The Inter@ctive Pager 900 was a clamshell-type device that allowed two-way paging. In all systems, clients send messages to pagers, an activity commonly referred to as "paging". AirTouch Paging was re-branded as Verizon Wireless Messaging Services. The debug address registers are effective whether or not paging is enabled. VOS systems support paging partitions and paging files. Paging messages can be used for information transfer. The Cisco IOS kernel does not perform any memory paging or swapping. Paging and mobile (cellular) telephones were adopted early and freely. Various methods of memory protection exist, including memory segmentation and paging. QQ was first released as a "network paging" real-time communications service. Steps were taken to reduce its memory usage through methods such as paging. The three different paging procedures generally adopted are sequential, parallel and selective paging. Marking algorithms is a general class of paging algorithms. These frequencies are often used for on-site hospital paging systems, including voice paging. It is possible to implement segmentation with or without paging.