Monday, July 29, 2013
Old Cell Phones Can Still Call 911
Have you ever wondered how a 911 call works? Even if your cell phone is old and no longer has service, due to a number of factors, you can still dial emergency numbers.
Here in the U.S. there are two primary cellular technology families: 'CDMA': IS-95, cdma2000, EV-DO; 'GSM': GSM, UMTS, HSPA, LTE. These technologies are completely incompatible from an air-interface perspective, as well as the core networks being different. This means your Verizon (CDMA) phone cannot place an emergency call on an AT&T (GSM) network. However, there is the exception that some new phones have chipsets that support both GSM and CDMA technologies. For example, a Verizon Galaxy S3 can communicate with GSM networks, although this was very uncommon not that long ago.
When your phone is in an idle state, it is constantly tracking cells in the area that it might be able to connect to. It can distinguish between cells that are part of its network, and those that are not, and will prefer cells on its home network. If a phone only sees one home network cell, it is likely to track other cells on other networks, so as to best provide service.
When a 911 call is placed, it is not the same as dialing 9-1-1. This number is actually never even dialed; instead, a call is initiated that is flagged by the phone as 'emergency,' which is why you can often dial other countries emergency numbers and have them routed. This call must be given ultimate priority, which means other calls should be dropped to keep that call in progress and power must be raised on the base station. However, even this may not be possible.
In the event the call cannot go through on your home network, your phone will attempt to associate to other networks in the area that it can communicate with. Assuming you have roaming off (or your home network does not have a roaming agreement with this network) these networks will tell your phone "you don't have service here" but they will still allow your phone to attempt to talk to the network for exactly this reason, even though you cannot route a call, it will allow you to place an emergency call, and will even kick the other network's subscribers off if that makes routing your call more expedient.
You can even place 911 calls without a SIM in a GSM phone. This process is interesting because without a SIM you don't have an IMSI, but emergency calls are supported.