are not unique to CDMA, and ALL carriers must use them.
Originally Posted by VCI_Cell
I used to be responsible for determining when we needed new MDNs and MSIDs for my markets and ordering them...
Your MDN (Mobile Dialed Number) is your phone number in the sense that that is the number people dial to call you, this is the number that you can port from carrier to carrier.
The MSID (Mobile Station ID) sometimes still called a MIN (Mobile Identification Number) is carrier specific. This is the number that is in carrier's routing tables that tell the switch how to route your calls and how other switches find you and send calls to you. This number has to change if you switch carriers.
The MSID was necessary and created simply because of WLNP. Now that WPLP allows us to take our number with us from carrier to carrier, something had to be done to identify a subscriber as belonging to a specific carrier. The method of doing that in the past was the MIN (think of it as the MDN and MSID mushed together). All the infestructure for call routing from carrier to carrier was already in using that 10 digit number (routing was/is based on the first 6 digits aka "NPA-NXX"). Each carrier was given/owned NPA-NXXs in blocks of 10,000 that was theirs alone (ex 555-555-0000 through 555-555-9999), and there was/is a database called the LERG that identified who owned every single NPA-NXX in the country along with the point code of their switch (kinda like an IP address for phone calls). Every carrier would use that database to setup routing tables in their switch that allow both calls between carriers and roaming. The breakup of the MIN into two 10 digit numbers allowed the existing system to work with only small sofware and procedural changes to that system. MSID takes over the routing duties, and MDN takes the dialing duties, both of which used to belong to the MIN.
It is now becoming more and more common for MDNs and MSIDs to be different not only because of WLNP, but also because of number pooling. When carriers request more numbers to sell (MDNs), they must now do it in increments of 1,000 instead of the 10,000 it used to be, except I think, in certain situations. And even then with the pool of numbers you receive, your MDNs and MSIDs often don't match (I know, I know, I always thought that was stupid too). Both numbers are equally important to complete calls.
There are 10 kinds of people in the world. Those who understand binary and those who don't.