Managed access systems are scaled-down versions of all cellular carrier technologies in the area. Cellular power is transmitted within the perimeter of the facility, forcing cell phones used in the facility to sync with the system. It permits only calls from authorized cell phones and all 911 calls to go through. It works by using the international Mobile Equipment Identity, cellular phone number, mobile device hardware ID, and electronic serial number. The managed access system is operative for the Metropolitan Transition Center in downtown Baltimore where there are a number of towers for cellular power. The managed access vendor must carefully balance the power levels, so there may be an area in the prison yard where signals get through at times; however, for inmates it is not worth the effort to find a weak spot because of the risk of being detected while searching. The system is constantly being tested by having officers try to make calls from various areas of the facility.