<unk>  is a wireless protocol developed by Motorola, used for two-way paging, messaging, and low-bandwidth data. It is based on the FLEX one-way paging protocol, adding capabilities for multiple forward channels, multiple return channels, and roaming. It originally came in two variants,<unk> 25 and<unk> 50.<unk> 50 was originally developed to support a messaging service launched by MTEL in the mid 1990s, while<unk> 25 was developed several years later to provide an upgrade path for traditional one-way paging carriers. The 50 and 25 signified 50 kHz and 25kHz channel spacing, although in reality both variants supported flexible channel configurations. The two variants were unified into a single protocol with version 2.7, which was released simply as<unk>  2.7. Devices compliant with<unk>  2.7 are backwards compatible with both<unk> 25 and<unk> 50 networks, with several new features to improve roaming, performance, and interoperability between different networks.<unk>  systems support forward channel speeds of 1600, 3200, and 6400 bits per second, and return channel speeds of 800, 1600, 6400, and 9600 bits per second. Like FLEX,<unk>  is synchronous, based on 1.875 second frames and 4-level FSK modulation. The Motorola PageWriter released in 1996 was one of the first devices to use the<unk>  network protocol. Although<unk>  now has limited viability in the commercial market, it is finding new uses in Automatic Meter Reading, public safety, and low cost/bandwidth M2M applications.
ReFLEX
ALECSAT
ELIXIR
SCoRS
DCAP-BTLS
DRACO
PVSRIPO
SimThyr