"Essential services remained in operation in some of these areas. In others, backup generation systems failed. Telephone networks generally remained operational, but the increased demand triggered by the blackout left many circuits overloaded. Water systems in several cities lost pressure, forcing boil-water advisories to be put into effect. Cellular service was interrupted as mobile networks were overloaded with the increase in volume of calls. Major cellular providers continued to operate on standby generator power.[citation needed] Television and radio stations remained on the air, with the help of backup generators, although some stations were knocked off the air for periods ranging from several hours to the length of the entire blackout."