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I am unsure what the IS-41 location registration process is. Could someone please explain this?

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IS-41 is a protocol used to manage many aspects of mobile (as in cellular phone) networks.

When someone places a call (or sends a text message or IP packet) to a subscriber of a mobile network, the mobile network needs to find out where the subscriber's phone (usually referred to as User Equipment, or UE) is located in order to deliver the call to the subscriber.

One way to do this would be to transmit a paging message to the called UE from every cell in the network. This approach would not be feasible for an even moderately large network, because all of the radio resources (airtime) in the network would rapidly be consumed by these paging messages, and there would be no airtime left in which to transmit actual phone calls / messages / data packets.

Instead, cellular networks using IS-41 organize groups of geographically adjacent cells into Location Areas (LAs). When a subscriber first powers on his or her UE, the UE scans for a cell belonging to the subscriber's network operator and decodes certain informational messages from that cell, including the cell's Location Area Code (LAC). The UE then performs a procedure to connect to the network which includes, among other things, informing the network of the LAC of its serving cell. Servers in the network store the subscriber UE's LAC for later use. (Search for HLR and VLR if you want more information on these servers / network elements.)

As the subscriber moves out of the area of one cell into another, the UE checks the new cell's LAC against the previous cell's LAC. If the old and new serving cells have the same LAC, then the phone takes no further action. If, however, the LAC of the new cell is different than the LAC of the old cell, the UE will perform the location registration process again to inform the network of its new serving cell's new LAC.

The purpose of this is so that, when the network needs to deliver a call (or other type of message) to a UE, it only has to transmit a paging message addressed to the UE on every cell in the UE's serving LAC instead of every cell in the entire network. This drastically cuts down on the amount of paging traffic and allows for the creation of large cellular networks.

There is a tradeoff regarding the number of cells in an LA. With more cells in an LA, each paging message consumes more radio resources because it has to be transmitted from more cells, but fewer radio resources are consumed by UE location update procedures, because UEs move between LAs less frequently. On the other hand, with fewer cells in an LA, each paging message consumes fewer radio resources because it has to be transmitted from fewer cells, but more radio resources are consumed by UE location update procedures, because UEs move between LAs more frequently.

Mobile network operators monitor paging and location update traffic and optimize the size and "shape" of LAs based on the observed traffic patterns.

Further (very specific) details on the IS-41 location registration and call delivery procedures can be found in the book Design and Analysis of Wireless Networks by Yi Pan and Yang Xiao, a relevant excerpt from which can be found at:

https://books.google.com/books?id=voJf4sF4UDEC&lpg=PA20&ots=8hbN5VIT-H&dq=is-41%20location%20registration&pg=PA21#v=onepage&q=is-41%20location%20registration&f=false

It should be noted that IS-41 is only used in certain cellular networks; another protocol called Mobile Application Part (MAP) is used in GSM and UMTS networks.

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